After more than six months of preparation, at long last the day had come for the building work to commence on my extension. At 7.15am our building contractor, Godfrey Rawlings, arrived accompanied by digger-driver Gary, who was perched astride a monster 5.5 tonne excavator, which had clawed its way up the overgrown embankment from the lane below, bursting into the garden.

The first job of the day was to start excavating the embankment to provide site access for materials and plant. The new driveway would need to span at least 6m to accommodate heavy trucks reversing onto the site. This was complicated somewhat as new broadband cables had recently been buried along the lane.

After numerous requests, someone from the internet provider Gigaclear eventually came out and pinpointed their location (buried less than a foot deep). As an additional precaution against any unwitting severing of underground pipes I’d managed to acquire utility charts showing the (approximate) locations of mains electric and water pipe runs in the lane.

Armed with this information, the giant digger made light work of demolishing embedded tree roots and scooping out large clumps of compacted earth. Its mechanical arm sculpted a gentle slope, with a heavy-duty culvert pipe embedded within for channelling surface water drainage. Smashed up chunks of hardcore from our patio were recycled onto the new driveway with a topping of MOT type 1 sub-base, all neatly encased between splayed edgings of prefilled sandbags.

The whole thing was completed in the space of about 45 minutes, not bad for the equivalent of several man-days’ hard labour. At this rate we’d be moving into the finished extension by the end of the month!

Buoyed by the lightning rate of progress so far, we retired to the kitchen for a well-earned cuppa and a preliminary site meeting. I took the opportunity to present the contractor with fresh copies of the drawings and to agree a draft programme to synchronise deliveries of materials (my responsibility) with the workforce on site.

Dealing with the Spoil

The oversite is cleared before the trenches are dug for the extension

The excavation lines are clearly marked for the diggers after the oversite has been cleared

Being fortunate enough to have a garden extending to nearly an acre, I’d been contemplating using the excavated spoil from the foundations for landscaping. Having already experimented with the earth from the archaeological trial pits we’d had to dig a couple of months ago, it was apparent that quite large amounts could be dispersed by spreading it in layers over the ground as topsoil.

But, rather than laboriously levelling large amounts of spoil across the entire plot, the decision was taken (with the permission of our friendly local farmer) to temporarily dump it in an adjoining field where it could later be removed by grab lorry.

As it happens, clean, uncontaminated clay-rich sub-soil is in demand for capping landfill sites, for land restoration and for raising ground levels in developments on floodplains. A few calls later and the troublesome spoil was conveyed away by a convoy of eight-wheeler trucks to be recycled by a civil engineering contractor. Thankfully, the resulting dent in our contingency budget turned out to be relatively minor.

The next task was to excavate the oversite footprint of the extension, which needed to be cleared to a minimum 300mm depth to provide a sufficient void under the beam and block floor. Some more deft digging exposed the full expanse of the incoming oil pipe and electric cable runs, together with the underground foul drains leading out from the kitchen, which it turned out were located directly above the blue polyethylene mains water supply pipe nestling in its shadow. With the locations of all the services now clearly identified we could get on with the serious work of excavating trenches.

Digging the Extension’s Trenches

Working from the measurements on the drawings, the precise lines of the foundations had been carefully marked using traditional string lines and profile boards. These were later revised to show the centre of the trench rather than the edges, at the driver’s request.

That evening after dinner, a sense of peace and calm descended on the now vacant site. I couldn’t resist popping outside to have a general mooch around and a precautionary measure-up of the trenches which were about half dug. At first I thought my measurements were out. Then my heart sank as it dawned on me that there was a glaring discrepancy. One of the trenches had veered away from the centre line.

Although standard trenchfill foundations with a generous 600mm width can accommodate a reasonable margin of error, at its zenith this was out by more than just a centimetre or so. Unfortunately, you can’t simply stick the spoil back in the hole and start again. The next day the skewed trench had to be re-excavated, making it significantly wider.

Hitting an Unexpected Hurdle

The trenches for the extension flooded

The digger’s bucket punctured an old soakaway, leading to flooded trenches and a mad dash to get them dry before the building inspector arrived

With the trenches rapidly taking shape, a visit from building control was duly booked in. The depth of the foundations on our existing house could now be ascertained more accurately so, to be on the safe side with building control, we decided to match their 1.4m depth — some 200mm deeper than we’d originally planned.

All went well until lunchtime when the digger bucket suddenly hit a gigantic pocket of subterranean gravel. To our amazement, the original developers in 2001 had constructed the most enormous soakaway as a means of discharging surface rainwater, serving both our house and the neighbouring property. But being sited in ground consisting largely of impervious clay, very little water was dispersed. Given the fact that the original plot was blessed with a large pond as well as a small stream in the lane outside, why they had opted to employ such an ineffective method puzzled us all.

As we pondered this conundrum, Godfrey suddenly let out an alarming wail; our freshly dug trenches were filling with gallons of trapped ground water seeping out of the punctured soakaway and were already a foot deep in rapidly rising groundwater! With building control due to carry out the inspection in a couple of hours we knew there was no way they’d pass the foundations if they couldn’t see the bottom of the trenches. Confronted with such an alarming sight they might, God forbid, rule out conventional foundations and require us to build an expensive raft or super-deep piles. Something had to be done, and quick!

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