Investigation of the Atterberg limits and undrained fall-cone shear strength variation with water content of some peat soils

Road construction in peatlands is challenging. The ability to make rapid estimates of the response of construction soils derived from natural peats to changes in water content is useful for pavement and geotechnical engineers. This paper details some laboratory test results on peat soils sourced from two sites in the South-west of England. The samples were sieved and the roots and natural fibres removed prior to laboratory testing. Water contents on the natural specimens were determined. The percentage of roots in the samples was determined. The thread rolling test was used to estimate the plastic limit of the peat soil material. A series of fall cone tests were conducted at varying moisture contents to determine the liquid limit of the peat soil as well as study the variation of fall cone undrained shear strength with the liquidity index, logarithmic liquidity index and the water content ratio. Both the liquidity index and logarithmic liquidity index are able to predict the fall cone undrained strength to within ± 40% around 90% of the time. When using the water content ratio to predict the fall cone undrained shear strength an accuracy of ± 40% is achieved around 85% of the time. The study concludes that the liquidity index and logarithmic liquidity index are better predictors of fall cone undrained shear strength but the water content ratio approach may be preferred if the engineer is less confident in plastic limit determination for peat soils.