In the last post, the dewatering crew mobilised and pre-drilled the spear holes along the sheet piling. This short clip shows the next step: jetting the spears into the ground. There's no voiceover, so here's what you're watching.
What's happening in the clip
The casing goes in with water. The excavator holds a long steel casing upright over the spear hole. Water is pumped down through it under pressure, and the jet at the bottom loosens the wet sand so the casing can be lowered down through it. The water comes back up around the outside of the casing, which is why the ground around the hole is flooded.
The spear goes inside. With the casing down to depth, the crew drops the spear down inside it. You can see the white riser pipe sticking out of the yellow jetting head. The spear is a pipe with a slotted screen at the bottom, and that screen is where the groundwater gets drawn in.
Filter gravel goes around it. The green kibble hanging off the excavator is full of filter gravel. One of the crew lets it run down a chute into the casing, around the spear, while water keeps flushing through. The gravel fills the gap between the spear and the sand. It acts as a filter: water flows through it freely, but the fine sand stays out of the spear and the pump.
The casing comes out. Once the gravel is in, the casing is pulled back out, leaving the spear and its gravel pack in the ground.
Why jetting suits this site
The ground here is wet sand, sitting just a metre or so below the surface. Jetting works well in sand: the water does the digging, so there's no big auger spoil and very little vibration, which matters with the neighbours' houses so close. The crew controls how hard they jet to limit the mess, because nobody wants to spray sand over the neighbours' places.
What happens next
Every second pan of the sheet piling gets a spear, through the voids we cast in the boundary footing back in Post 32. Once they're all in, the spears are connected to a header pipe that runs along the wall to the pump. Then pumping starts, the water gets tested, and the filtration gear is set up to treat the water before it goes back into the canal. How the system draws the water table down is covered in Post 36.
