Page 1 Page 2 Page 3 Page 4 Page 5 Page 6 Page 7 Page 8 Page 9 Page 10 Page 11 Page 12 Page 13 Page 14 Page 15 Page 16 Page 17 Page 18 Page 19 Page 20 Page 21 Page 22 Page 23 Page 2412 An introduction to pipe jacking and microtunnelling 3 SITE INVESTIGATION AND INFORMATION REQUIRED ON SOIL CONDITIONS Tunnelling methods depend on the stability of the ground. Where unstable conditions are likely to be met, the face of the tunnel must be controlled to prevent ground loss, and to enable excavation to take place safely. A controlled face can be achieved by using suitable tunnelling methods such as full face earth pressure balancing machines, or slurry/soft ground TBMs. Alternatively a stable face when pipe jacking can be achieved using the following geotechnical processes: • Dewatering by well points or deep wells • Stabilisation by grouting with cement, cement/bentonite, or chemicals • In extreme cases, ground freezing The choice of process is a function of the nature of the ground, the water content, and in particular the particle size analysis. The table below gives an indication of the process applicable to various conditions, but because of the specialist nature of such activity, detailed advice should be sought from a geotechnical engineer. UNSTABLE GROUND CONDITIONS GROUND CLAY Fine Medium Coarse Fine Medium Coarse Fine Medium Coarse Cobbles SILT SAND GRAVEL ROCK PARTICLE SIZE (mm) FACE SUPPORT METHODS GROUND TREATMENT PERMEABILITY 0.001 0.002 0.01 0.06 0.1 1 2 10 60 100 EARTH PRESSURE BALANCE MACHINE (EPBM) CUTTER BOOM CUTTER BOOM OPEN FACE HANDSHIELD/BACKACTER May require a ground treatment method Blasting Rock head Rock head TUNNEL BORING MACHINE (TBM) Compressed air – Slurry shield – Crushing capacity MICRO TUNNELLING (Machine type dependent on ground conditions) Cementitious grouts Suspension grouts – clay filler grouts Low viscosity grouts – resins Chemical grouts – silicates Replacement grouting – jet grouts Ground freezing Compressed air Wellpoint dewatering Shallow and deepwell dewatering 63 159 212 300 425 600 1.18 2 3.35 5 6.3 10 14 20 28 37.5 50 63 75 BS test sieves 1 -1 -2 -3 -4 -5 Kcm/sec = d102 (Hazen Formula) 3c) Face support and ground treatment methods for varying ground conditions µm mm Note: Whilst open face handshield and compressed air are referred to above, these are only used in special circumstances. Reference The Work in Compressed Air Regulations 1996 and British Tunnelling Society (BTS) Guidance on the Work in Compressed Air Regulations 1996.