Automated forward geological prospecting mounted on hard rock TBMs

Direct by Design

Automated forward geological prospecting mounted on hard rock TBMs Starting from the premise that hard rock TBMs have poor adaptability to adverse geology, unknown faults, karst caves, groundwater bodies and fractured rock zones, the research team at Shandong University in China has developed an automated geological forward prospecting technique. The authors describe how the system for mounting on TBMs, offers an effective approach for locating and imaging adverse geology for up to 100m ahead of the face to ensure safe and efficient TBM tunneling. wave information. The velocity analysis and migration methods are adopted to calculate the distribution and imaging of faults, fractured zones and other anomalies.

geological forward-prospecting technique was applied 139 times and successfully detected 61 major water-inrush sources.

As an example, the TIP detected a low resistivity area and the seismic method identified a strong reflection (Fig 2).

As a result, a large-scale water-bearing structure about 20m ahead of the tunnel face was successfully recognized. The construction routine was adjusted in time to deal with the 1,500m3/hr of waterinrush that was revealed by advance of the excavation to ensure safe construction.

The TBM operation, as assisted by the technique, was safeguarded and the tunnel project was completed nine months ahead of the anticipated schedule.

On the Gaoligongshan highspeed railway tunnel project in Yunnan Province, the TBM was expected to tunnel through 19 faults and several strong-weathering zones that present high risks of water-inrush and collapse. The automated geological forward-prospecting technique on the TBM was used 98 times and detected 43 sources of potential geo-disaster.

On one occasion, two fractured zones ahead of the face were identified (Fig 3).

The first detected fractured zone was verified by the advancing TBM excavation.

Subsequently, although the rock condition became slight better, the second fractured zone was revealed at the rear of the detection range. The two fractured zones

Underground in Action U

Time domain Tunnel Induced Polarization (TIP)

Consisting of a measuring electrode array installed on the cutterhead and current supply electrodes installed on the shield and tunnel sidewalls, the time domain TIP detection technique can image water bodies within 30m ahead of the cutterhead. Driven by hydraulic cylinders, the electrodes can reach out and achieve electric field transmission and measurement automatically under the control of the host computer. The new observation mode with excitation and moving current electrodes improves the detection range to 30m ahead with water bodies imaged by data inversion.

Actual applications

Of the more than 60km of on-site applications, installation on the fourth section of the Yinsong water supply project in Jilin Province, China, was for a 7km TBM drive through limestone with a high risk of water inrushes. The automated

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etection of adverse geological conditions ahead of hard rock TBM drives is an important prerequisite for the safe and efficient construction of TBM tunnels. Many TBM drives over the years have encountered geological disasters, resulting in TBM blockage or damaged, heavy economic loss and in some cases personnel casualties. Forward prospecting of adverse conditions ahead of the cutterhead is of crucial importance for project success.

To address this requirement, a technique has been developed to detect geological anomalies ahead of an advancing hard rock TBM tunnel face. There are traditional geological prospecting methods used with drill+blast excavation, but these present great challenges when applied to TBM drives: • TBMs provide little space availability for prospecting; • Electromagnetic and vibration interferences are severe and affect the effectiveness of signals; and • TBM tunnelling is five to 10 times faster than traditional drill+blast, requiring faster detection techniques.

To address these problems, the automated geological forward prospecting technique is developed to be mounted on TBMs and is based on seismic prospecting to detect karst caves, voids, faults and fractured rock zones and time domain tunnel induced polarization for detecting water bodies ahead of TBM excavation faces (Fig 1). Automated data acquisition is recorded via remote control and presented to TBM operators in a graphic visualization mode. The technique and system has been applied over 1,300 times on 21 TBM engineering projects without missing any major geo-disaster sources.

Shucai Li, Bin Liu, Lichao Nie, Yong Li, Xinji Xu, Zhengyu Liu, Lei Chen and Yuxiao Ren of the Shandong University, China

Fig 1. Mounting of the prospecting technique on a hard rock TBM

Seismic Ahead Prospecting (SAP)

There are 10-16 vibrators installed on the upper deck of the TBM about 50m from the tunnel face. Seismic waves can be excited by servo control of hydraulic vibrators with the reflections recorded by 10 to 16 geophones installed on the tunnel sidewalls.

The observation system has offsets in the tunnel axial, vertical and horizontal directions, to collect a wealth of seismic

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Fig 2. Forward geological prospecting applied on the Yinsong project TBM project (left) detected a large-scale water-bearing structure 20m ahead of the tunnel face that the continuing TBM excavation confirmed with an inrush of about 1,500m3/h were both consistent with the detection result. Based on the detection results, the contractor took precautions and a collapse and TBM blockage geo-disaster was avoided.

On the Hanjiang-Weihe water supply project in Shaanxi Province through the Qinling Mountains and beneath an overburden of up to 2,000m, predicted an extremely complex geological environment.

As a result of 20 times of detection, the technique detected five major water body and faulted fracture zone sources of geodisaster. When the TBM in the northern Qinling Mountain drive became stuck and had not moved for more than five months, the prospecting technique detected the location and scale of the faulted fracture zone around the TBM and provided the geological survey information for development of a rescue plan. A pilot tunnel was excavated to release high geo-stresses and allow for pre-consolidation of the surrounding rock.

Recovery of the TBM was achieved 21 days ahead of the estimated schedule.

Functions and benefits

New functions are being added to allow the technique to be more accurate, more automatic and more intelligent and thereby

better support the safe operation of TBMs in long hard rock drives. In summary, the main functions of the technique are as follows: • The TIP technique can image water bodies within a range of 30m; • The SAP technique can image faults and fractured rock zones within 100m ahead; • Fully automatic detection equipment mounted on TBM achieves automated data acquisition by remotely controlling 20 to 28 IP (induced polarization) electrodes and 10 to 16 seismic vibrators or exciters; • The detection results are presented directly on the TBM operating console; • By detecting adverse geologies in advance, safety warnings are issued to construction teams and reasonable construction plans to prevent geodisasters can be developed, ensuring the safety of the project and the personnel; and • The technique provides information to avoid geo-disasters and improve the safety, efficiency, economic and intelligence of TBM tunneling as well as the long-term safety of the operational tunnels.

Based on the technique and its ability to achieve automated detecting, positioning and imaging of geological structures, reasonable excavation schemes can be planned and executed. Applications of the system have indicated that the technique has helped avoid major geo-disasters and has assisted to improving excavation efficiency. Combined with artificial intelligence, the decision-making method for TBM tunneling parameters is improved, making TBM excavation faster, more economical, smarter and safer.

To date, the technique has been granted six USA patents and more than 20 patents in China. Its application has been included in several Chinese tunneling industry standards for guiding the geological forward-prospecting in TBM tunnels and in 2020, the system was awarded the Technical Project Innovation of the Year of the annual International Tunnelling and Underground Space Association Brunel Awards.

In future, TBMs will face greater challenges. The technique can support the safe construction of TBM excavations in high-risk geological formations. n

Fig 3. Detection of geological anomalies (right) ahead of the hard rock TBM drive for the Gaoligongshan railway tunnel project

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