A New Foundation Technique Using Pi les Sealed By Cement Grout Under High Pressure

Abstract

ABSTRACT The full-scale tests carried out on steel piles have shown that the injection of cement under high pressure over the body of a pile allows one to attain very high bearing capacities, two to three times those found for a conventional pile. The resulting reduction in the pile's length implies several advantages for offshore works (platform foundations, diverse anchorages, port works, etc.). INTRODUCTION The construction of high structures (such as high-rise buildings, chimneys, fixed offshore drilling platforms, etc.), large span frame s (Viaducts), port works for high dead-weight vessels (dolphins, anchor buoys, etc.) poses more and more foundation problems. The solutions proposed at the present time are cumbersome, since the axial loads (in compression or tension) and horizontal loads are very high. The classical techniques of deep foundations do not permit one to reach high capacities, and therefore the eventual solutions seem hardly appropriate to the problems that arise. One can, in effect, orientate oneself either toward a large number of piles (but the foundation is penalized by the group effect) or try to attain high individual capacities. As the classical methods do not permit one to employ a high average side friction, large pile lengths in the soil are necessary for floating piles or alternatively a resistant substratum is sought after. If the foundation is subjected to high pull-out loads, one can only rely on side friction. Following recent experience we have shown that with conventional piles (driven, or cast in situ) the side friction reaches the following limiting values for a general soil classification and average working pile lengths:for coarse sands and gravels, 1 to 1.5 bar;for fine silty sands, 0.7 bar;for clays, 1.0 bar. However, 'one must add that the specifications or accepted methods of calculation, either French or Anglo-Saxon, recommend the limitation of the side friction to values near to those found experimentally and quoted above2- 6 when the piles are constructed by conventional methods. Confronted with the problems of side friction for the foundations of offshore drilling platforms, the Bureau Commun d'Etudes Marines (ELF-AQUITAINE) asked us to propose a method whereby one could increase the bearing capacity of metal piles placed in a borehole. An offshore drilling platform in the North Sea may exert, for example, under the effect of the waves, loads varying from a pull-out of 1,700 tons to a compressive load of 3,500 tons on a single pile. The driving of a casing 20 m into the sea bed, then the boring, lead to piles with an embedded depth of 100 m under the sea bed, with an average side friction of 1 bar (limiting value). However, the ground penetrated by the pile has an undrained cohesion of 5 to 8 bars. It is therefore obvious that the present methods of carrying out foundations penalize above all the side friction. This is why we have proposed a new method that allows one to increase the sealing of a pile to the ground by the injection of cement under pressure.