Earth Retention Systems For Deep Excavations
Earth retention is necessary where space is not available for sloped excavations. Depending of depth of excavation, soil conditions, ground water level and proximity of nearby structures and underground utilities, there are a number of different techniques that can be utilized for shoring works. Ammico has extensive experience in installing various types of earth retention systems and can offer economical value engineered solutions that are most suited to the project conditions.
Projects involving Earth Retention Systems For Deep Excavations include:
Secant piles are over cutting bored cast-in-situ piles that serve for both earth retention and cut-off and are an economical solution for excavations up to twelve to fifteen meter depth. Due to tolerance limitations, the reliability of secant piles against ingress of ground water may become questionable once excavation depths exceed fifteen meters.
Contiguous Piles and Concrete Soldier Piles
Where lateral ingress of ground water is not a serious concern, contiguous piles can be a fairly economical solution. These are piles that are touching each other or in close proximity to adjacent piles.
Concrete soldier piles are similar to steel soldier piles except that the shoring piles comprise of bored cast-in-situ concrete piles rather than structural steel beams. Due to their higher stiffness vis-à-vis steel soldier piles, the concrete soldier piles result in lesser ground movement compared to steel soldier piles.
In stable grounds e.g. soft rocks, shotcrete may be used rather than concrete panels or wooden lagging between the concrete soldier piles to provide a plain and stable surface.
Steel Soldier Piles
Steel soldier piles comprise of structural steel beams or double channels installed into pile boreholes drilled with hydraulic rotary rigs and concreted from toe level to excavation level. The center to center spacing of soldier piles is typically 1.2m to 2m. During excavation, concrete planks or wooden lagging are inserted in the space between adjacent soldier piles. This shoring system is effective where loads from adjacent structures are not significant and where a higher level of ground movement is acceptable.
Cutter Soil Mixing (Csm) With Steel Soldier Piles
For earth retention of soft soils where the shoring is also required to act as a cut-off against lateral inflow of ground water, CSM can be an effective solution. Rectangular panels are drilled with a pair of cutting wheels mounted on a kelly attached to a piling rig. As the cutting wheels rotate and penetrate the soil, cement-bentonite slurry is injected between the two wheels and mixed with the in-situ soil, the rotary motion of the cutting wheels aiding both penetration into the soil as well as mixing of the in-situ soil with the slurry.
When the CSM cutter wheels reach the required toe level, the CSM frame is withdrawn from the panel and a structural steel beam is lowered into place with a vibratory hammer. As the soil-slurry mix sets and gains strength, the structural steel beam gets grouted in position and transforms into a soldier pile capable of carrying the required lateral loads.
In cases where sloped excavation is possible (and therefore earth retention is not needed) and the requirement is only to provide a cut-off against lateral ingress of ground water, CSM can be used without soldier beams to provide a cost effective cut-off.
Shotcrete With Rock Bolting
In medium to strong rocks, it may not be necessary to install a shoring system as such rocks are generally stable under a near vertical cut to reasonable depths. In such strata, however, to avoid local failures, it is prudent to install rock bolts and cover the rock face with a steel wire mesh and subsequently place shotcrete over the wire mesh.