4.4.1 Soldier piles

The commonly used types of soldier piles in excavations are the H steel，I steel，and rail piles.Concerning the dimensions and properties，the books on steel structures or the AISC Specification can be referred to.Types of rail pile are usually classified in terms of weight per length（kg／m）.The rail pile，having a smaller section and is therefore easier to be driven into soils，is mostly used in hard soils or cobble⁃gravelly soils.

The dimensions of soldier piles and the distance between them are determined based on the results of stress analysis.Then take the maximum bending moment（Mmax）from the typical bending moment envelope（Figure 4.18）.According to the ASD method，we can obtain the section modulus of the soldier pile as

where

σa＝allowable stress of the steel；

λ＝short⁃term magnified factor of the allowable stress，which can be found from the country building codes.

The dimensions and spans of rail piles can thus be selected according to the computed section modulus.Basically，under a certain stress，the longer the span is，the larger the required dimension of the soldier pile and the thicker the lagging should be.On the contrary，the shorter the distance is，the smaller the required dimension of the soldier pile and the thinner the lagging could be.The numbers of soldier piles are then increased.To compute the thickness of the laggings，we usually assume the lagging to be the simply supported beam on the soldier piles.The computed thickness of the lagging often comes out larger than the commonly used laggings in general excavations，which are around 3⁃4 cm thick.Considering the lateral earth pressure on the back of the wall is not necessarily uniformly acting on the laggings，sometimes it is centering on soldier piles，which are of higher rigidity，and sometimes the pressure is less than expected due to the effect of soil arching.The 3⁃4 cm thick lagging is often adopted if the excavation is shallow.