Tunnel Face Stability & New CPT Applications - Geo-Engineering

200 190 180261702. Stability Analysis of the Tunnel Face160 150 TBM x gauge140 130 120 11005 10distancetogaugexinm15 20 25 30 35Figure 2.24: Pore pressure measurements in front of TBM at 2nd Heinenoordtunnel [38]porepressurepinkPas’(kPa)302010010 −3high density slurrylow density slurry∆s = 40kPa∆s = 30kPa∆s = 20kPa10 −2 10 −1 1k(cm/s)Figure 2.25: Relationship between permeability and effective slurry pressure [120]during standstill of the TBM and the lowest points of the spikes coincide with the pore pressuresat rest. As these measurements will be used in the evaluation of the stability model, they will bediscussed in more detail in section 2.4.Hashimoto et al. [75] report similar measurements from a field case, where excess porepressures were recorded up to 10m in front of the tunnel face. They show a different pressuredistribution with distance from the face for three different slurry types in similar soil conditions,but do not give the actual slurry or soil properties involved. Mori et al. [120] report laboratorytests, numerical modelling as well as field observations on this phenomenon in sandy soils.Field measurements show excess pore pressures up to 40kPa and a visible influence at least30m in front of the tunnel face. Based on laboratory tests and numerical models they concludethat within the investigated margins a decreasing permeability significantly reduces the effectiveslurry pressure as excess pore pressures dissipate more slowly. They also conclude that thiseffect can be countered by using a slurry with a higher fines content. Their results are presentedin figure 2.25.Especially for sandy soils laboratory investigations and field experience have shown that, if