Welcome. This article shows the construction of deep and large diameter shafts for mining and other civil work applications such as the one shown in this article.
These shafts were constructed to enable the passage of gas pipelines which come from an offshore gas field. The first step of the shaft construction is to build a return pit, with enough storage to contain 2 to 3 times the water volume of the shaft that will be excavated. Once this is completed a grouting campaign is initiated in the periphery of the shaft to be constructed.
This has a dual purpose, it gives an indication on possible difficulties in the drilling of the larger diameter shaft due to unforeseen geology (i.e. faults) and it lowers the permeability of the rock, so that minimizes loss of circulation during the drilling of the shaft. After drilling is completed, grouting takes place in stages (3m to 10m) through the utilization of inflatable packers.
Normally the campaign starts with the drilling and grouting of 4 holes and then from the volume injected at each stage, it is evaluated if further drilling is required. Drilling is accomplished with a DTH (Down The Hole) hammer and a 6″ inch diameter drill bit As soon as the grouting is finalized, a rotary rig is utilized to install a steel casing all the way to the top of bedrock.
This allows the drilling of the shaft to take place only with water. Other techniques are also available to overcome the initial soil interface It is important that the casing is correctly installed on the soil-rock interface. Casing can be installed in sections, welded and lowered into the excavated shaft. A concrete plug is then poured at the bottom, as to lock the casing into place. Once this operation is finalized, if drilling with bentonite or polymer slurry, pump it out and correctly dispose it In this case we utilized a vacuum truck to pump out the bentonite slurry from within the casing.
To guarantee verticality of the shaft, given that these shafts are 600 meters deep, a pilot hole is drilled. This will be utilized to guide the excavation of the larger diameter shaft. The drilling is accomplished with a down the hole motor, that has special flaps, which automatically, through a series of sensors, keep the drilling vertical. In 600 meters, tolerance deviation was a maximum of 1 meter, to ensure that when inserting the pipeline, we would not have problems lowering it into the excavated shaft.
The next step is to assemble a pendular rig, on top of the concrete slab. The rig is then secured onto the slab. The bottom hole assembly (BHA) which is composed of a drilling bit, weights and stabilizers is lowered into the pre-excavated shaft. The weight of the BHA is a function of the number of cutters on the drill bit and the load that each cutters requires for an efficient cutting of the rock. The drill bit has a stinger, which inserts into the pilot hole. This ensures that the enlargement of the shaft stays vertical. The drilling is accomplished through reverse circulation drilling. Cuttings are sucked into the drill bit through specific openings go up thorugh the drill pipe, and into the return pit.
These are then left to sediment, while the water, is then pumped back into the shaft, as to ensure that the excavation is always full of water. Reverse circulation is accomplished through the injection of compressed air within the drill pipe, which creates a differential pressure. This is enough to lift the cuttings from the drilling interface. Finalized the drilling of the shaft, a pipeline is inserted inside.
This is accomplished by welding and lowering it in sections. Hydrostatic pressure test guarantees that there are no leaks. And the annulus, the space between the pipe and the drilled shaft, is then filled with concrete. This has a dual purpose. Protects the pipe from corrosion and holds it into position once the pendular rig is disconnected.
Equipment is then disassembled and site location is returned to its original state. This view shows the tunnel that was built that connects the pipelines to the drilled shafts. This tunnel was built concurrently to the excavation of the shafts. For this reason the technique is called blind shafts, given that no tunnel was there when they were constructed.
There are other techniques to build shafts, such as raise boring, if a tunnel is already built. A ventilation shaft keeps the tunnel with fresh air.