Woodward, et al., (1972) commented on the empirical design of piers: "Many piers, particularly where rock bearing is used, have been designed using strictly empirical considerations which are derived from regional experience". They further stated that "when surface conditions are well established and are relatively uniform, and the performance of past constructions well documented, the design by experience approach is usually found to be satisfactory."
The principle of drilled piers is to provide a relatively inexpensive way of transferring the structural loads down to stable material or to a stable zone where moisture changes are improbable.
There should be no direct contact between the soil and the structure with the exception of the soils supporting the piers.
Straight-shaft Piers in Expansive Soils
Figure 18.21 (a) shows a straight-shaft drilled pier embedded in expansive soil. The following notations are used.
L1 = length of shaft in the unstable zone (active zone) affected by wetting.
L2 = length of shaft in the stable zone unaffected by wetting
d = diameter of shaft
Q - structural dead load = qAb
q - unit dead load pressure and
Ab = base area of pier
When the soil in the unstable zone takes water during the wet season, the soil tries to expand
which is partially or wholly prevented by the rough surface of the pile shaft of length L1 . As a result
there will be an upward force developed on the surface of the shaft which tries to pull the pile out of
its position. The upward force can be resisted in the following ways.
1. The downward dead load Q acting on the pier top
2. The resisting force provided by the shaft length L2 embedded in the stable zone.
Two approaches for solving this problem may be considered. They are
1. The method suggested by Chen (1988)
2. The O'Neill (1988) method with belled pier.
Two cases may be considered. They are
1. The stability of the pier when no downward load Q is acting on the top. For this condition a factor of safety of 1.2 is normally found sufficient.
2. The stability of the pier when Q is acting on the top. For this a value of Fs = 2.0 is used.
Equations for Uplift Force Qup
Chen (1988) suggested the following equation for estimating the uplift force Qup
The depth (L1) of the unstable zone (wetting zone) varies with the environmental conditions.
According to Chen (1988) the wetting zone is limited only to the upper 5 feet of the pier. It is possible for the wetting zone to extend beyond 10-15 feet in some countries and limiting the depth of unstable zone to a such a low value of 5 ft may lead to unsafe conditions for the stability of structures. However, it is for designers to decide this depth L1 according to local conditions. With regards to swelling pressure ps, it is unrealistic to fix any definite value of 10,000 or 5,000 psf for all types of expansive soils under all conditions of wetting. It is also not definitely known if the results obtained from laboratory tests truly represent the in situ swelling pressure. Possibly one way of overcoming this complex problem is to relate the uplift resistance to undrained cohesive strength of soil just as in the case of friction piers under compressive loading. Equation (18.15) may be written as
It is possible that the value of αs may be equal to 1.0 or more according to the swelling type and environmental conditions of the soil. Local experience will help to determine the value of αs
This approach is simple and pragmatic.
The length of pier embedded in the stable zone should be sufficient to keep the pier being pulled out
of the ground with a suitable factor of safety. If L2 is the length of the pier in the stable zone, the
resisting force QR is the frictional resistance offered by the surface of the pier within the stable
zone. We may write
For a given shaft diameter d equations (18.18a) and (18.18b) help to determine the length L2 of the pier in the stable zone. The one that gives the maximum length L2 should be used.
Piers with a belled bottom are normally used when large uplift forces have to be resisted. Fig. 18.21(b) shows a belled pier with all the forces acting.
The uplift force for a belled pier is the same as that applicable for a straight shaft. The resisting force equation for the pier in the stable zone may be written as (O'Neill, 1988)
Figure 18.21 Drilled pier in expansive soil
Table 18.8 Values of N.
Figure 18.22 Grade beam and pier system (Chen, 1988)
For a given shaft diameter d and base diameter db, the above equations help to determine the value of L2 The one that gives the maximum value for L2 has to be used in the design.
Fig. 18.22 gives a typical foundation design with grade beams and drilled piers (Chen, 1988). The piers should be taken sufficiently below the unstable zone of wetting in order to resist the uplift forces.