An Appraisal Of The Existing Practice Of Determining The Axial Load Capacity Of Deep Penetration Piles In Calcareous Sands
Authors: Datta Manoj , Gulhati Shashi K. , Rao Guda V. 
Published: October 2008

Particle crushing and cementation in calcareous sands are known to affect the axial load capacities of piles in such sands, Particles of these sands differ in their nature, shape and form and exist both in cemented and uncemented states. The current practice for design of piles does not consciously consider these differences. For uncemented sands, this paper presents laboratory triaxial test data to demonstrate their different susceptibilities to crushing and consequently their different shear behaviour. Further, experimentally determined values of steel - to - sand skin friction are presented. For cemented sands, a review of results of load tests in cemented calcareous sand strata and chalk are presented. This data and information is used to appraise the existing design practice.

This appraisal highlights the fact that existing practice is conservative for uncemented calcareous sands which do not crush significantly whereas it is probably somewhat unsafe for calcareous sands prone to high crushing under stress. For cemented deposits it indicates that current practice does not appear to take full advantage of the unit end bearing likely to develop where as it probably over-estimates the unit skin friction.

INTRODUCTION

The axial capacity of piles is usually determined from results of load tests, from dynamic formulae or from the static method. For the design of deep penetration piles being used for foundations of jacket-type fixed offshore platforms, the static method is generally adopted. The current practice for design of piles in calcareous sands is often limited to using reduced values of limiting unit skin friction and limiting unit end bearing in relation to the values used for design of piles in non-calcareous (i.e. quartzitic) sands

Calcareous sands have grains which are of widely diverse nature, shape and form, depending upon whether they are skeletal or non-skeletal in origin. Recent studies (Datta et al (1979)9) have shown that this difference in origin markedly affects the response of these sands to stress. Further, some deposits of such sands are encountered in a cemented state which also influences their behaviour under stress.

This paper presents a study undertaken to assess, whether or not, the existing practice of determining axial capacity of piles in calcareous sands is appropriate for all types of such sands. This assessment is based on a review of field test results and on results of a laboratory investigation currently being conducted at I. I.T. Delhi (India). This appraisal is confined to driven, open ended cylindrical steel piles.

EXISTING DESIGN PRACTICE

The ultimate bearing capacity, Q, of a pile driven in sand is determined as the sum of the total skin friction resistance, Qf, and the total end bearing resistance, Qp' offered by the sand, i.e.

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The magnitudes of f and q have been shown in soil mechanics literature to depend upon soil type, pile type and pile installation procedure. For offshore piles, general recommendations of the American Petroleum Institute (API) are widely used for design purposes. Hence attention is focused on these recommendations as well as on the specific recommendations of some investigators/consultants.