INTRODUCTION TO DEEP FOUNDATION

1. Types of Piles

Piles may be classified according to their composition or function as follows:

a. Composition of Piles

(1) Timber Plain or treated with preservative.(2) Concrete Precast or cast – in – place.(3) Composite Commonly timber or steel (Lower portion of the

pile)and Concrete (Upper portion of the pile).(4) Steel H-pipe pile or sheet pile.

b. Function of Piles

(1) Bearing Piles May be point bearing or pedestal type bearing piles. These are used for bridge or building foundation when:

Soil strata immediately underneath is weak and load is to be transferred to a strong strata e.g., rock.

There is danger of upper strata being scoured away by current or wave action.

Bearing capacity is drawn totally from bearing strata.

(2) Friction Piles Friction piles are supported by friction between pile and surrounding soil. Clay loses strength initially due to pile driving but regains strength by the

time piles are loaded. Pile capacity in granular soils depended on internal friction angle and

coefficient of lateral earth pressure. Pile driven through several layers of soil derives bearing capacity from the

supporting strength of each layer.

(3) Tension Piles or Uplift

Anchor down the structure subjected to uplift due to hydrostatic pressure. Resist overturning of wall and dams.

(4) Laterally Loaded Piles Used for retaining walls, bridges, dams and wharves i.e., sheet piles and seepage cut off walls.

(5) Bearing and Friction Piles These are used in soils where support capacity of pile is drawn from bearing and skin friction.

(6) Compaction PilesThese are used to compact granular soil, thus increasing their bearing capacity.

(7) Fender Piles are DolphinsThese are used to prevent water front structures against impact from ships or other floating objects.

(8) Batter PilesThese are used to resist large horizontal or inclined forces.

2. Static Pile Capacity

All static pile capacities can be computed by the following equations (Bowles 1988):

PU= Ppu + Psi (Compression)

Tu = Psi + W (Tension)

Where

Pu = Ultimate pile capacity in compression.Tu = Ultimate Pullout capacityPpu = Ultimate point capacityPsi = Total skin (or shaft friction) resistance

Contribution from several strata penetrated.Neglect for “end bearing” piles.

W = Weight of pile.

The allowable pile capacity Pa or Ta is obtained by applying a suitable factor of safety (F.O.S) such as:

Pa = Pu/F.O.S

Ta = Tu/F.O.S

a. Ultimate Static Pile Point Capacity

The ultimate static pile capacity can be combated as follows:

PPu=A p(c N 'c+q N 'q+ 12γB N 'r)

WhereAp = Area of pile point effective in bearing.c = Cohesion (or undrained shear strength Su)B = Base width of pile.N′c = Bearing capacity factor for cohesion adjusted

for shape and depth. When Φ=0, we have c=Su and Nc is often taken as 9.

N′q = Bearing capacity factor for overburden effect q=γ L and include shape and depth effects.

N′r = Bearing capacity factor for base width.q = Effective vertical stress (overburden pressure) at pile point.

The N’r term is usually neglected as the pile base width B is not large. It may not be neglected where an enlarge pile base is used. Neglecting the N'r tern, the pile capacity can be computed as:

PPu=A p ¿ )

This equation gives for c=Su and Φ=0, the N′q = 1 and ultimate point capacity is:

PPu=A p(9 Su)

The allowable pile capacity is obtained by dividing the ultimate capacity by a suitable safety factor which usually varies between 2.5 and 4.

Meyerhof (1951, 1976) has proposed bearing capacity factors N′c and N′q as shown in Figure 7-2. The factor incorporate the effect of shape and depth effect (e.g., from N q to N′q and Nc to N′c). Meyerhof used a critical depth ratio Lc/B which basically means that if the embedment depth exceeds the critical depth ratio, the bearing capacity reaches a limiting value. The limiting point bearing in cohesionless soil is given for L/B > Lc/B as:

PPU=APq N ' q≦ A p (50N ' q ) tan Ø (KN)

Meyerhof also uses 9Su as the limiting resistance for clay (ϕ=o). To determine Meyerhof N factors from Figure 7-2, proceed as follows:

Compute R1 =L/B and obtain R2 =Lc/B from the critical depth curves at the given ϕ angle of figure 7-2.

Enter the curves: if R1 > 0.5R2 and ϕ is less than 30°, obtain factors from the upper N1 curves. If R1 <0.5R2 use al liner ratio between the lower and upper Ni

curve ,e.g.,

N 'C=NC '+(N ' '¿¿C−NC)

R1

0.5 R2¿

If ϕ > 300 , and depending on L/B, project to the reduced curves shown in the upper right part of Figure 7-2 and interpolate as necessary.

b. Skin Resistance Capacity

The skin resistance capacity, Psi of a pile can be given as:

Psi = Σ As fs (Units of fs)

Where

As = Effective pile surface area on which fs acts and is computed as:As = Perimeter x embedment increment ΔL.

Circular Piles As = π D ΔLSquare Piles As = 4 B ΔL

ΔL = Increment of embedment length (to allow for Pile shaft variations and soil stratification). In homogeneous soil and pile with uniform cross-section, ΔL is equal to the length of pile embedded in soil.

fs = Skin resistance

Tomilson (1971) proposed method for computing skin resistance fs which is known as α method.

α Method

The skin resistance using α method (Tomilson method) can be computed as follows.

fs = αc + σv k tanδ (units of c ,σv )Where

α = Coefficient given in Figure 7-3 and Table 7-2c = Average cohesion (or Su) for the soil stratum of interest.σv = Average , effective vertical stress on element ΔL. If γ is

the effective unit weight of soil then:σv = 0.5 γ′ΔL

In case a uniform surcharge q also acts, then the total effective stress can be given as:

σv = 0.5 γ′ ΔL + q

k = Coefficient of lateral earth pressure ranging from

ko to about 1.75 k0 depending on volume of displacement and initial soil density, etc. Values close to ko are generally recommended because of long-term soil creep effect even where the pile has a large volume. ko for piles is most commonly computed as:

ko = (1- sinϕ) OCR

In which OCR is an over consolidation ratio.

δ = Effective friction angle between soil and pile material (Table 7-3). Use δ=0 when ϕ=0.

American Petroleum Institute (API) also suggests using α method with factors as shown in Figure 7-3.

Table 7.2 Values of adhesion factors for piles driven into very stiff cohesive soil for design (Bowles 1988).

Case Soil ConditionsPenetrationRatio, PR

AdhesionFactor

1Sands or sandy gravelsOverlying stiff to very stiffCohesive soils

<20

>20

1.25

Fig 7-3

2Soft clays or silts overlyingStiff to very stiff cohesiveSoil

8<PR<20

>20

0.40

Fig 7-3

3Stiff and very stiff cohesiveSoils without overlying strata

8<PR<20

>20

0.40

Fig 7-3* Penetration ratio, PR = Depth of Penetration/Diameter of pile

Table 7.3 Friction angle between various foundation material and soil and rock. (Bowles, 1988 Page 498).

Interface materialFrictionAngle, δDegrees

Mass Concrete or masonry on the following Clean sound rock

Clean gravel, gravel- sand, coarse sandClean fine to medium sand, silty medium to Coarse sand, silty or clayey gravelClean fine sand, silty or clayey fine to sandFine sandy silt, non plastic siltvery stiff and hard residual or pre-consolidated clayMedium stiff and stiff clay and silty clay

Steel sheet pile against:

Clean gravel, gravel-sand mixture well graded rock fill with SpallsClean sand, silty sand-gravel mixture, single-size hard-Rock fillSilty and gravel or sand, gravel or sand mixed with silt or ClayFine sandy silt, non plastic silt

Formed concrete or concrete sheeting against:

Clean gravel, gravel-sand mixtures, well graded rock fillWith spallsClean sand, silty sand –gravel mixture, single size rock fillSilty sand, gravel or sand mixed with silt or clayFine sandy silt, non plastic silt

Various structural materials:Masonry on masonry, igneous and metamorphic rocks:Dressed soft rock on dresses soft rockDressed hard rock on dresses soft rockDressed hard rock on dresses hard rockMasonry on wood (dross grain)Steel on steel at steel-pile interlocks Wood on soil

3529-3124-29

19-2417-1922-26

17-19

22171411

22-2617-221714

353329261714-16

3. Pile Groups

a. General

Pile foundation rarely consists of a single pile. Generally, there will be a minimum of two or three piles under a foundation element or footing to allow for misalignments and other inadvertent eccentricities. Building codes may stipulate the minimum number of pile under a building element. The load capacity, setting and individual pile loads associated with pile groups is the concern of this section Figure 7-4 presents some typical pile clusters for illustrative purposes only, since the designer must make up the group geometry satisfy any given problem.

b. Pile –Group Considerations

When several piles are clustered, it is reasonable to expect that the soil pressure produced from either side friction or point bearing will overlap as idealized in Figure 7-5. The superimposed pressure intensity will depend on both the pile load and spacing and if sufficiently large, the soil will fail in shear or the settlement will be excessive. The stress intensity from overlapping stressed zones will obviously decrease with increased pile spacings; however large spacing are often impractical since a pile cap is cast over the pile group for the column base and/or to spread the load to the several piles in the group.

Spacing of piles in pile group usually varies between 2.5 to 3.5 D or 2 to 3 H for vertical loads. Here D= pile diameter; H = diagonal of a rectangular or H pile. For groups carrying lateral and/or dynamic loads, larger pile spacings are usually more efficient. Maximum pile spacings are not given in building codes but spacings as high as 8 or 10 D have been on occasion.

c. Efficiency of Pile Groups

When several piles attached to a common slab member, termed a pile cap, make up a group, a question arises of whether the cap load, or group capacity, can be taken as the sum of the several individual pile design loads or as some lesser amount. If the capacity is the sum of the several individual pile contributions the group efficiency, Eg = 1.0.

There are mixed opinions on pile group efficiency as:

Eg= GroupcapacityNumber of piles x individual pile capacity

A realistic guideline as follows:

Friction piles in granular soils at the usual spacing of s=2 to 3D will have Eg≥1 (this is because granular soil densifies in the vicinity of a driven pile from the initial soil parameters used for capacity computations).

Friction piles in cohesive soils., the block shear (side friction) + point bearing of the group in plan is used as the capacity but in no case is the group capacity to be considered greater than the single pile capacity times the number of piles in the group.

At present the converse-Labarre equation [see Moorhous and Sheehan (1968)], has been widely used to compute group efficiency in the past. The ASSHTO Bridge Specifications still give it as a “suggestion” for friction piles. The converse-Labarre equation is:

Eg=1−θ (n−1 )m+ (m−1 )n90mn

Where m , n and D are shown on Figure 7-6 and θ= tan -1 D/s in degrees. This equation is limited to rectangular groups with identifiable values of m x n.

d. Group Capacity When a concrete pile cap is poured directly on the ground as the most common case, the

group capacity is at least the block capacity based on the shear around the perimeter of the group defined by the plan dimensions plus the bearing capacity of the block dimension at the pile points. The only exception is point bearing piles founded in rock where the group capacity would be the sum of the individual point capacities.

When the pile cap is above ground as is common for offshore structures, the group capacity will be the lesser of:

Block capacity based on a perimeter on defined by the exterior piles (see distance L1 of Figure 7.6) plus bearing capacity of block at the pile points, or

Sum of the capacity of the individual piles. This often controls for large s/D ratios.

4. Pile Load Test

Many variables influencing the behavior of a pile under load have led to the practice of carrying out load tests on one or more piles at the site of important projects. The test piles are preferably of the same type and are driven by the same equipment to the same requirement as anticipated for the job. In some instance, several alternatives are investigated to permit refinement in design. All pertinent details of the equipment and procedure are recorded during the driving of a test pile, including the blows per inch of penetration, preferably throughput the entire embedded length of the pile. Any pause in driving, as for breakdown of equipment or for splicing the pile, in noted.

The Load is usually applied in increments by means of a hydraulic jack reacting against a dead weight or against a yoke fastened to a pair of anchor pile, Figure 4.7. Under each increment the settlement of the head of the pile is observed as a function of time until the rate of settlement become very small. A new increment is then applied. As the capacity of the pile is approached the size of the increment is decreases in order to permit better definition of the load at which the capacity of the pile is reached.

The results of typical load tests are shown in Figure 4.8 in which the total load is plotted as a function of the settlement of the pile head. Curve “a “represent a pile that slipped or plunged suddenly when the load reached a definite value termed the ultimate pile load or pile capacity. Curves “b” and “c” on the other hand, show no well defined breaks; consequently, the determination of the ultimate pile load is to some degree a matter of interpretation.

A reasonable procedure (Division, 1997) that takes account of the significant variables is

illustrated in the figure. The elastic deflection of the pile is computed by the mean of the expression PL/AE and plotted on the load settlement diagram as line 00’; for the best interpretation the scales of the diagram should be chosen so that the slope of 00’ is about 20 degree. The line CC, is drawn parallel to line 00, with an intercept on the settlement axis equal to (0.15+0.1d) in, where d is the diameter of the pile in feet. The intercept is measure of the tip settlement required to develop the capacity. The ultimate load is defined as the load at which the line cc intersects the load- settlement curve. This criterion is applicable to load settlement records obtained from tests in which each load increment is held for periods not exceeding 1 hr. It is overly conservative, however, for load settlement records based on loading periods of 24 hr or longer because creep or consolidation settlements may then become significant portions of the total settlement.

5. Negative Skin Friction

When a fill is placed on a compressible soil deposit, consolidation of the compressible

material occurs. A pile driven through (or into) the compressible material (either before or after fill placement) before consolidation is complete, the soil will move downward relative to the pile. This relative movement will develop skin friction between the pile and the moving soil termed negative skin friction. According to measurements reported by Bjerrum et al. (1969), Bozozuk (1972), and Bozozuk et al. (1979), the negative skin friction can exceed the allowable load for pile sections. Fellenius (1972) has also reported large values of measured negative skin resistance.

The principal effect of negative skin resistance is to increase the axial load in the lower fixed portion of the pile. It may result also in increased pile settlements due to the shortening of the pile under the increased axial load. “Negative” skin friction can produce large tension stresses when the effect is from expansive soils; especially if no, or insufficient, gap is left between soil and pile cap and the soil expand against the cap. Negative skin friction can develop from:

a. A cohesive fill placed over cohesion less soil deposit; the fill develops shear resistance between the soil and pile from lateral pressure/flow effects and drags the pile downward as the fill consolidates. Little effect is produced in the underlying soil except that the weight of fill increases the lateral pressure.

b. A cohesion less fill placed over a compressible, cohesive deposit. In this case there will be some down drag in the fill zone, but the principal down drag will

occur in the zone of consolidation. For point-bearing piles, any settlement of the group will be due to axial shortening of the pile. For floating piles, additional penetration with matching settlement will occur unless the pile is sufficiently long that the bottom portion can develop enough positive skin resistance to balance the additional load developed by negative skin resistance. Lowering of the groundwater table with resulting ground subsidence.

c. Pile- driving (and load-test) operations which produce negative stresses in the upper shaft when the load is released and the pile shaft expands upward. The resulting slip and negative skin resistance must be balanced by a positive skin resistance in the lower shaft and/or point load [Vesic (1977)].

Example 1Estimate the ultimate pile capacity of a round concrete pile that is 23 m long and has a diameter of 300 mm. The pile is in offshore environment and 23 m is the only part embedded in the soil. The soil profile indicates an average unconfined compression strength of 24 kPa. The submerged unit weight of soil is estimated as 8.15 kN/m3. Assume α = 1.25.

Example 2Estimate the pile length required to carry 150 kip axial load for the pile soil system shown in the figure. The 18 inch pile to be filled with concrete after driving. Neglec pile weight. Use Alpha method to find skin friction.