Determination Of Subgrade Bearing Capacity Of Shallow Foundation

The subgrade bearing capacity is one of the three classical problems at issue in soil mechanics. Ensuring the capacity rationally is the primary content of the geotechnical investigation and foundation design and bring the greatest economy and security benefit to engineering. In view of this, the thesis makes the following:(1) several in-situ test methods are given and the advantages and disadvantages of the various methods were pointed out .The empirical formula of various regions in China are obtained, which can calculate the bearing capacity by using the datum of in suit tests and provide a convenience and reference for the construction of similar projects in different regions.(2) Based on the Mohr-Coulomb strength theory, a new formula of critical load of subsoil with K₀≠1 is deduced. This method avoids artificial overestimated results due to the assumed K₀≠1,which can reflect the essentials of subgrade bearing capacity more accurately.The critical load based on Mohr- Coulomb strength criterion did not analyze effect of intermediate principal stress on yield so that it be not considered that the strength of foundation can be utilized effectively. Based on the unified strength theory, a new formula of critical edge pressure and critical load of subsoil with taking account of the effect of the intermediate principal stress( 2σ) and K 0≠1 is deduced. This method can reflect the essentials of subgrade bearing capacity more accurately and be considered that the strength of foundation can be utilized effectively ,which is compared against those based on the Mohr-Coulomb criterion.A thorough study of the intermediate principal stress coefficient b has been made through statistical analysis for 177 groups of soil index and the loading test data of different foundation soils in Hebei province. The result proves that the value of b changes from 0 to 0.50 for the silt and clay soil foundation, and the relative error between the theoretical value of the foundation bearing capacity and the measured values of loading test is generally less than 5%.And many methods to get the coefficient of lateral earth pressure at rest of different kinds of soil is given,which is on the guiding role of new formula .(3)Information concerning with the bearing capacity of footings on mogeneous foundation and non-homogeneous foundation was collected and evaluated. Based on the regulations on ultimate bearing capacity in"Specification for geotechnical investigation of tall buildings"(JGJ 72-2004)and traditional formula of critical load of subsoil,a great number of datum of bearing capacity in Hebei area are collected and the choice method of safety coefficient is given, which is useful for the project which is under construction.In a word,there is not is the only reliable method for ensuring the bearing capacity which is determined by regular patterns or by the comprehensive evaluation of geological conditions, test data, foundation and superstructure with engineering experience.