Presented in this thesis is a study of the effect of compactive effort and long-term soaking on strength, stiffness and deformation characteristics of a gypsiferous subgrade soil from Kirkuk city.

The tested soil is sandy lean clay (CL) according to the Unified Soil Classification System and A-6 (6) soil according to AASHTO soil classification system. The soil contains about 35% total soluble salts (T.S.S.) and about 33% gypsum.

The compaction characteristics of the tested soil indicate that the AASHTO compaction curves are of double peaks having two maximum dry unit weights and two optimum moisture contents.

To study the effect of compactive effort and long- term soaking on the properties of the tested soil, ninety-six CBR soil samples were prepared, fourty eight CBR samples for each moulding optimum moisture content of 2% (dry optimum moisture content DOMC) and 11.5% (wet optimum moisture  content WOMC). For each moulding moisture content and each soaking period of (0, 4, 7, 15, 30 and 120 days), eight CBR samples were prepared, two for each case of the four chosen compactive efforts of 12, 25, 56 and 70 blows/layer.

Unconsolidated undrained and consolidated undrained triaxial tests (with pore water pressure measurements) were carried out on samples extracted from CBR samples as well as on samples compacted in manufactured mould at WOMC to determine the cohesion and friction of tested soil using the triaxial cell.

For samples compacted at DOMC, various unconsolidated undrained shear box tests were carried out to determine c and ø of the tested soil.

The full load-penetration curve obtained from CBR test was used to determine the resilient modulus. In addition, the ultrasonic tests were carried out on CBR samples to estimate the resilient modulus, and good agreement between both approaches was noticed especially for samples compacted at WOMC.

Vertical swelling / settlement measurements were carried out on each CBR sample during soaking using 0.002 mm dial gauge.

The moisture content determinations were carried out in this study to determine the effect of compactive effort on the distribution of moisture along the CBR soil sample during soaking of samples compacted at WOMC and DOMC. Similarly, the total soluble salts tests (T.S.S.) and the gypsum content tests were carried out for the same purpose.