ABSTRACT
O'Neill and Reese (1999) recommend the chart's trend line given in Fig. 17.15 for designing drilled piers. The suggested relationships are:
a =0.55 for cu / Pa :::; 1.5 (17.25a)
(17.25b)
Cohesive Intermediate Geomaterials Cohesive IGM's are very hard clay-like materials which can also be considered as very soft rock (O'Neill and Reese, 1999). IGM's are ductile and failure may be sudden at peak load. The value of fa (please note that the term fa is used instead of Is for ultimate unit resistance at infinite displacement) depends upon the side condition of the bore hole, that is, whether it is rough or smooth. For design purposes the side is assumed as smooth. The expression for fa may be written as
(17.26) where, qu = unconfined compressive strength
fa = the value of ultimate unit side resistance which occurs at infinite displacement. Figure 17.16 gives a chart for evaluating a. The chart is prepared for an effective angle of
friction between the concrete and the IGM (assuming that the intersurface is drained) and Sf denotes the settlement of piers at the top of the socket. Further, the chart involves the use of an /pa where an is the normal effective pressure against the side of the borehole by the drilled pier and Pais the atmospheric pressure (101 kPa).
