As stated earlier, one of the effective methods to retain a soil mass is to install a vertical wall which consists of long thin element such as steel, concrete or wood that are driven in the ground.
The elements are usually connected by interlocking joints. Sheet piles in comparison with massive type of walls (concrete or stone) are considered as flexible structures, in which bending moments are developed by the produced lateral loads. Therefore they should be designed so that, they are able to resist the maximum bending moments.
Some of their common applications are shown in Fig. 1.
Figure 1: Application of sheet pile wall
Cantilever sheet pile walls
Cantilevered sheet piles are usually used for the height of about 6m or less than dredge line. (Fig.1-a)
In geotechnical practices, cantilever embedded retaining structures are specifically used for protecting permanent and temporary excavations, for highway constructions, and sanilation of landslides.These stuctures are mostly sheet walls as temporary retaining structures, and pile walls and diaphragms as permanent retaining structures.
Anchored sheet piles
When the height of sheet pile is less than 6m, it is economical to use sheet pile which is anchored near its top. (Fig.1-b) Anchoring the sheet pile cause less penetration depth and also less moment to the sheet pile.
Well constructed anchor walls undergo less lateral deflection than braced walls and so provide a better control of back-slope subsidence. Anchor installation only requires a small excavation to allow equipment access.
Anchored walls are always pre stressed which essentially removes the slacks from the system. The anchor will maintain their load through the excavation sequence unless creep occurs.
The anchors also place the entire soil mass between the anchors and wall in compression, thus creating a very large gravity wall.
There are two causes of subsidence for anchor systems:
1) Caving of the anchor holes prior to grouting
2) Flow of cohesionless material into the excavation through wall opening made for anchor installation.
Another disadvantage includes possible downward movement of the wall due to the vertical component of the anchor forces.
Sheet piling systems comprises developed sheet pile sections with either an overlapping joints or an interlocking one (Fig.1-c). The sections can either be cantilevered to support the excavation, depending on the imposed loads, or they can be restrained by a specially designed mechanism with anchor. During excavation ground anchors are progressively installed to restrain the sheet piling. This creates a ‘reinforced earth zone’ behind the sheet piling to form a retaining wall structure around the excavation. Hence, deep excavation can be supported in a wide range of ground conditions.
A cofferdam is a temporary structure designed to keep water and soil out of the excavation in which a bridge pier or other structure is built. (Fig.1-d) When construction must take place below the water level, a cofferdam is built to give dry work enviroment.sheet piling is driven around the work site, seal concrete is placed into the bottom to prevent water from seeping in from underneath the sheet piling.
This article comes from EJGE edit released