Shallow or Domestic Excavations about Trench Safety

The images above and below were taken on a couple of larger projects i have been involved with however the majority of domestic drainage is in the great scheme of things fairly shallow, and you are unlikely to be using trench boxes and trench sheets if you are a DIYer.

There is as much danger involved in excavating the first metre (depth) of a dig as there is the next three or four, one of the main reasons is that the majority of things that are likely to spoil your day such as gas, water, electric and cable services will be within that depth.

If you have pulled up on site with an excavator, trench box and banks man you would assume that these hazards have been taken into account it is however the guy with the pick axe and wheel barrow looking to repair a defective gully pot that will not have considered where the water or gas main enters the property.

There are of course strict guidelines as to where and at what depth any service pipe, cable or duct should be buried both outside and inside of the curtilidge of any property, unfortunately the gangs that installed many of these services were paid by the yard and corners were often cut, it is not uncommon to find gas, water and even electric cables concealed within concrete hard standings or within the sub-base that supports tar-mac or flagged driveways.

In the last 15 years there has been a massive installation program with regard to cable communications beneath our highways and footpaths and to some degree these were monitored by various bodies, however once the installation entered the home owners side of the boundary anything went and we find cables inches beneath flower beds and lawns or pushed into joints between flag stones and then covered with sand and cement pointing, we even find the cables lay above ground beneath hedgerows but concealed with leaves and debris. Cutting through one of these cables will not be life threatening but the cost of renewing a section (they apparently can not be repaired so you have to renew complete lengths) will make your eyes water especially if you are not insured.

Another issue with domestic excavations is where to store the spoil, if you excavate a metre deep trench and store the spoil at the side you may as well be in a 1.5mtr deep hole when the trench sides collapse and the spoil slides in. There is an old saying used by the canal navies `dig deep, throw well back` presumably for this very reason.

Trench Shuttering & Shoring

Trenches collapse for numerous reasons and excavations should therefore be shored for safety reasons, a trench does not have to be in excess of 1200mm deep to be dangerous particularly if you are bending down or crouching in the excavation in order to joint a pipe or pick up the tape measure you have dropped.

Loose and granular soil is naturally unstable as are most ground types when affected by ground water or inclement weather, trenches that cross disturbed or infill ground are also prone to collapse and vibration from site plant and vehicles can also disturb previously stable ground.

It is therefore necessary to use a proprietary trenching system such as hydraulic walling frames, trench sheets, trench boxes and drag boxes

Other techniques used for trench safety include battering back the sides of the excavation to a safe working angle and stepping the sides of the dig, though these techniques are usually used on larger projects.

(This article comes from editor released)

Trench Sheet Extractor

Trench sheet extractors are used for removing trench sheets from an excavation one at a time through the pre-drilled hole in the end of the trench sheet, reducing the risk of damage during removal.

There are two types. The standard one has a hinged jaw arrangement with a pin that locates through the pre-drilled hole in the end of the sheet & the other is a quick release one that allows single operation lifting with a rope attached to release the sheet from ground level.

  • Standard = s.w.l of 5 Tonne
  • Quick release = s.w.l. of 2 Tonne

(This article comes from Pro Mech Ground Support Systems editor released)

Working Procedure —— Traditional Sheet Pile Shoring System For Trench Protection

1. Make sure that the trench line is carefully and professionally protected on both sides to avoid any accidents.

2. Check the trench line for existing underground utilities (cables and cable ducts, water and gas pipes, sewerage pipes, etc) and remove them before commencing any other work.

3. Remove any pavement (concrete or asphalt) and any other obstacles.

4. Provide trench sheets and/or sheet piles of sufficient length, section modulus and quantity along the trench and drive them by the selected hydraulic vibrator or impact hammer to the designed installation depth.

5. Start excavation with a suitable hydraulic excavator of sufficient operating weight and boom length. Excavation depth in this first step will be done ~300 mm deeper than the first layer of horizontal waler beams.

6. Lift the waler beams of designed section modulus and length into the trench at both sides and fix each beam by two safety chains (which are hooked to the top of the trench sheets/sheet piles) to the correct position and the designed trench depth. Observe that the waler beams (H- beams) are perfectly placed horizontally in longitudinal and crosswise trench direction).

7. Lift the trench struts or other bracings of designed buckling strength into the trench and fix them carefully between the beams. Each pair of beams has to be supported by at least two bracings according to the design. Make sure that the struts or bracings are correctly extended and fixed to the beams.

8. Continue to excavate the trench to the final depth and repeat steps 5, 6 and 7 in case that more than one layer of waler beams and bracings are required according to the approved shoring design.

9. After laying the pipe or culvert the trench has to be filled-back and compacted in layers according to project specifications whereby also the bracing system (waler beams, trench struts and safety chains) will be removed in steps.

10. After reaching the top backfill level extract the trench sheets or sheet piles by a suitable hydraulic vibrator (add-on vibrators, Mobilram piling rig or crane-suspended vibrator).

11. Clean the locks of interlocking sheet piles and store them carefully for next use. If piles are damaged (at the top by the vibrator clamp and at the bottom by stones or bolders during driving) cut the damaged length and make sure, that the locks can interlock again.




(This article comes from IMECO Austria editor released)

Traditional Sheet Pile Shoring System For Trench Protection


The traditional sheet pile shoring system for open-cut trenches, manholes and shafts need the following three main items:

1. Vertically installed trench sheets (overlapping) and/or light and heavy sheet piles (interlocking) which are driven and extracted by a suitable pile driving equipment.

2. The bracing system which includes one or more layers of horizontally placed H-beams (waler beams), cross bracings (mostly trench struts or beams with spindle heads) and safety chains to secure the H-beams in horizontal position.

3. The equipment to drive and extract the trench sheet or sheet piles which can be

a) Hydraulic add-on vibrators which are mounted to an excavator boom instead of the bucket and connected to the hydraulic system of the base machine (hammer line).

b) Telescopic piling rigs type Mobilram or equivalent consisting of an excavator-type carrier, a telescopic mast with automatic verticality control and a hydraulic vibrator.

c) Hydraulic vibrators with power pack suspended from a crawler crane.

If hard layers are encountered (SPT>50) a hydraulic impact hammer may be used to drive the trench sheets or sheet piles through the hard layer and get the piles to the designed level. In some cases it might be necessary and/or feasible to pre-drill hard soil to enable pile driving.

All parts of the sheet pile shoring system have to be designed according to actual site conditions (i.e. pipe length and diameter, excavation depth, soil and groundwater condition, additional side loads, etc) and approved according to international standards (EN, DIN or BS) to withstand the active earth pressure and to provide maximum safety for anybody working inside the trench.

In most cases a suitable ground water control system (submersible pumps, vacuum wellpoints or deep wells) must be foreseen to lower the ground water level temporarily to provide a clean and dry working environment at the trench base and to reduce the water pressure to the shoring system.

(This article comes from IMECO Austria editor released)