IMPROVEMENT OF GYPSEOUS SOIL BY PRE-WETTING

Abstract

Gypseous soil is one of problematic soils facing civil engineer.  The problem appears when constructing heavy buildings or hydraulic structures on these soils after wetted by water from rainfall or from the raising of water table level from any source.           On the other hand gypseous soil is considerably strong and has good properties when it is dry. The dissolution of gypsum depends on many factors: gypsum content, temperature, atmospheric pressure and others. Number of Remedies for these soils was carried out by many investigators, some of these methods are not workable, and the others are expansive.            A new method of improving the collapsibility of such soil was presented by Prewetting Gypseous soil with water many cycles using laboratory and field models with different footing stress, which reduces the collapsibility of this soil primary. A laboratory model of 320 mm diameter and 472 mm height made from thick plastic was used.  Two soils with different gypsum content 50% and 70%, brought from Aldor and Balad, in Salah Al Deen government in Iraq, was used.  A footing of circular base (50 mm diameter), applies 45 kN/m2 and 100kN/m2 stresses with the aid of fix weights placed on it.  The results of laboratory tests on the model samples shows a considerable reduction percent in the deformation ratio (Settlement/width of footing (S/B)) of 63% achieved from the 3rd wetting cycle by water for sample contain 50% gypsum.  The reduction percent was 91% at the 3rd cycle of prewetting for sample containing 70% gypsum tested at stress level up to 45kN/m2, while it is 86% for the laboratory model with 100kN/m2 applied stress, with the same gypsum content.  The results of field test for the soil containing70% gypsum tested at 100kN/m2 stress shows a pronounce improvement in the S/B value which gives a reduction percent reaches 90 %, which shows a considerable improvement in the collapsibility of this problematic soil.