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UBC Theses and Dissertations

Evaluation of the Tri Star vibrocompaction probe Brown, David F.


The Franki Tri Star method for deep compaction of saturated sands has been used on a project on Annacis Island, Vancouver, B.C., Canada. The objective of soil compaction was to stabilize a deep saturated sand deposit, which was susceptible to liquefaction during strong earth quakes. The project involved the compaction of a 750 feet long by 15 feet wide strip of sand deposits down to 10m depth. The sand was overlain by a fill and a clayey silt layer of 2 to 5m thickness. Because of the silt layer and the presence of thin silt and clay seams in the sand, the soil was judged marginal for vibrocompaction. Therefore, extensive tests were performed on site to develop the optimal compaction procedure (vibration time, frequency of compaction and grid spacing). The compaction process was monitored by vibration measurements on the ground surface. In this way, it was possible to determine the time required for densification and to establish the optimal vibration frequency. In order to monitor the Tri Star probe the contractor carried out testing with the help of the site investigation firm, ConeTec, and in collaboration with the University of British Columbia (U.B.C.). U.B.C. monitored the long term effects, primarily with the Piezometer Cone Test (CPTU) and investigated the use of the Flat Plate Dilatometer (DMT) and Seismic Piezometer Cone Test (SCPTU). In addition, the Lateral Stress Piezometer Cone Test (LSCPTU) was used. All testing yielded before and after treatment results, with the CPTU giving progressive time and distance effects. The contractor and consultant also carried out Standard Penetration Tests (SPT), CPTU testing and measured ground settlement before, during and after treatment. The testing has proven that the Tri Star system is an efficient method to compact soil deposits which are susceptible to liquefaction. In particular an increase in penetration resistance of between 200 and 400% was recorded and the probe's zone of influence was found to be of 2m radius. The DMT and CPTU provided similar results and on comparison to the LSCPTU suggested that increases in relative density and lateral stress contributed approximately equally to the soil improvement. The SCPTU provided relatively inconclusive results.

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