CHAPTER 5. RETAINING STRUCTURES

5.1 Introductory notions. Classification

Retaining structures are construction works for which the main load is represented by the earth pressure. The aim of a retaining structure is to support the side of an excavation or a fill or, in the case of cofferdams, to support the pressure of the water.

According to their destination, retaining structures can be grouped in temporary works and permanent works.

Temporary works serve to support the sides of excavation in which a foundation or an underground structure is built or, in the case of cofferdams, to create an area in which work below water level can be carried out in dry.

Based on the construction system, the temporary retaining works can be classified as:

- works with recoverable elements (timbering, sheet piling)- works with non-recoverable elements (diaphragm walls)

Permanent works can be retaining structures above the ground level (retaining walls, quays etc.) or underground structures (basement walls, subway lines and stations etc.).

5.2 Timbering

Timbering implies the support of the sides of excavation by timber boards, the stability being maintained by means of horizontal struts or, in the case of large excavations, by raking struts or shores.

There are three main variants of timbering:- timbering with horizontal boards,- timbering with vertical boards or runners,- timbering with soldier piles and horizontal laggings.

5.2.1 Timbering with horizontal boards

The use of this kind of temporary support is restricted to soil having a cohesion large enough to enable the side of the excavation to stand unsupported for a certain length of time until the elements of the timbering are put in place. In this category enter firm to stiff cohesive soils.

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There are three main elements which form this kind of timbering: horizontal boards, walings, placed vertically, struts.

The fig. 5.1 shows the phases of the construction of a timbering using horizontal boards:

- I: the untimbered excavation is performed, on a depth depending on the cohesion of the soil;

- II: boards, walings and struts corresponding to the first step of excavation are put in place; the struts are tightened by cutting them slightly too long and then driving one end with the other held in position, until they are at right angles to the waling;

- III: a new untimbered excavation is performed, followed by the installation of boards, walings and struts.

1 – board; 2 – waling; 3 – strutFig. 5.1

The cycle is repeated until the final excavation level is reached.In the case of large excavations, in order to prevent the buckling of long struts, vertical piles are used (fig. 5.2).

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1 – vertical pileFig. 5.2

Another solution, which has the advantage of improving the working space, is to use raking struts (fig. 5.3).

1 – raking strutsFig. 5.3

5.2.2 Timbering with vertical boards or runners

This kind of support is used in non-cohesive soils (sands, gravels) and in soft clays and silts. Unlike the case when horizontal boards are used, in these ground conditions the timbering is placed in position ahead of the excavation.

Fig. 5.4 shows the phases of the construction of a support using vertical boards.

- I: On the ground surface a first row of frames is placed, consisting on guide walings and struts. Timber runners are pitched behind the walings. The usual dimensions of these boards are 175 mm by 38 mm or 175 mm by 50 mm in lengths up to 4.8m. After driving the runners at the depth of the first step, wedges are placed between them and the walings.

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- II: The excavation begins in panels of 1…1,5 m, observing strictly the following rule: before proceeding to the excavation of a panel, the runners belonging to the respective panel must be lowered one step further. For that purpose, the wedges which kept the runners tight against the walings are removed and the runners are driven. Then, a second row of frames is installed and wedges between the walings and runners are placed. The procedure is repeated for the next panel and so on, until the second row of frames is unstalled on the entire area.

- III: The same cycle is used for the new step of excavation and then repeated until the final level of excavation is reached.

Using the described procedure, the depth of the excavation is limited by the length of the vertical boards (usually no more than 4…5 m). If the excavation has to be taken deeper than the runners, a second setting of runners is placed within the walings of the top setting and an inner guide waling is also placed. The new runners are then driven down and bracing frames of walings and struts are put in place as the excavation proceeds. If still deeper excavation is required, a third setting of runners can be used. In this way, the excavation at the top setting will be about 1 m wider than at the third setting, leading to an increase in the volume of excavation and in the consumption of timber (fig. 5.5).

1 – frame; 2 – puncheons ; 3 – vertical boards; 4 – wedges;5 – contour of the excavation for a panel of 1…1,5 m

Fig. 9.4108

Fig. 5.5

Another procedure suitable for deep excavation is the one using poling boards, known also as the “marciavante” procedure. Construction phases using “marciavante” method are shown in fig. 5.6.

- I the first set of bracing frames is put in place, on whose contour poling boards of 1.5-2 m in length are driven; between them and the walings of the bracing frames are put wedges;

- II excavation is performed up to a level of 0.3-0.4 m above the tip of the poling boards; the second set of bracing frames is installed; between the poling boards and the second set of bracing frames are placed guiding wedges which will insure the required inclination of the second set of boards;

- III poling boards of the second set are driven; between them and the second set of bracing frames are placed wedges; then, phases II and III are repeated until the final level of excavation is reached.

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1 – poling boards; 2 – horizontal frames; 3 – puncheon ; 4 – guide wedges; 5 – wedges

Fig. 5.6

For large excavations (fig. 5.7) a bracing system made of struts and liners is needed, in order to increase the stiffness of the structure, to reduce the span of the elements working in bending (walings) and the buckling length of struts.

1 – vertical boards; 2 – wedges;3 – horizontal frames with liners; 4 – clamps

Fig. 5.7

For narrow trenches, of relatively small depths, such as those needed for installing pipes, sewers etc., a movable support can be provided consisting generally of vertical sheeting members and struts adjustable either by

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hydraulic rams or screw jacks, allowing sheeting members to be forced tightly against the soil (fig. 5.8).

a – transversal section; b – detail of the horizontal steel frame;1 – vertical board; 2 – horizontal steel frame; 3 – clamping sleeve;

4 – keyFig. 5.8

5.2.3 Timbering with soldier piles and horizontal laggings

This is a method normally applied to deep excavations and which is some times called “Berlinese method” since it was first used at the construction of the Berlin subway. The soldier piles are rolled – steel H – section profiles, driven before excavations from ground level to a level 1-2 m below the bottom of the excavation. As the excavation is taken down, timber boards are inserted horizontally between the flanges of the piles and held against the face by wedges (fig. 9.9). The system is completed with walings and struts. The distance between soldier piles is usually between 2.5 and 4 m.

1 – horizontal board; 2 – waling; 3 – strut; 4 – steel H profileFig. 5.9

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5.3 Sheet piling

Sheet piles are elements made of timber, steel or reinforced concrete, installed in ground by driving or vibrating, used for the construction of walls which, besides the strength and stability requirements, should also fulfill the condition of water tightness.Sheet piles can be used in temporary retaining structures or in permanent structures, as for example the harbour quay shown in fig. 5.10.

1 – sheet pile; 2 – piles; 3 – reinforced concrete slabFig. 5.10

5.3.1 Timber sheet piles

Timber sheet piles are made of fir or oak, representing boards of 5…10 cm thickness or planks up to 25 cm thick and 20…30 cm width.

When good water tightness is not required, contiguous (fig. 5.11a) or overlapped boards (fig. 5.11b,c) can be used. When watertightness is required, various kinds of joints are used such as in half-wood joint (fig. 5.11d) in birdsmouth joint (fig. 9.11e) or tongue and groove joint (fig. 5.11f). The tongue and groove type of joint may be obtained by bolting together three boards (fig. 5.11g).

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Fig. 5.11

Sheet piles are inserted into ground by driving. In order to ease the penetration and insure the closing of joints, timber sheet piles are sharpened at the lower end on the groove side (fig. 5.12).

Fig. 5.12

Sheet piles are driven with the tongue in front and the groove sliding in the tongue of the previously installed sheet piles. Otherwise, the groove could be stucked with soil grains obstructing the penetration of the sheet pile and causing the opening of the joint.

To make better use of the driving capacity of the available equipment, in some situations 2 or 3 timber sheet piles are driven simultaneously, after joining them by means of clamps (fig. 5.13).

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1 – tongue; 2 – grooveFig. 5.13

1 – pile; 2 – sheet pileFig. 5.14

To support the walls of large excavation by using timber sheet piles, at intervals of 4…4.5 m and, in any case, at the corners of the wall, timber piles are first driven, serving as guides for the sheet piles (fig. 5.14). To facilitate the driving of the sheet piles in vertical direction, external guide walings made of two boards each are placed at the ground level and 1 m above the ground level and bolted to the piles (fig. 5.15).

1 – guide piles; 2 – tongs; 3 – set of 2-sheet piles; 4 – wedge; 5 – timber piles; 6 – wedges; 7 – clamps; 8 – spacer

Fig. 5.15

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In order to avoid the opening of the joints between the sheet piles in case obstacles are met in the ground, the driving is not done for the entire depth for each sheet pile but in “ladder”, observing to keep the difference in level of the tips of two consecutive sheet piles (or bunches of 2-3 sheet piles) less than 1…1.5 m.

Timber sheet piles are manufactured of green wood, because the dry wood in contact with water would expand, deforming the wall.

The advantages of the timber sheet piles are: easiness in manufacturing; reduced weight; easy to be installed; low cost. The disadvantages are: limited length (max. 6…8 m, from which 3...4 m the embedment; complete recovering is practically not possible; cannot be driven deeply into dense granular soils or stiff clays without risk of splitting.

5.3.2 Steel sheet piles

Steel sheet piles are used when deep penetration or penetration in hard soils are required. There are many types of rolled steel sections available for sheet piles, which differ among them by the shape and by their interlocking sections. The most common types are:

U type of steel sheet piles, at which joints are placed along the wall axis at each intersection of the axis with the profile (fig. 9.16a);

S type of steel sheet piles, at which joints are also along the wall axis but at every second intersection with the profile (fig. 9.16b);

Z type of steel sheet piles, at which joints are placed outside the wall axis, alternating from one side to the other (fig. 9.16c).

1 – sheet pile; 2 – interlocking sectionFig. 5.16

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At equal mass and identical disposition of the material against the vertical plane, sheet pile walls made of the three types of sections (profiles) have different stiffnesses, as a result of the position of joints. Thus, at the U – type of walls, the principal axis x of each element is parallel to the axis x-x of the wall. Slight relative rotations of the sheet piles under the pressure normal to the x-x axis are possible, hence the real stiffness, considering the interlocking, should be diminished. When stresses in the wall are checked, one should consider in this case , where wx is the resistance modulus against the x-x axis and is the reduced resistance modulus, considering the joints. At the S-type of walls, the principal axes x of each element are parallel among them but inclined in respect to the axis x-x of the wall. The possibilities of rotations are smaller than in the first case and

. At the Z-type of walls, the principal axes x of adjacent sheet piles are normal one to the other, the rotation tendencies of the adjacent sheet piles are cancelling; in this case .

There is a great diversity of interlocking sections (fig. 5.17). The joints or interlocking sections must be strong enough to support the tensile stresses occurring in exploitation, must be watertight and must insure an easy penetration and extraction of the sheet piles.

Steel sheet piles can reach lengths up to 30 m.

Fig. 5.17

5.3.3 Reinforced concrete sheet piles

Reinforced concrete sheet piles are precast elements, of square or rectangular cross-section, with sides of max. 50…60 cm and thicknesses of 10…50 cm. Their length is limited to 18…20 m, because of the large weight which would make the driving very difficult or impossible.

The interlocking system can be similar to the one used at timber sheet piles. In fig. 5.18 a is shown a reinforced concrete sheet pile having a tongue and groove type of joint. In order to reduce the friction during the driving process, the tongue is provided only in the lower third of the sheet pile, in the upper

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part there are two grooves. The gap between grooves is filled with jute and mortar (fig. 9.18b).

Fig. 5.18

9.4 Embedded walls

Embedded walls made of reinforced concrete are intensively used as retaining structures for deep excavations, particularly in urban, congested areas. Unlike sheet piles walls, embedded walls made of reinforced concrete cannot be recovered and reused.

From the constructive point of view, embedded walls can be formed by bored piles or by panels.

5.4.1 Embedded walls made of bored piles

There are two types of pile walls:- secant pile walls- contiguous pile walls

Secant pile walls are of two kinds:- hard/hard secant pile walls- hard/soft secant pile walls.

Hard/hard secant pile walls consist of overlapping concrete bored piles. The secant pile wall is constructed in two stages (fig. 9.19). All piles constructed during stage 1, defined as primary piles, are spaced at specified spacing. All piles constructed during stage 2, defined as secondary piles, are positioned between the primary piles and secant with the primary piles.

Fig. 5.19

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Guide walls are placed at the ground surface to ensure the designed position and the verticality of each pile.

Fig. 5.20

Usually, only the secondary piles are reinforced (fig. 9.20). However, primary piles can also be reinforced with reinforcing cages of rectangular shape or with steel sections (fig. 5.21a,b).

If proper construction is insured, hard/hard secant pile walls offer a high water tightness.

Hard/soft secant pile walls (fig. 5.22) are constructed in a similar way as the hard/hard secant pile walls. The difference is that primary piles are formed of a low strength cement/bentonite mix, where bentonite is an active clay rich in montmorillonite.

This kind of walls are used when the conditions for watertightness are less severe.

Fig. 5.21

Fig. 5.22

Contiguous pile walls

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When the wall is not required to retain water, contiguous bored piles are used (fig. 5.23). The piles are constructed at centres equal to the pile diameter plus an allowance for temporary casing width and tolerance which can vary between 70 and 150 mm. Guide walls may be used at ground level to ensure positional tolerance.

Fig. 5.23

5.4.2 Embedded walls made of panels

Embedded walls made of panels will be named in what follow as diaphragm walls.Panels are rectangular trenches with lengths depending on the excavation equipment and on the position within the wall.

The common feature of all types of diaphragm walls is that the excavation of the trench is made under the protection of a supporting fluid or slurry. This is a clay suspension of 1.03…1.10 g/cm3 density, formed by mixing water with bentonite. Additives are also used to improve the flow characteristics of the fluid and the gelling or blocking action of the fluid. The clay suspension infiltrates through the sides and the bottom of the trench, filling on a certain distance the pores of the soil. Through this layer of soil with low permeability only water can pass, while clay particles are accumulating at the face of the layer forming a shield called cake. The layer of soil enriched in clay together with the cake form a screen. The presence of this screen and of the pressure exerted by the suspension on the sides of the trench ensure the stability of the walls if the required properties of the suspension are met, such as the density, viscosity, shear strength, pH, sand content etc.

The corresponding tests and compliance values are specified in various codes.Based on the criterion of the material met in a vertical section and of the role played by the wall, embedded walls made of panels (diaphragm walls) can be divided into two categories:

- homogeneous walls

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- composite walls

Homogenous walls are diaphragm walls at which both the material used and the role played are unchanged along the same vertical.In function of the material, homogeneous walls are classified in:

- cast in situ concrete diaphragm walls- precast concrete diaphragm walls

Composite walls are diaphragm walls at which the functions of strength and water tightness are separated on the vertical.

Cast in situ concrete diaphragm walls

The basic sequences of the construction of a panel are:I. construction of guide-wallsII. excavation, with a bentonite suspensionIII. placing the reinforcementIV. concretingV. trimming

A distinct phase is represented by “forming the joints”. Its position between the above specified basic sequences depends on the type of installation used for excavating the trench.

I. Guide walls are small, parallel temporary walls, usually made of reinforced concrete (fig. 5.24).

They have multiple roles: to materialize on the ground surface the position of the wall; to provide a guide for the excavating tool; to secure the sides of the trench against collapse in the vicinity of the

fluctuating level of the supporting fluid; to provide temporary support for the reinforcement cages; to provide reaction for the hydraulic jacks when extracting the tubes

used for forming the joints; to serve as rolling tracks for some installations used for the

excavation.

The distance between the parallel guide walls is equal to the width of the excavation tool, plus 5…10 cm. In order to keep this distance, both in front and behind the excavation tool, struts are placed.

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Fig. 5.24

II. Excavation of panels- Excavation with the “Kelly-type” installations. The characteristic

feature of these installations, which are attached to an existing equipment (such as a crane or excavator) is the presence of a heavy rod named “kelly” having at its end a hydraulically operated grab. In fig. 5.25 is shown a “kelly-type” installation, equipped with a hydraulic grab made in Romania. The grab has two jaws which are opened and closed by means of hydraulic jacks. The lifting and the lowering of the kelly is done by cables. For the excavation, the grab with open jaws is brought above the guide-walls and is inserted slowly in the trench to avoid waves of the slurry which could affect the walls. Under its own weight and the weight of the kelly, the grab is let to fall from a height of 1…3 m above the bottom and penetrates in the soil. Loaded with soil, the grab is then lifted above the guide-wall and, after a rotation of the excavator (or crane), is discharged in a truck. During the excavation, the level of the supporting fluid is kept permanently 0.5…1.0 m below the ground level, but at least 1.0 m above the groundwater table. Samples are taken periodically from the slurry and tested in the laboratory at the construction site to determine the properties of the suspension, which are checked against the required ones, to ensure that the suspension does not become excessively diluted or contaminated by soil particles.

With kelly-type installations, trenches of 0.60…0.80….1.0 m width can be excavated, at depths up to 35 m. The minimum length of a panel is equal to the maximum opening of the jaws and varies between 2.20 and 2.80 m. The maximum length of a panel should not exceed 7 m.

In fig. 5.26 is shown the excavation of a 7.0 m length panel, using the grab E.S.G.H. made in Romania. In the first two phases, two shafts are excavated at both extremities of the panel, with a length of 2.80 m each. Between the shafts remains a core of soil which is excavated in the last phase. The reason

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for doing so is the following: during the excavation, the stability of the installation, whose center of gravity is high due to the very long kelly, is ensured by the reaction opposed by the soil encountered by the jaw of the grab; if this reaction is not uniform, there is a risk for the installation to rotate and overturn; if the shafts would have been excavated in sequence, one of the jaw would rest on the ground while the other would enter in the shaft previously excavated and filled with slurry, situation which should be avoided.

1 – grab; 2 – Kelly; 3 – guide; 4 – telescopic arm; 5 – rollera - equipment ESGH 20-30; b - grab

Fig. 5.25

Fig. 5.26

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1 – cup; 2 – mast; 3 – winch; 4 – transporting binFig. 5.27

- Excavation with the installations E.L.S.E. The work of this installation, of Italian origin, is based upon the principle of the excavator with straight cup; the cup makes both a translation movement in vertical direction by descending along a heavy mast and a translation movement around a point of support (fig. 5.27). The installation uses the guide-wall as rolling tracks. The moving mast is placed at the end of the panel to be excavated, the excavation being performed by the retirement of the installation. The cup in vertical position bolted in the mast, is lowered together with the mast, with teeth downward, and penetrates in the soil. Then, the cup is brought in horizontal position and lifted along the mast above the ground level, where is discharged. The cycle is repeated until the excavation level specified in the design is reached. The length of the excavation in one stage, dictated by the amplitude of the cup’s movement, is 3.80 m. For the next stage, the installation is retired further, in order to ensure the excavations of the rest of the panel. The length of the panel is usually taken 5…7 m, which means a two-stage excavation. The width of the excavation made with E.L.S.E. installation is of 0.6..0.8…1.0 m and the depth up to 30 m. Soils suitable to be excavated with E.L.S.E. are non-cohesive or weak cohesive soils. In soils with high plasticity and cohesion (clays), both the excavation and the discharge of the soil are difficult, due to the large adhesion between the soil and the cup. Also, E.L.S.E. cannot excavate in hard ground, such as a sandstones, limestones, etc.

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- Excavation with the installation C.I.S. Soletanche. This installation, of French origin, is using the reverse-circulation of the suspension. The slurry, mixed with the excavated material (detritus) is absorbed through the pipe of the installation; after the coarse particles are removed, the slurry is sent back in the trench. Usually, the excavating tool placed at the end of the pipe is performing a rotary drilling. However, when the material to be excavated is hard, a percussion type of drilling can be used, and the excavation tool is adopted in consequence. The excavation of a panel by using C.I.S. Soletanche installation begins with the excavation at one of the extremities of the panel of a shaft with a diameter equal to the width of the panel. Then, moving on the guide-walls, used as rolling tracks, to the other extremity, the installation excavates a second shaft. The excavation of the core between the two shafts is made in layers of 30…50 cm in thickness, by the displacement back and forth of the installation (fig. 5.28). With the installation C.I.S. Soletanche, trenches of 0.40…1.0 m width and up to 50 m in depth can be excavated in any kind of ground, including in hard or very hard materials. Due to this advantage, the installation C.I.S. Soletanche can be used in combination with the installations Kelly and E.L.S.E. for the socketing of the wall into the bedrock.

1 – engine; 2 – winch; 3 – slurry pump; 4 – stem for boring and absorption; 5 – excavating tool; 6 – slurry

Fig. 5.28

III. Placing the reinforcement

Panels are reinforced with reinforcement cages which include vertical and horizontal bars, forming two parallel nets linked with stirrups and inclined bars (fig. 5.29). Each cage is provided also with suspension and lifting bars,

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bracing bars to improve the stiffness for the handling operations and spacers, usually made of rollers, to ensure that the correct concrete cover is maintained. On the vertical, the cage can be made of one piece or of several pieces assembled by welding as the cage is lowered in the trench. When two cages are installed in horizontal direction, the minimum distance between them shall be 200 mm.

1 – longitudinal bars; 2 – horizontal bars; 3 – bracing bars; 4 – lifting bar; 5 – spacers

Fig. 5.29

The vertical length of a reinforcement cage shall be such that the distance between its base and the bottom of excavation is at least 0.2 m. The cage shall not rest on the bottom, but shall be suspended from the guide-walls by means of the suspension bars.

IV. Concreting

Concrete is placed beneath the supporting fluid through one or more concreting pipes or tremie pipes which are pipes equipped with a hopper at the top but may also be pipes connected directly to concrete pumps. The inner diameter of the concreting pipe shall be at least 0.15 m and 6 times the maximum aggregate size. Its outer diameter shall be such that it passes freely through the reinforcement cage. For panels with length less than 5 m, one concreting pipe can be used.

A concrete of low consistency is used. The consistency of the fresh concrete just before concreting shall correspond to a slump value between 180 mm and 210 mm. Retarding admixtures are used to prolong the workability as required for the duration of the concreting process.

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When starting concerting, the supporting fluid and the concrete in the concreting pipe shall be kept separate by a plug of material of by other suitable means. To start concreting, the concreting pipe shall be lowered to the bottom of the trench and then raised approximately 1 m. After concreting has started, the concreting pipe shall always remain immersed in the fresh concrete. The minimum immersion should be 2 m.

Since the top of the cast concrete is contaminated because of the contact with the slurry and may not be of the required quality, sufficient concrete shall be placed in the panel to ensure that the concrete below the cut-off level pass the specified properties. This is achieved by providing an additional height of concrete above the cut-off level.

Fig. 5.30 shows the operations of placing of the reinforcement cages in the trench and of the concreting of a panel.

1 – reinforcement cage; 2 – slurry; 3 – joint tube; 4 – previously concreted panel; 5 – concrete poured in the panel using the tremie pipe

Fig. 5.30

V. Trimming

Trimming of the concrete to cut-off level (removing the concrete of poor quality, in excess) shall be carried out using equipment which will not damage the concrete or reinforcement. Final trimming to cut-off level shall only be carried out after the concrete has gained sufficient strength to avoid damage.

Forming the joints

The method used to form the joints depends on the type of equipment used for excavating the trench.

In the case of Kelly-type and C.I.S. Soletanche equipments, the joints are normally formed by using steel stop ends or joint tubes.

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The steel stop ends are steel tubes with a diameter equal to the width of the trench which are introduced in vertical position at the ends of a panel, penetrating 0.5…1.0 m below the bottom of the trench in order to have ensured the stability. The tubes are lowered into the trench with cranes and extracted by use of hydraulic jacks. In the case of stop ends which are extracted vertically, it is essential to define the optimum time for starting this operation. If the tube is extracted too soon, the fresh concrete behind the tube will flow in the space left by the tube. If, on the contrary, the extraction is done too late, the tube can be sticked to the concrete and the recovering becomes impossible. In order to avoid sticking, small rotations should be applied to the tube before commencing the extraction and then the extraction should be made gradually during the setting of the concrete. Usually, the extraction starts 4…6 hours after the concreting is finished. A method to prevent the contact between the concrete and the tube is to provide at the extremity of the reinforcement cage a shield made of thin steel plate, on the entire depth of the trench. In the case of tubes which are extracted laterally, the extraction shall be made upon the completion of the excavation of the adjacent panel.

Instead of recoverable steel tubes, non-recoverable precast elements can be used as stop ends (fig. 5.31).

Fig. 5.31 1 – previously concreted panel;2 – panel under excavation

Fig. 5.32

In the case of using E.L.S.E. installations for the excavation of the trench, the joints are formed by cutting into the concrete of the previously cast adjacent panel. Cutting is done by the teeth of the excavating tool (fig. 9.32).In special cases, water stops can be incorporated into the joints.

Phases of the construction of a cast in situ diaphragm wall made of panels

The wall is made by a number of panels. The panels’ disposition, their dimensions in plane, the construction sequence, are established in the design, taking into account the peculiarities of the job, the excavating installations etc.

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When using E.L.S.E. installations, the wall is formed of consecutive panels, for each panel the construction cycle (excavation, lowering of the reinforcement cage, concreting) being complete.

When using Kelly-type or C.I.S. Soletanche installations, there are two variants for the construction of the walls:a. wall made of primary and secondary panels (fig. 5.33)

I – excavating of the primary panels and placing at their extremities the joint tubes;

II – lowering the reinforcement cage in the primary panels;III – concreting the primary panels;IV – extraction of the joint tubes;V – excavation of the secondary panels; the excavating tool is adapted in

order to properly clean the semi-circular joints between the secondary and primary panels;

VI – lowering the reinforcement cage in the secondary panels;VII – concreting the secondary panels.

1 – excavated primary panel; 2 – joint tube; 3 – reinforcement cage; 4 – concreted primary panel; 5 – excavated secondary panel;

6 – concreted secondary panelFig. 5.33

b. wall made of a starter and intermediate panels (fig. 5.34)I – excavation of the starter panel; placing at its extremities the joint

tubes;II – lowering the reinforcement cage in the starter panel;III – concreting the starter panel;

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IV – excavation of the intermediate panel; placing at its extremity of a joint tube;

V – lowering the reinforcement cage in the intermediate panel;VI – concreting the intermediate panel.

Phases IV…VI are repeated for each intermediate panel until the whole length of the wall is reached.

1 – excavated primary panel; 2 – joint tube; 3 – reinforcement cage; 4 – concreted primary panel; 5 – excavated intermediate panel

Fig. 5.34

Diaphragm walls made of precast concrete

These walls are made of precast elements lowered into a trench containing a self-hardening slurry.

Fig. 5.35 shows a wall made of precast panels which imitate the timber sheet piles with tongue and groove. Fig. 5.36 shows a wall which imitate a Berlin-type wall, with the difference that the horizontal lagging boards are replaced by continuous vertical precast sheets. Fig. 5.37 shows a solution used at a wall inside an industrial hall in Bucharest, designed by the Center for Geotechnical Engineering of the Technical University of Civil Engineering Bucharest. The length of the panel had to be reduced to a minimum, representing the opening of the jaws of the grab (2.20 m). Three double T precast elements were used, provided with steel profiles H and U attached at the web of the precast elements, serving as guides and to improve the watertightness.

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1 – panel; 2 – guide wall; 3 – trench filled with slurryFig. 5.35

1 – joint panel; 2 – field panel; 3 – guide wall; 4 – trench filled with slurry; 5 – self-hardening slurry

Fig. 5.36

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Fig. 5.37

The phases of the construction of a diaphragm wall made of precast concrete are:

- I - excavation of the trench; this is usually done under a bentonite suspension but, with a carefully prepared slurry, can be done also directly under a self-hardening slurry;

- I bis - when excavation is made under bentonite suspension, after completing the excavation the bentonite suspension is replaced by the self-hardening slurry using the tremie pipes, as in the case of concreting;

- II - lowering into the self-hardening slurry of the precast elements.

When the excavation under the protection of the wall is performed, the hardened slurry on the exposed face of the wall is trimmed.

The characteristic feature of the diaphragm walls made of precast concrete is the use of self-hardening slurries.

A self-hardening slurry is a bentonite suspension in which a certain amount of cement and additives are added. The self-hardening slurry is setting and then hardening, like a plastic mortar, in the excavated trench, producing a firm binding between the precast element and the surrounding soil and closing the joints between the elements.

The receipt of the self-hardening slurry is established by laboratory tests. Requirements are different in the case when the slurry is used also as a supporting fluid in the excavation phase, as compared to the slurry which is replacing a bentonite suspension.

The retarder additive should ensure the starting of the setting after the lowering of the precast elements into the slurry. After that, the hardening process should be quick enough, in order to ensure a good binding between the precast elements and the soil.

Composite walls

The self-hardening slurry is widely used in the case of composite walls. Indeed, in the lower part of a wall for which only the water tightness is

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required, the reinforced concrete (cast in situ or precast) can be replaced by the self-hardening slurry.

1 – grab; 2 – hose for sending the self-hardening slurry; 3 – pump for removing

the bentonite suspension; 4 – plastic mortar; 5 – concrete; 6 – tremie pipe; 7 – reinforcement cage

Fig. 5.38

Fig. 5.38 shows the phases of the construction of a panel in a composite wall with the bottom in an impervious layer:

a. excavation under bentonite suspension to the final level; b. b replacing the bentonite suspension in the lower part of the trench

with the self-hardening slurry with higher density (1.20…1.25 g/cm3);c. excavating the self-hardening slurry, after hardening, on a depth of

1.0 m; d. lowering the reinforcement cage in the upper zone of the panel and

concreting with a tremie pipe of the upper zone of the panel.

The use of embedded walls as retaining structures

There are two main methods in the construction of underground structures with the use of embedded walls:

- the open excavation or the cut and cover method- the top-down or Milanese method

Embedded walls for underground structures constructed in open excavation

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Fig. 5.39

Fig. 5.39 shows the main stages for the construction of an underground structure (for instance a subway gallery):

- construction of the embedded walls;- first excavation phase of excavation;- placement of the first row of struts;- second phase of excavation;- placement of the second row of struts;- third phase of excavation;- placement of the third row of struts;- concreting the base of the gallery;- after the hardening of the concrete in the base of the gallery,

dismantling the lower row of struts;- concreting the walls and the slab of the gallery;- after the hardening of the concrete in the gallery, dismantling the

intermediate row of struts;- gradual filling of the space above the gallery, including the

dismantling of the upper row of struts.

Embedded walls for underground structures constructed with the top - down method

This method is sometimes called “Milanese method”, since it was first applied at the construction of the Milano subway. It consists in construction from the beginning, near the ground surface, a reinforced concrete slab connected to the embedded walls, in which holes are left for the access of people and equipments and evacuation of the excavated soil. The excavation takes place under this roof. The advantage of the method is the possibility to resume activities at the ground surface (car traffic etc.) before completing the

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underground structure. When the span between the embedded walls is large, intermediate supports for the slab can be built, represented by steel columns founded on barrettes, which are cast in situ reinforced concrete blocks, constructed with the same technique as the cast in situ diaphragm walls.

1 – embedded wall; 2 – guide wall; 3 – short trenche excavated under slurry;4 – steel column; 5 – reinforcement cage; 6 – foundation of the

steel column (barrette); 7 – ballast; 8 – reinforced concrete slab; 9 – mat;10 – wall; 11 – intermediate slab; 12 – column; 13 – fill;

14 - pavementFig. 5.40

Fig. 5.40 shows the main stages of the construction of a subway station constructed with the top-down method:I – construction of the embedded walls;II – construction of interior, shorter trenches, under bentonite suspension;III – lowering of the steel columns connected at their lower part with the reinforcement cage of the barette;IV – concreting with the tremie pipe of the barrette and filling with gravel the rest of the short trench; V – constructing the slab near the ground level;VI – excavating under the slab;VII – construction of the underground structure;VIII – placing the fill above the upper slab of the underground structure.

5.5 Design elements for braced excavations and for embedded walls

Supporting the sides of deep, narrow excavations made by timbering or sheet piling with struts acting across the excavation is called bracing. A basic

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problem for the design of a braced excavation is to establish the appropriate diagram of earth pressure.

5.5.1 Earth pressure diagrams

In the chapter 8 it was shown that for the development of the active earth pressure behind a wall is necessary for the wall to move away from the soil. This is not the case of a braced excavation. When the first row of struts is installed, the depth of excavation is small and no significant yielding of the soil mass will have taken place. As the depth of the excavation increases, yielding of the soil before strut installation becomes significant but the first row of struts prevents yielding near the surface. Deformation of the wall will be of the form shown in fig. 5.41, being negligible at the top and increasing with depth. The deformation condition of the Rankine theory is not satisfied and the theory cannot be used for this type of wall. Failure of the soil will take place along a curved surface as shown in fig. 5.41. Only the lower part of the soil wedge within this surface reaches a state of plastic equilibrium, the upper part remaining in a state of elastic equilibrium. Based on numerous measurements performed on bracing systems with multiple supports, the following simplified conventional earth pressure diagrams for various types of soils are recommended in practice:

for non-cohesive soils in fig. 5.42 a for cohesive soils of low consistency in fig. 5.42 b for cohesive soils of high consistency in fig. 5.42 c

Fig. 5.41

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a b cFig. 5.42

5.5.2 Design of a timbering

The structural design of a timbering involves the following phases:- predimensioning the timbering by proposing dimensions for the

elements;- selecting the appropriate earth pressure diagram;- establishing bending moments, shear forces, axial loads in the

elements of the timbering;- checking the sections proposed for the timbering elements (boards,

walings, struts). Some assumptions are usually taken in the design of a timbering:- a conventional earth pressure diagram, such as previously given, is

adopted;- the continuity of the boards and walings over the supports is

disregarded; boards and walings are treated as simply supported elements.

In what follows is given, as an example, the analysis of a timbering with vertical boards in a non-cohesive soil (fig. 5.43).

Fig. 5.43

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The analysis of the vertical boards is done as for simply supported beams of span l1, the distance between two consecutive walings or the vertical distance between struts. For a width b of the board, the load per unit length on the board is:

(5.1)

The bending moment in the board is:

(5.2)

To check for the board section:

(5.3)where is the allowable strength for the material in the board and d is the thickness of the board, which will result from the relation (5.3).

The analysis of the walings is done as for simply supported beams of span l2, the horizontal distance between struts. The waling takes the reaction from boards pertinent to a field l1. The load per unit length of the waling is:

(5.4)

The maximum bending moment for the waling is:

(5.5)

If e is the known width of the waling and f is the thickness to be determined:

(5.6)

From the relation (5.6) is determined f.The analysis of struts is done as for elements subjected to compression, with due consideration for the buckling.

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For the most loaded struts of the timbering in the fig. 5.43, the compression load is:

(5.7)

To check the struts in compression the relation (5.8) is used:

(5.8)

where A is the section of the strut;w is the buckling factor; is the allowable strength of the timber in compression parallel to

the timber fibres.

To check the struts in crushing normal to the fibres at the strut-waling contact:

(5.9)

where is the allowable strength in compression normal to the fibres of the timber

Usually, timbering are using fir or pine, for which the following values of the allowable strengths can be used:

- for bending = 120 daN/cm2

- for compression along the fibres = 120 daN/cm2

- for compression normal to the fibres = 18 daN/cm2

If instead timber, steel elements are used, in the design relations previously given, appropriate values for and w should be introduced.

5.5.3 Analysis of sheet pile walls and embedded walls

Sheet pile walls and embedded walls can be grouped, based on the criterion of the statical system, in two categories:

- walls forming statically determined systems (cantilever walls; anchored or propped walls with one level of anchor or prop in the upper part and free earth support at the bottom);

- walls forming statically undetermined systems (walls with one level of anchor or prop in the upper part and fixed earth support at the bottom; walls with two or more levels of anchor or prop in the upper part and free or fixed earth support at the bottom).

The analysis of the sheet pile walls and embedded walls has two objectives:

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- to determine the depth of penetration of the wall in the soil, taking into consideration various failure modes;

- to determine the structural design of the wall to resist bending moments, shear forces and prop or anchor forces derived from equilibrium calculations.

In fig. 5.44 are shown various kinds of failure for these walls.

In what follows, several types of walls pertaining to the two categories will be considered.

Fig. 5.44

a. Cantilever walls

These walls are free at the upper part and derive their equilibrium from the lower, embedded part of wall. Two situations can occur:

- wall acted upon by a horizontal force H;- wall retaining soil of a height h.

In the first case, the load can be, for instance, the resultant of the pressure exerted by water on the free upper part of the wall (fig. 9.45 a). Subjected to the force H, piles bends and rotates. If the deformations by bending are disregarded, the wall can be treated as an infinitely stiff plate rotating around a point O (fig. 9.45 b). On the front face of the wall, above the point O, the wall induces a compression on the soil and conditions for developing a passive resistance are present, while below the point O on the same face the

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wall moves away from the soil which is relaxing and the active earth pressure develop. On the face behind the wall, the situation is opposite.

By neglecting the friction between wall and soil, the pressures diagrams (fig. 5.45 c) have, according to Rankine, the slopes and where:

Fig. 5.45

Fig. 5.46

By computing along the embedment t the difference between the passive and active earth pressure, a resultant diagram is obtained (fig. 5.46 a), with ordinates limited on both faces by two lines of inclination.The diagram thus obtained is physically not possible, since there are two pressures in the same point O. In reality, the transition from the left to the right part of the pressure diagram cannot be done by a jump like in the fig. 9.45.a, but gradually, along a curve passing through the point O (fig. 5.46 b) and which can be approximated by a line (fig. 5.46 c).

The stability of the wall is insured by the couple of forces Ep and , representing the resultants of passive pressures developed on the two sides

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of the wall is in equilibrium with the overturning moment produced by the force H.

In order to find the embedment t required for insuring the stability, two methods can be used:

- in the first method, the final diagram in fig. 5.45 c is used, involving three unknowns: t, d and e, for which only two equilibrium equations are available, the horizontal forces equation and the moment equation. A trial and error approach is then used.

- an embedment depth t is proposed;- the two equilibrium equations are written:

(5.10 a) (5.10 b)

Equation (5.10 a) represents the horizontal projections of horizontal forces, while equation (5.10 b) expresses that the moment about point C is zero.

- the system of equations is solved, to find the unknowns d and e;- the computation is repeated for a new value of t, until the following

condition is fulfilled:

(5.11)

where FS is a factor of safety which can be taken 1,5…2.

— In the second method, the diagram in fig. (9.46 c) is replaced by the simplified diagram in fig. (9.47 a), where the passive resistance on the posterior face of the wall was replaced by the unknown force Ep. There are two unknowns, t and Ep, which can be found with the equilibrium equations. In fact, only t is of interest, which is obtained by taking the moment about the point D.

MD = 0

(5.12)

Equation (5.12) leads to the following equation:

(5.13)

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The solution of eq (9.13) is obtained by trials in order to obtain to. For the first trial, values of the ratio to/h in function of given in the table 9.1 can be used.

Table 5.120 25 30 35 40

to/h 1.6 1.2 0.9 0.7 0.5

The embedment depth should be larger than to:

t = (1.20….1.25)to (5.13)

In order to find the maximum bending moment, the pressure diagram used for the computation of t is taken (fig. 5.45 c or fig. 5.46 a) and the depth zo at which the shear force is zero, corresponding to Mmax, is found. For instance, when the diagram in fig. (5.47 a) is applied:

(5.14)

(5.15)

(5.16)

Fig. 5.47

In the case of the cantilever wall retaining a soil of height h, the approach is similar as in the previous case, with the difference that the horizontal action is produced by the active earth pressure (fig. 5.48). The corresponding diagrams are given in fig. 5.49 a, fig. 5.49 b and fig. 5.49 c. The bending moment diagram is shown in fig. 5.50.

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Fig. 5.48

Fig. 5.49

Fig. 5.50

b. Walls with one level of anchor or prop in the upper part and free earth support at the bottom

When the cantilever wall cannot take the loads or when the ground conditions do not allow to obtain the embedment required by the fixed-end condition, an additional support is provided in the upper part of the wall by means of an anchor (fig. 5.51 a) or prop (fig. 5.51 b). The second support is represented by the embedment depth t, as a free earth support.

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Fig. 5.51

Fig. 5.52

It is assumed that by the elastic displacement of the prop or anchor and by the rotation of the wall around the base, conditions for development of passive earth pressure in front of the wall are met. The two pressure diagrams can be drawn (fig. 5.52). The problem is statically determined, there are two unknowns (the embedment depth and the load in the strut or anchor RA) and two available equilibrium equations ( . A factor of safety FS of 1.5…2 can be introduced by dividing the passive pressure force to FS.

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Fig. 5.53Field measurements have shown that the real active earth pressure diagram is different from the diagram with linear distribution resulting from the Rankine’s theory. As a result of the bending of the wall and of an arching effect, a redistribution of the pressures is occurring, leading to a overloading of the supports and to a discharge of the field (fig. 5.53). To take into account this effect, a coefficient of reduction of the maximum bending moment corresponding to the triangular diagram is introduced. The German Code EAU 77 recommends to take 0.67 Mmax.

c. Walls with one level of support in the upper part and fixed earth support at the bottom

Unlike the previous case, the point of rotation is located above the base, the wall changes in curvature within the embedment depth, leading to the development of passive resistance on both sides of the penetration depth (fig. 5.54). The pressure diagram is constructed as in the case of the cantilever wall (fig. 5.43 c). The theoretical diagram can be replaced by the design diagram in fig. 9.55 and the embedment depth t by the reduced depth to.

Fig. 5.54

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Fig. 5.55

An approximate method which can be used in this case is named “the method of replacing beam”. It is considered known the depth y of the inflexion point of the deformed shape of the wall. Thus, for , y = 0.25 h and for , y = 0.08 h, where h is the height above the excavation level. Taking the inflexion point as a hinge, the wall is divided into two simply supported beams: BC and CD. The equilibrium equation for the upper beam BC leads to the reaction at the level of the strut or anchor RA and the reaction in the support C, RC. To obtain the value of to, the condition of moment O in the point D is written for the lower beam CD. Then, t is taken as: (1.20…1.25)to.

5.5.4 Solutions for the support in the upper part of embedded walls

From the point of view of the support in the upper part, embedded walls can be classified in two categories:

propped walls, where the support is provided by struts (fig. 5.51 b) anchored walls, where the support is provided by several means,

such as:- tendons or tie rods transferring the load to a steel or reinforced

concrete plate or to a concrete block called deadman (fig. 5.51 a); - tendons or ties transferring the load to a group of raking piles, from

which one pile works in tension T, the other in compression C (fig. 5.56 a);

- anchor piles (fig. 5.56 b);- ground anchors (fig. 5.56 c).

The deadman may be formed of isolated elements (fig. 9.57a) or by a continuous plate (fig. 9.57b).

a b c

Fig. 5.56

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Fig. 5.57

The stability of the deadman is insured by the passive resistance of the soil in front of the plate or block (fig. 9.58). The stability condition is:

(5.17)

where Pa and Pp are the active and passive forces and FS a factor of safety which can be taken 1.5.

(5.18)

(5.19)

where l is the distance between tendons;d – height of the deadman;e – distance from the upper edge of the deadman to the ground level.

Relations (5.18) and (5.19) are used in the case of continuous plates and in the case of isolated plates of width b, when , and for .

As for the position of the deadman, the following procedure is used (fig. 9.58): from the bottom of the wall (in the case of free earth support) or from the point of inflexion at the depth y (in the case of fixed earth support) the failure plane

inclined with the angle ( in respect to the horizontal. Between this

plane and the wall the prism I is formed, where is strictly forbidden to place the deadman. At the same time, the passive zone in front of the deadman should not interfere with the active soil zone behind the retaining wall.

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Therefore, from the point C a failure plane with inclination of ( in

respect to the horizontal is drawn, defining the prism II.

The deadman should be located above this plane.

Fig. 5.58

5.6 Ground anchors

A ground anchor is an installation capable of transmitting an applied tensile load to a load bearing stratum.

Ground anchors can be permanent, when required to ensure the stability and satisfactory service performance of the permanent structure or excavation being supported or temporary, when used during the construction phase of a project to whitstand forces for a known short period of time, usually less than 2 years.

Fig. 5.59 shows several examples of permanent structures using ground anchors: a retaining wall (fig. 5.59 a), a high rise building with a multiple-level basement, founded below the water table and subjected to uplift forces of the water (fig. 5.59 b), a tower for an electrical line (fig. 5.59 c).

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Fig. 5.59

Examples of temporary ground anchors are given in the fig. 5.60.

Fig. 5.60

A ground anchor consists basically of an anchor head, free anchor length and fixed anchor.Fig. 5.61 shows a typical ground anchor.

Fig. 5.61

The main phases in the construction of a ground anchor are:

1 – boring of a boreholeSpecial equipments are used, able to bore holes at any inclination; the kind of boring (cased or uncased; under a slurry etc.) as well as the boring tool are adapted in function of the ground conditions.

2 – introduction in the borehole of a tendon

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Tendons usually consist of steel bar, strand or wire, either singly or in groups.

3 – grouting

The most common grouts used for ground anchors are cemetious grouts which usually are water/cement mixes, with a ratio lying in the range of 0.35 to 0.60. Admixtures to improve the properties of the grout, such as the workability or durability, or to increase the rate of strength development are sometimes used.

The main functions of the groutings are:- to form the fixed anchor length (Lfixed), which is the designed length of

the anchorage over which the tensile load is capable of being transmitted to the surrounding ground;

- to reinforce the ground in the close vicinity of the fixed length, in order to increase the capacity of the anchor;

- to increase the watertightness of the ground in the close vicinity of the fixed length in order to reduce the losses of the grout.

Grouting is usually made through a grouting tube; the separation between the fixed anchor length and the free anchor length is made by using a device called packer, which is an expandable rubber ring surrounding the grouting tube.

4 – stressing

Stressing is required to fulfill the following two functions:- to ascertain and record the load carrying behavior of the anchor;- to tension the tendon and to anchor it at its lock-off load.

Stressing equipment is similar to the one used for prestressed concrete elements. Stressing should not be carried out until a sufficient hardening of the grout in the fixed length has been achieved, which normally requires seven days.

The component of the ground anchor which transmits the tensile load from tendon to bearing plate or structure is called anchor head.A special problem is the corrosion protection.

All steel components which are stressed (tendon bond length, tendon free length, anchor head) shall be protected against corrosion for their design life (less than two years for temporary ground anchors, more than two years for permanent ground anchors).

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The load capacity of ground anchors depends primarily on the ground in which the fixed anchor length is located. Common values are of 1000….2000 kN in sands and gravels and 200…300 kN in cohesive soils. In rocks, the load capacity can reach much larger values, of 5000….10.000 kN.

The load capacity of the ground anchors can be estimated by computation, but it is compulsory to be then checked by field tests performed in advance of the working anchors. Stressing each ground anchor represents by itself a test.

Due to the special character of the work, the construction of the ground anchors shall be assigned only to contractors with proven experience in this field.

5.7 Retaining walls

Retaining walls are permanent retaining structures used along the roads or railroads in hilly or mountain areas, along navigation canals, behind buildings on a slope etc. Their role is to support the soil placed behind them, thus enabling a transition on a very short distance between two levels, when it is not possible to insert a slope between the two levels.

In the past, natural stones were used in the construction of retaining walls. At present, concrete and reinforced concrete are materials most used for these retaining structures.

There is a great variety of retaining walls. In the following, some of the most used types of retaining walls will be presented.

5.7.1 Gravity walls

5.7.1.1 Mass concrete retaining walls

Mass concrete walls are suitable for small retained heights, usually up to 3…4 m. Fig. 5.62 shows a section through a gravity retaining wall made of concrete and the forces which are acting:

- active earth thrust Pa on the back of the wall;- passive earth force Pp on the face of the wall, below the ground level;- weight G of the wall;- reaction R on the base.

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Fig. 5.62

As a rule, the passive force Pp, whose development requires, as shown in the chapter 8, large displacements, is disregarded.

In the fig. 5.62 are also given recommendations for a preliminary selection of the dimensions of the wall.

In fig. 5.63 are shown other forms of mass concrete walls.

Fig. 5.63

A filter of coarse permeable material is desirable behind a retaining wall to prevent the development of high pore water pressures within the backfill. To allow the water to percolating into the filter to drain out, weep-holes are provided in the wall (fig. 5.64).

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Fig. 5.64

The design of the wall has three aspects:- structural design

Normally mass concrete walls should be designed on a no-tension basis under the design earth pressure. At least two sections should be checked (fig. 9.62):

at the middle of the elevation AB at the joint BC between the elevation and the foundation.

- foundation design

The pressure on the soil under the base of the wall should be checked to ensure that it does not exceed the allowable bearing pressure pall.

(5.20)

where N is the total normal force on the base and e =

N = Pav + G (5.21)

Pav being the vertical component of the active earth thrust.

Three conditions shall be fulfilled:

(5.22)

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(5.23)

(5.24)

Relation (9.24) expresses the condition that the resultant of forces Pa and W

is located within the middle third of the base ( ).

- stability checks

Stability against sliding

The friction force S on the base (the component along the base of the reaction R) is compared with the horizontal component Pah of the active earth thrust. The friction force S is equal to the normal force N multiplied by the friction coefficient between the base and the ground.

(5.25)

where ml is a factor of safety equal to 0.8.

When values for obtained by field tests are lacking, values given in tab. 5.2 can be used, at least for a preliminary design.

Tab. 5.2 – Values of the friction coefficient μGround type μ

Clays:- Ic< 0.75- Ic≥ 0.75

0.250.30

Sandy clays and clayey sands

0.30

Fine, silty sands 0.40Coarse sand, gravels 0.50Rocks 0.60

When the base resistance to sliding is inadequate, this can be increased by either widening the base, inclining the foundation (fig. 5.65 a) or providing a shear key (fig. 5.65 b).

Stability against overturning

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The following condition shall be fulfilled:

(5.26)

where Mr is the overturning moment

Mr = Pa a (5.27)

a being the arm of the force Pa in respect to the toe of the wall D

MS is the stability moment

MS = G d (5.28)

d being the arm of the force G in respect to the point Dmr is a factor of safety equal to 0.8.

Fig. 5.65

5.7.1.2 Gabions

Gabions are large cages or baskets usually of steel wire or square welded mesh, rectangular in shape, filled with stone and used as gravity retaining walls or anti-erosion works (fig. 5.66).

The permeability and flexibility of gabions make them suitable where the retained material is likely to be saturated and the bearing quality of the soil is poor.

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Fig. 5.66

Gabion walls are designed on the same principle as a gravity mass wall. In fig. 5.67 are given examples of gabion retaining walls. The density of the stone fill can be taken as 60% of the solid material.

Fig. 5.67

5.7.1.3 Crib walls

Crib walls are built of individual units assembled to create a series of box-like structures containing granular free draining fill, to form a gravity retaining wall. The units should be so spaced, that the fill material contained within the crib is not affected by climatic changes and acts in conjunction with the crib work to support the retained earth.

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The individual units can be made of timber or of precast concrete.

In fig. 5.68 is shown an example of assembling the individual units. In fig. 5.69 is given a vertical section on a crib wall.

Fig. 5.68

Fig. 5.69

This kind of gravity walls is indicated for use on compressible soils having the ability to adapt to differential settlements.

5.7.2 Reinforced concrete walls

5.7.2.1 Cantilever walls

Cantilever or T walls are made of a vertical or inclined slab monolithic with a slab base (fig. 5.70).

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Fig. 5.70

The advantage of this wall is the use of the weight of the retained material, resting on the base slab, together with the weight of the wall, in order to ensure the stability against sliding and against overturning. Various structural elements such as the slabs AB, BC and DE, working as cantilevers, are designed to resist bending.

For the stability checks of a cantilever wall, the active earth pressure should be defined. There are two approaches to compute the earth pressure:

- on the polygonal surface AFCD, in which FC represents the failure

surface with an angle in respect to the horizontal. In this

case, the soil prism FBC is considered as part of the wall,- on the vertical plane CH. In this case, the soil prism AHCB is

considered as part of the wall.

For heights up to about 8 m, a cantilever wall is generally economic. For greater heights a counterfort wall is more appropriate.

5.7.2.2 Counterfort walls

Counterfort walls are made of a vertical or inclined slab supported by counterforts monolithic with the back of the wall slab and base slab (fig. 9.71).

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Fig. 5.71

5.8 Reinforced earth

The idea of using the soil itself for structures aimed to resist to the active earth pressure, appeared in the construction of crib walls, gabions, cantilever or counterfort walls, is best expressed by the reinforced earth, a system patented by the French engineer Henri Vidal and introduced in practice in the years 60’s of the 20th century.

A compacted soil mass is stabilized as a result of frictional forces developed between the soil and the tensile reinforcing elements, usually in the form of horizontal strips made preferably of galvanized steel, but also of aluminium alloys, plastic or geotextiles. The stresses within the soil mass are transferred to the elements which are thereby placed in tension. The soil used as the fill material should be predominately coarse-grained and be adequately drained to prevent it from becoming saturated.

A reinforced earth retaining structure should be made in such a way that several basic conditions are fulfilled: the facing should resist to the earth pressure and be sufficiently flexible to withstand any deformation of the fill; the length L of the strips should be long enough in order to ensure, by the friction developed on the top and bottom surface of each strip in contact with the soil, the stability of the structure; reinforcing elements should be able to carry out the tensile stresses.

The facing is attached to the reinforcing elements to prevent the soil from spilling out and to satisfy aesthetic requirements. Two of the most used solutions are the facing consisting of precast concrete units (fig. 9.72 a) or of pliant U – shaped steel sections arranged horizontally (fig. 9.72 b).

In what follows, a simplified method of analysis is presented.

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a bFig. 5.72

For given d (distance between strips in the horizontal plane) and H (distance between strips in the vertical plane), the total earth thrust on the facing, pertaining to a strip at the depth z, is:

(5.29)

where K is the appropriate earth pressure coefficient at depth z.

The frictional resistance available on the surfaces of the element is given by:

(5.30)

where L is the length of the element, b is the width of the element, the angle of friction between soil and element.

In order to obtain the minimum length of the element, Lmin, which is the length of the element beyond the failure surface AC (fig. 9.72), P should be multiplied by a factor of safety FS which should not be less than 2.

(5.31)

From the relation (9.31) results that Lmin is constant and independent of z.

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The thickness of the element is obtained knowing the tensile force P and the width b, from the condition of tensile resistance of the material. The thickness of the facing is determined in function of P, , d and the strength characteristics of the material used for facing.

The anchoring length Lmin should be ensured outside the failure surface AC, where the shear stresses on the surfaces of the element are acting in wards (fig. 9.73). Sometimes, in order to facilitate the construction process, instead of a linear variation with depth of the length, a step-wise variation can be adopted (fig. 9.73 b).

Fig. 5.73

When the fill is made of granular material, displacements of the wall are sufficient to develop the active limit state condition and K in relations (5.29) and (5.31) can be taken as Ka.For a preliminary design, the following values can be used: d = 0.7 m; = 0.25…0.30 m; b = 75 mm.

In the design of the earth reinforced earth structure, the external stability must also be considered.

Although behaving as a relatively flexible structure, a reinforced earth structure should be designed, from the point of view of external stability, as if it were a gravity wall. The back of the wall should be taken as the vertical plane through the inner end of the lowest reinforcing element. The total active thrust on this plane is calculated by the Rankine theory. The factor of safety against sliding between the reinforced fill and the foundation soil should not be less than 2. The pressure distribution on the base must be wholly compressive and fulfill conditions (5.22…5.24) as for a gravity wall.

A basic problem for a permanent structure made of reinforced earth is the durability of reinforcing elements. Data available on the rate of corrosion of

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galvanized steel in soils indicate that elements of this material are likely to have a minimum service life of 120 years.5.9 Cofferdams

Many engineering works must be constructed in rivers or in still water or in the areas prone to flood along rivers. This is the case of bridge foundations, of docks, locks and other hydraulic structures etc. To make possible the construction in the dry of the respective object, this should be surrounded by a temporary structure, creating an area from which the water is removed. The temporary structure is called cofferdam.

5.9.1 Cofferdams made of earth and rock fill dikes

5.9.1.1 Earth dikes

Earth dikes as cofferdams are used for shallow waters (2…3 m) and rates of the flow of the river under 0.5 m/s. Due to the relatively large transversal area, required by the slopes, the earth dikes lead to a significant narrowing of the river section, hence they are rarely used for the bridge piers located in the river. Instead, they can be used for bridge abutments, for which the needed area for work in the dry is obtained by linking the dike with the bank of the river (fig. 5.74).

Fig. 5.74

The soil for the construction of the earth dike should fulfill some requirements: to ensure watertightness, to be compactable, to avoid being easily eroded by the water flow. Clayey sands with about 25% clay fraction meet these requirements. But even dikes made of sand could be a reliable solution, counting on a watertightness reached in short time, as a result of the filling of the voids by the fine particles carried by the water. When the dike is constructed on the land or in dry in periods of low waters, clay can be also used, if excavated in dry form, spread and compacted in thin layers. The clay

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fill should not be used for the construction of dikes by discharge under water, since the soil is softening and forming an unstable fill.

At flow rates larger than 0.1 m/s, slopes of the dikes should be protected to prevent erosion.

5.9.1.2 Rock fill dikes

Rock fill dikes, made of large blocks of stone, have the advantage of steeper slopes in comparison with the earth dikes and of resisting better to erosion forces caused by the water flow. Instead, they are permeable and require special measures to ensure watertightness, such as concrete cores (fig. 5.75 a) or clay cores (fig. 5.75 b), when the construction of the dike is made in dry, or sheet pile walls (fig. 5.75 c), when dike is built in water. In all cases, the watertight element should penetrate into an impervious layer of soil.

Fig. 5.75

5.9.2 Sheet piles cofferdams

5.9.2.1 Single skin cofferdams

These are formed by one-line timber or steel piles, working as cantilever walls (fig. 5.76 a) or as walls with free earth support and raking struts (fig. 5.76 b), when ground conditions do not allow to drive the sheet piles to a depth required in a fixed-end support.

Fig. 5.76

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Other means to ensure the stability of the single skin cofferdams are represented by earth fills placed in front of the wall (fig. 5.77 a) or on both sides of the wall (fig. 5.77 b).

Fig. 5.77

5.9.2.2 Earth-filled double-wall cofferdams

Double-wall cofferdams consist of two parallel lines of steel piling connected together by a system of steel walings and tie rods. The space between the lines of piling is filled with coarse cohesionless materials such as sand, gravel or broken rock.

The width b of the cofferdam should be not less than 0.8 of the retained height h of water (fig. 5.78). The penetration of the piling into the soil below the bottom of the river should be sufficient to develop the necessary passive resistance and prevent horizontal sliding of the cofferdam as a gravity structure.

Fig. 5.78

A major disadvantage of double-wall cofferdams is that if failure takes place in a certain point, it will propagate on a relatively large distance along the wall.

5.9.2.3 Cellular cofferdams

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Cellular cofferdams are self-supporting structures, constructed using straight web steel sheet piles driven to form cells of various shapes and filled with sand, gravel or broken rock. They can be founded on rock, sand or stiff clay and utilized as either temporary or permanent structures to retain considerable heights of soil and/or water.

The stability of a cellular cofferdam depends upon the tensile strength of the sheet piling, the properties of the filling, the shape and size of the cells and the foundation materials. The outward pressure of the filling produce a high circumferential tensile forces in the piling, which the straight web piles are designed to resist, unlike through shaped piles sections which are unsuitable.

Fig. 5.79

In fig. 5.79a is shown one of the most utilized type of cellular cofferdam, with circular diaphragm cells. The main advantage of this type is that each cell is a self-supporting unit, which means that the loss of stability of one cell does not imply the progressive failure of the entire wall. Also, each cell can be filled independently of adjacent cells. Circular diaphragm cells can be constructed in rough and flowing water of maximum velocity about 1.3 m/s, and at large depths of water, reaching 20…25 m.

A good example of the use in Romania of this kind of cellular cofferdam is represented by the works at the navigation and hydro energetic system at the Iron Gates, on the Danube (fig. 5.80).

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Fig. 5.80

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