1 Trench Support Methods A) Traditional Methods include the following types: i) Sheeting, waling and struts ii) Soldier piles iii) Staged trench excavation

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Trench Support Methods

A) Traditional Methods include the following types:

i) Sheeting, waling and struts

ii) Soldier piles

iii) Staged trench excavation

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Trench Support Methods

B) Proprietary Support Systems -

developed by the relevant manufacturers for more labour saving construction, as well as more conscious desire for site safety.

They are grouped into three broad types:

i) hydraulically strutted shores and walings

ii) shields and boxes

iii) plate lining systems

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Sheeting, walings and Struts (Fig. G-1)

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Sheeting, walings and Struts

Materials available Sheeting: timber boards, steel trench

sheets Walings: timber, steel sections, aluminium

sections Struts: timber, steel trench struts(screw

type), hydraulic struts, heavy duty steel struts, purpose made tubular struts

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Design Methods

CIRIA Report 97 (Ref.)Use of SheetingDesign of walings and Struts: Either by Calculation Methods; orby Quick Reference Tables (Table G)

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This method may not give the most economical solution.

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Precautions in Design

1. The lateral earth pressure acting on the trench support will tend to increase with time for cohesive soils. Increased pressure should be allowed in the design for trenches which are expected to be left open for a long period

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Precautions in Design

2. To avoid any weakness or excessive pressure at the cantilevered ends of the sheeting:

a) The first waling should be placed not lower than 500 mm below the ground surface;

b) The sheeting should be tied in at the base

c) The waling should extend approx. 300 mm beyond the last strut to ensure even spread of strut load

d) Timber wedges driven between waling & sheeting to ensure uniform support between the soil face and the support system

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Advantages of Sheeting, Waling, Strut Support

Systemconvenient and versatile support system components cheap and easily obtainedlight and easily dismantled by operativesflexible to accommodate changes in trench profile, local obstruction, crossing services and construction features e.g. wider excavation for manholesensure safe working in all stages during hand excavation

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Disadvantages of Sheeting, Waling, Strut

Support Systemtrench walls left unsupported when full depth of the trench is excavated by backhoe/backacterswide variations in the design ,i.e. the arrangement and spacing of struts and walings which may require attention of more competent site supervisorsLonger pipe lengths and/or plants installation may be obstructed by the closely spaced struts inside the trench of deeper depth in particular

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Soldier Piles

Materials

1. H-section soldier piles ( between 200x200 and 300x300 mm ), with or without timber walings and

2. the sheeting

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Soldier Piles

Method

H-section piles driven at 2.5 metres spacing to a depth below the trench bottom prior to trench excavationTimber boards wedged against the inner flanges of the H-section piles or the timber walings (Fig G-2)

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Staged Trench Excavation

The excavation could be split in stages for very deep trench.

Care must be taken not to disturb the strutting of the upper stages and to allow space excavation and lowering pipes etc. between the more complex pattern of struts.

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Proprietary Support Systems

Hydraulically Strutted Shores and Walings Method

Most systems available are aluminium made to save weight.The struts and palings are pre-assembled in rectangular forms which can be lowered into the trench by the excavator between previously placed sheeting

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Proprietary Support Systems

Hydraulically Strutted Shores and Walings

Method (Cont’d)

The hydraulic struts are then expanded by remote operation from ground level.

Frames up to 5 metres long walings

Struts could be used for trenches up to 5 metres wide.

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Hydraulically Strutted Shores and Walings

Advantages

The operatives need not enter the trench during strutting.

do not require continuous sheeting (allow gap between sheetings for crossing services)

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Shields and Boxes

MethodSimple two sided rectangular structures placed in the trench to provide a safe environment for operatives to workGenerally can be extended in width and height.Sometimes the walls can be jacked Vs trench sides but for most occasions being a device to protect worker s Vs trench collapse only.

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Shields and Boxes

Advantages

The loose fit makes it easier to drag the box (by the excavator) in progress with the works along the trench.

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Shields and Boxes

DisadvantagesIt is important to consider the loss of ground between adjacent boxes.Do not provide significant support to trench sidesInconvenient to use with high water tables or where services cross the line of the trench.Weight of the box/shield could be important (could be up to 3 tonnes)

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Plate Lining Systems(Fig G-5)..\Teaching_Notes\TEMP_WK\multimedia\Cofferdam-Plate-lining1.jpg

MethodThe system consists of rectangular plate supports with one/two adjustable struts at each end.The plates are supplied in vertical modules to accommodate a wide range of trench depths and to enable the upper section of the trench to be supported as excavation proceeds.Steel-made unit sized up to 5 metres long by 2.5 metres high.Resisting external pressures up to 40 kn/sq.m.

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Plate Lining Systems

Disadvantages

Special care are required :

for setting out the line for the system

when dealing with crossing services and manhole positions

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Working Clearances for Trench Works

Trench Width (Fig G-6)..\Teaching_Notes\TEMP_WK\multimedia\Trench_Width.jpg

to allow access for installing services, space for any sumps required, and sufficient access for compaction plant during backfilling

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Recommendationsfor Trench Widths

Pipe diameter

Minimum Width between Walings

D (mm) Trench up to 3 m deep

Trench 3 m to 6 m deep

< 300 700 1000

300 - 850 D + 400 1000

> 850 D + 600 D + 1000

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Safety hazards in trench excavation

Trench walls collapse

Install trench support

Avoid superficial load from construction vehicles

Keep stockpiling at least 1.5 m from the trench side

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Falling objects to trench operatives

Operatives to wear safety helmets

Install guard rails at both trench sides

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Misuse of lifting heavy objects by excavators ( gravity fall in case of hydraulic failure )

Check whether the machine has been certified for lifting operations or

Use other suitable lifting plants equipped with check valves

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Hazards from existing underground services

Damage live public utilities e.g. water, gas, electricity

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Obtain the most updated location information of the services from the relevant authorities prior to excavation

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Hazards from existing underground servicesLocate ferrous and power cables by electrolocation equipment

Pilot trench holes ahead of main excavations

Crossing services to be supported by hangers suspended at ground surface or propping from beneath

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Hazards from existing underground servicesGround movement resulted from trench works can cause brittle cast iron pressure pipes fracture and burst (sometimes some time after reinstatement of the trench due to adjusted movement of the adjoining soil)

May require replacement of existing services by new installations through agreement with the utility authorities

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Hazards to existing buildings

Deep excavation may undermine the foundations, settlement damage or collapse of adjacent buildings

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Hazards to existing buildings

Backfill the trench with concrete up to the level at which the perimeter line of the supporting wedge cuts the trench (foundation stress at a spread of 45 degrees, and additional 150 mm of concrete above the 45 degree line are assumed)

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Hazards to the public at the surface

Danger to vehicles driving for carriageway trenching works

Provide properly guard rail protection (at least 1 metre above ground), traffic signs, lighting and illumination (at night) in accordance with the appropriate traffic signs design manual.

The arrangement has to be agreed with the highway authority and the police

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Site Safety Regulationsfor Excavation Works

Under the Contraction Sites (Safety) Regulations of Hong Kong, the Contractor carrying out excavation work shall be responsible for:

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(1) Safety of excavation:-

(a) erecting structures made by suitable material in connection with the operations of the work as soon as may be necessary after their commencement so as to prevent workmen from being endangered by a fall or displacement of earth rock or other materials adjacent to or from the side of the excavation,

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(b) ensuring every part of the excavation to be examined by a competent person at least once per week after commencement of the excavation work until it is completed. Such a report will be made in approved forms which include a statement that further excavation is safe and secure. No further work is allowed until the report is released.

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(2) Fencing of excavation wherever there is risk that a person will fall a distance of more than 2 metre by:

(a) erecting a suitable barrier as close as practicable to the edge of the excavation

(b) the excavation is securely covered

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(3) No material is placed/stacked close to the edge of excavation. No load, plant is placed/moved close to the edge of excavation which is likely to cause the side of the excavation to collapse.(4) Whenever there is apparent danger to persons employed in the excavation area from rising water or from an irruption of other material, adequate means of emergency escape must provided for such persons.

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Documents

1 Trench Support Methods A) Traditional Methods include the following types: i) Sheeting, waling and struts ii) Soldier piles iii) Staged trench excavation