Before construction can start on a house with a deep excavation, there's a lot of preliminary work to make the project safe. For this site, that covers four main areas: retention, dewatering, vibration management and bulk excavation.
Our geotechnical engineer is preparing a Dewatering Management Plan (DMP) and a Vibration Management Plan (VMP), along with recommendations for our sheet piling retention system (steel sheets driven into the ground to hold back the soil) to suit this site's soil.
Dewatering: getting the water out
The main challenge here is the high groundwater table, just 1.6 metres below the surface. If you've ever dug a hole in wet sand, you know it collapses. To deal with it, we'll use spear point dewatering: the perforated pipes you often see on large commercial sites, connected to pumps that draw water out of the ground.
Each spear is a pipe pushed into the ground with a perforated tip at the bottom. A header pipe joins the spears together, and a pump pulls water up through them. Our geotechnical engineer wants the water table held at least 1 metre below the bottom of the excavation, so the sand the slab sits on can be properly compacted. With groundwater about 1.5 metres down and a dig of up to 3 metres, that means drawing it down by about 2 to 2.5 metres, using spears around the perimeter and wells inside the dig if needed.
The Dewatering Management Plan sets the rate of water extraction for the site's conditions and, importantly, how the water is treated before disposal. We can't simply pump it into the nearby canal. It has to be treated to suit the soil's mineral and chemical profile so it meets environmental standards. Post 31 shows what can happen when this goes wrong.
Managing vibration
The Vibration Management Plan covers the vibration created when the sheet piles go into the ground. It sets out the precautions to take for this soil and gives the sheet piling contractor clear limits to work within.
Protecting the neighbours
We've already taken several steps to protect neighbouring properties. Surveyors have installed reference marks on the adjoining houses at the front and back and recorded their reduced levels (RLs, surveyed heights), so any settlement can be detected.
Before sheet piling starts, the geotech engineer will install vibration monitors on these houses. They monitor in real time, and if vibration reaches an unacceptable level, work stops. This protects the neighbours' homes and services, and gives us and our insurer clear evidence if a damage claim is ever made.
Inspecting existing services
The final piece of preliminary work is checking existing services. A large, ageing concrete stormwater pipe runs along the site. It's been damaged by tree roots and ground movement in the past and was relined by council decades ago.
Before we start work, a specialist contractor will run colour camera (CCTV) inspections of this stormwater pipe and the sewer lines. That gives us a documented record of their condition before construction, so if a dispute ever arises, there's clear video evidence.
The pipe exits at the rear of the property. Part of our strategy to protect it is to keep the existing piered fence as long as possible, so we don't disturb the pipe and pay for repairs we didn't need.
What's next
We're waiting on the final DMP and VMP reports. We need them to engage the sheet piling and dewatering contractors, and to secure our construction risk insurance, because they show how we'll limit the risk of damage.
Once the retention wall is in, the area behind it will be backfilled in compacted layers with imported soil, as the geotech recommends, to keep it stable.
If you’re planning a basement
Management plans, dilapidation reports, survey monitoring and CCTV pipe inspections all cost money and time before a single slab is poured. Make sure they're in your budget and program from the start.
