Statistical Analyses of Shear Strength Properties of Cemented Coarse Grained Alluvium, a Case Study: Tehran, Iran

Soil cohesion is one of the main parameters in designing mitigation methods for shoring to support deep excavation. Therefore, construction cost of tall buildings with deep excavation in large cities is greatly dependent of cohesion. Because of problems encountered with undisturbed sampling, in-situ testing has an important role in determining soil parameters. Cementation generally increases soil cohesion. On the other hand, new geotechnical design codes propose statistical methods as the rational method for analysis and representation of soil data in order to capture soil parameters uncertainties. This paper aims to explore the characteristic values of coarse-grained alluvium based on experimental results of in-situ direct shear and plate load at the edge of trench tests. The site location in in Tehran, capital of Iran, with coarse-grained soil consisting of gravely sand to sandy gravel with some cobbles and boulders, mostly cemented by carbonaceous materials. Statistical analysis has been done between cohesion and angle of internal friction. Also, probability distribution functions have been fitted to soil parameters. The obtained results indicate that friction angle has a truncated normal distribution but cohesion can be presented with lognormal distribution. The sample test show correlation coefficient of 0.45 between c′ and φ′ stating a positive correlation. Furthermore, characteristic values of these parameters are determined based on different methods and the results have been compared with each other. The characteristic value of cohesion and friction angle calculated in the range of 25 to 50 kPa and 30 to 35°, respectively. Global stability analyses with different calculated characteristic values are also performed. It is presented that characteristic value evaluation is an important step in geotechnical practices.