Geotechnical Properties of Peat in San Joaquin Delta

The San Joaquin Delta region in Central California covers about 780 square kilometres (300 square miles) of reclaimed islands and tracts and over 650 km (400 miles) of waterways and 1300 km (800 miles) of levees. Its peaty soils are some of the riches farm land in the world, but also have very undesirable geotechnical properties. The geotechnical problems facing the islands which are related to the peat are oxidation, seepage and erosion loss, low strength, light weight, and high compressibility. The oxidation, compressibility, and wind erosion of the peat and other soils cause subsidence at a rate of 6 to 8 cm (2.5 to 3 in.) per year, and as a result the islands presently are as much as 6.5 m (20 ft) below sea level. The investigations, which were performed to evaluate the hazards to the water aqueducts passing through this area, included over 50 borings and 100 cone penetration test (CPT) probes. Laboratory testing included classification, strength, and consolidation tests of peat. The geotechnical properties of the peat and their statistical variations are given, and the important parameters of the strength and deformation are analyzed in detail. The results indicate that peat in this area has a mean dry density of 0.32 ton/m³ (20 lb/ft³) and a water content of 320%. The effective angle of internal friction was 28 deg; however, the maximum strength was reached at strains well beyond 15 to 20%. In triaxial undrained tests the developed pore pressure often equalled the cell pressure for sample strains beyond 5 to 10%.