SOIL COMPRESSIBILITY AS INFLUENCED BY SEWAGE-SLUDGE INCORPORATION

The effect of incorporating sewage sludge on the compressibility of fo ur Trinidadian agricultural soils was investigated over a range of str esses from 0 to 1000 kPa. Air-dried sewage sludge was applied at four levels (0, 4, 8 and 12% by mass of dry soil) to the soils (two sandy l oams, clay loam and clay) and these were tested at their optimum compa ction moisture contents. Compression curves (void ratio versus log app lied stress) for each soil were almost linear over the range of applie d stress. Mean values of void ratio for all soils at four representati ve stress levels increased with increasing sewage sludge content and d ecreased with increasing applied stress and clay content. Significant interaction effects were observed between the experimental factors, wi th the interaction effect between soil type and applied stress being t he most significant. Sewage sludge increased soil compressibility in a ll cases. Soil compressibility was quantified using two compression in dices: C-c defined as the slope of void ratio versus log applied stres s and C defined as the slope of bulk density versus log applied stress . C-c and C tended to increase with increasing sewage and clay content s, but C-c was identified as the more sensitive index, particularly in the 10-100 kPa stress range. An equation was derived to relate C-c to initial bulk density before compression (rho(i)), particle density (r ho(s)), strain difference (epsilon(2)-epsilon(1)) and the correspondin g applied stresses (sigma(1) and sigma(2)). The equation is C-c = rho( s)(epsilon(2)-epsilon(1))/rho(i) log(sigma(2)/sigma(1)). (C) 1996 Sils oe Research Institute