SHOULDER PILE (I-BEAMS)
I-Beams are normally used for a maximum excavation of 5.0 m and where ground water table is at deeper depth compare to maximum depth of excavation. I-Beam shoring does not provide water tight system.
In case of loose to dense sand, I-Beams can be installed into the ground by driving it by vibro hammer. In case of hard strata, initially pre-boring shall be carried out and then the beam shall be inserted in to it and the hole shall be back filled with gravels or any other suitable material from the excavation level to the required depth of beam.
Length, section and span of I-beam are determined by design requirements to ensure stability of I-beams against lateral pressure of soil.
After installation of I-Beam excavation will be done and simultaneously concrete panel or plywood / timber planks will be inserted between two I-Beams.
After completion of construction works (retaining wall) within the excavated area, temporary I-Beams shall be extracted using vibro hammer and crane.
Contiguous piles are normally used if ground water table is at deeper depth compare to maximum excavation level, also if adjacent to shoring system there is no any existing structure then also contiguous piles may be used even if ground water table is at shallower depth to maximum excavation level. It work as a cantilever member wherever its possible.
This shoring system consists reinforced pile of adequate diameter to meet design requirement along the shoring perimeter. In between two adjacent piles normally 500 mm gap is provided, so it does not give water tight shoring system.
General procedural steps for casting of contiguous pile are as follows
Marking the position of piles on the ground and then installing reinforcement concrete or I beam guide wall according to approved design and drawing.
As per site condition, method to install contiguous bored piles will be decided. Piles may be installed by using temporary casing and /or bentonite slurry to support the soil or by CFA.
After completing installation of piles, heads should be chipped up to cut off level and then capping beam shall be cast.
After that site will be ready for desired excavation
Normally secant pile is used if ground water table is at shallow depth compare to maximum excavation depth. If depth of maximum excavation is more then irrespective of ground water table condition secant piles are used, in UAE maximum up-to 2B contiguous piles are allowed and if excavation is more, then we must opt for secant pile.
Procedure for secant pile and contiguous pile is same. In secant pile system, there are two types of piles are cast namely primary or soft pile and secondary or hard pile. Primary piles are of low grade of concrete and cast without any reinforcement. Secondary piles are of high grade of concrete and it has adequate reinforcement to carry lateral pressure. Initially soft piles are cast at site and then hard piles are cast in-between two successive soft piles. Thus, this piling system provides water tight shoring system.
SUPPORT SYSTEM OF SHORING
In some situation cantilever shoring system is not adequate to support the lateral pressure. In case of deeper excavation and space limitation for shoring system additional support is required to meet the requirement and economizing shoring system.
There are two types of shoring supporting system mainly used, anchor and internal strut (Prop).
Anchors consist of high strength strands. It is installed by drilling through ground (normally at angle of 300 to 450 with horizontal) to required length and then grouted for minimum fixed length. Anchors are either supported in wale system or in capping beam of shoring wall.
Anchors can be used above ground water table only and if adjacent to shoring (Retain) side allow, since anchors are spread on retain side the shoring.
Due to non-accessibility of adjacent property if anchorage can not be done for a shoring wall then internal strut (prop) system to be provided. Prop is a horizontal strut supports in between two shoring wall or an inclined vertical strut supported one end on earth and another on shoring wall.