ABSTRACT

A design procedure is presented for predicting the frictional Capacity of grouted piles in sand. 'The premises underlying current design practice as they apply to the frictional capacity of piles in sand are reviewed, especially the empirical and theoretical basis for limiting skin friction for driven and grouted piles in sand. In the proposed new procedure, soi1-to-grout friction is predicted as a function of the grout fluid pressure, increasing hydrostatically with depth and with effective pile-soil load transfer limited only by the bond stress between the grout and pile. The changes in lateral stresses during installation and the influence of dri11ing mud on the development of skin friction are discussed.

INTRODUCTION

The variety of soil conditions encountered in the investigations for offshore structures has increased with the expansion of exploration in such areas as Alaska, Africa, India, the North Sea, the Arabian Gulf, Southeast Asia, extending recently into waters more than 1000 ft deep. Since 1947, most of the experience with driven piling for the support of offshore platforms has been accumulated in the Gulf of Mexico off Texas and Louisiana, where the sub soils primarily consist of normally and underconso1idated clays with scattered beds of sand. Conventional driving procedures developed for pile installation in the Gulf of Mexico frequently are inadequate to achieve the required penetrations in other areas. Pile driving difficulties are usually attributable to strata of dense sand, stiff to hard clays, cemented sand, and rock which occur in the offshore environments outside the Gulf of Mexico.

When the soil conditions disclosed by a site specific geotechnical investigation suggest that adequate pile capacities cannot be achieved by driving alone, grouting of an insert pile into an oversized drilled hole extending below a driven primary pile, as sketched on Fig. 1, is a potential design and construction choice. This pile system has been frequently chosen as the means of achieving high capacity piles with substantial penetrations in hard clays, dense sands, or rock. Grouted pile penetrations are known to have reached at least 400 ft 23.Readily available information about the design, installation and performance of grouted piles has been meager to nonexistent, and as a result, the design of grouted piles has followed criteria developed for driven or bored piles, without special consideration being given to the unique characteristics of the grouted pile installation.

The objectives of this paper are to (1) review current criteria and previous research related to predicting the frictional capacity of grouted piles embedded in sand, (2) discuss the unique characteristics of grouted piles and the factors affecting their frictional capacity, and (3) propose rational design criteria for the frictional capacity of grouted piles embedded in sand. Current criteria for predicting the end bearing capacity of grouted piles embedded in sand are considered appropriate for design and therefore are only referenced and not discussed in detail herein.

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