Recently, the water-stop curtain project at Tianshan Yilu Station on Qingdao Metro Line 15, in which Bauer CSM double-wheel cutter deep mixing equipment participated in construction, was successfully completed.

Tianshan Yilu Station is located at Tianshan Yilu and the planned Xincheng Erlu in Qingdao, laid out along the planned Xincheng Erlu in an east-west orientation; in the long term it will be an underground three-level island-platform station with a passage transfer to Line 23. The main structure foundation pit of the station is about 159.8 m long, about 24.3-29.3 m wide, and about 29.5 m deep. The overall foundation pit structure is constructed by the open-cut method, with bored cast-in-place piles in a suspended-footing configuration plus anchor cable support, and steel pipe piles plus anchor cables plus anchor rods for the lower support; the foundation pit water-stop curtain adopts the CSM double-wheel cutter deep mixing process. The project is under general contracting by China Construction Port and Waterway Engineering Group Co., Ltd. The project department, taking "building a world-class metro" as its work deployment and "no seepage, no leakage" as its quality goal, carried out careful planning in all aspects and conducted comparative selection of the construction unit and construction equipment. The figure below shows the planned entrance/exit layout of Tianshan Yilu Station and the surrounding environment.

The CSM water-stop curtain at Tianshan Yilu Station was constructed by Tianjin Jinkan Geotechnical Engineering Co., Ltd. using Bauer CSM double-wheel cutter deep mixing equipment, with a wall thickness of 800 mm, a maximum design depth of 25.1 m, a total of 166 panels, and the double-wheel cutter overlapping cutter joint process. The strata penetrated by the water-stop curtain construction include: plain fill, silty clay, medium-coarse sand, fully weathered andesite, strongly weathered andesite, and moderately weathered andesite, finally embedding into slightly weathered andesite. The figure below is a typical geological cross-section of the project, showing that the bottom bedrock is andesite.

There are four difficulties during the construction of the CSM water-stop curtain: 1. The upper clay layer easily causes bit balling and mud wrapping, affecting cutting efficiency; 2. The characteristics of the strata create risks of bit sticking and bit burying during equipment construction; 3. The strata are characterized by large undulations in burial depth and unclear interfaces between soft and hard strata. To ensure the water-stop effect, the water-stop curtain design requires embedding into slightly weathered andesite; 4. Due to the requirement of entering rock, tool wear is relatively large. Therefore, this places high demands on the construction capability of the equipment, the understanding of geological conditions, and the professional technical level and management capability of the construction unit.

Tianjin Jinkan Geotechnical Engineering Co., Ltd. introduced the first domestic Bauer guide-rod CSM double-wheel cutter deep mixing equipment in the early period, and the completed cutoff wall depth reached a maximum of 47 m, with cumulative completion of CSM double-wheel cutter cutoff wall and retaining wall projects totaling hundreds of thousands of square meters of work volume. In terms of important technical indicators of the wall, such as permeability coefficient and wall strength, all met the design code requirements, and the company has relatively rich engineering practice experience.

The figure below shows Jinkan Geotechnical Company using Bauer CSM double-wheel cutter deep mixing equipment at the Tianshan Yilu Station site of Qingdao Metro Line 15 for the rock-socketed water-stop curtain project.

The Bauer CSM double-wheel cutter deep mixing equipment adopts a crawler chassis, and the trenching layout as well as equipment travel and positioning are all very flexible. Each configured gearbox has a torque of up to 100 kNm; in addition to general geological conditions, it can also cut hard soil layers, dense sand-gravel layers, and strata requiring a certain degree of rock penetration, further ensuring the water tightness of the wall bottom. The mixing wheels have a maximum speed of 35 revolutions per minute, which helps quickly mix and liquefy the soil, making the wall quality more uniform. The figure below shows the mixing wheels configured for the Bauer CSM double-wheel cutter deep mixing equipment that can cut hard strata.

Due to the characteristics of the CSM process, there is no need to pre-build guide walls, which saves reinforced concrete materials and time. During construction, the in-situ soil is fully utilized, and the amount of returned slurry is small, saving more materials and being relatively more environmentally friendly. The figure below shows that during double-wheel cutter deep mixing construction, there is no need to build guide walls and no need to provide a reinforced concrete construction platform.

The B-tronic system configured for the Bauer CSM double-wheel cutter deep mixing equipment helps operators control the verticality of the wall and precisely control the injection volume of cement slurry, ensuring compliance with design requirements.

The CSM water-stop curtain at Tianshan Yilu Station began trial wall construction in late March 2023 and successfully completed all work volume early this month. Due to proper on-site management by the participating units, the solid and reliable professional competence of the construction technicians and operators, and the adoption of reasonable construction measures, all the above difficulties were overcome. While ensuring construction quality, construction efficiency was improved, and the CSM double-wheel cutter deep mixing water-stop curtain project was finally successfully completed under the difficult geological environment in the Qingdao area, also providing schedule assurance for later foundation pit excavation.