Not much looks different on site, but behind the scenes we've been busy with planning, coordination and early site preparation.
Tie-downs and small jobs
We've installed tie-down straps and screw pegs at each corner of the container, with a spare screw peg stored underneath. We used two-tonne straps and put a few twists in the front strap ends, which stops them vibrating and humming in the wind. One strap is buried and the top of another is visible.
We also top-dressed the lawn with sand to level it out, which tidied up the area around the site.
Procurement and plans
A lot of our time has gone into talking with contractors and finalising technical documents.
- Sheet piling. We're in regular contact with the sheet piling company. Sheet piles are interlocking steel sheets driven into the ground to hold back soil and water while we dig the basement. Once we submit the final information we can book their mobilisation (getting their crew and rig to site). There's currently a four-week lead time.
- Dewatering and vibration management plans. We've been waiting months for the geotech's dewatering and vibration management plans. Dewatering means pumping groundwater out so the basement can be dug and built in the dry. The plans were due last week but haven't arrived, and we need them before we can finalise the sheet piling details.
- Development Approval (DA). We're expecting the DA this week, which will set off coordination with three or four contractors.
Power for the dewatering
Dewatering needs a reliable power supply. We have a temporary meter box on site and plan to have an electrician install a 32 A three-phase connection to power the dewatering plant. We're confirming the size of the plant and how close it needs to be to the power source. We may also need a custom extension cable.
We're talking through the options with the electrical engineer and generator suppliers. The house design includes a large generator (originally specified as 120 kVA with a 60 kVA ATS, or automatic transfer switch, which swaps to generator power when the mains fails). For dewatering, we need a smaller three-phase generator on site with its own ATS. If the dewatering loses power overnight, the basement floods quickly, so automatic backup is essential.
Checking the groundwater with a test pit
We dug a test pit down to the groundwater to see how the site behaves. The sides have been caving in a little, so it's not as deep as it was. Over two days, we measured the water level every hour between high and low tide with a laser level.
What we found:
- The water level in the pit was consistently higher than the water level in the nearby canal.
- That suggests the water in the site isn't draining freely to the canal. It's held in the ground and drains more slowly, most likely because of the soil.
That's good news. It means the tide isn't constantly topping up the groundwater under the site, so once the dewatering is running it should be easier to keep the site dry.
Survey work and reference points
We've raised a purchase order for an iDent (identification) survey, which re-establishes the boundary pegs. Ours cost about $2,950, and fees vary by lot and area. The surveyor will also do searches and set reference points. We're using reference marks in the kerb and across the street rather than relying on boundary fences that may be in the wrong place. Accurate, independent reference points prevent disputes with neighbours and make sure the house goes in the right spot.
When the surveyor sets benchmarks across the street, we'll finalise the top level of the sheet piling (its RL, or reduced level: the height relative to a fixed datum). That avoids cutting piles later if they're driven too high. After sheet piling and dewatering, we'll fix timber along the sheet piles and set internal benchmarks. Those set the slab heights, basement levels and pit depths: basically the whole internal layout.
We explain reduced levels with a sketch in Post 21.
Costs and decisions
Dewatering is a big cost: it's likely to run about $7,000 to $8,000 a week. The goal is to get it in and out as quickly as possible.
Sheet piles or secant piles?
There are two common ways to hold back the ground around a basement on a site like this, and they look very different in the ground.
Our estimates at this stage were:
- Sheet piling: roughly $220,000 to install, plus dewatering at around $7,000 a week for a minimum of 6 weeks. Total for this option is around $400,000 to $450,000.
- Secant piles (overlapping concrete piles that form a continuous wall): typically start at about $450,000 for the piling, plus dewatering on top. Total is around $650,000 to $700,000.
That put the difference at roughly $200,000 in favour of sheet piling at the time. Once the work was done, the final saving came out at around $300,000 (see Post 30).
Talking it through with the engineer
There have been robust discussions with the structural engineer. The engineer prefers secant piles and has designed for them, but for this residential site we're leaning towards sheet piling because:
- This isn't a high-rise project.
- The neighbouring houses are far enough away to limit vibration.
- We believe vibration controls and monitoring can manage the risk.
We're waiting on the geotech's vibration management plan and will install vibration monitors on the neighbouring houses. If vibration goes over the limits, work stops.
We're also trialling a new polymer with the sheet piling company to help measure and possibly reduce vibration. This will be the product's first field trial.
Contract administration and budget
Since the last update we've been firming up quotes and reviewing the budget. I've been entering the December estimate into our contract administration software, checking the line items and allowances, and making sure there are no obvious gaps.
The short version
Progress may look slow on site, but this planning and procurement is what decides how fast and smoothly construction runs.
Coming up
- Final geotech plans
- DA approval
- Final submission to the sheet piling contractor
- BA (building approval)
- Construction insurance
- Site generator, ATS and backup power for the dewatering
- iDent survey and benchmarks
- Power installed and dewatering equipment delivered
- Mobilisation and sheet piling installation
- Bulk excavation
More updates soon.
