Recent Model Tests With Suction Piles in Clay and Sand

Abstract

ABSTRACT The beneficial effects of suction on the break-out resistance of short, hollow piles (I.e. suction piles) in clays as well as sands have been verified by model tests. For sands increased break-out resistance is proportional to the developing suction inside the pile. For clays the increase in break-out resistance is due to a suction induced transition from local failure along the shaft to a reversed bearing capacity failure. INTRODUCTION As a part of a research programme on the utilization of suction piles as foundation for small light-weight platforms of mono tower type model tests were carried out in connection with the development of a design procedure. This paper presents model test results and their preliminary interpretation. Steel tubes closed at the top and installed in the seafloor by the differential water pressure obtained when pumping from the pile interior is termed suction piles. The literature on suction piles deals primarily with the conditions for installing and anchoring by the differential water pressure created from continued pumping. The possibility of utilizing the differential pressure created by the piles themselves, when subjected to uplift is investigated in this paper. TEST PROGRAMME The total test programme analyzed includes tests on Kaolin l, Nivaa Clay and G12 sand2 . Variable test parameters comprise:Pile diameterEmbedment/diameter ratioSoil typeUndrained shear strength of claysLoad type (monotonic, sustained)Regeneration time prior to loadingRate of monotonic breakoutMagnitude of sustained breakout load THEORETICAL CONSIDERATIONS Breakout from clays The short term breakout capacity of embedded objects has been suggested determined by the use of plasticity theory on general shear failure mechanisms. Model tests at embedment ratios less than 1 support this3, 4. At larger embedment ratios 3 different failure mechanisms, (i)-(iii), have been recognized 1,2 . as illustrated in Fig. 1:General shear failure in terms of a reversed bearing capacity failure is formulated [1]: (available in full paper) The value of N which is determined at the time of maximum breakout force F, and inserting a as the ultimate value in [II, is termed N1 The value of N that fulfils the vertical equilibrium equation [2] for the clay plug at the time of maximum breakout force, (inserting a as the ultimate value), is termed N2 (available in full paper)Local tension failure occurs when the inside soil-wall friction and suction exceed the tensile strength of the soil [3]. A clay plug inside the pile is then torn apart from the seafloor leaving a hole: (available in full paper) (iii) If no suction is allowed to develop inside the pile, the soil-wall friction cannot exceed the tensile strength of the soil and failure develops as a local shear failure along the pile shaft [4]:(available in full paper) Notations used in [1],[2],(3) and (4) are outlined in Fig.2.