The Elastic Properties of a Dense Glacial Till Deposit

Dense, heavily preconsolidated glacial till is a relatively incompressible soil that occurs throughout most of Canada. When loaded, it undergoes very small settlement, most of which is elastic. For the average structure, these elastic compressions are too small to be of concern and are usually ignored. However, for some structures they can be critical and their magnitude must be estimated prior to construction. To make the necessary analyses requires knowledge of the elastic properties of the in situ glacial till.This paper presents the results of field and laboratory tests that were made on a dense glacial till deposit to determine its modulus of elasticity, in connection with the design and construction of a 100 ft. high combined earth and concrete dam. In the field, in situ loading tests were made against the walls of a 50 ft. deep test shaft. The modulus of elasticity was computed, using elastic equations applicable to the case of a rigid circular plate pressed against a semi-infinite elastic solid. Moreover, during construction of the project, measurements were made of the elastic rebounds and settlements that occurred under known conditions of unloading and loading. Steinbrenner’s approximate solution for computing settlement due to loads acting on the surface of an elastic layer was then used to compute the apparent modulus of elasticity. In the laboratory, unconfined compression tests and repetitive triaxial tests were made on undisturbed samples. The modulus of elasticity was estimated from the stress-strain relationships obtained.The data presented in the paper indicate that the apparent, in situ modulus of elasticity of the glacial till deposit is very high, being in the order of 150,000 lb./sq. in. Reasonable agreement exists between modulus of elasticity values computed from the in situ plate bearing tests and those computed from observed rebounds and settlements. However, modulus of elasticity values computed from unconfined compression and repetitive triaxial tests in the laboratory are apparently too small, being only a fraction of those values obtained by the field procedures. Sample disturbance is thought to be a major factor affecting laboratory test results.Grain size characteristics, density, natural water content, and strength properties of the glacial till deposit are presented in the paper. These data provide a comprehensive description of the material and permit comparison with glacial till deposits encountered at other areas.