Double Acting Brace System Installation Guide

How to assemble, install and remove the Double Acting Brace System, the modular hydraulic bracing frame we hire out for four-sided excavations such as manholes and pumping chambers.

The legs are laid out on timber bearers, pinned together into a frame and pumped out to size. In the Dig & Push method, two frame levels are lifted into a shallow pit, then the sheets are pushed around them in stages as the excavation goes down, with a temporary frame restraining the bottom of the sheets. Removal is the reverse of this procedure. We also cover pre-driving the sheets, excavations with three or more frame levels, and casting a concrete base slab.

If you'd like a printable copy, download the Double Acting Brace System User Guide (PDF). For full specifications, see the Double Acting Brace System product page.

Equipment Details

Installation Sequence - Dig & Push

1. Preparation

Lay first Double Acting Brace leg down on level ground on suitable timber bearers. Connect lifting hooks through the lifting points on the outer body.

If preferred, the legs can be assembled within the excavation at the level specified in the Temporary Works Design. Follow steps 1-3 within the excavation.

2. Assemble and pump out the frame

Lay remaining legs down as Stage 1 and insert a 40Ø Pin & R-Clip in each corner (Detail A). The frame legs should be fully retracted when delivered. Ensure that the pump is full of shoring fluid. Attach the two single hoses supplied to either side of the pump. Connect the hoses to the male connectors contained within the housing near the end of the frame inner. Open the lock-off valves and pump out the frames to the required size. Close the lock-off valves to maintain pressure.

Please note that pumping out of all four legs may need to be undertaken incrementally, moving from leg to leg to ensure that the frame doesn't bind when out of square.

Ensure that the shoring fluid level is maintained. Additional fluid is likely to be required during pumping out of 4 legs / one frame.

3. Excavate and lift in the frames

The following steps describe the 'Dig & Push' installation method for an excavation with 2no. frame levels (F1 & F2, upper and lower frames). Alternative methods below.

Check for services and then excavate to the underside of Frame F1 (upper), using a stepped or battered edge if required.

Using correctly rated lifting chains, lift two frame levels into the shallow pit and lay on timber bearers.

4. Push the sheets

Using the correct Driving Cap for the sheet type, push sheets around the perimeter of the excavation until refusal, or about half the required dig depth. The Double Acting frames will act as a guide. Ensure that the restraint chains are hooked onto the sheet piles.

Temporary use of shorter sheets can be used to provide a digging window if required.

5. Install Temporary Frame B

Excavate to the midpoint between the upper and lower (F1 & F2) frames' final levels. Install the 'dummy' Temporary Frame B to restrain the bottom of the sheets.

The frame can be installed partially retracted and then pumped out to the required size in the excavation. If this is not possible, the frame can be installed leg-by-leg.

Remove the Temporary Frame A that was stacked above Frame F1 in Stage 3.

6. Install Frame F2

Progress the excavation to the underside of Frame F2 level by digging and pushing the sheets incrementally.

Install the lower Frame F2 and attach restraint chains.

Temporary Frame B can now be removed.

Continue 'digging & pushing' the sheets until the final formation level is reached, installing additional frame levels where required.

7. Final sheet levels and edge protection

Ensure that all the sheets are pushed to their final levels, normally the formation level.

Install a suitable access and edge protection system.

The removal of the equipment is the reverse of this procedure, reinstalling the temporary frame levels where required as backfilling progresses.

Alternative Installation Methods & Removal

Alternative Installation: Pre-driving

1. Pre-drive all sheets to the final toe level as identified in the Temporary Works Design using suitable plant.
2. Excavate to the underside of the upper Frame F1.
3. Install Frame F1 to the level specified in the TWD.
4. Excavate to the underside of the lower Frame F2.
5. Install Frame F2 to the level specified in the TWD.
6. Excavate to formation level.
7. Unless a slab is poured (see adjacent), the excavation must be backfilled to the underside of each frame prior to its removal.

Dig & Push Installation with three or more frame levels

In situations where three or more frames are required, Stage 6 (Installing Frame F2) is repeated with further temporary frame levels installed to ensure the bottom of the sheets is always secured.

Excessive sheet cantilever below the lowest frame must be avoided.

Two-Stage Construction - Casting of concrete base slab

1. Prepare the formation adequately to enable a concrete slab to be poured.
2. Cast a suitable concrete base slab ensuring contact with the sheet piles around the entire perimeter. Use plywood or other suitable separation membrane to enable extraction of the sheets.
3. Once the slab can sustain the load identified in the Temporary Works Design, remove the lowest frame level. Ensure that the weight of the equipment is adequately considered in the Lift Plan (by Contractor). The frame must be fully supported by suitable plant whilst the hydraulics are retracted and adequate access provided.
4. Backfill the excavation in adequately compacted layers to the underside of the next frame level.
5. Remove the next frame level and continue this process until ground level.
6. Extract the trench sheets and reinstate accordingly.

Please ensure all legs are fully retracted prior to collection.

Residual Risks

The following residual risks must be addressed in the Contractor's RAMS:

1. Locate all potential services prior to excavation in accordance with NRSWA Regulations.
2. The excavation may be classified as a confined space. The Contractor must assess this risk and provide suitable gas detection and rescue equipment where appropriate.
3. The Contractor should always adequately consider the surrounding environment and account for adjacent structures, slopes, roads etc.
4. Additional or independent edge protection may be required during the installation.

Do's and Don'ts

• Ensure an adequate Lift Plan is incorporated into the method statement (by Contractor) and pay close attention to the weights of individual legs (shown above) and whole frame levels.
• Carefully inspect all components and hydraulics prior to use. Contact SEL for advice where damaged components are found.
• Ensure all operatives are Trained & Competent and have been briefed on the requirements of the Temporary Works Design where necessary.
• Install suitable edge protection, access system and Davit system where required.
• Ensure appropriate vertical support is provided to the legs during installation and removal.
• Allow a minimum of 100mm extension on the rams to enable retraction when removing the equipment.
• Don't enter an excavation before adequate shoring is in place.
• Don't use hanging chains for lifting.
• Don't use faulty or damaged equipment.

Edge Protection & Access

Please contact Site Equipment for info on our full range of Access & Edge Protection accessories.