CHAPTER 6 - THE CONTROL OF GROUNDWATER

The occurrence of groundwater on site is considered as one of the most difficult problem in excavation work.

Reduction of groundwater within an excavation is needed for access by workers and machines and for performing construction works in dry conditions.

Methods of control of groundwater in excavations can be grouped in two main categories:

Methods of dewatering by pumping water Methods of restraint of flow

In this chapter, methods of the first category will be dealt with.

6.1 METHODS OF DEWATERING

6.1.1 Sump pumping

Water accumulated in the excavation is collected in a perimeter trench and pumped out of the excavation for discharge. In the case of unsupported excavation (fig. 3.1) or of supported excavation with a impervious wall, water access is possible both through the sides and through the bottom. When the sides of the excavation are protected by impervious walls, for instance a closed sheet piling braced for lateral support (fig.6.2a), water access is possible only through the bottom, which is provided with slight inclinations and trenches to conduct the water to one or several sumps (fig. 6.2b). To prevent piping, the sump can be protected by an inverted filter (fig. 6.3).

Fig. 6.1

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1 – sheet pile wall; 2 – sump; 3 - pumpFig. 6.2

The greatest risk associated with the sump pumping is the occurrence of the quick sand condition (or piping, boiling). In the case of sheet piling, upward seepage of groundwater into the excavation can produce soil liquefaction, when the vertical effective stress within the soil reaches zero (fig. 6.4). When the quick sand condition is occurring, the pumping must immediately cease. To reach the required level of the excavation, one of the following solutions can be adopted:

- recharging water into excavation to the original level and, then, performing the excavation and the concreting under water;

- lengthening the seepage path by driving the sheet piles to a deeper penetration;

- reducing the head of the water causing seepage, by pumping from wellpoints or bored wells placed at or below the level of the bottom of sheet piles (see p. 6.1.2).

1 – sump; 2 – inverted filterFig. 6.3

Both silty or sandy clays, due to their low permeability, and coarse sands and gravels, due to the large dimension of both particles and voids, are unlikely to be subjected to piping. Soils most likely to be subjected to piping are soils with no cohesion or very low cohesion, with grain size small enough to be disturbed by the seepage forces, and permeable enough to allow seepage

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through them. Loose fine sands, silty sands and sandy silts meet these requirements.

Fig. 6.4

If the depth of the excavation is not too big, pumps are placed at the ground surface, as in fig. 6.1. The aspiration height or suction lift is limited to 6…7 m.

In order to determine the total discharge, the following relation can be used:

Q = q. A (m3/h) (6.1)

where A is the surface of the bottom of the excavation and q is a specific discharge which can be taken: 0.16 for fine sands; 0.24 for medium sands; 2.00 for coarse sands.

When the excavation is supported by sheet piles (fig. 6.2), the following relation can be used:

(m3/h) (6.2)

where H is the head, in m; k coefficient of permeability in m/h; U the perimeter of the wall; q is the specific discharge given in the table 6.1 in function of the ratios (H+t)/l and H/(H+t) where t is the embedment and l the distance from the ground water table to the impervious layer.

Table 6.1H/H+t(H+t)/l

0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 0,90 0,95

1,00 1,39 1,13 0,98 0,88 0,78 0,70 0,61 0,52 0,42 0,360,75 1,20 0,95 0,81 0,70 0,61 0,53 0,46 0,39 0,30 0,230,50 1,12 0,89 0,74 0,64 0,56 0,48 0,41 0,34 0,27 0,220,25 1,08 0,84 0,70 0,60 0,52 0,45 0,39 0,32 0,25 0,210,00 1,02 0,80 0,67 0,58 0,50 0,42 0,38 0,31 0,24 0,20

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6.1.2 Groundwater lowering

6.1.2.1 General conditions

The main methods used for groundwater lowering are the wellpoint systems and the bored wells.

The scheme for the groundwater lowering is illustrated in fig. 6.5.

Fig. 6.5

Wellpoints or bored wells systems are installed prior to excavation inside or outside the excavation area. Once in function, they will cause groundwater to flow away from the excavation, improving the stability to its side batters and base and allowing construction works to proceed in the dry.

Another use of groundwater lowering is illustrated in fig. 6.6, and is intended to prevent the hydraulic failure of the base of the excavation. Ground conditions on the site are characterized by the presence of two water layers, the upper one with free level and the lower one under pressure, separated by a layer of impervious soil, a clay. When the excavation reaches the clay layer, this one is subjected to a pressure , corresponding to the difference in the elevations of the two water layers. If the thickness h of the clay layer below the base of the excavation is not sufficient, there will be a heave of the layer of clay followed by its rupture under the pressure . The phenomenon is called hydraulic failure of the base of the excavation. In order to prevent it, the lowering of the groundwater is necessary.

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1 – excavation; 2 – impervious layer; 3 – pervious layer

Fig. 6.6

In fig. 6.7 is put into evidence the advantage of using the groundwater lowering with bored wells or well points, as compared to the sump pumping, in the case of an excavation with a vertical soil support. When the sump pumping is used (fig. 6.7a), the wall should be impervious and to resist both the active earth pressure and the water pressure. There is a risk of piping, particularly in soils such as fine sands, silty sands or sands silty. When the groundwater lowering is done (fig. 6.7b), water is drawn away from the excavation and, being filtered as it is removed from the ground, carries little or no soil particles with it, once steady discharge conditions have been attained. At the same time, the wall is subjected only to the active earth pressure and should not be impervious.

There is, however, a shortcoming of this method, too, namely the occurrence of settlement due to an increase in density as result of the ground lowering of the water table. Indeed, in a point A adjacent to the dewatering system (fig. 3.5), the effective overburden pressure before lowering the ground water table is:

(6.3)

After ground water lowering, peff becomes:

(6.4)

The increase in pressure is:

(6.5)

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Fig. 6.7

6.1.2.2 Bored wells

A 40…60 cm diameter borehole is performed, under the protection of a casing, until the impervious layer is reached or, if this is not possible, deep enough below the bottom of the excavation. Inside the casing a tube of 15…30 cm diameter is inserted, provided with a perforated screen over the length where dewatering of the soil is required and it terminates in a 2…3 m length of unperforated pipe to act as a sump to collect any fine material which may be drawn through the filter. After the well casing is installed, graded gravel filter material is placed between it and the outer borehole casing over the length to be dewatered. The outer casing is withdrawn in stages as the filter material is placed and the remaining space above the screen is backfilled with any available material.

Pumping from bored wells can be undertaken by surface pumps, with their suction pipes installed in bored wells. The depth of draw-down in this case is maximum 8 m. When a great depth of water lowering is required or when an artesian head must be lowered in permeable strata at a considerable depth below excavation level, electrically powered submersible pumps are used, with a rising main to the surface. In the case of submersible pump, there is no limitation on amount of draw-down as there is for suction pumping. A pump in a 350 mm borehole can raise 7500 l/min. against 30 m head.

In fig. 6.8 is shown a complete installation of a bored well.

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1 – inner casing; 2 – rising main; 3 – submersible pump; 4 – silt, collecting in sump; 5 – mesh filter screen; 6 – outer well casing withdrawn;

7 – soil backfill; 8 – graded filter materialFig. 6.8

The design of a dewatering system using bored wells is based on relations established in hydraulics of underground works, of the kind shown in the chapter 3, in connection with the determination of the coefficient of permeability k by a well-pumping test.

As it was shown, when the well reaches the impervious layer (see ch. 2), the equation of the draw-down curve is:

(6.6)

At a distance R from the axis of the wells, named radius of influence, there is no effect of the dewatering, the draw-down curve meets the original groundwater level. By replacing x = R, z = H in the relation (3.3):

(6.7)

If the draw-down at the well face is so:

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h = H - so

so = H – h (6.8)

Replacing (3.5) in (3.4):

(6.9)

Equation (6.6) gives the relation between the discharge q of water pumped from the bored well and the resulting lowering so.

By experiments, it was found the following relation for R:

R = 3000 so k1/2 (6.10)

where so is in m and k in m/s.

For an individual well of radius rw, the discharge quantity given by Darcy’s law is:

(6.11)

where hw is the height of well screen and ie is the average entry gradient. According to empirical findings, ie should not exceed 1/(15 k1/2) to avoid turbulence and filter unstability. Thus, the capacity of an individual well should be limited to:

(6.12)

1 – original ground water level; 2 – lowered groundwater level; 3 – bored well; 4 – fictitious well

Fig. 6.9

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The relation (6.12), established for the pumping from a bored well was extended to the case of pumping simultaneously from several wells, assuming that these wells are located around a perimeter of a circle of radius

R1, where , B and L being the length and the width of the

rectangular excavation. It is also assumed that the cumulated effect of the wells is equal to the one of a fictitious wells of radius R1 and having the same radius of influence R as the single well (fig. 6.9). By this way is obtained the total flow Q needed for the required lowering so:

(6.13)

The number of wells is n, where

(6.14)

The required pump capacity is:

(6.15)

where is the density of water to be pumped, and is the system efficiency which, considering friction loss in the delivery pipe work is usually in

the range 0.3 to 0.5. For ,

(6.16)

where Q is in litres/s and h is in metres

6.1.2.3 Wellpoints

A wellpoint consists of a 1 m long and 50 – 75 mm diameter gauze screen surrounding a central riser pipe.

Wellpoints are jetted down by water at a pressure of up to 15 bar, penetrating in the ground by their own weight and requiring only to be guided by the

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worker (fig. 6.10). Once the desired level is reached, the jetting water supply is cut down to a low velocity sufficient to keep the hole around the point open. A coarse sand is then fed around the annular space to form a supplementary filter around the point, after which the water is cut off.

1 – water jet; 2 – impervious seal of clay; 3 – header

Fig. 6.10

In fig. 6.11 are given details of the tip of a wellpoint. A particular feature is a rubber ball acting as a valve, which is lowered when jetting (fig.6.11a) and raised when pumping (fig. 6.11.b).

Wellpoints act most effectively in sands and sandy gravels of moderate permeability. The draw-down is slow in silty sands but these soils can be also effectively drained with wellpoints.

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1 – riser pipe; 2 – gauze screen; 3 – rubber ball by jetting; 4 – rubber ball by pumping; 5 – natural filter

Fig. 6.11

In soils of lower permeability, the effectiveness of the wellpoints installation is increased by including a vacuum pump which creates in the wellpoints a negative pressure of 0.7…0.8 daN/cm2. The groundwater which is at the atmospheric pressure is drained forcefully to the wellpoints where the pressure is lower. When using the vacuum, wellpoints have to be provided with a clay seal, in order to maintain a high vacuum at the well screen (fig. 6.12).

1 – lowered groundwater level by gravitational dewatering; 2 – lowered groundwater level by vacuum dewatering; 3 – impervious seal of

clay; 4 – wellpointFig. 6.12

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In soils of finer particle size, such as silts, a further improvement of the effectiveness of wellpoints can be obtained by using the drainage by electro osmosis.

For that purpose, between the well points and at close distance are driven in the ground steel rods connected to positive pole of a source of electrical current, becoming anodes, while the header main of the well points installation is connected to the pole minus, making the wellpoints to act as cathodes. The water particles surrounded by cations flow through the pores in the soil and are collected at the cathodes, increasing the rate of flow obtained by gravitational drainage. A typical layout of an installation combining gravitational drainage and electro osmosis is shown in fig. 6.13.

1 – wellpoint as cathode; 2 – steel rod as anodeFig. 6.13

In fig. 6.14 is given an example of the use of dewatering by electro-osmosis in the case of a battered excavation. The anodes are placed nearest to the excavation causing the ground water to flow away from the slopes, which effectively stabilizes them and permits steep slopes.

Wellpointing equipment comprises usually wellpoints, header mains, centrifugal pump, vacuum pump, electrical engine etc. Wellpoints are normally spaced from 1 to 4 m apart, depending on soil conditions and drawdown requirements.

1 – wellpoint as cathode; 2 – steel rod as anode; 3 – stream linesFig. 6.14

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Unlike bored wells, which can be equipped with submersible pumps, wellpoints are operating with central pumps at the ground surface providing a limited suction lift. A lowering of 5 - 5,5 m below pump level is a practical limit. For deeper excavation, the wellpoints must be installed in two or more stages.

In fig. 6.15 is shown a cross-section of a large open excavation with a wellpoint installation applied in two stages.

Fig. 6.15

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