BAUER hydraulic trench cutter system, the most advanced excavation system, has opened new horizons for the use of the diaphragm wall technique in terms of quality, safety and speed.The constant or even exponential increase of population density in urban agglomeration areas is a global trend.One result of this development is the need of utilising more and more underground space and to go deeper into the ground and increase the space required for providing public amenities, parking and the requirement of improving or introducing public transport systems.A key element to prevent the collapse of traffic is the construction of underground metro lines.Specific challenges for planning and executing underground retaining structures in urban areas are:•Limited working space for equipment setup and dense traffic conditions•Heterogeneity of soil including old foundations•Underground obstructions and utilities•High groundwater table•Sensitivity of adjacent building structures against construction-induced settlements and vibrations •Noise restrictionsDiaphragm walls are typical, effective and proven construction elements for underground structural walls commonly used for retention systems, permanent foundation walls or deep groundwater barriers. They provide strong soil and hydraulic support, especially where watertightness and a stiff retaining structure are required to control and protect the surrounding environment against settlement and water drawdown. For these reasons, diaphragm walls are becoming increasingly popular especially in more difficult conditions.Diaphragm wall applications in metro projects:•Underground structures (retaining walls in station areas)•Cut and cover structures (along the railway lines)•Circular shafts (as ventilation shafts, emergency exit shafts or starter shafts for TBM)The need for a diaphragm wall requires deployment of the best construction methodology and equipment in order to achieve the best result in terms of safety, quality, time, and environment. The most advanced excavation system is the BAUER hydraulic trench cutter system (BC). It opened new horizons for the use of the diaphragm wall technique in terms of quality, safety and speed.Major advantages of the BC system are:•The possibility of adjusting the system to depth (30-150 m) and thickness (600-1,500 mm).•It works very well in mixed soil conditions from loose sand to cemented layers and even in rock layers.•High productivity is a major advantage of the trench cutter system.•The capacity of working within stringent verticality tolerances and the possibility of forming watertight joints between the panels by overcutting are important quality features.•The excavated material allows visual control of embedment in impermeable soil at the toe of the wall.•Work may be carried out right against existing structures and the line of wall can be adjusted to any shape in plan.BC Trench Cutter SystemThe trench cutter is an excavating machine that operates on the principles of reverse circulation. It is made up of a heavy steel frame (1) with two gear boxes (2) mounted at the bottom of the frame. Cutting wheel drums fitted with a series of various teeth optimised to the prevailing soil conditions are fixed to the gearboxes; they rotate in opposite directions, break up the soil and mix it with the bentonite suspension (3). As the cutter penetrates, soil, rock and bentonite are conveyed towards the openings of the suction box (4). From where they are pumped by a centrifugal pump (5), located right above the cutter wheels, through the slurry pipe incorporated in the cutter’s frame, via the mast head into the slurry conveying system to the desanding plant. Here, solid soil and rock particles and liquid bentonite are separated and the latter is pumped back into the trench.The torque output of the cutter wheels in combination with the weight of the cutter is sufficient to cut into any type of soil and to crush cobbles, small boulders or rock. The verticality of the trench cutter and thus the trench alignment are generally measured on two axes by means of two independent inclinometers (6). Adjustment of verticality in two directions is carried out by a system of steering plates (7).The development of the hydraulic trench cutter opened new horizons for the use of the diaphragm wall technique.A statement of the specialist foundation contractor Intrafor (Hong Kong) highlights the capacity of the BAUER trench cutter system:“Ground is excavated by cutter: hard teeth mounted on wheels excavate the trench while materials are taken away by circulating bentonite. A state-of-the-art cutter can reduce the excavation time by up to 40 per cent when properly deployed by a trained operator while ensuring extremely tight tolerances."(quoted from company magazine “ Intrafor Ground Engineering”)Experience in India with BAUER trench cutter BCA breakthrough for this method in India could be reached with the construction of the first cut-off wall for the 280 MW Dhauliganga hydroelectric power plant in 2002. Several other cut-off wall projects have been completed successfully with the BAUER cutter system in the Himalayas since then.For the construction of inner city structural retaining walls it was and still is standard practice in India, to excavate diaphragm walls with grabs or install walls with other methods like pile walls.BC trench cutters on the Chennai Metro Rail projectIt is on the huge and prestigious Chennai Metro Rail (CMRL) project that cutter-based diaphragm walls were introduced at a large scale.Chennai Metropolis has been growing rapidly and the traffic volumes on the roads have also been increasing enormously. Hence the need for a new rail based rapid transport system has been felt and towards this objective the Government of Tamil Nadu have decided to implement the Chennai Metro Rail project. Major areas of CMRL Corridor 1 and CMRL Corridor 2 will be constructed as underground lines with 19 underground stations.Four major Indian construction companies are relying on the BAUER cutter technologies additional to the use of grabs and rotary drilling rig. They are using the cutters currently on four stations.All of these station sites have one fact in common: The walls have to penetrate very hard soil or rock. This is a typical situation in which the capacity and power of the BC trench cutter system can be fully utilised.Afcons Infrastructure is working on the biggest station Chennai Central.Trench depth ranges there from 23 to 30 m with a trench width of 1,000 mm.Rock layer starts at 17 m below ground and very hard rock is encountered below 26 m.Al Ghurair Construction - FoundationsThey are engaged in the construction of diaphragm walls at the station “Government Estate”.The walls are scheduled to a depth of 22-26 m with a trench thickness of 1,000 mm.From a depth of 20 m below ground level onwards rock is encountered with a compressive strength of about 70 MPa.Larsen & Toubro (L&T) is working on several lots of the CMRL project. One of the lots is the construction of diaphragm walls at the station LIC.As rock is encountered at a depth of 19 m, while the 1,000 mm thick wall has to reach a maximum depth of 25 m, L&T is deploying a BAUER trench cutter with a special hose handling system and cutter wheels with tungsten carbide round shank chisels.The fourth example of trench cutter use on the CMRL Corridor Phase 1 is the installation of 1,000 mm thick diaphragm walls to a depth of 22-26 m for station retaining walls at the station “Gemini”. The works are executed by HSSG, a Dubai-based contractor who works as subcontractor for Gammon India. Similar to all other examples, the ground conditions are very difficult. Grade 5 rock is encountered at a depth of 19 m, followed by Grade 2 rock below 22m.The job references on the CMRL project give a strong indication that the latest, reliable and efficient methodology for constructing diaphragm walls in Metro Rail construction has reached the specialist foundation industry of India.Working with the cutter technology on the current CMRL projects does not only prove the general superiority of the method, it specifically increases the quality and speed of the joint construction between the panels. The possibility of forming a watertight joint by simply cutting into the concrete of adjacent panels makes the installation and removal of temporary stop end structures obsolete in the future. It is a good example how the efficiency of wall construction in urban areas can be increased for further projects.