2.2 Empirical Design Methods for the Stability of Excavation

2.2.1 Strut loads

One of the most important aspects in excavation support design is to estimate the strut load. To date, many design methods for estimating the strut load are empirical and based on limit equilibrium of wedge (Terzaghi, 1941) and the apparent earth pressure diagrams (Terzaghi, 1943; Peck, 1969). The apparent pressure diagrams developed by Peck (1969) and those developed by Tschebotariof (1973) are some of the typical approaches to estimate the maximum strut load during excavation. The apparent pressure diagrams developed by Peck shown in Figure 2.4 are empirical estimation derived from some field observations and measurements. The deformation control is implied in the factor of safety proposed by Terzaghi (1943). However, deformation cannot be determined directly by using this empirical apparent earth pressure approach.

2.2.2 Base stability

Another important consideration for a safe excavation support system is the effect of base heave. In this aspect, Bjerrum and Eide (1956) studied the stability against base heave and proposed the use of stability numbers. The factor of safety against base heave is defined by considering the surcharge, average undrained shear strength of soil within the zone of influence, and the bearing capacity factor of the ground. He recommended that a minimum safety factor of 2 should be achieved against base heave.