Page - 1409 - in Book of Full Papers - Symposium Hydro Engineering

3. THE DIFFERENT BETWEEN THE "IN-SITU" AND LAB TEST IN NEOGEN FORMATION With regard to the results of in-situ and laboratory tests, following points should be noted in this regard: A. Laboratory tests on disturbed samples of any depth are actually face with some cracks due to vibrations and rotation of the drilling bit and due to movements and transportation of samples. Indeed, these samples cannot represent the condition of natural soil on that location where the samples are taken and they show considerably lower strength against the natural state. Cracked soil or normal cracks have considerably lower modulus of elasticity compared to the in-situ soil, and due to natural cementation in the materials. B. Soil obtained from mudstone, siltstone and marlstone intercalations originally, has higher cohesion among their particles compared to the disturbed or cracked soils, even if the cracks are so tiny, due to their old origin, related to millions of years ago, and to their natural compaction during all these years. Thus, any kind of disturbance or formation of cracks not only decreases or diminishes cementation among particles, but also causes lower cohesion and modulus of elasticity compare to the ones obtained from in-situ tests. The results obtained from Pressure Meter Test provided in table 1. In a considerable number of tests, even in saturated status of the test place, modulus of elasticity is between 1200 to 2000 kg/cm2. Based on the engineering classification of the material strength, as it can be seen, clay, silty clay, grave-sand and clay soils can be classified as hard and very dense soils and their higher modulus of elasticity is an indicative of cementation among the particles. However, triaxial tests (CU or CD) on disturbed soil samples provides a modulus of elasticity of 120 to 230 kg/cm2, as the maximum value the saturated compacted clay. This is while in pressure meter test within boreholes, different values were obtained such as higher value than 330 and 410, and even 930, 1200 and 1880 kg/cm2, which shows cementation and high cohesion of more than the obtained values in triaxial tests. Meanwhile, it is known that the stress of compressive strength of fine-grained soils has direct relation with modulus of elasticity. C. Plate loading in-situ test results provided in table 2. As it can be seen, the elasticity modulus of 250, 300 and 500 kg/cm2a belong to upper layers (5 to 7 m depth) and it will be increase by depth, unless the layer material is changed. Therefore, this situation was considered in mathematical model of stability analysis. 1409