Trench Excavation Works
- a Different Way of Thinking

A 40 m trench box system enables long lengths of polyethylene pipe to be laid at a site in downtown Auckland

“Slide rail systems have begun to be the system of choice for larger diameter and deeper pipe installations. Traditional methods such as sheet piling still have a place, but for many situations, the slide rail system is very much faster and comes at a much lower cost to the contractor.”

When European shoring systems first made their appearance in New Zealand in 2007, the concepts involved in their efficient use were little understood. Many contractors either tried to avoid shoring altogether by benching or battering, or took the risk of doing nothing at all. At best, backyard fabricated trench shields were used. As well, many specifiers and council engineers had scant knowledge of the safety requirements associated with trench excavations.

So much has changed in the three years since.

What is now required?

Most contract specifications and schedules of quantities for works involving trench excavations now have specific requirements for safe shoring methodology to be used. Typically, any trench excavations over 1.5 m deep are singled out, and tenderers must show how they will safely undertake these works with special shoring equipment or other safe trench methodologies.

The Department of Labour has been active in encouraging this change, and no doubt lives have been saved due to their focus on this issue and the subsequent increase in awareness throughout the industry.

Shoring systems which completely eliminate subsidence and provide a safe and efficient working environment for in-trench construction activities are now freely available throughout New Zealand and are frequently used by smart operators.

This concept involves positive shoring, rather than digging a trench then dropping something in, to avoid harm to workers. At all times the trench walls are fully shored as these systems are dug in as the excavation progresses.

Going to great lengths

The KS100 trench box system from Krings International in France can safely shore to 3.5 m deep and is available in lengths of 4 m or 2.5 m. Increasingly, contractors are realising the benefits of opening up longer lengths of trench.

Efficiencies can be gained by doing this. For example, one crew can concentrate on excavation and shoring installation while another safely lays pipe, without delays and without any risk to people in the trench. Longer lengths of pipe can be laid.

During a recent project in Auckland, the contractor elected to open up 40 m of trench using 10 x 4 m KS100 units in line to enable long lengths of PE pipe to be laid in a hostile environment which included contaminated ground and twice-daily tidal inflows.

Slide rail systems have begun to be the system of choice for larger diameter and deeper pipe installations. Traditional methods such as sheet piling still have a place, but for many situations, the slide rail system is very much faster and comes at a much lower cost to the contractor.

Initially, the Krings International France DGVP, or double slide rail system, was the only one available in New Zealand through Trench Shoring New Zealand (TSNZ). This enables excavations to 6 m deep and almost any width specified. However, there are many pipe installations of diameters up to 2 m or so and at depths of up to 4 m where the EGVP, or single slide rail system, can be more effectively used.

TSNZ has imported the first set of EGVP single slide rail and this is now available to hire with a choice of 3 m or 4 m long panels.

The single slide rail system is like the double, with the transverse frame assembly able to slide up and down to provide working clearance. The systems can be configured to enable long lengths of PE, up to 15 m, to be inserted into the open trench. This methodology will soon be employed on Auckland’s South Western Interceptor pipe line.

Deeper and deeper

The Krings International France DGECK, or double corner slide rail, has been used many times in the past year or so for pit excavations to 5.5 m in depth. This equipment is especially suitable for large diameter manholes or pump stations. Now, the DGECK can be used for depths up to 7.5 m. Panel lengths are either 3 m or 4 m, giving more options as to the size of the pit.

A subset of the DGECK system is the combination of the DGECK and the single or double slide rail systems. This configuration is increasingly being used for tank and chamber installations, which require pits which are longer than the standard 3 m square or 4 m square DGECK. By adding intermediate slide rails, together with external walers, open pits of 8 m long x 4 m wide can be provided. The transverse frame assembly is removed once the walers have been installed. Engineering designs have been completed to provide certified shoring for a range of such configurations to 6 m depth.

And, there’s more to come

In response to increasing demand, TSNZ has recently ordered additional slide rail equipment which will arrive in New Zealand towards the end of the year. TSNZ will then have the capacity to service multiple projects and in much longer configurations. A significant amount of EGVP single slide rail will be part of this order, as well as another set of DGVP double slide rail and a second set of DGECK double corner slide rail.