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Excavation support or earth retaining structures are required where excavation is deep and required slope cannot be provided. Types of excavation supports are discussed.Contents:
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.
Soil type | Unconfined compressive strength (psf) | Cohesion (psf) | Safe height (ft) |
Very soft | Less than 500 | <250 | <5 |
Soft | 500-1000 | 250-500 | 5-10 |
Medium | 1000-2000 | 500-1000 | 10-20 |
Stiff | 2000-4000 | 1000-2000 | 20-40 |
Very stiff | 4000-8000 | 2000-4000 | 40-80 |
Hard | >8000 | >4000 | >80 |
Types of Excavation Supports
Generally, there are two most commonly adopted methods of providing excavation supports:- Soldier pile and lagging
- 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. 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.


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. For soil nailing, firstly shallow cut is excavated and then the cut is coated with shotcrete. Shotcrete is nothing but concrete which is sprayed through pipes with some force. On the shotcrete area, soil nails are driller and grouted. 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.
