| |
i am working on a project where i am stuck with the concept of critical depth for pile capacity.
The soil conditions within the top 30 consist of silts, lean clays with interlayered sand layers with a generalized c = 200 and phi = 31 based on driect shear tests. From about 30 to 40 feet are well-graded/poorly graded sands with N values of about 40 to 60. Direct shear tests yield c = 200 and phi = 37. Below 40 feet to about 70 feet is lean clay strata with c = 200 and phi = 31.
Out of all pile foundations, the client selected 14-inch PCC driven pile. The PDA and CAPWAP data indicated high capacity for pile legths of about 30 to 40 feet tipping in dense to very dense sand...but lower capacities for longer piles tipping in clays below 40 feet..
Intially we recommended a 50-foot pile tipping in lean clay, howevever since the CAPWAP indicated low capacity, we are thinking about using 40-foot pile tipping in dense sands.
but , based on the critical depth concept, assuming that the tip resistance remains constant below 20(D) which is 23 fee for this case, i will be loosing a lot of end bering capacity if i follow criical depth concept. and i have to lengthen the pile to 6o to 70 feet.
so, i am stuck with a judgement call as to whether the critical depth concept is really valid or not. if it is not, the 40-foot design pile length would significantly save foundations costs.
i would really appreciate if anybody could shed some light and provide some advice on this controversial concept.
also, please forward this article if you have it
FH Kulhawy, "Limiting Tip & Side Resistance - Fact or Fallacy", Analysis & Design of Pile Fndns., Ed. JR Meyer, ASCE, New York, Oct 1984, 80-98.
thanks in advance..
|
|
Subject:
critical depth - fact or fallacy? 
