The model test about silty clay’s deformation as water pressure decreasing

Abstract

The main compression layer of Xi’an is over-consolidated. Based on the property, the physical model is designed as a steel cylinder which is 2.5m diameter,3m high. The soil layers are inter-bedded with silty clay and medium-fine sand. The consolidation pressure and overburden pressure is 295KPa. The process of soil deformation was simulated as the water falling by decreasing the water pressure in the medium-fine sand layer during the silty clay in expansion. Data monitoring test reveal that the silty clay layer continued to expand for 220 hours. After expansion, the silty clay has a larger compression. Based on the study of this test, we could predict that larger land subsidence of Xi’an will occurrence again as the confined water level decline by over pumping of ground waters. Introduction Land subsidence of Xi’an city occurred mainly in urban areas and inner suburban areas. A wide range of level survey suggests that, from 1959 to 2000, the areas which the accumulated subsidence over 200mm reached to 150km2. around the center of land subsidence areas, such as Hujiamiao, Nanshapocun, Dayantashizi, Xiaozhai, Dongbalicun, the accumulated subsidence were over 2000mm . The previous researches reveal that the main compression layer of Xi’an is overconsolidated and the excessive exploitation of confined water is the main reason to the city’s land subsidence. By the end of last century, the confined water head stop declining and even rising, due to the sharp reduce on exploitation of ground water. Meanwhile, settlement velocity, although is slowing, but the annual velocity of settlement still reached at 18.0mm/a-40.0mm/a. In some areas, such as Dianzicheng, A large amount of groundwater was extracted because of water scarcity. Therefore, the large scale physical model tests was designed to study the relation between the ground water level and over consolidation soil layers’ deformation, the ultimate goal of study is to provide theory basis for formulating the reasonable planning of groundwater exploitation. This paper is a continuation based on my previous paper named “The model test about over-consolidated soils’ stress and deformation as water pressure increasing” . The Test of over consolidation soil The purpose of the test The purpose of the test is to study the stress (such as, earth pressure and pore-water pressure) and deformation of silty clay and medium-fine sand when the water pressure is increasing in medium-fine sand. The test equipment and method [6] A closed steel cylinder with 300cm high and 250cm in diameter was used (Fig. 1)in test. The soil in the cylinder is silty clay and medium-fine sand by hydraulic fill. Fig.2 shows the thickness of each layer, and each layer fit with pressure sensors and displacement sensors which horizontal distance to the cylinder centre is 75cm (Fig. 2, Tab 1). The water inlet in the top of cylinder links to International Conference on Materials, Environmental and Biological Engineering (MEBE 2015) © 2015. The authors Published by Atlantis Press 1086 a water tank located on the 9th floor, via a tube to supply the pressure needed for soil layer consolidation. Table 1. The postion of sensors Double diaphragm pressure cell Pore pressure gauge Serial Number Postion Serial Number Postion 5-1 Top of silty clay 1 5-9 43.46cm above the bottom silty clay 1 4-2 Top of medium fine sand 2 5-5 50.92cm above the bottom silty clay 1 Displacement meter 3-7 interspace of medium fine sand 2 5-2 Silty caly1 F Consolidation and overburden pressure Soil in the test material [6] The medium-fine sand(Q3) was fetched from the deep foundation pit in the west suburbs of Xi’an city. And silty clay (Q2) is fetched from another deep foundation pit of city’s metro line 2 construction site in Xiaozhai district. The initial physical parameters showed in Tab 2, Fig. 3. Test preparation After 68 hours’ consolidation under pressure F of 295KPa(Fig.4, F is the consolidation pressure and being hold on as the overburden pressure through the test), each soil layers deformation for two consecutive hours is less than 0.01mm. The soil thickness after consolidation is showed by Fig.2. And the water pressure in medium-sand layer 2 (Gauge 3-7,Fig.4)increased from 50 to 150KPa Table 2. Initial physical parameter of soil layer Indicator Silty clay Medium-fine sand Specific gravity of solids 2.7024 2.675 Mass density(g/cm) 1.789 1.360 Water content(%) 41.14 Plastic limit 18.6 Liquid limit 31.16 Plasticity index 11.56 grain size distribution curve 0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 100.00 0.01 0.1 1 10 grain size,mm p e r c e n t f i n e r ( % ) Fig3.Grain size distribution curve of medium-fine sand Fig. 1 Test equipment Fig. 2 Schematic diagram of experimental model and thickness of each layer Consolidated for 68 hours (cm)