Question

We have discussed several types of shoring. If we have a certain project, when should we do the shoring using sheet piles, Contiguous Piles, Secant Piles, Soldier Beams and Lagging, or Diaphram Walls? Compare between these different types and list the criteria at which we choose the appropriate shoring system for any project.We have discussed several types of shoring. If we have a certain project, when should we do the shoring using sheet piles, Contiguous Piles, Secant Piles, Soldier Beams and Lagging, or Diaphram Walls? Compare between these different types and list the criteria at which we choose the appropriate shoring system for any project.

Answer 1

Most of the construction jobs in countries like Dubai, deep excavations must be made before the structure can be built. Excavation support systems are temporary earth retaining structures that allow the sides of excavation to be cut vertical or near vertical. This is done to maximize the size of an excavation when the price of real estate is high or space is limited by property lines or existing structures. Some of the commonly used shoring techniques and their features and applicability are discussed as follows:

1. Sheet-pile walls: Sheet-pile walls are widely used for both large and small waterfront structures. The vibrations caused during the driving can restrict the use of sheet pile walls in urban settings, except when special static driving methods are used. This can act as a water tight steel wall in soil. They might encounter difficulty in very dense soils or stiff soils with boulders. The sheet pile walls should be able to resist lateral earth pressure as well as ground water pressure. This is most applicable for medium dense sand. Bad interlocking during the driving can lead to leakage under water table.

2. Contigous piles: Contiguous piles serving as retaining walls are popular since traditional piling equipment's can be employed for their construction. They are considered more economical than diaphragm wall in small to medium scale excavations due to reduction in cost of site operations. These piles are connected with a capping beams at the top, which assists equitable pressure distributions in piles. These retaining piles are suitable in areas where water table is deep or where soil permeability is low. Contiguous piles are suitable in crowded urban areas as these piles restricts ground movements on the backfill side. The Contiguous wall can only be used where ground water is not a hazard or where grouting or jet grouting is used can be used to remedy leakage between the piles. Mostly suitable for cohesive soils.

3. Secant piles: Secant pile walls are formed by constructing intersecting piles. Secant bored pile walls are formed by keeping spacing of piles less than diameter. They are used to build cut off walls for the control of groundwater inflow and to minimize movement in weak and wet soils. It is somewhat a water tight wall and more economical compared to diaphragm wall. This can be used as a permanent water retaining wall and hence suitable for all types of soils.

4. Soldier beams: Soldier beams consist of vertical members built at suitable centers with a system of ground support spanning between them. The piles are first installed along the perimeter of the proposed excavation. Sheeting, supporting the ground is placed in position as excavation proceeds. Soldier or king piles may be used to support deep, shallow or wide excavations in various materials including clays and sands. Excavation in water-bearing strata may require special attention as this method is unsuitable for the exclusion of water. If soil is washed out from behind the sheeting, unacceptable settlement may occur to adjacent structures or services. Soils with some cohesion and without water table are suitable for this type of shoring. Soldier piles are driven or placed in bored holes in urban areas. This can resist lateral pressure alone.

5. Diaphragm walls: Diaphragm walling is a technique of constructing a continuous underground wall from the ground level. These walls provide structural support and water tightness. In this method, excavation is made possible by filling and keeping the wall cavity with bentonite -water mixture during excavation to prevent collapse of vertical excavated surfaces. These retaining structures find following applications:

• Earth retention walls for deep excavations, basements and tunnels
• High capacity vertical foundation elements
• Retaining wall foundations
• Water control

This can resist both lateral earth pressure and ground water pressure. This is suitable for all types of soil.