Embed Size (px)
Citation preview
i
Chapter 5
PILES
CHAPTER 5
5.1 Piles
For piles the length to width (diameter) ratio i.e. Lp/d 4 , where Lp is the pile length and d is pile diameter.
The basic situation for a pile foundation is where soft soil exists near the ground surface which underlain by rock formation e.g.
Figure 5-1 Pile foundation resting on hard stratum underlying a soft soil
layer
5.2 Uses of Piles
Piles are commonly used for the following purposes (Figure 5-2).
To carry superstructure loads into or through a soil stratum. Both vertical and lateral loads may be involved.
Building
Soft Soil
Piles
Firm Soil
2
To resist uplift or overturning forces such as for basement mats below the W.T. or to support the tower legs subjected to overturning from lateral loads such as wind.
To compact loose, cohesionless deposits through a combination of pile volume displacement and driving vibration, thus increasing their bearing capacity.
To carry the foundation through the depth of scour to provide safety in the event the soil is eroded away.
To stiffen the soil beneath machine foundations to control both amplitudes of vibration and the natural frequency of the system.
In offshore construction to transmit the loads above the water surface through the water and into the underlying soil. This case is one in which partially embedded piling is subjected to vertical (and buckling) as well as lateral loads.
Figure 5-2 (a) Tension pile to resist overturning movements in tall buildings (b) Shear pile to
resist horizontal forces or movements Friction pile (c) raking piles in harbor and river
(a) (b)
(c) tension compression
Wind
3
5.3 Classification of Piles
5.3.1 Classification according to the mechanism of load transfer
End/Point Bearing Piles
If a bedrock or rocklike material is present at a site within a reasonable depth, piles can be extended to the rock surface (figure 5-3(a)). In this case, the ultimate bearing capacity of the pile depends entirely of the underlying material; thus the piles are called end or point bearing piles. In most of these cases the necessary length of the pile can be fairly well established.
Instead of bedrock, if a fairly compact and hard stratum of soil is encountered at a reasonable depth, piles can be extended a few meters into the hard stratum.
Friction Piles (figure 5-3 b)
When no layer of rock or rocklike material is present at a reasonable depth at a site, point/end bearing piles become very long and uneconomical. For this type of subsoil condition, piles are driven through the softer material to specified depth. These types of piles are called friction piles because the load on the pile is resisted mainly by skin/friction resistance along the side of the pile (pile shaft). Pure friction piles tend to be quite long, since the load-carrying capacity is a function of the shaft area in contact with the soil.
In cohesionless soils, such as sands of medium to low density, friction piles are often used to increase the density and thus the shear strength.
Friction cum end bearing piles
In the majority of cases, however, the load-carrying capacity is dependent on both end-bearing and shaft friction (figure 5-3 c).
4
5.3.2 Classification of piles according to their method of
installation (figure 5-4)
Driven or displacement piles
They are usually preformed before being driven, jacked, screwed or hammered into ground. This category consists of driven piles of steel or precast concrete and piles formed by driving tubes or shells which are fitted with a driving shoe. The tubes or shells which are filled with concrete after driving. Also included in this category are piles formed by placing concrete as the driven piles are withdrawn.
Bored or Replacement piles
They require a hole to be first bored into which the pile is then formed usually of reinforced concrete. The shaft (bore) may be cased or uncased depending upon type of soil.
Soft
ground
hard
Soft to
firm
Soft to
firm
Firm to
hard
Soft
Figure 5-3(a) End bearing pile (b) Friction pile (c) friction cum end bearing pile
(a) (b) (c)
5
5.3.3 Classification of Piles according to Materials
Timber piles
Timber piles are made of tree trunks driven with small end as a point
Maximum length: 35 m; optimum length: 9 20m
Max load for usual conditions: 450 kN; optimum load range = 80240 kN
Disadvantages: difficult to splice, vulnerable to damage in hard driving, vulnerable to decay unless treated with preservatives (If timber is below permanent W.T. it will apparently last for ever), if subjected to alternate
(a) (b) (c) (d) (e) (f)
Figure 5-4 Principal types of pile: (a) precast RC pile (b) steel H pile (c) shell
pile (d) concrete pile cast as driven tube withdrawn (e) bored pile (cast in
situ), (f) under-reamed bored pile (cast in situ)
6
wetting & drying, the useful life will be short, partly embedded piles or piles above W.T. are susceptible to damage from wood borers and other insects unless treated.
Advantages: comparatively low initial cost, permanently submerged piles are resistant to decay, easy to handle, best suited for friction piles in granular material.
Steel Piles
Max length: practically unlimited,
optimum length: 1250 m
load for usual conditions = maximum allowable stress x-section area,
Optimum load range = 3501050 kN
The members are usually rolled HP shapes/pipe piles. Wide flange beams & I beams proportioned to withstand the hard driving stress to which the pile may be subjected, In HP pile the flange thickness= web thickness, pipe piles are either welded or seamless steel pipes, which may be driven either open ended or closed end. Closed end piles are usually filled with concrete after driving. Open end piles may be filled but this is not often necessary.
Advantages: easy to splice, high capacity, small displacement, able to penetrate through light obstructions, best suited for end bearing on rock, reduce allowable capacity for corrosive locations or provide corrosion protection.
Disadvantages: Vulnerable to corrosion, HP section may be damaged/deflected by major obstruction
Concrete Piles
Concrete piles may be precast, prestressed, cast in place, or of composite construction.
Precast concrete piles may be made using ordinary reinforcement or they may be prestressed.
7
Pecast piles using ordinary reinforcement are designed to resist bending stresses during picking up & transport to the site & bending moments from lateral loads and to provide sufficient resistance to vertical loads and any tension forces developed during driving.
Prestressed piles are formed by tensioning high strength steel prestress cables, and casting the concrete about the cable. When the concrete hardens, the prestress cables are cut, with the tension force in the cables now producing compressive stress in the concrete pile. It is common to higher-strength concrete (35 to 55 MPa) in prestressed piles because of the large initial compressive stresses from prestressing. Prestressing the pile tends to counteract any tension stresses during either handling or driving.
Max length: 1015 m for precast, 2030 m for prestressed
Optimum length: 1012 m for precast, 1825m prestressed
Loads for usual conditions 900 for precast, 8500 kN for prestressed
Optimum load range: 3503500 kN
Disadvantages: difficult to handle unless prestressed, high initial cost, considerable displacement, prestressed piles are difficult to splice.
Advantages: high load capacities, corrosion resistance can be attained, hard driving possible
Remarks: cylinder piles in particular are suited for bending resistance.
Cast in place concrete piles are formed by drilling a hole in the ground & filling it with concrete. The hole may be drilled or formed by driving a shell or casing into the ground.
Disadvantages of Concrete piles: Concrete piles are considered permanent, however, certain soil (usually organic) contain materials that may form acids that can damage the concrete. Salt water may also adversely react with the concrete unless special precautions are taken when the mix proportions are designed. Additionally, concrete piles used for marine structures may undergo abrasion from wave action and floating debris in the water. Alternate freezing & thawing can cause concrete damage in any exposed situation.
8
Composite piles
In general, a composite pile is made up of two or more sections of different materials or different pile types. The upper portion could be cased cast-in-place concrete combined with a lower portion of timber, steel H or concrete filled steel pipe pile. These piles have limited application and are employed under special conditions.
5.4 Load Capacity of Piles
Three general methods are available to establish load capacity:
(1) Static Analysis (2) Dynamic Analysis (3) Load Testing (4) Correlation with field tests (SPT, CPT etc)
Dynamic formulae are used for driven piles. Static formulae are used both for bored and driven piles. Load testing is the most reliable method to determine the load capacity of the pile in the field. They should be performed on all piling projects. However, they are considerably more expensive than the other methods used to determine pile capacity, and economic consideration sometimes preclude their use on projects. Field tests like SPT, CPT are also used to correlate to load carrying capacity particularly for cohesionless soils.
5.5 Driven Piles
5.5.1 Dynamic Pile Formulas
Piles are usually forced into the ground by a pile driver or pile hammer. In medieval times piles were driven by men manually swinging hammer, which consists of a weight raised by ropes or cables and allowed to drop freely striking the top of the pile. After the drop hammer came the single acting hammer, double acting hammer, differential acting hammer, diesel pile hammer, vibratory driver.
Dynamic pile formulas are widely used to determine the static capacity of the driven pile. These formulas are derived starting with the relation
Energy Input = Energy Used + Energy Lost
The Energy used equals the driving resistance (Pu) the pile movement (s).
9
Energy lost is due to friction, heat, hammer rebound, vibration and elastic compression of the pile, the pacing assembly, and the soil.
Energy News Record (ENR) Formula:
This formula takes into account the energy lost due to temporary compression (C) resulting from elastic compression of the pile. Thus
Energy Input = Energy Used + Energy Lost
Wr h = Pu s + Pu C
Pu = Wr h / (s + C)
Where Wr = weight of the ram, h= height of fall of the ram, s = penetration of pile per hammer blow, Pu = average resistance of soil to penetration.
C= 25 mm (1 inch) for drop hammer, and
C= 2.5 mm (0.1 inch) for steam hammer (single acting/double acting)
Pa = Pu / SF where Pa = allowable load on pile, and SF(factor of safety) = 6
For single/double acting hammer, the term (Wr h) can be replaced by hE where h=hammer efficiency (table-1) and E=rated energy of hammer. Thus
Pu = hE / (s + C)
Table-1: Hammer efficiency h
Table 5-1 Hammer efficiency h
Hammer type Efficiency h
Single and double acting hammer
0.7-0.85
Diesel hammers 0.8-0.9
Drop hammers 0.7-0.9
10
Example: A precast concrete pile 12in 12 in in cross section is driven by a steam hammer. The maximum rated hammer energy = 26 kips-ft, hammer efficiency = 0.8 and the number of blows for the last 1 inch of penetration = 5. Estimate the allowable pile capacity by using (a) ENR formula using SF=6.0
Solution:
ENR formula:
h= 0.8, E=26 kip-ft, s=1/5 = 0.2 inch/blow. SF=6.0
SFCs
EPa h
)(
= 0.8 26 12 / [ (0.2 + 0.1) 6 ] = 836 kips
5.5.2 PILE DRIVING EQUIPMENT
Piles are installed by a special pile driving device know as a pile hammer. The hammer may be suspended from the boom of a crawler crane, supported on a large frame called a pile driver or carried on a barge for construction in water. In all cases, the hammer is guided between two parallel steel members called leads. The leads may be adjusted at various angles for driving vertical and batter piles.
Several types of hammers are in use and each of which are different sizes. The hammer types are:
Drop hammer
The drop hammer consists of a heavy ram in between the leads. The ram is lifted up to a certain height and released to drop on the pile. This type is slow and therefore not in common use. It is used in the cases where only a small number of piles are driven.
Single-acting hammer
In single acting hammer a heavy ram is lifted up by steam or compressed air but dropped by its own weight. The energy of a single acting hammer is equal to the weight of the ram times the height of fall.
Double-acting hammer
The double-acting hammer employs steam or air for lifting the ram and for accelerating the downward stroke. The energy of a double-acting hammer is
11
equal to the (weight of the ram + mean effective pressure x the effective area of ram) x times the height of fall.
Diesel hammer
The diesel hammer is a small, light weight and highly mobile. They use gasoline for fuel. To start the operation, the ram is raised, and the fuel is injected. As the ram is released, the ram falls and compresses air and fuel. The air and fuel becomes hot because of the compression and the air-fuel mixture is ignited. The resulting explosion (1) advances the pile and (2) lifts the ram. If the pile advance is very great as in soft soils, the ram is not lifted by the explosion sufficiently to ignite the air-fuel mixture on the next cycle, requiring that the ram be again manually lifted.
Vibratory hammer
The principle of the vibratory driver is two counter-rotating eccentric weights. The driving unit vibrates at high frequency and provides two vertical impulses-one up and one down. The downward pulse acts with the pile weight to increase the apparent gravity force. These hammers have reduced driving vibrations, reduced noise, and great speed of penetration.
5.5.3 HAMMER SELECTION
Generally the size of hammer is more important factor than type of hammer.
A heavy pile should be driven by a heavy hammer delivering large energy. Preferably the weight of the hammer should be at least on-half the total weight of the pile, and the deriving energy should be at least one foot-pound for each pound of pile weight.
Each type of hammer has its use under suitable conditions. The advantages and disadvantages of each type are summarized below:
Single-acting hammer
They are advantageous when driving heavy piles in compact or hard soils; the heavy ram striking at low velocity produces least damage due to impact. The disadvantages are low driving speed and large headroom requirement.
Double-acting hammer
12
They are generally used to drive piles of light or moderate weight in soils of average resistance against driving. This type of hammer can drive piles at fast speed, requires less headroom and can be used to extract piles by turning them [i.e. the double-acting hammer] upside down.
Diesel hammer
They are similar in application as double-acting hammers, but driving may become difficult in extremely soft ground.
Vibratory hammer
They have fairly good results in silty and clayey deposits. They are used in heavy clays or soils with appreciable numbers of boulders. These hammers have reduced driving vibrations, reduced noise, and great speed of penetration.
5.6 STATIC PILE FORMULAS
The ultimate load which can be carried by a pile is equal to the sum of the base resistance and the shaft resistance (figure 5-5).
Pu + Wp = Abqb(gross) + As qs
Pu is the ultimate load that can be carried at top of pile, qb ultimate (gross) bearing capacity at base level, Ab = base area of pile, qs=ultimate shearing/skin resistance per unit area, As= perimeter area of pile, and Wp = weight of the pile.
Subtracting Ws from both sides of the equation. Where Ws is effective soil weight replaced/displaced due to pile volume. Ws = LAb where is the effective weight of soil, and L is pile length.
Pu + (Wp – Ws)= Abqb(gross) + As qs -Ws
Pu + (Wp – Ws) = Abqb(gross) + As qs - LAb
Pu + (Wp – Ws) = (qb(gross)- L)Ab + As qs
Pu + (Wp – Ws) = (qb(gross)- o)Ab + As qs
Where o=L is effective vertical stress at pile base
Pu +(Wp – Ws) = Abqb + As qs
13
Pu = Abqb + As qs – (Wp – Ws)
Pu = Abqb + As qs – W
Pu is the ultimate load that can be carried at top of pile, qb ultimate (net) bearing capacity at base level, Ab = base area of pile, qs=ultimate shearing/skin resistance per unit area, As= perimeter area of pile, W= Wp – Ws = weight of the pile – effective weight of soil replaced. In most cases Wp Ws and hence W0.
Pu = Abqb + As qs
However in the case of under-reamed piles (figure 5-4 f) the reduction in pressure on the soil at base level due to the removal of soil is greater than the subsequent increase in pressure due to the weight of the pile and hence use equation-1 (i.e. do not assume that Wp Ws)
5.6.1 COHESIONLESS SOILS
End bearing Resistance (qb)
qs
qb
WP
Figure 5-5 Free body diagram of a pile
Pu
14
The ultimate B.C. and settlement of a pile depends mainly on the relative density of sand. However, if a pile is driven into sand the relative density adjoining the pile is increased by compaction due to soil displacement (except in dense sands, which may be loosened). The soil characteristics governing ultimate bearing capacity and settlement, therefore, are different from the original characteristics prior to driving. This fact, in addition to the heterogeneous nature of sand deposits, makes the prediction of pile behavior by analytical methods extremely difficult.
The ultimate (net) B.C. at base level can be expressed as
qb= cNc + o Nq + ½ B N -o [o is subtracted to get net value of qb]
Where o is the effective overburden pressure at base level of pile.
qb= cNc + o(Nq-1) + ½ B N
c=0 for sand and 1/2BN term can be neglected because the B (width/diameter of pile) is small compared to the length of pile. so
qb= o (Nq -1) oNq (Nq -1) Nq [because reduction of Nq by 1 is a substantial refinement not justified by estimated soil parameters]. Nq is a B.C. factor (figure 5-6)
15
Figure 5-6: Values of Nq for pile formulae (After Berezantsev et al. 1961)
16
Friction/Shaft Resistance (qs):
The average value of skin resistance (qs) over the length of pile embedded in sand can be expressed as
tan'' cqs
Where '' osK and c’=0
tan'oss Kq
Ks a coefficient of earth pressure dependent largely on the relative density of the soil, ' = average effective pressure in the layer perpendicular to qs (i.e. horizontal). '
o = average effective vertical overburden pressure in the layer, and
=angle of friction between the pile and the soil.
Table 5-2 Ks and values
Pile Type Ks
Loose Sand Dense Sand
Concrete 3/4 1.0 2.0
Steel 20 0.5 2.0
Wood 2/3 1.5 4.0
Variation of qb and qs with depth
The above equations for qb and qs indicate a linear increase with depth of qb and qs. However, tests on full scale and model piles have shown that these equations are valid up to certain depth called critical depth (zc). Below this depth zc(=15d to 20d, conservatively take zc = 15 d), base (qb) and shaft/skin (qs) resistances do not develop linearly [i.e. become constant]. This is because the vertical effective stress adjacent to the pile is not necessarily equal to the effective overburden pressure (away from the effect of pile) but reaches a limiting value at critical depth zc (figure 5-7).
17
qb and qs from SPT Test
Ultimate base resistance qb
Due to the critical depth limitation and to the difficulty of obtaining values of the required parameters, the above equations are difficult to apply in practice. It is preferable to use empirical correlations
The following empirical correlations have been proposed by Meyerhof for driven piles in sand.
qb = (40N55)Lb/B 400N55 (kN/m2)
N= is the value of standard penetration resistance in the vicinity of the pile base. Use any applicable SPT N corrections discussed in earlier chapter-3.
B = width or diameter of pile point
Lb = the length of pile embedded in sand
Skin friction resistance
W.T.
d
oc
d
oc
zc=15 d zc=15d
Figure 5-7 Effective vertical stress distribution diagram adjacent to pile
18
Skin friction resistance is Nqs 2 [kN/m2]
Where N is the average value of standard penetration resistance over the embedded length of the pile within the sand stratum.
The values of qs should be halved in the case of small displacement piles such as steel H piles. For bored piles the values of qb and qs are approximately 1/3 and 1/2, respectively, of the corresponding values for driven piles.
5.7 COHESIVE SOILS
5.7.1 Driven Piles
In the case of driven piles, the clay adjacent to the pile is displaced both laterally and vertically. Upward displacement of the clay results in heaving of the ground surface around the pile and can cause a reduction in the bearing capacity of adjacent piles already installed. The clay in the disturbed zone around the pile is completely remoulded during driving. The excess porewater pressure set up by the driving stresses dissipates within a few months as the disturbed zone is relatively narrow (of the order B): in general, dissipation is virtually complete before significant structural load is applied to the pile. Dissipation is accompanied by an increase in skin friction. Thus the skin friction at the end of dissipation is normally appropriate in design.
5.7.2 Bored Piles
In the case of bored piles, a thin layer of clay (of the order of 25 mm) immediately adjoining the shaft will be remoulded during boring. In addition, a gradual softening of the clay will take place adjacent to the shaft due to stress release, pore water seeping from the surrounding clay towards the shaft. Water can also be absorbed from wet concrete when it comes in contact with the clay. Softening is accompanied by a reduction in shear strength and a reduction in skin friction. Construction of a bored pile, therefore, should be completed as quickly as possible. Limited reconsolidation of the remoulded and softened clay takes place after installation of the pile.
Base Resistance
19
The relavent shear strength for the determination of the base resistance of a pile in clay is the undrained strength at base level. The ultimate bearing capacity is expressed as
qb= cuNc + o Nq + ½ B N -o [o is subtracted to get net value of qb]
1/2BN term can be neglected because the B (width/diameter of pile) is small compared to the length of pile and for u=0, Nq=1, we get
qb = cu Nc where Nc = 9, and cu is undrained shear strength at pile base
Skin Resistance
Total Stress (Undrained Conditions u=0)
)tan( uas cq [where ca is average adhesion]
0)tan( u as u=0
Hence uas cα cq is a coefficient depending on type of clay, the method of
installation, and the pile material. The appropriate value of is obtained from load tests. Values of range from 0.3 to 1. uc is the average undrained shear strength. One difficulty with this approach is that there is usually a considerable scatter in the plot of undrained shear strength against depth and it may be difficult to define the value of uc . See figure 5-8 for different values of .
Effective Stress (drained conditions)
An alternative approach is to express skin friction in terms of effective stress. The zone of soil disturbance around the pile is relatively thin, therefore dissipation of the positive or negative excess pore water pressure set up during installation should virtually be complete by the time the structural load is applied. In principle, therefore, an effective stress approach has more justification than one based on total stress. In terms of effective stress the skin friction can be expressed as
)'tan(' ' oss Kcq
)'tan(' oss Kq [c’=0 for saturated clay under drained conditions]
20
Where Ks is the average coefficient of earth pressure and 'o is the average
effective overburden pressure adjacent to the pile shaft. Failure is assumed to take place in the remoulded soil close to the pile shaft, therefore the angle of friction between the pile and the soil is represented by the angle of shearing resistance in terms of effective stress (’) for the remoulded clay: the cohesion intercept for remoulded clay will be zero. The above equation can also be written as
'osq [where )'tan( sK ]
Approximate value of can be deduced by making assumptions regarding the value of Ks, especially in the case of normally consolidated clays. However, the coefficient is generally obtained empirically from the results of load test carried out a few months after installation. Correlations with loading tests have shown that for soft clays falls within a narrow range of values (0.25 to 0.4), irrespective of the clay type.
21
Figure 5-8 Relationship between the adhesion factor and undrained
shear strength su (cu).
5.8 FACTOR OF SAFETY
The base resistance requires a larger deformation for full mobilization than the shaft resistance, therefore different values of load factor may be appropriate for the two components, the higher factor being applied to the base resistance.
In the case of large-diameter bored piles, including underreamed piles, the shaft resistance may be fully mobilized at working load and it is advisable to ensure a
22
load factor of 3 for base resistance, with a factor of 1 for shaft resistance, in addition to the specified over all load factor (generally 2) for the pile.
5.9 NEGATIVE SKIN FRICTION
When piles are driven through a layer of fill material which slowly compacts or consolidates due to its own weight, or if the layers underlying the fill consolidate under the weight of the fill, a downward drag is imposed in the pile shaft (figure 5-9).
The skin friction between the pile and soil therefore acts in a downward direction. The force due to this downward or negative skin friction is thus carried by the pile instead of helping to support the external load on the pile.
Negative skin friction increases gradually as consolidation of the clay layer proceeds, the effective overburden pressure gradually increasing as the excess pore water pressure dissipates.
Pu + qsNAs = qbAb + qsAs where qsN is negative skin friction (downward), AsN is the corresponding surface area of pile, qs is the skin friction
qsN
qb
qsN
qs
Fill: consolidating under own weight
Soft clay: consolidating under weight of fill
Firm or Hard bearing layer
Figure 5-9 Negative skin friction
Pu
23
(upward) and As is the corresponding surface areas of pile, Ab is the end bearing area of pile.
To calculate negative skin friction equation 'oNsq can be used. In normally
consolidated clays, a value of =0.25 represents a reasonable upper limit to negative skin friction for preliminary design purposes.
It should be noted that there will be a reduction in effective overburden pressure adjacent to the pile in the bearing stratum due to the transfer of part of the overlying soil weight to the pile: if the bearing stratum is sand, this will result in a reduction in bearing capacity above the critical depth.
5.10 PILE GROUP
Rarely is the foundation likely to consist of a single pile. Generally, there will be a minimum of two of three piles under a foundation element or footing to allow for misalignments and other inadvertent eccentricities.
The group of piles is installed fairly close together (typically 2B-4B where B is the width or diameter of a single pile) and joined by a slab, known as the Pile Cap, cast on top of the piles.
The cap is usually in contact with the soil in which case part of the structural load is carried directly on the soil immediately below the surface. The group of piles in this case is called piled foundation. If the cap is clear of the ground surface, the piles in the group are referred to as free-standing (figure 5-10).
24
5.10.1 Load Distribution in Pile Group
It is generally assumed that the load distribution between the piles in an axially loaded group is uniform.
However experimental evidence indicates that for a group in sand the piles at the center of the group carry greater loads than those on the perimeter.
In clay, on the other hand, the piles on the perimeter of the group carry greater loads than on those at the center.
5.10.2 Efficiency of Pile Group
In general the ultimate load which can be supported by a group of N piles is not equal to N times the ultimate load of a single isolated pile of the same dimensions in the same soil. Where N is the number of piles in a group. So
[in general] Ultimate load of Pile Group N Ultimate load of a single pile
The ratio of the average load per pile in a group at failure to the ultimate load for a single pile is defined as the efficiency of the group ().
Average load per pile in a group at failure = Ultimate group load / N
= ( Ultimate group load ] / ( N Ultimate Individual load )
Pile Cap Pile Cap
(a) (b)
Figure 5-10 (a) A group of free-standing piles (b) A group of piled foundation
25
5.10.3 Pile Group in Cohesionless Soils
Driven Piles
The driving of a group of piles into loose sand or medium-dense sand causes compaction of the sand between the piles, provided that the spacing is less than about 8B: consequently the efficiency of the group is greater than unity. The maximum efficiency is reached at a spacing of 2 to 3 diameters and generally ranges between 1.3 to 2. It is recommended that in this case the design value of =1 be taken.
In the case of piles driven into dense sand, the group efficiency is less than unity due to loosening of the sand and the overlapping of zones of shear (figure 5-11).
Bored Piles
However, for a group of bored piles the efficiency may be as low as 2/3 because the sand between the piles is not compacted during installation but the zones of shear of adjacent piles will overlap.
26
Figure 5-11 Stress surrounding a friction pile and the summing effects of a
pile group
5.10.4 Pile Group in Cohesive Soils
A closely spaced group of piles (spacing = 2B to 3B) in clay may fail as a unit, with shear failure taking place around the perimeter of the group and
Four piles contributing to this stress zone
Three piles contributing to this stress zone
Two piles contributing to this stress zone
27
below the area covered by the piles and the enclosed soil. This is referred to as Block Failure. (figure 5-12)
The ultimate load in the case of a pile group which fails as a block is given by
usgbbgug cAqAP
Abg = base are of the group = Bg Lg; Asg= perimeter area of the group = 2D(Bg + Lg); cu = undrained shear strength at depth D
uc = average undrained shear strength between 0 and D below the ground.
qb=cuNc where Nc=5.14 (1+0.2Bg/Lg) [ 1 + (0.053D/Bg) ] 9
)(2 gguggucug LBDcLBcNP
Design Ultimate Load
Piled Foundation
The ultimate load should be taken as the lesser of the
Block Failure value (2) The sum of the individual pile values
Free Standing Group of Piles
The ultimate load should be taken as the lesser of the
(1) Block Failure value (2) 2/3 of the sum of the individual pile values
5.11 Settlement of Pile Group
The settlement of pile group is always greater than the settlement of a corresponding single pile, as a result of the overlapping of the individual zones of influence of the piles in the group. The bulbs of pressure of a single pile and a pile group (with piles of the same length as the single pile) are of the form illustrated in figure 5-13.
In order to estimate settlement for a pile group, it is assumed that the total load is carried by an “Equivalent raft” located at a depth of 2L/3 where L is the length of piles (figure 5-14). It may be assumed as shown in figure 5-14 that the load is spread from the perimeter of the pile group at a slope of 1 horizontal to 4 vertical to allow for that part of the load transferred to the soil by skin friction. The vertical stress increment at any depth below the equivalent raft may be
28
estimated by assuming in turn that the total load is spread to the underlying soil at a slope of 1 horizontal to 2 vertical.
29
Pile Cap Pile Cap
(a) A group of free-standing piles (b) A group of piled foundation
Lg
Bg
D D
Lg
Bg
D
Figure 5-12Block failure of pile group in clay
(c ) Dimensions of Failure block
30
Figure 5-13 Bulbs of pressure for a single and a pile group
31
Figure 5-14 Equivalent raft concept
5.12 Pile Load Test
The loading of a test pile enables the ultimate load to be determined directly and provides a means of assessing the accuracy of predicted values.
Tests may also be carried out in which loading is stopped when the proposed working load has been exceeded by a specified percentage.
Figure 5-15 shows a schematic diagram of the pile load test arrangement for testing in axial compression in the field. The load is applied to the pile by a hydraulic jack. The load is applied in suitable increments, allowing sufficient time between increments for settlement to be substantially complete. According to ASTM D1143, the test pile is loaded in eight equal increments up to a maximum load, usually twice the predetermined working (allowable) load. Unloading stages are normally included in the test program. This testing procedure is Maintained load test (or Controlled load test).
In constant rate of penetration (CRP) test the pile is jacked into the soil at a constant speed, the load applied in order to maintain the penetration being continuously measured. Suitable rates of penetration for tests in sands and clays are 1.5 mm/min and 0.75 mm/min respectively.
Another type of pile load test is cyclic loading, in which an increment load is repeatedly applied and removed.
Driven piles in clays should not be tested for at least a month after installation to allow most of the increase in skin friction (a result of dissipation of the excess pore water pressure due to the driving stresses) to take place. Load tests on piles in sand can be carried out immediately after the piles are driven.
5.12.1 Ultimate Load
Figure 5-15 shows load settlement diagram obtained from fried loading and unloading. For any load Q, the pile settlement can be calculated as follows. When Q=Q1,
net settlement, snet(1) = st(1) – se(1)
32
When Q=Q2
net settlement, snet(2) = st(2) – se(2)
and so on….
Where snet = net settlement
se = elastic settlement of the pile itself
st = total settlement
These values of Q can be plotted in a graph against the corresponding net settlement, snet as shown in figure 5-15 (c). The ultimate load of the pile can be determined from this graph. Pile settlement may increase with load to a certain point, beyond which the load settlement curve becomes vertical. The load corresponding to the point where Q-snet becomes vertical is the ultimate load, Qu, for the pile. It is shown by curve 1 of the figure 5-15 (c ).
In many cases, the latter stage of the load-settlement curve is almost linear, showing large degree of settlement for a small increment of load; it is shown by curve 2 in figure 5-15 ( c). The ultimate load for such cases is determined from the point of the curve where this steep linear portion starts.
5.12.2 Disadvantages
The performance of single pile does not correspond to actual conditions of performance underneath the structure within the entire group of piles. (2) The loading test must be performed at the actual construction site and under real conditions of the blueprint conditions which are often difficult to fulfill and to execute. (3) This method of test requires specially heavy, sturdy equipment and platforms, precise settlement measuring devices, large quantities of dead load, or powerful hydraulic jacks. (4) The aforementioned conditions and factors make this kind of pile bearing capacity test very expensive.
33
34
Figure 5-15 (a) Schematic diagram of pile load test arrangement; (b) plot
of load against total settlement (c) plot of load against net settlement
35
Figure 5-16 Schematic setup for applying vertical load to the test pile using
a hydraulic jack acting against an anchored reaction frame
36
Problem 5-1Single Pile in Sand
A 12m long, 305mm square section pile is to be embedded in sand. Water table is encountered at 3m depth below the ground surface. Sand has the following properties: =16.8 kN/m3 above WT, sat=18 kN/m3, =35. Angle of friction
between soil and pile is taken to be =0.6, lateral earth pressure coefficient Ks=1.4. Calculate the ultimate compressive load.
Solution:
End-bearing resistance:
qb= oNq from figure 5-6, L/B=12/0.305=40, and =35, Nq=42
qb= 54.742=2297.4 kPa
Friction resistance:
tan'oss Kq
First find average vertical effective stress along the pile length, it is
equal to the area under vertical effective stress distribution diagram
divided by the length of pile.
2' kN/m 46 = 4.575)-(12 54.7+2)-(4.575 54.7)/2+(33.6 + 2)/2 (33.612/1 okPakKq oss 7.24)356.0tan(464.1tan'
L=12m
zc=150.305 =4.575 m
16.82=33.6 kPa
(18-9.81)(4.575-2)+33.6 =54.7 kPa
o=54.7 kPa
2m
37
Ultimate compressive load (Pu)
Pu = Abqb + Asqs
Ab=0.3052= 0.093 m2, As= 40.30512= 14.64 m2
Pu= 0.0932297.4 + 14.6424.7 = 213.6 + 361.6 = 575.2 kN
Example 5-2 Single Pile Capacity in Sand using SPT
A precast concrete pile 450 mm square in section and 9 m long is to be driven into a river bed which consists of a depth of sand. The standard penetration resistance (N) at the pile base is 24, and the average value of N along the pile length is 13. Calculate the ultimate compressive and tensile load carrying capacity of the pile.
Solution
N = 24
13N
Lb = 9.0 m
Ultimate compressive load capacity = Abqb + As qs
Ultimate tensile load capacity = As qs
qb = (40N)Lb/B 400N (kN/m2)
qb = 40 24 9/ 0.45 =
Nqs 2 [kN/m2]
=2 13 = 26 kN/m2
Problem 5-3, Single Pile in Clay
L=12
38
A 400mm, square section concrete pile is driven to an embedded depth of 12m in a cohesive soil, which has the following properties, u=0, =20 kN/m3 both above & below W.T., cu at 12m depth is 85.4 kPa. The water table is at a depth of 3m. Assume =0.4. Calculate safe load capacity for the pile adopting a FOS
of 3 for the base shear and factor of safety of 2.5 for skin resistance.
Solution
End bearing resistance
kPacNq cb 6.7684.8599c
Skin/friction resistance:
osq
Let us find average effective overburden pressure
kPao 9.86)]312(2/)607.151(2/)603[(12/1
kPaq os 7.349.864.0
Ultimate & Allowable Compressive loads:
ssbbu qAqAP
kNPu 7922.6668.1257.3412)4.04(6.786)4.04.0(
L=12m
203=60 kPa
(20-9.81)(12-3)+60=151.7 kPa
3m
39
kNPa 3085.2
2.666
3
8.125
Problem 5-4Pile Group in Clay
Determine the safe load capacity for a square group of 9 piles in cohesive soil. Safety factor 2.5 against block failure. =20 kN/m3, cu at the base 85.4 kPa, average undrained shear strength =60.2 kPa, u=0. Pu for single pile is equal to 819 kN.
Given Data
No of piles = n = 9, =20 kN/m3, cu (base) = 85.4 kN/m2, cu (avg.) = 60.2 kPa
Pu (single pile) = 819 kN
Required
Safe Load Capacity of pile group = Pug
Solution
ssbbgpu qAqAP )(
2/2.60 mkNcq us
cub Ncq
Find Nc by using the relation:-
9053.012.0114.5
gg
gc B
D
L
BN
925.2
12053.01
25.2
25.22.0114.5
cN
945.9 cN
So take 9cN
kPaNcq cub 6.76894.85
40
20625.525.225.2 mAb
depthParameterAs
210812)25.225.2(2 mAs
kNP gpu 103922.601086.7680625.5)(
Ultimate load for piled foundation (Pile cap resting over group).
(a) Base Failure Value=10392 kN (b) Such as of induced pile cap=8199=7371 kN
Minimum of (a) & (b) is selected for Pu(group)= 7371 kN
kNFOS
PP u
allowable 4.29485.2
7371
12 m
2.25 m
2.25 m
