Abstract
This investigation is carried out to study the phenomenon of collapsibility that occurrence in the city of Tikrit in Salahaddin Governorate at located 170 km north of Baghdad . The physical indexes and engineering properties tests were to used determine the natural water content, Atterberg limits, grain size distribution of soils, shear strength and consolidation test .The grain size distribution of soil showed the silt percentage is higher than those of clay and sand. They are composed of silt that represents ( 52 - 60 )% of the soil and ,Clay represents ( 34 - 45 ) % of the soil. The liquid limits(L.L) values ranges between ( 23 – 37 )%, While the plastic limit (P. L.) values ranges between ( 21-31 )% and Plasticity index (P.I) values ranges between ( 6 - 7)%.The moisture content values between (1.9 - 2.2) , are classified low, due to high gypsum percentage, which absorbed most of the amount of water, and incorporated into its molecular structure. Direct shear test shows that the cohesion values (C),range between ( 40 – 51) kN/m2, While the internal friction angle( Ø°) of the soil ranges between ( 25°- 33°),The consolidation parameters showed that the compression index (Cc)values ranges between (0.040-0.055),swelling index (Cr) values ranges between (0.003-0.015),and Pre- consolidation pressure (Pc) values ranges between (125-275) kN/m2. The Collapse potential (Cp*100) values ranges between (1.41 – 1.85) kN/m2, as a moderate trouble. The chemical tests of the soil showed that the percentage of Total soluble salts (T.S.S)range between (24.24 - 56.63)%,While the gypsum content range between%(22.35 -55.15)%,which play a role in soil properties fluctuation Therefore, gypsum soils should be studied during investigation phase, to assess the soils ability to prevent failure in future.