A comparison of four instruments for measuring the effects of organic matter on the strength of compacted agricultural soils

Abstract

Soil strength estimates from shear vane, Proctor and drop cone penetrometer were obtained alongside cohesion and angle of internal friction measured with the triaxial test. Two organic materials (peat and farmyard manure) were incorporated at the rates of 0%, 4%, 8% and 12% to two soils (one sandy loam and the other clay), and compacted with 25 blows of the Proctor hammer. For the shear vane and the penetrometers, the soils were tested at moisture contents ranging from 5% to 55%, while for the triaxial tests, the soils were tested at three moisture states (5% below optimum, optimum and 5% above optimum moisture content). Although organic matter addition decreases soil strength of compacted soils at lower moisture contents, the effect decreases as moisture content increases, and there is a small increase in strength at the highest moisture contents. Organic matter decreased the strength of the soils at the three moisture states by decreasing the angle of friction rather than soil cohesion. Proctor penetrometer strength estimates were the highest followed by those from the drop cone, shear vane and the triaxial test. The shear vane overestimated the cohesion in the soil when compared with the triaxial test measurement. The penetration resistance measured with the Proctor and the drop cone penetrometers were correlated as were the shear strength measurements from the triaxial and the shear vane instrument. The Proctor penetrometer was the quickest to use, followed by the drop cone penetrometer, shear vane and the triaxial test.