Trench Support System

TRENCH SUPPORT SYSTEM 

Describe the factors affecting the method of selections of trench support system

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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 

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Classification of Excavations 

Shallow excavation – up to 1.5m

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Medium excavation – 1.5m – 3.0m

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Deep excavation – over 3.0m

Battered Trench 

Usually cheap and quick to excavate in open ground

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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)

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The slope employed is generally stable but there are odd pockets of less stable ground

TRENCH SUPPORT SYSTEM 

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Classification of Excavations 

Shallow excavation – up to 1.5m

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Medium excavation – 1.5m – 3.0m

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Deep excavation – over 3.0m

Battered Trench 

Usually cheap and quick to excavate in open ground

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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)

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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

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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

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Traditional trench support system 

Planking and strutting was originally the accepted method of trench shoring

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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

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Purpose of excavation – minimum width, minimum depths and placing of support members to provide reasonable working space within excavation

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Presence of ground water which need interlocking timbering, sump pits and pumps or dewatering

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Position/location of excavation – restrictions or police requirements especially when excavating in public road

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Non-availability of the right type of plant

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Presence of large number of services which restrict the use of machinery

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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

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Open boarding: Suitable in moderately firm ground. When erected with ½ sheeting the method is commonly known as hit and miss

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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

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Waler Unit

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Manhole Brace

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Shields and boxes

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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

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Installation and removal are carried out above ground level enhances safety

Waler Units 

Used in ground conditions that required sheeting

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Centre strut can be released temporarily and swung aside to allow greater access

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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

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Installation and removal can be carried out by one or two workman giving greater value in speed of operation

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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

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Provide clear working space with no obstructing braces

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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

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Also known as drag boxes or saddles

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Designed as safety device against collapse without an attempt to support the trench side

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Shield or boxes are pushed into the ground as the excavator digs out the loosened soil

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between the plates

TRENCH SUPPORT SYSTEM 

Plate Lining System 

Consists of rectangular plate support with adjustable struts at each end

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Plates are slot into vertical H-section strutted guide rails

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Available in a wide variety of units sizes up to 5 m long and 2.5m high

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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

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After an unexpected fall of rock, earth or material

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After explosive charges have been fired

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Within the preceding seven days