Excavation support or earth retaining structures are required where excavation is deep and required slope cannot be provided. Types of excavation supports are discussed.
What Are Excavation Supports?
In general, to construct a structure either above the ground or below the ground, excavation of soil is necessary in the ground.
The excavation may be shallow or deep based on our requirement. But when deep cuts are made in soil, the soil from the sides of excavated area may collapse due to low stability.
To prevent this and to make the cut accurate vertical we need some temporary earth retaining structures called excavation supports.
The failure of soil cut arises when the limit of excavation exceeds its safe height limit and it depends upon unconfined compressive strength of soil and cohesion.
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Bracing of an excavation
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Following table provides the safe height limit for different soil types based on their unconfined compressive strength and cohesion limits.
Types of Excavation Supports
Generally, there are two most commonly adopted methods of providing excavation supports:
(i) Soldier pile and lagging
(ii) Soil nailing
Soldier Pile and Lagging
Soldier piles are nothing but I-sections which consists web between two flanges. These I-sections are drilled into the ground around the periphery of an excavation. The piles are driven to the point slightly below the required excavation depth.
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| Soldier pile & lagging |
Weak concrete is used for grouting of piles. The spacing between every two adjacent piles is limited to 10 feet. After that exaction is done and the surrounded soil is well supported by the flanges of piles.
In between the piles, wood lagging is used. The lagging consists long timber sheets or planks placed behind the front flanges of piles to support the soil between the piles.
The timber is of 2 to 4 inches thickness, so, some manual excavation behind the front flange is needed. To offer strong bond between horizontal wood sheets and front flange, welded studs, clips etc. are used.
For narrow cuts which have limited width and more depth, excavation bracings are provided in the form of struts. Struts are connecting member which connect the one side cut wall to another cut wall horizontally.
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| Soldier pile & lagging |
To install struts, horizontal member called waler is arranged in front of wooden lagging. If the excavation is very wide, then inclined struts called rakers are used.
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| Soldier piles & lagging for wide excavation with horizontal struts |
Soil Nailing Method of Excavation Support
Soil nailing is another technique of protecting excavated ground. It is economical and rapid process. In this case, soil is reinforced in situ to make it stable.
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| Soil nailing method of earth retention |
The Group of nails installed on a grid holds the soil in a stable manner. The same process is repeated until the required depth is reached.
Soil nailing is more effective and easier if it is done with subsequent layers of installation. Even though it is rapid in construction it needs skilled persons to who are having knowledge on geo technology and craftsmanship. Soil nailing should be more effective above the ground water table level.
Soil nailing is mostly used for rail or road embankments, tunnel excavations, retaining structures for high rise buildings etc. Soil nailing structures provides good resistance against seismic activities and deflections are also with in the tolerable limits.
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| Schematic diagram for soil nailing method of earth retention |
Sometimes this excavation bracings or rakers may disturbs the field operations. To overcome this, tie back system is developed. In this case, tie back bars or anchor bars are installed in the soil cut that is behind the excavation support.
So, these tie backs holds the excavation supports and they are grouted to the rock below the ground. From this lateral earth pressure also prevented and the load from the structure is carried to rocks through this tie backs.
The angle of inclination of anchors is decided based on the soil behavior and availability of hard stratum but In General 10 to 20-degree inclination is provided for anchors.