Foundation Interview Questions and Answers

The choice of a large diameter pile suffers from the disadvantage that serious consequences would occur in case there is setting out error of the pile. Moreover, in terms of cost consideration, for the same load capacity, the cost of group of small diameter piles is generally lower than that of a large dimeter pile. On the other hand, for small diameter piles i.e., mini-piles, they are advantageous in site locations with limited headroom and space. In addition, in some structures with only a few piles, it is uneconomical because of its high mobilization cost. (Reference is made to Dr. Edmund C Hambly – 1979)

Shear strength of the soil is the resistance offered by soil grains against the shear deformation. A soil may drive its shear strength from the following parameters.

• Interlocking between particles (Sand and clay drive their strength from this).
• Friction between particles: due to sliding and rolling (Sand, Gravel and Silt drive their strength from friction).
• Intermolecular attraction i.e., adhesion and cohesion (Silt and clay drive their strength from this).

Piles installed in freshly placed fills of soft compressible deposits are subjected to a downward drag, a consequence of the consolidation of the strata after the piles are installed. This downward drag on the pile surface, when the soil moves down relative to the pile, adds to the structural loads and is called negative friction. This is in contrast to the usual shaft friction which is mobilesed when the pile moves down relative to the soil. Thus, negative skin friction has an effect of reducing the allowable load on the pile.

The rational methods of foundation design which being used now to reduce or prevent the effects of swelling can be grouped in three categories, namely

1. Isolating the structure from the swelling soil
2. Designing a structure to withstand the effects of swelling
3. Preventing the swelling.

Belled piers which are popular in USA for use in expansive soil conditions are isolated structures on swelling soils. The under-reamed pile beam construction is similar to that of belled piers and are designed for use under Indian conditions. The principle involved is to transfer the load of a building through the piles to a depth beyond the zone of seasonal variation in moisture content. The under-reamed piles are bored cast-in-situ piles with their lower portions enlarged or reamed in the form of a bulb. The piles are connected at their top by plinth beams of reinforced concrete which support the super-structure.

Designing a structure which is strong enough and rigid enough to withstand the effects of swelling may prove to be highly uneconomical except in the case of very small structures where even if the loads are supported by a central area or peripheral area much smaller than the plan area, the bearing pressures are within limits. Swelling can often be controlled, if not eliminated, by providing an impervious apron around the structure. By providing an apron, the moisture gradient between the center of the structure and its edges is minimized and hence the differential swelling is controlled.

Pile foundations may be considered appropriate for bridges in the following situations:

1. When the founding strata underlies deep standing water and soft soil.
2. When the foundation level is more than 30 m below the water table, so that pneumatic sinking of wells is difficult.
3. When suitable founding strata is available below a deep layer of soft soil and
4. In conditions where pile foundations are more economical than wells.

Cast-in-place concrete piles are constructed in their permanent position by filling with concrete the holes which have been formed in the ground in various ways for the purpose. There are two types:

1. The shell pile, in which a steel shell is first driven with a mandrel and concrete is placed, leaving the shell in place, and
2. The shell-less pile, in which the pipe and mandrel used for making the hole are removed as the concrete is filled in.

Well foundation (open caisson) is the most commonly adopted foundation for major bridges in India. This type evolved in India, and has been adopted for the Taj Mahal. Since then, many major bridges across wide rivers have been founded on wells. Well foundation is preferable to pile foundation when the foundation has to resist large lateral forces, the river bed is prone to heavy scour, heavy floating debris are expected during floods and when boulders are embedded in the substrata.

The shape of wells may be circular, double-D, square, rectangular, dumb, bell, etc. The circular well has the merit of simplicity for construction and sinking. Double-D, rectangular and dumb-bell shapes are used when the bridge has multi-lane   carriageway. For piers and abutments of very large size used in cantilever, cable stayed or suspension bridges, large rectangular wells with multiple dredge holes of square shape may be used.

The following factors are to be considered while determining the thickness of the steining:

1. It should be possible to sink the well without excessive kentledge.
2. The wells should not get damaged during sinking.
3. If the well develops tilts or shifts during sinking, it should be possible to rectify the tilts and shifts without damaging the well.
4. The well should be able to resist safely the earth pressure developing during a sand blow that may occur during sinking.
5. At any level of the steining, the stresses under all conditions of loading that may occur during sinking or during service should be within permissible limits.

The steining is normally of reinforced concrete. The concrete used for steining kg/m3 of concrete with water/cement ratio not more than 0.45.

A bottom plug is essential to transfer the load from the well steining to the base soil. It is usually provided for a thickness of about half the diameter of the dredge hole. In practice the bottom plug is provided up to a height of 0.3 m above the top of well curb* The concrete used is of M20 grade, the richness of the mix being necessitated by the possibility of loss of part of the cement due to under-water placing of the concrete.

Caisson foundations are of two types:

1. Open caissons (also known as well foundations in India)
2. Pneumatic caissons.

An open caisson is one that has no top or bottom cover during its sinking. It is more popularly known as well foundation. A pneumatic caisson is a caisson with a permanent or temporary roof near the bottom so arranged that, men can work in the compressed air trapped under it. Pneumatic caisson can be used for a depth of about 30 m below water level, beyond which pile foundations would have to be resorted to.

Well Curb. The well curb carries the cutting edge for the well and is made up of reinforced concrete using controlled concrete of grade M25. The cutting edge usually consists of a mild steel equal angle of side 150 mm. The angle will have one side projecting downward from the curb for soils where boulders are not expected. In soils mixed with boulders, the angle will have the vertical leg embedded in the steining in such a manner that the horizontal leg of the angle is flush with the bottom of the curb.

Well sinking is a specialized operation requiring considerable skill. When a concrete well is to be sunk on shore or with shallow water depth, it is usual to sink the well up to about 6 m by excavating the soil in the dredge hole by employing skilled divers, and after dredging, pumping out water from the sump to induce sinking. A tripod and a mechanical grab operated by a power winch may also be used. The sinking of the well through the soil is resisted by skin friction along the external surface of the well and by bearing on the cutting edge at the bottom. These resistances are overcome by the dead weight of the well steining reduced by the buoyancy of the submerged portion.