Janaka J. Kumara

BASE RESISTANCE OF OPEN-ENDED PILES EVALUATED BY VARIOUS DESIGN METHODS

The design bearing capacity of an open-ended pile depends largely on the accuracy of the design method. Although the knowledge of shaft resistance has been understood quite well, the base resistance has not yet been completely understood due to the effects of soil plugging. The mechanisms of soil plugging is yet to be fully understood particularly for large diameter and long length piles installed in large construction projects. This paper compares the base resistance of two open-ended field piles constructed in the Tokyo Bay project evaluated by various design methods including cone penetration test (CPT)and standard penetration test (SPT)-based methods. In total, five design methods including the conventional American Petroleum Institute (API) approach were included. In Japan, SPT-based design methods are used in practice. The CPTbased design methods, which are not popular in Japan were also included to evaluate their effectiveness. The CPT-design methods discussed in this paper classify open-ended piles into plugged or unplugged modes. The results reveal that the Fugro (i.e., CPT-based) and the API design methods overestimate the base resistance. In contrast, the ICP (i.e., CPT-based) and Port and Airport Research Institute (PARI) (i.e., SPT-based) design methods underestimate the base resistance. Based on the results, we can recommend the University of Western Australia (UWA) design method (i.e., CPT-based) as it produces the nearest results to the actual base resistance measured from the field load tests.

Effects of the lateral stress on the inner frictional resistance of pipe piles driven into sand

The inner frictional resistance of pipe piles depends on the degree of soil plugging. Many factors including pile diameter, lateral stress at the pile tip and geometrical conditions of piles could influence the soil plugging. In this paper, the effects of inner sleeves attached at the pile base on the inner frictional resistance are discussed, particularly highlighting the lateral stresses at the sleeve using small-scale steel pipe piles penetrated into a medium dense sandy ground. A closed-ended pile of the same diameter was also tested to compare it with similar open-ended piles. The results of incremental filling ratio (IFR) and plug length ratio (PLR) were also discussed. A simple method was also proposed to evaluate IFR and PLR for the sleeved piles since they have originally been defined for non-sleeved piles. The results of the IFR indicate that all the piles penetrated under partially-plugged or unplugged state producing smaller penetration resistance than a similar closed-ended pile. The results of the corrected IFR give a better indication of the soil plugging of the sleeved piles, particularly at shallow penetration depths. The results also suggest that the inner frictional resistance increase with the sleeve height. The results of the coefficient of lateral earth pressure, Kh also indicate that Kh increases with the sleeve height. The effects however become less significant at higher sleeve heights. Therefore, we can recommend the use of the inner sleeve as an improvement method to increase the bearing capacity of open-ended piles installed in sandy grounds.

Effects of Dry Density and Water Content on Mechanical Properties of Sand-Bentonite Buffer Material

Mechanical properties of the buffer material of deep geological repository are influenced by various factors. The engineered barrier system of a deep geological repository is subjected to local groundwater flow after it is decommissioned. The changes in water content of buffer material could affect deformation and suction properties. In this study, the influence of water content and dry density on the mechanical properties of sand-bentonite buffer material were investigated. The triaxial compression tests were performed on sand-bentonite specimens of 1400, 1600 and 1800 kg/m3 of dry density and 6, 12 and 18% of water content. The volumetric strains of specimens were evaluated using a newly built double-cell type triaxial testing apparatus. Total suction of specimens was measured using the chiller-mirror hygrometer technique. Total suction was measured on identical specimens prepared for suction measurement and triaxial compression tests. The results indicate that water content reduces deviator stress while dry density increases it. The results also suggest that water content changes strain-softening behaviour of relatively less saturated specimens into strain-hardening behaviour when water content is high. A high confining pressure (of 1.0 MPa) inclines towards strain-hardening behaviour than a small confining pressure (of 0.1 MPa). Water content also increases strain-hardening behaviour. In contrast, dry density reduces strain-hardening behaviour, particularly under a small confining pressure. The results also indicate that dry density reduces the magnitude of volumetric expansion. A high confining pressure encourages volumetric expansion. While water content decreases frictional behavior, it increases cohesive behaviour under a relatively low dry density (of 1400 and 1600 kg/m3). In contrasts, water content reduces cohesive behaviour under a high dry density (of 1800 kg/m3). Thus, a micro-scale analysis would produce more insights on this. While water content has huge influence on total suction, both dry density and confining pressure have no effects on it.