ANALYSIS AND DESIGN OF EXCAVATION SUPPORT SYSTEM IN SOFT CLAY

Conventional design procedures for deep excavations in soft clay in Singapore are not always adequate. Consequently, difficulties such as large strut loads and excessive ground movements are often encountered. Therefore, a method was proposed for the design of excavation support system in soft clay. The method provides a suitable approach to treat the problem of net active forces acting on the sheetpile below the level of the excavation. Having some of its basis in the plastic theory of structures, the method enables the minimum required moment capacity and corresponding embedded depth to be obtained for assurance against soil failure. A composite pressure diagram was suggested that would provide an adequate envelope of maximum strut loads to be used in design. The behaviour of an 11 m deep strutted excavation in soft clay was monitored for construction control and the field measurements were used to verify the method's assumptions. The soft marine clay at the site extends to a depth of about 23 m and sheet piles were driven to about 30 m deep for adequate embedment. From the inclinometer data it was found that the point of maximum deflection is below the level of the excavation and that ground movement develops in a cumulative manner. Rate of movement varied from l. 5 to 7.0 mm/day during excavation and O to O. 5 mm/da)' during the period of wale and strut installation. The higher rates were probably caused by the formation of extensive plastic zones in the soil. Preloading was found to be important in restricting lateral ground movement and the development of loads in struts several levels higher. Time dependant development of strut loads of approximately 2 kN/m/day appears to occur where the soil is very soft. It was found that the proposed model agreed well with measured strut loads both in magnitude as well as in the trend. In the model ensures against soil and structure collapse but it does not ensure that the ground movements will not result in damage to nearby structures or utilities. To enable widespread use of the proposed method, simplified methods of design were suggested that can be performed manually or with a microcomputer.

Author dc:creator

• CHUA LAI HENG

• Deep Excavation
• Soft Clay
• Ground Movement
• Plastic Design
• Earth Pressure
• Sheetpile
• Internal Bracing