Forward probing utilizing electrical resistivity and induced polarization for predicting soil and core-stoned ground ahead of TBM tunnel face

전기비저항과 유도분극을 활용한 TBM 터널 굴착면 전방 토사지반 및 핵석지반 예측 기법

Abstract

It is essential to predict ground conditions ahead of a tunnel face in order to successfully excavate tunnels using a shield TBM. This study proposes a forward prediction method for a mixed soil ground and/or a ground containing core stones by using electrical resistivity and induced polarization exploration. Soil conditioning in EPB shield TBM is dependent upon the composition of mixed soils; a special care need to be taken when excavating the core-stoned soil ground using TBM. The resistivity and chargeability are assumed to be measured with four electrodes at the tunnel face, whenever the excavation is stopped to assemble one ring of a segment lining. Firstly, the mixed ground consisting of weathered granite soil, sand, and clay was modeled in laboratory-scale experiments. Experimental results show that the measured electrical resistivity considerably coincides with the analytical solution. On the other hand, the induced polarization has either same or opposite trend with the measured resistivity depending on the mixed ground conditions. Based on these experimental results, a method to predict the mixed soil ground that can be used during TBM tunnel driving is suggested. Secondly, tunnel excavation from a homogeneous ground to a ground containing core stones was modeled in laboratory scale; the irregularity of the core stones contained in the soil layer was modeled through random number generation scheme. Experimental results show that as the TBM approaches the ground that contains core stones, the electrical resistivity increases and the induced polarization fluctuates.

토사지반과 핵석지반에서 EPB 쉴드 TBM을 통한 성공적인 터널 시공을 위해서 굴착면 전방의 지반 정보를 정확히 파악하는 것이 필요하다. 본 연구에서는 전기비저항 탐사와 유도분극(induced polarization) 탐사를 함께 활용하여 복합토사지반과 핵석지반에 대한 전방 예측 방안을 제시하고자 하였다. 토사지반의 구성은 EPB 쉴드 TBM에서 첨가재 선택에 필수요소이며, 핵석지반은 기계화 시공에서 난이도가 높은 지반이기 때문이다. 탐사는 TBM이 굴진을 멈추고 세그먼트 1링을 조립할 시에 커터헤드에 설치된 4개의 전극을 활용하여 수행된다고 보았다. 토사지반의 경우 화강풍화토, 모래, 점토로 구성된 복합지반에 대해 축소모사하여 실내실험을 수행하였다. 실험 결과 전기비저항은 복합지반 이론해와 상당히 일치하였으며 유도분극은 경우에 따라 전기비저항과 경향성이 일치하거나 완전히 상반되었다. 이러한 결과를 토대로 실제 현장에서 적용 가능한 토사지반 예측방안을 제시하였다. 핵석지반의 경우 균질지반에서 핵석지반으로 굴착해 나가는 상황을 축소모사하였으며 핵석의 불규칙성을 난수를 통해 모사하였다. 실험결과 전기비저항은 핵석지반에 접근할수록 증가하였고 유도분극은 불규칙하게 오르내림을 거듭하는 경향을 나타내었다.

Fig. 1. Typical electrode array used in electrical resistivity surveys

Fig. 2. Front view of the Wenner configuration to a vertical fault

Fig. 3. Theoretical resistivity profiles across a vertical fault, Wenner configuration (Van Nostrand and Cook, 1966)

Fig. 4. Voltage decay curve in time domain IP (Park et al., 2018)

Fig. 5. Conceptual pore model inducing chargeability in water saturated sands (Park et al., 2016)

Fig. 6. Laboratory-scale modelling of field condition

Fig. 7. Relationship between risk level and boulder size (Hunt and Nero, 2011)

Fig. 8. Direction of movement of the electrode for boundary effect checking

Fig. 9. Test results for checking boundary effect

Fig. 10. Measured resistivity and chargeability while TBM passes through the mixed soil ground (α→β)

Fig. 11. Measured resistivity and chargeability while TBM passes through the mixed soil ground (β→α)

Fig. 12. Forward probing guideline for predicting soil grounds composed of weathered granite soil, sand, and clay

Fig. 13. Measured resistivity and chargeability while TBM passes through the ground containing core stones (α→β)

Fig. 14. Current flow line in a ground containing core stones

Table 1. Experimental cases

Table 2. Properties and electrical characteristics of soil samples