TRENCH SUPPORT SYSTEM
Describe the factors affecting the method of selections of trench support system
Identify various types and functions of trench support system
TRENCH SUPPORT SYSTEM
Introduction
Purpose of excavating a trench is to allow installation
of public utilities, sewers, foundation etc.
The excavation of the trench be either done by hand or
machine
TRENCH SUPPORT SYSTEM
Classification of Excavations
Shallow excavation – up to 1.5m
Medium excavation – 1.5m – 3.0m
Deep excavation – over 3.0m
Battered Trench
Usually cheap and quick to excavate in open ground
Appropriate proprietary equipment can be used in battered trenches primarily to
protect personnel. Such protection would be particularly useful where: - the safe temporary slope of the ground is difficult to determine or is unusually shallow
The ground may become unsafe if any delay occurs (e.g. in clay soils)
The slope employed is generally stable but there are odd pockets of less stable ground
TRENCH SUPPORT SYSTEM
Classification of Excavations
Shallow excavation – up to 1.5m
Medium excavation – 1.5m – 3.0m
Deep excavation – over 3.0m
Battered Trench
Usually cheap and quick to excavate in open ground
Appropriate proprietary equipment can be used in battered trenches primarily to
protect personnel. Such protection would be particularly useful where: - the safe temporary slope of the ground is difficult to determine or is unusually shallow
The ground may become unsafe if any delay occurs (e.g. in clay soils)
The slope employed is generally stable but there are odd pockets of less stable ground
TRENCH SUPPORT SYSTEM
Vertical Cut Trench
Where the overall working space is restricted, particularly in urban areas, the trench faces will usually have to be vertical
The effect of the Construction Regulations is to require trenches over 1.2m in depth to be supported unless the trench is in stable ground
Traditional trench support system
Planking and strutting was originally the accepted method of trench shoring
Close boarding for applications in running soil and generally poor ground. Less planking may be possible in more stable conditions
TRENCH SUPPORT SYSTEM
Factors affecting the method of excavation and timbering
Nature of the subsoil to determine plants or tools required and amount of timbering needed
Purpose of excavation – minimum width, minimum depths and placing of support members to provide reasonable working space within excavation
Presence of ground water which need interlocking timbering, sump pits and pumps or dewatering
Position/location of excavation – restrictions or police requirements especially when excavating in public road
Non-availability of the right type of plant
Presence of large number of services which restrict the use of machinery
Disposal of excavated spoil may restrict the choice of plant
TRENCH SUPPORT SYSTEM
Method of timbering
Pinchers: only suitable in stable ground and can spaced up to 1.8m in shallow excavation
Open boarding: Suitable in moderately firm ground. When erected with ½ sheeting the method is commonly known as hit and miss
Close boarding: Suitable in soft or loose ground. Trench has to be excavated in stages
TRENCH SUPPORT SYSTEM
Proprietary Trench Support Systems
Hydraulically strutted shores and walings
Vertical Shore
Waler Unit
Manhole Brace
Shields and boxes
Plate lining systems
TRENCH SUPPORT SYSTEM
Hydraulically strutted shores and walings
Vertical Shore
Installation and removal can be carried out by one or two workman giving greater value in speed of operation
Installation and removal are carried out above ground level enhances safety
Waler Units
Used in ground conditions that required sheeting
Centre strut can be released temporarily and swung aside to allow greater access
In situation where excavation is capable to stand for a short period, trenching and timbering are carried out in sections of length of the waler units
Installation and removal can be carried out by one or two workman giving greater value in speed of operation
In poor ground conditions, runners is necessary to provide continuous support throughout the stages of excavation
TRENCH SUPPORT SYSTEM
Manhole Brace
Designed to support rectangular or square excavations
Provide clear working space with no obstructing braces
On large span frames, single cross bracing may be needed to accommodate loading
TRENCH SUPPORT SYSTEM
Shields and Boxes
Consists of two sided rectangular structures placed in trench to provide safe working environment
Also known as drag boxes or saddles
Designed as safety device against collapse without an attempt to support the trench side
Shield or boxes are pushed into the ground as the excavator digs out the loosened soil
between the plates
TRENCH SUPPORT SYSTEM
Plate Lining System
Consists of rectangular plate support with adjustable struts at each end
Plates are slot into vertical H-section strutted guide rails
Available in a wide variety of units sizes up to 5 m long and 2.5m high
Slide rails and panels are pushed into the ground as soil is excavated between panels
TRENCH SUPPORT SYSTEM
Hydraulically strutted shores and walings
Safety
Trench should be examined:
At least once a day if persons are employed therein
After an unexpected fall of rock, earth or material
After explosive charges have been fired
Within the preceding seven days