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From the GMA Techline
Subject: Geogrid-reinforced soil mat I am considering a geosynthetic-reinforced soil mat to be placed under a building structure with conventional footings. The purpose of the soil mat would be to limit differential settlement across the building footprint in event of seismically induced liquefaction settlement. That is, instead of a pre-stressed concrete slab or a grade-beam type mat foundation, we would use a geogrid-reinforced mat. The mat would “mask” abrupt settlements caused by sand boils, or similar, beneath the building.
Are there any design resources forthis application?
(Matt, Washington)
Reply: The design procedure that applies to your situation is just now emerging from the research arena. It comes under the heading of “Basal Reinforcement Over Deep Foundations in Soft Soils.” Here, a high-strength geotextile is placed over the deep foundations and supports the overlying surcharge and live load. Critical in the analysis is the spacing of the piles. The required strength of the geotextile is very sensitive to this variable.
Many papers [on this subject] are now becoming available. The 8-ICG conference in Yokohama, Japan, last September had several … and [several] manufacturers are actively involved.
Your situation is more difficult since you have to estimate the size of the liquefied soil zone. While your loads are light in comparison to the problem I just mentioned, the large spacing promises to be troublesome.
Good luck.