Mass Stabilization for Settlement Control of Shallow Foundations on Soft Organic Clayey Soils

The site for a new two-story classroom building was underlain by a 4.57 m thick layer of natural marsh deposit consisting of very soft organic clay. The specialty contractor provided a design-build solution of mass stabilization, a dry soil mixing technique, to increase the shear strength and decrease the compressibility of the organic clay insitu. This method allowed the structure to be founded on shallow spread footings without soil replacement or deep foundations. Total settlement of the building, on shallow foundations designed for an allowable bearing capacity of 192 kPa, was limited to 25.4 mm, with differential settlement on the order of 12 mm. Dry Portland cement was pneumatically pumped to a high-speed mixing tool mounted on track hoe arm. The clay and cement were systematically mixed insitu to proportions determined by a pre-production laboratory mix design program. Post-treatment testing consisting of laboratory Unconfined Compressive Strength, Cone Penetrometer and Column Penetration testing confirmed that the required strength was achieved. This paper presents the design, construction, and quality control testing of the mass stabilization program. A pre and post-treatment test data comparison shows the extent of the improvement of the treated soil. No significant settlement was measured during or after construction.