6.1 INTRODUCTION

In the construction of various types of foundations, piles are generally used to transmit downwardly-directed load to a stronger soil at a greater depth. They are also used to resist lateral load imposed on a foundation. During the last three to four decades, several theoretical and experimental studies were conducted by various investigators to evaluate the downwardly-directed and lateral load-bearing capacity of single and group piles embedded in sandy and clayey soils.

Piles can also be used in the construction of foundations subjected to uplifting forces. The uplift force is resisted by skin friction developed at the soil-pile interface (Figure 6-1). At the present time, limited studies are available to estimate the uplift capacity of piles. Only a few laboratory model study results are available regarding the efficiency of group piles subjected to uplifting forces.

Figure 6-1: Pile subjected to uplifting load

The net ultimate uplift capacity of a single pile can be expressed as (Figure 6-1)

The net ultimate uplift capacity of a pile embedded in sand is primarily a function of the following parameters:

1. length of embedment, L;

2. pile diameter, D;

3. roughness of the pile surface;

4. soil friction angle ? and its relative density; and

5. nature of placement of the pile (driven, bored or cast-in-place).

In a similar manner, the magnitude of Q _u for a pile embedded in saturated...