Foundation Analysis

Questions:

1. Discuss the following cases of group piles a. In cohesion less soil b. In cohesive soil c. On hard rock

2. Write short notes on settlement of pile group. 3. What are the basic considerations of pile cap design? 4. Discus the cases of negative skin friction briefly.

Solution of Question no – 1:

Piles group in cohesion less soil:

Driven piles: If piles are driven into cohesion less soil like loose sands and gravel, the soil around the piles to a radius of at least three times the pile diameter is compacted. When piles are driven in a group at close spacing, the soil around and between them becomes highly compacted. When the group is loaded, the piles and the soil between them move together as a unit. Thus, the pile group acts as a pier foundation having a base area equal to the gross plan area contained by the piles.

Where

n = number of piles in the group = bearing capacity of an individual pile = bearing capacity of pile group

Bored Pile Groups in Sand and Gravel: Bored piles are cast-in-situ concrete piles. The method of installation involves

• Boring a hole of the required diameter and depth. • Pouring in concrete.

There will always be a general loosening of the soil during boring and then too when the boring has to be done below the water table. Though bentonite slurry (sometimes called as drilling mud) is used for stabilizing the sides and bottom of the bores, loosening of the soil cannot be avoided. Cleaning of the bottom of the bore hole prior to concreting is always a problem which will never be achieved quite satisfactorily. Since bored piles do not compact the soil between the piles, the efficiency factor will never be greater than unity. However, for all practical purposes, the efficiency may be taken as unity.

Pile Groups in Cohesive Soils The effect of driving piles into cohesive soils (clays and silts) is very different from that of cohesion

less soils. When piles are driven into clay soils, particularly when the soil is soft and sensitive, there will be considerable remolding of the soil. Besides there will be heaving of the soil between the piles since compaction during driving cannot be achieved in soils of such low permeability. There is every possibility of lifting of the pile during this process of heaving of the soil. Bored piles are, therefore, preferred to driven piles in cohesive soils. In case driven piles are to be used, the following steps should be favored:

1. Piles should be spaced at greater distances apart. 2. Piles should be driven from the center of the group towards the edges, and 3. The rate of driving of each pile should be adjusted as to minimize the development of pore water

pressure. Experimental results have indicated that when a pile group installed in cohesive soils is loaded, it may fail by any one of the following ways:

1. May fail as a block (called block failure). 2. Individual piles in the group may fail.

When piles are spaced at closer intervals, the soil contained between the piles move downward with the piles and at failure, piles and soil move together to give the typical 'block failure'. Normally this type of failure occurs when piles are placed within 2 to 3 pile diameters. For wider spacing’s, the piles fail individually. The efficiency ratio is less than unity at closer spacing’s and may reach unity at a spacing of about 8 diameters. The equation for block failure may be written as ……………………… (1) Where,

c = cohesive strength of clay beneath the pile group = average cohesive strength of clay around the group

L = length of pile = perimeter of pile group

A = sectional area of group = bearing capacity factor which may be assumed as 9 for deep foundations

The bearing capacity of a pile group on the basis of individual pile failure may be written as

……………………… (2)

Where,


The bearing capacity of a pile group is normally taken as the smaller of the above two equations.

Pile Groups in Rock The stability of a pile group bearing on a rock formation is governed by that of the individual pile. For example, one or more of the piles might yield due to the presence of a pocket of weathered rock beneath the toe. There is no risk of block failure unless the piles are terminated on a sloping rock formation, when sliding on a weak clay-filled bedding plane might occur if the bedding is unfavorably inclined to the direction of loading. The possibility of such occurrences must be studied in the light of the information available on the geology of the site. The bearing capacity of a pile group on the basis of individual pile failure may be written as

Where


Solution of Question no – 2: The settlement of pile groups may be viewed as the result of four separate causes:

1. The axial deformation of the pile. 2. The deformation of the soil at the pile-soil interface. 3. The elastic or immediate deformation of the soil between the piles 4. The consolidation deformation in the stratum below the tips of the piles.

The settlements reflected by cause 1 and 2 are relatively small and are, therefore, usually neglected. The settlement described by cause 3 is relatively small. The settlement reflected by cause 4, however can be estimated by treating the pile group as an equivalent footing and computing the consolidation theory. The total settlement is the sum of 3 and 4. Elastic Settlement of pile groups: The elastic settlement is the immediate settlement of the pile group due to the deformation of the soil mass. The elastic settlement of pile group under service working loads per pile increases with the width of the group ,

∆

Where, = the elastic settlement of the entire pile group = the width of the pile group section

s = the spacing of the piles to each other (center-to-center) d = the width or diameter of each pile in the group

= the elastic settlement of each pile at their service load

Consolidation settlement:

The stress increase P at the middle of layer in the soil mass due to the group load Q is,

∆

and the settlement at that level can be given as a function of the voids ratio

∆ ∆

Therefore, the total consolidation settlement of the pile group is,

∆ ∑ ∆

23 L

L1

Layer 1

Layer 2

Layer 3

Hard Strata

21

21 L2

L3

Z

L

Q g

It is well recognized that the settlement of a pile group can differ significantly from that of a single pile at the same average load level. However, the settlement of piled structures must be estimated on the basis of the group action of the piles, not on a single pile test.


Unless a single pile is used, a cap is necessary to spread the vertical and horizontal loads and any overturning moments to all the piles in the group. The cap is usually of reinforced concrete, poured on the ground unless the soil is expansive. Pile caps are almost invariably made of reinforced concrete and are designed as individual footings subjected to the column loads plus the weight of the pile cap and the soil above the cap. Under a concentric load, all piles in the same group are assumed to take equal axial loads. The soil under the pile cap is not assumed to offer any support. Under an eccentric loading or a concentric loading plus a moment, the pile cap is designed in accordance with the following consideration:

1. Pile cap is perfectly rigid. 2. Pile heads are hinged to the pile cap, therefore, no bending moment is transmitted from the pile

cap to the piles. 3. Piles are short and elastic columns, therefore, the deformations and the stress distribution are

planar. 4. The pile cap is in contact with the ground. 5. The piles are all vertical. 6. Load is applied at the center of the pile group. 7. The pile group is symmetrical and the cap is very thick (or rigid) 8. Each pile carries an equal amount of the load for a concentric axial load on the cap; or for n piles

carrying a total load Q, the load per pile is

9. The combined stress equation (assuming a planar stress distribution) is valid for a pile cap non-centrally loaded or loaded with a load Q and a moment, as

∑ ∑

Where,

M , M momnet about x and y axis, respectively

,

∑ , ∑

.


Negative skin friction is downward drag acting on the piles due to relative movement between the piles and the surrounding soil. When piles are driven trough compressible soils, and the site has a newly placed fill or will be filled in the future, the possibility of negative skin friction should be investigated. Soft to medium clays, soft silt, peat, mud etc are compressible soils. Lowering of ground water level in such compressible soils may also bring about negative skin friction. If there is any doubt as to the compressibility of the soil, consolidation test should be made. If the results indicate that the layer will settle under the weight of the fill, even only a small amount, the pile capacity should be reduced to compensate the drag due to negative skin friction.

Some of the conditions where negative skin friction can occur are given below:

1. When the pile is installed in a fill, which will undergo consolidation. 2. In a soil which will be disturbed of remolded thoroughly during the pile installation. 3. In piles installed in soft clay with surcharge loading on it. 4. In soil where lowering or variation of ground water can occur, thus leading to significant

settlement of soil strata around the pile. 5. In cases where piles are driven through a strata of soft clay into firmer soils and the soft clay tends

to settle relative to the pile. 6. In piles in a clay stratum which undergoes shrinkage settlement.

Settlement due tocomppression ofcompressive soil

Compressible Layer

Hard Stratum

New Fill

Q Q

AA

Perimeter

Area

Section AA

Figure: Negative Skin Friction

Estimation of negative skin friction:

The conditions which cause negative skin friction on piles are shown in figure. Under the weight of the new fill, or due to lowering of ground water to a level below the compressible layer, the compressible layer settles gradually for a long period of time. Consequently all soil strata above this layer settle with it. The downward movement of the soil tends to drag the piles by shear along the pile surfaces. For a single pile this downward force is equal to the shear resistance times the surface area of the pile. For a pile group, the soil between the piles may be held by friction or adhesion of the pile surface, and hence, there is no relative movement between the piles and the soil in between. Therefore the negative skin friction, on pile and pile group is

For Single pile,

.

For Single pile group,

.

(Whichever is smaller)

Where,

tan

,

,

Methods of Mitigating Negative Skin Friction

The following methods are used to mitigate skin friction in piles:

1. Coat the surface of the precast pile with thick coat of special bituminous paint which have been proved to reduce skin friction as much as 90% of the theoretical value.

2. Drive the piles inside a casing. In the top negative friction height, the space between pile and casing filled with a viscous material and the casing is withdrawn after installing the pile.

References:

1. Michael Tomlinson and John Woodward, “Pile design and construction practice” Fifth Edition 2. VNS Murthy, “Advanced Foundation Engineering, Geotechnical Engineering Series” , First

Edition. 3. Bowles, Joseph E., “Foundation Analysis and Design”, Fifth Edition. 4. John N. Cernica, “Geotechnical Engineering Foundation Design”. 2005. 5. Wayne C. Teng. “Foundation Design”. 1992

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Foundation Analysis