SHAFT SINKING

Methods of Sha, Construc2on

Conven0onal Drilling and Blas0ng

Sha; Boring Machine with Mucking

V-Mole with Pilot Hole

Blind hole Drilling

The Sha; CuEng Machine or Excava0on

Raise boring Methods

Horadium Method

Stepwise Benching Alimak Raising

Rock bol2ng and meshing

The easiest way to line a sha; is with rock-bol0ng and meshing. A wire mesh is fastened to the walls with evenly spaced rock bolts. For ver0cal sha;s ring shaped beams called ribs are usually also added to support increasing horizontal stresses as the sha; moves down. Rock bol0ng is a commonly used, cheap method. The rock-bolts increase normal stresses on joints so that shear failure along joints becomes more dicult.

Down-the-hole Lining Methods

Cast-in-place lining

It is possible to cast concrete rings as the sha; sink progresses. This is commonly done in combina0on with a V-mole or sha; boring machine. Generally temporary rock bolts and mesh are s0ll required close to the cuEng head. Then, on a second oor of the sha; boring machine, an L-shaped casing is posi0oned which is then lled from a hose transpor0ng concrete from a surface plant. This method provides a smooth, water0ght and permanent lining for the sha;. The casing can be reinforced to cope with horizontal stresses (i.e. ring-shaped reinforcement) making the casing elements more economical, and in this case the reinforcement is well protected by the concrete. Some aYen0on should be given to connec0ng the elements with rubber proles to prevent water inows.

Forepoling or Boodex method

The Italian ground treatment manufacturer Soilmec has successfully used the forepoling method in rock for the stabiliza0on of tunnels. It has come into common prac0ce for tunnelling, also given the name Boodex by Atlas Copco.

In the case of Soilmec, the method uses Tubex technology to drill a ring of piles about 12 metres long at a slight incline around the tunnel face. The ring forms an umbrella to support the next nine metres of excava0on, before a new ring of piles was installed.

The method described above is typical for tunnelling, but has also been used ver0cally for sha; drilling. The limited length of piles means the forepoling or Boodex machine has to be used from within the sha; along with conven0onal drilling and blas0ng or mechanical excava0on for the removing rock and soil.

Remote Lining Methods

Floated pre-cast lining segments

Sha;s up to 4.5m have been commonly drilled around the world with pre-cast lining posi0oned during or a;er workings. A large diameter steel pile is o;en used because it is strong and smooth and can easily be lowered or driven into the ground. Concrete can then be poured behind the walls to create the lining. It is common to insert the steel pile during drilling in clay soils which tend to collapse, but also have low fric0on making pile driving easy.

In sands, mudstone and sandstone, steel, pre-stressed concrete or composite liners with a smaller diameter than the sha; are lowered a;er drilling out the hole. The space behind the piles is then lled with grout to ensure a close t with the surrounding soils.

Caisson sinking

Wall elements that are smaller than the sha; diameter can be dropped into the sha; as it progresses, or upon comple0on. However, this means the sha; must be supported by mesh or drilling uids during construc0on. As an alterna0ve, it may be possible to push down a lining as work progresses, in combina0on with sha; excava0on, and only lubricate behind the sha; wall.

Wall elements are added from the top avoiding the need for any workers down the sha;, and providing easy access to the work. A slip form lining can be used, allowing a con0nuous poor of concrete, or pre-cast elements can be added piece by piece.

Horizontal rings of reinforcement can easily be installed to withstand horizontal pressures. Because the horizontal pressures increase as the sha; goes down, the amount of reinforcement can be scaled to the expected loads. However, as the hole deepens, wall fric0on will con0nuously increase, un0l the casing will not slide down any further.

To solve this, some addi0onal measures may be applied: Use of mud or drilling uid to lubricate between the sha; wall and rock. Use of a series of steel cables that are unrolled from the boYom of the sha; as it proceeds downwards. Use of water jets to decrease eec0ve stress, lubricate the wall and push aside any obstruc0ons

Grout treatment

The most problema0c unstable areas in overburden are commonly encountered near the bedrock (hard rock base). Con0nuous ground water ow and movement of rock layers results in abundant fracturing and weathering at this loca0on. Grou0ng is tradi0onally performed from inside the sha; in combina0on with conven0onal sha; sinking. To get workmen out of the sha;, it is also possible to grout from the surface. Rotary percussive drills are used to get to the required depth where packers or tubes-a-mancheYe are used to inject grout at high pressure.

The aim of this grou0ng is to: Decrease permeability and thus stop water ows Increase normal stress on joint planes, thereby increasing shear strength. Increase cohesion on joint surfaces by replacing so; materials and forming a chemical bond with the rock surface.

Pre-reinforcement for Raiseboring

Freezing

Instead of crea0ng a grout curtain wall, an freezing provides an alterna0ve method to harden the ground. This is common prac0ce in Germany, North America and Canada. The primary factor aec0ng ground freezing is the speed of groundwater ow. Other factors are water temperature and salinity of the groundwater. Groundwater ows are usually limited in greeneld sites, but exis0ng mine developments usually have some form of groundwater pumping in eect. Because of the heat transport that accompanies groundwater ow, it will cost extra 0me to freeze the groundwater, and it may even be impossible to create a complete ice curtain.

The problem with groundwater ow is increased if the groundwater table is very low. To create a frozen curtain wall at low depths, the groundwater table should be raised. Unfortunately the raising of the groundwater table implies groundwater ows away from the sha;, making freezing dicult. The ow rate may be limited in soil strata because of the large amount of pore space par0al satura0on will be enough to strengthen the soil. In fractured rocks full satura0on will be required, and pumping will induce high ow veloci0es, making freezing impossible.