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Innovative trench support systems have been developed within the past two decades. New proprietary systems over the traditional methods have been used widely. These systems are more commercially demanded as they save the labor construction cost, site safety and time. The proprietary support system provides facility of rapid installation and the removal facility. This helps in avoiding the problems caused due to the lack of skill of labors and the risk in operations. The principle types of proprietary system based on the Construction Industry Research and Information (CIRA) Report 97 are mentioned below: 1. Hydraulic Frames
2. Sliding Panels
These systems mentioned above can be implemented in a stable ground. This means that the stable ground will stay unsupported for a desirable period of time till the lowering of the members into the trench is done. Hence no work must be carried out by the operators within the unsupported excavation. These are more reliable when working with areas with damp sands, stiff clay, gravel area or with weak weathered rocks. For areas like water bearing gravel sands, sandy silt, and sand, these systems can be used, given that the ground water has been removed by means of a bore well system or a well point system.Hydraulic Frames Trench Support System
These are also called as wall frames supporting systems. This system is a modest extension of traditional methods. Here the struts and the wings are pre-assembled in the form of a rectangle. This is then lowered into the trench by the excavator. In this system, based on the stability of the ground under consideration, trench sheets or runners are set in the excavation. The extent of stability of the ground decides the spacing at which these runners have to be placed. The arrangement is shown in the figure-1. The sheets are pushed into the soil at a depth which is below the bottom of the trench. This makes the sheets to be self-supporting for a temporary period of time. The next procedure involves the lowering of the lower frame. This lower frame consists of a pair of wings. These are connected by struts that are hydraulically operated. The pressure hoses are attached to the pump placed on the ground surface as shown in the figure-2. The pump is operated to expand the struts. This makes the sheets to get closer against the trench sides. The lower frame installation is complete. This is followed by the upper frame, which have the same procedure. The arrangement is shown in figure-3.
Fig.1: Excavation of the trench in full depth. Then the trenches are placed in sheets. They are pierced in the soil as a temporary support
Fig.2: The hydraulic struts is connected to the pump and the lower frame is lowered into the trench. The struts are inflated and the hydraulic hoses are removed
Fig.3: Installation of upper frame by following the similar procedure
As the sheeting is placed from the ground level, there is no need for the worker to enter inside the trench. This is an added benefit of the system. Experience have showed that a single banks man is enough for the complete operation. There are varying sizes of frames offered by the manufacturers. For waling, there is size up to 5m and over 2m deep sizes for the vertical shores. All these systems will have hydraulic struts that can extend up to or greater than 5m wide support trenches.
Fig.4: A figure of Hydraulic Strutted Shoring System in Mech plant Limited
Special frames are made to support large excavations. Within the telescopic members of the frame support, these arrangements have hydraulic jacks which helps in having maximum clear opening to carry out construction. Most of the components are made of aluminum to save weight.Shores as Trench Support
This system has an arrangement similar to the hydraulic frames, except the use of wings are removed and instead a pair of trench sheets is forced against the wall. This is inserted by a single hydraulic sheet. Ground areas, where there is less need for ground support, the shores are used.Boxes as Trench Support
This system involves the assembling in the ground level. The assembly is later lowered into a trench that is either pre dug or partly dug. It is then sunk as a caisson to the final level of the trench. Choosing a suitable strut length helps in adjusting the width. Stacking of box one over the other help in the use of box in deep trenches. The figure-5 shows the arrangement of boxes in the direction of progress of work.
Fig.5: Arrangement of Boxes in Trench
Sliding Panels as Trench Support
The sliding panels are assembled by pushing two pairs of side rails into a trench. This trench may be partly dug or it can be pushed from the unstable ground surface. Sliding panels made of steel are set in between the rails. This is then pushed down progressively as the excavation goes deeper. For deep trenches, the panels will get stacked one above the other.Shields or Drag Boxes as Trench Support
Shield or drag boxes are not really used as support methods. These are used to be pulled along with the trench once they are excavated. This is carried out by the digger to protect the workmen from the fall of the earth. This type of box formation is rigid in nature. When compared to the width of the trench, these have lesser width. It is very essential that the excavator used must have sufficient power in order to pull the box along.
Fig.6. Drag Box Support
Planning Behind the Use of Proprietary Systems
The proprietary system equipment and their capacities and service are mentioned by the manufactures. Along with this, the choice of best proprietary system for the construction can be made with the help of the recommendation given by CIRA No:95. The third edition of this technical note also gives an idea about the specifications and details of manufactures of the specific equipment also. Among all the systems mentioned above, the hydraulic systems are used in situations where the service is complex and difficult for safe support.Read More:Movable Excavation Supports for Trench and Foundation ConstructionTrenchless Renewal Methods for Replacement, Renovation of PipelinesTrenchless Construction Methods and Their Details and UsesSelection of Excavation Dewatering System for Construction WorksExcavation Monitoring System -Types and Objectives for Safety in Construction