BTSYM | SOIL TRANSITION MODELS


Chamber pressure dissipation approaching soil transition 1


Right, figure 6: Rate of dissipation of EPBM chamber pressure as determined from the analytical fit to chamber pressure drop during EPBM standstill, in proximity of (a) soil transition 1 and (b) soil transition 2. Vertical lines indicate the GBR soil transition and probabilistic approach results


0.1 0.2 0.3 0.4 0.5


0 250 260 Clays Transition (GBR) Transition (p95 ≥ 50% Sand) Sands


270


280


290 Ring # Chamber pressure dissipation approaching soil transition 2


0.1 0.2 0.3


0 500 510 520 Sands Transition (GBR) Transition (p95 ≥ 50% Clay) Clays


300


310


320


330


340


530


540


550 Ring #


560


570


580


590


600


varying soil proportions within the tunnel envelope.


The difference in probabilities from the ECDF curves helps to evaluate the uncertainty in the soil transition locations in terms of CI. For a ring#, therefore, the ECDF is helping to


interpret the probability of occurrence of a specific soil proportion. P95


observations from the ECDF curves for soil


transition 2 indicate that the G3/G4 soil proportion within the tunnel envelope increases from 10% to 50% over a longitudinal distance of approximately 80 rings (or 144m with ring length = 1.8m). The G3/G4 proportion increases from 30% to 50%


over about 50 rings. The family of ECDF curves for soil transition 2 are


relatively more spaced out, indicating a slow and gradual transition from G3/G4 to G1/G2 soil. It is observed that for 90% CI (P95


-P5 ), the width of


the uncertainty band for soil transition 2 is larger due to the lack of boreholes and knowledge of the ground conditions. For the transverse plane, ECDF curves are generated


to quantify the probability of encountering mixed-face conditions for mixed-ground transition plane elevations. As shown in Fig 4, the intersection of the ECDF curve with the elevation of the TBM crown, invert and springline is used to interpret the respective probability of occurrence of mixed-face conditions – which are, here, about 83% and 17%, respectively. The probabilistic approach offers superior and


quantitative insights into the expected ground conditions within the tunnel envelope compared to the deterministic soil profile from the GBR. Key outcomes of the probabilistic approach include:


(a) knowledge of the spatial locations; and, (b) quantification of the probability of varying cohesive/ cohesionless soil proportions within the tunnel envelope. The results aid in quantifying the soil transition location uncertainty expressed in terms of CI.


48 | Fall 2023


The geostatistical modeling-based probabilistic


approach can be applied during the tunnel project’s procurement stage to estimate the volume of a soil type to be excavated, an important element in planning the management of the excavated material.


SOIL TRANSITION LOCATION UNCERTAINTY - Transverse Direction ECDF curves for mixed-face conditions quantify the probability of encountering mixed-face conditions and also the probabilistic distribution of the mixed-ground transition plane’s elevations. For each ring#, the intersection of the ECDF curve


with TBM crown, springline, and invert elevation is used to interpret the respective occurrence probability of mixed-face conditions. The occurrence probability is calculated as the


difference in the probabilities of mixed-ground occurring at the respective elevations. On the ART project, the analysis producing the


transverse direction ECDFs for ring #325 and ring #1100 show: at ring #325, the probability of encountering mixed-face conditions between the TBM springline and crown is about 13%, whereas that between the TBM springline and invert is about 80%; at ring #1100, the figures are about 83% and 17%, respectively. An integrated review of the results from the


geostatistics based-methodology provides quantitative insights on the occurrence of mixed-face conditions and the tunnel excavation environment compared to the ground profiles from the GBR. For example, the results from the probabilistic


approach could be interpreted as showing that between ring #200 and ring #300, the probability of occurrence of mixed-face conditions varies between 60% and 90%. The mixed-face conditions are expected to occur between the TBM springline and the invert with the transition plane occurring between 3m and 5m (elevations with respect to TBM crown) for 50% CI.


α [KPa/min]


α [KPa/min]


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