Cantilever sheet piling walls depend on the passive resisting capacity of the soil below the depth of excavation to prevent overturning. The depth of sheet piling walls below the bottom of the excavation are determined by using the difference between the passive and active pressures acting on the wall. The theoretical depth of sheet pile penetration below the depth of excavation is obtained by equating horizontal forces and by taking moments about an assumed bottom of sheet piling. The theoretical depth of penetration represents the point of rotation of the sheet piling. Additional penetration is needed to obtain some fixity for the sheet piling. Computed sheet piling depths are generally increased 20% to 40% to obtain some fixity and to prevent lateral movement at the bottom of the sheet piling.
The cohesive value of clay adjacent to sheet pile walls approaches zero with the passage of time. Design and analysis for clay soil conditions must generally meet the conditions of cohesion less soil design if the sheet piling support system is to be in use for more than a month. For those few cases where a clay analysis will be appropriate, reference is made to the USS Steel Sheet Piling Design Manual.
It is possible to have negative pressure values with cohesive soils. Since cohesive soil adjacent to sheet pile walls loses its effective cohesion with the passage of time it is recommended that negative values be ignored. Do not use negative pressure values for the analysis of sheet piling systems. Any theoretical negative values should be converted to zero.
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