Foundation – Stability Analysis

The maximum load that can be sustained by shallow foundation elements due to the bearing capacity is a function of the cohesion and friction angle of bearing soils as well as the width B and shape of the foundation. The net bearing capacity per unit area, qu, of a long footing is expressed as:


(alpha)f= 1.0 for strip footings and 1.3 for circular and square footings
cu= Un-drained shear strength of soil
(sigma) vo = effective vertical shear stress in soil at level of bottom of footing

(beta)f = 0.5 for strip footings, 0.4 for square footings, and 0.6 for circular footings
gamma =unit weight of soil

B=width of footing for square and rectangular footings and radius of footing for circular footings
Nc, Nq, N=bearing-capacity factors, functions of angle of internal friction (phi)

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Groups of Piles

Group of piles means when we have more than 1 pile in a row. Many factors influence the pile group stability. The major factors are Geometry of the group, soil conditions and direction of loads.

The efficiency factor Eg is defined as the ratio of the ultimate group capacity to the sum of the ultimate capacity of each pile in the group. It is this factor which is mostly used to express the ultimate load considerations.
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Toe Capacity Load

In case of piles being driven in cohesive soils, the ultimate load is alculated by using the followinf formula


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Laterally Loaded Vertical Piles

When ever we are studying about a vertical pile, we need to understand that the flexural stiffness of the shaft and stiffness of the bearing soil in the upper 4D to 6D length of shaft are the two main factors on which the resistance to lateral loads of vertical pile depends.

Nondimensional solutions of Reese and Matlock help us plot the lateral-load vs. pile-head deflection relationship but the basic assumption with this is that the soil modulus K increases linearly with depth z

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Allowable loads on piles

The most commonly used formula to determine the allowable static load on a pile is the Engineering News formula.

Case-1 The allowable load for piles driven by a drop hammer

Case-2 The allowable load for piles driven by a steam hammer

Pa=allowable pile load, tons (kg)
W= weight of hammer, tons (kg)
H= height of drop, ft (m)
p= penetration of pile per blow, in (mm)
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