Performance of spread footings on subgrades reinforced with geogrids and geojacks

Document Type

Conference Presentation


Civil Engineering

Conference Title

Engineering geology and geotechnical engineering. Proceedings of the 34th symposium, Logan, April 1999.

Date of Presentation



Biaxial geogrids have been shown to be an effective method of improving the ultimate bearing capacity of cohesionless soils. However, the amount of settlement required to mobilize tension in the geogrid is significant and hence, there is little difference in the initial portion of the bearing pressure versus settlement curve for unreinforced sands and those reinforced with biaxial geogrids. For example, Adams and Collin (1997) showed that using a single layer of reinforcement, the pressure producing a settlement of 0.50% of the footing diameter, B is between 92% and 119% of that for the unreinforced case. In this study, a newly developed strain-controlled loading system was used to investigate the performance of cohesionless soil reinforced multi-oriented geosynthetic inclusions, or geojacks placed over a biaxial geogrid. The investigation used 152-mm diameter rigid footings in a test 1.37-m diameter test pits. The soil was a uniformly graded 16-30 sand (>98% passing No. 16 sieve, <1% passing No. 30 sieve). As this is a preliminary study prior to full-scale tests, the geogrid-type, depth of footing (not presented herein), and number of layers of reinforcement. The results indicate that the combined reinforcement of biaxial geogrids and geojacks improves the ultimate bearing capacity even beyond that obtained with a geogrid alone. Additionally, the settlement required to mobilize tension in the geogrid (and thereby enhance performance of the foundation system) is substantially reduced. Specifically, the pressure required to produce a settlement of 0.50% of the footing diameter, B is 230% of that using a geogrid alone and about 300% of that measured in the unreinforced case.

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