For projects involving tunnel boring machines (TBM) to be successful, we need to know what lies beneath the surface. The consistency of the ground is as critical as its type and composition; preferably the tunnel will be in a homogenous ground type but that seldom happens and frequently we encounter several different ground types in the tunnel face and along the profile—different consolidated and unconsolidated soils, soft rock, fractured rock, and hard rock. We may also encounter shear zones and varying ground water regimes with high ground water flow and high water pressures, even in some cases artesian conditions. What happens when we are faced with mixed face and difficult ground conditions along the tunnel alignment is the subject of this article.
Depending on the type of project, knowledge of the ground conditions will determine the best choice of TBM to use. Tunnel alignments may sometimes be developed to minimize unfavorable mixed ground or difficult ground conditions or limit the transitions between different ground types; however, the functional use of the tunnel often dictates the tunnel profile and horizontal alignment. Tunneling through mixed ground conditions often cannot be avoided therefore mitigation measures should be adopted, such as subdividing the alignment into different tunnel reaches of relatively uniform ground conditions, thereby allowing different tunneling methodologies to be applied to different sections of the tunnel.
Combined Sewer Overflow (CSO) storage tunnels or water conveyance force mains have more flexibility in their alignment and profile depth than transportation tunnels. CSO tunnel profile may be adjusted from the discharge point to suit better geological conditions as water in the CSO are pumped up into a wastewater treatment plant from temporary storage in the tunnel or holding tanks. Tunnels that are typically governed by fixed control points, such as highway, transit tunnels or drainage/sewer tunnels that rely on gravity flow are not as flexible in their profile as CSO tunnels. Transit tunnels have portals and transit stations to connect to and have controlled grades along the alignment that cannot be significantly modified. Interceptor and drainage or sewer tunnels must intercept flows at fixed locations to convey them by gravity either to outfalls’ or pumping stations.
Mixed Ground Conditions
Mixed ground conditions occur when there are two or more geological materials simultaneously present in the tunnel face and profile with significant differences in material properties (e.g., strength ratio of 1/10 or less between that of the weakest and strongest material). As a result, TBM specifications will be significantly influenced, as well its operation method, and the design of the tunnel support system installed behind the TBM.
Mixed ground may consist of any ground types e.g glacial deposits, alluvial soils or residual soils with or without rock possibly interfaced with nested boulders, saprolite or highly weathered rock. Mixed ground may also consist of boulder-soil matrix or layered-banded mixed ground of vastly different properties. In these occurrences tunneling may be difficult and in particular tunneling through the transition zones between soft ground and hard rock.
Figure 1 (a representative of the geological profile) shows a large variety of geologies that were encountered along the recently completed tunnel alignment for the Blacklick Creek Interceptor Sewer Project (BCIS) in the City of Columbus, Ohio, for which AECOM was the designer of record. The tunneling excavation went through soil zones of glacial tills and outwash deposits, as well as hard shale bedrock. The project Geotechnical Baseline Report (GBR) defined the highly variable geology along the tunnel alignment containing 30 interfaces between the soft ground and the bedrock, with each interface consisting of transition materials of potential highly weathered rock often with glacially deposited clusters of cobbles and boulders.
Despite the extremely challenging geological investigations, it was critical that the baseline geotechnical subsurface profile accurately represented and predicted the anticipated ground conditions to allow for adequate tunnel design, TBM specifications, and planning and monitoring of the TBM operation and performance. For the BCIS Project, AECOM’s team prepared a baseline geological profile included in the GBR which during excavation proved to be quite accurate, despite the complexity of the soil/rock interfaces along the alignment.
TBM Design and Parameters
Because TBMs will run into different ground conditions in an alignment, several methods of handling the conditions may be considered. Each have their advantages and disadvantages, depending on the ground conditions and the design requirements. Mitigation for mixed ground may be incorporated into the choice and the specifications of the TBM.
TBM Choice: Typically, TBMs are single mode either open face or closed face EPB or Slurry and are adequate for tunnels where ground conditions are similar for the entire tunnel length. Dual-Mode or Hybrid TBMs have been developed for tunneling in mixed ground conditions of soils and rock, with ground support typically being provided by gasketed liner of precast concrete segmental rings. Dual-mode TBMs can be operated in both open or closed mode or in some closed TBM’s switching between EPB and slurry. The TBM would operate in closed mode through the soft unconsolidated ground and transition zones and in open mode through stable ground and rock. In open mode probing and pre-excavation grouting are often specified to control groundwater inflows when tunneling through rock, or when tunneling through transition zones to control potentially unstable ground.
Slurry TBMs with dual mode capability may also be used. However, approximately 80-90 percent of pressurized face TBMs nowadays are EPB. EPB TBMs have a lower capital and operating costs compared to equivalent size slurry TBMs, mainly due to the additional cost of slurry processing plants that are not required for EPB TBMs. With advances in EPB TBM design and operating techniques they may be applied in a wider variation of ground types and the design lends itself better to conversion to open mode for a dual mode TBM; the screw conveyor in an EPB machine can be easily adjusted to operate in closed and open mode, compared to a slurry dual mode TBM where a separate auger mucking system is needed in addition to the slurry system for running the TBM in open mode. However, such TBM is suitable to run in highly unstable pressurized flowing sands, gravels and silts without ground treatment that an EPB TBM is unable to tunnel safely without substantial ground treatment, which may be costly and problematic. For the Blacklick Creek Interceptor Sewer Project, AECOM as the Designer specified a dual-mode EPB TBM. See Figure 2.
With the new generation of multi-mode TBMs, it is possible to smoothly switch between the different modes (EPB and slurry) as the ground changes at the face thus maintaining permanent and full control of face pressure. This new generation machine is called Variable Density Multi Mode (VDMM)TBM and it provides four modes of operation with open mode, EPB and two slurry modes with seamless switching between the modes to provide maximum flexibility and safety of operating in highly variable ground and ground water conditions without significant delays in switching from one mode to another. Figure 3 is an illustration of a VDMM TBM and Figure 4 is the VDMM TBM for the Shatin-Central Metro in Hong Kong.
Cutterhead Design: Apart from the TBM mode type, the cutter head design is critical to excavation in mixed face conditions. To excavate a wide variety of ground ranging from soft soils to hard rock, a mixed face cutterhead normally uses a combination of soft ground tools, such as scrappers, picks, rippers and bits; in addition to disk cutters for excavating through rock and hard materials. The cutters, cutterhead wear surfaces and screw conveyor are usually reinforced with hard metals, such as chromium carbide plating, hard facing and tungsten carbide bits to ensure the cutterhead extended life in abrasive environment in mixed ground condition. This also minimizes the need for cutterhead maintenance in difficult ground that could require hyperbaric intervention. Wear detection mechanisms may be built into the cutterhead tools to automatically detect when disc cutters are worn and require changing.
Backloading disk cutters and excavation tools are becoming the standard for large diameter TBMs subjected to high ground and water pressures in order to allow easier and safer tool replacement under atmospheric conditions. Large diameter TBMs over 30ft in diameter may also have some cutters replaceable by a system that does not require entry into the cutter chamber at all via robotic tools. Other important factors in cutterhead design for mixed ground is the balance between an optimal opening ratio with a robust structure and adequate number and arrangement of the cutting tools. In EPB TBMs the screw conveyors must also be designed with the knowledge of the maximum face pressure that is likely to be encountered and the probable presence of boulders and maximum size so as not to block the auger.
Tunneling Through Transition Zones: Transition zones can be difficult for TBMs to operate in an optimized condition. These difficulties include high cutter wear, cutter skidding, tunnel face instability and loss of pressure, high volume pressurized ground water inflow at the ground interface, and the need for tool replacement in hyperbaric conditions. These difficulties cause delays in excavation and could potentially impact the stability and the safety of the tunnel. When the cutter tools traverse from soft soil to hard rock in a mixed face, very high dynamic or shock loads can be imposed, in some cases with loads exceeding disc bearing capacity. The disc bearings will also be subjected to high side loads and overturning moments causing abnormal cutter ring and cutter bearing failure. The cutter rings may develop chipping or radial cracks and flat wear or multi-flat wear. In addition, the dynamic loads may impact the TBM main bearing and subjected it to undue stresses. The cutter disc failures will contribute automatically to an unusual increase to loads on the cutterhead and the main bearing and increased bearing wear and could lead to main bearing failure if not properly addressed.
In a mixed face, the soft material is often not capable of generating enough rolling force to keep the cutters rotating, so they skid with frequent stoppages causing flat spots to develop. And in abrasive soils, the spoil paste of soil and ground rock can then lead to accelerated cutter ring and tool wear, this will further hinder the cutter rotation in hard rock leading to additional wear and disc failure. These cutter wear issues are exacerbated further when the space between the cutter housing and the head plate is packed with spoil paste preventing the cutters from rotating. The pressure from the spoil trapped between the rock surface and the cutterhead will further increase the rate of abrasive wear of the face plate, bucket lips and drag bits and mounts. To alleviate these issues the design, layout, spacing, size and type of cutter discs and drag bits installed on the cutter face is critical for maintaining excavation advance rates and optimum tool life.
High hydrostatic pressure during mixed face tunneling with granular materials can result in unstable face conditions, creating an almost fluid soil in the cutter chamber and potential loss of balancing pressure and thus loss of ground at the face. This condition will also make hyperbaric interventions problematic with the potential loss of the compressed air in the cutterhead chamber. Large ground settlements could potentially occur in mixed face tunneling because of over excavation where the volume of muck removal of the soft ground lying above hard ground exceeds the penetration speed in the hard ground. This could also cause face instability and possibly ingress of groundwater.
Diﬃculties in TBM steering frequently occurs in mixed ground, in some cases due to the tendency of the TBM to ride on the harder rock surface. When possible, we should strive to tunnel from harder material into weaker material, but this is not always possible especially if the tunnel profile emerges from soft ground into hard rock. Also, in mixed ground if there is relative flexibility in the horizontal alignment, we should avoid passing beneath structures that may be sensitive to settlement at the ground surface in the transition zones. When such situations cannot be avoided, strict control and monitoring of TBM performance parameters and muck excavation volume and weight must be strictly adhered to.
In order to mitigate the above risks, not only the tunneling method and TBM design should be selected to suit the ground conditions, but the ground may also need conditioning to suit the tunneling method. Ground conditioning is generally aimed at making the ground more stable and less permeable, by introducing additives to maintain a smooth flow of muck through the cutterhead, thereby maintaining consistent earth pressure balance. Additives consisting of water, polymer, foam and bentonite slurry have been used in various combinations and proportions depending on the ground conditions.
The use of ground conditioning at the cutterhead has further been shown to prevent clogging; help maintain ground stability, reduce cutterhead torque, cutter wear and increase overall advance rates. Studies have also indicated that the single factor that have had the strongest correlation to machine performance appears to be a quality plan for ground and soil conditioning based on anticipated ground conditions prior to tunneling. A good ground conditioning regime can be equally as important as the machine design and logistical aspects on any TBM project.
For the Blacklick Creek Interceptor Sewer Project, AECOM as the Designer specified a dual-mode EPB TBM with a man lock for hyperbaric tunnel face and cutterhead access for maintenance and intervention purposes. For tunneling through the wide range of geological conditions outlined above, the Contractor used pressure compensated mono-block disc cutters typically used in hard rock excavations, with the longevity of tungsten carbide inserts frequently found on soft ground cutters. The carbide’s high abrasion resistance performs well in the abrasive soil materials, while the pressure compensation helps reduce bearing failure in high water head situations and to resist the impact or shock loading inflicted by boulders and rocks. The tungsten inserts help to stop the disc skidding and to keep the disc rotating even in soft ground conditions, and therefore minimizing uneven wear on the discs. Redundant gauge cutters were installed to prevent or limit loss of the overcut and reduce the likelihood of a hyperbaric intervention. Overall the tool selection and cutter reinforcement measures proved effective as no unplanned emergency tool replacement interventions were needed on the project, and all cutterhead tool replacements were able to be performed in atmospheric conditions in safe havens in stable ground.
The Blacklick tunnel project contract required the TBM to operate in closed mode while tunneling through soils, transition materials and short stretches of possible fractured rock with high permeability. But it could operate in open mode through long stretches of the tunnel in rock with anticipated low permeability. Probing and pre-excavation grouting were prescribed in case of higher than threshold groundwater inflow is encountered in the open tunnel heading. However, rather than performing the probing and grouting, the contractor elected to operate the TBM in semi-open mode in all the rock tunnel reaches, where a nominal face pressure was maintained continuously to balance the external hydrostatic pressure.
The TBM key mechanical performance indicators were identified and continuously monitored during tunneling, which allowed the Contractor to correlate the TBM performance and behavior against the vastly different geological conditions encountered along the alignment. When confronted with unexpected TBM performance, the past performance indicators were then compared to that of previously encountered ground conditions to assist in optimizing the ground conditioning approach and other operating parameters. The project was completed successfully in 2019 on schedule and on budget, with only a single incident of loss of face pressure while mining through a difficult transition zone, which was quickly rectified by adjusting the ground conditioning. Figure 5 shows the TBM completed the excavation.
Where we go from here
With the ongoing advancement of TBM capabilities, the ability to tunnel through mixed face ground conditions has been improved dramatically and the risks have been reduced significantly. However, the real issues are not the suitability or the capabilities of the TBM to navigate through difficult or mixed ground, but it is what leads to the selection of the right TBM; it is the development of a robust geotechnical investigation, the development of an accurate geotechnical baseline profile and geotechnical parameters, the preparation of suitable TBM specifications, the appropriate design of the TBM to deal with the ground conditions, and the optimization of the TBM operation during tunneling. These make the difference between a successful tunneling through mixed and difficult ground conditions or failure. It is important to recognize that a failure of TBM in mixed ground conditions may carry significant consequences including potential water inundation, major settlement, and even tunnel collapse. Having the right geotechnical assessment, the suitability of the TBM to the ground condition and the vigilant performance of the TBM operators to handle the ground conditions is the formula for a successful project.
Dr. Irwan Halim is AECOM Chief Engineer for water tunnels and has 30 years of experience in geotechnical and underground projects in the US, Canada, and abroad. He has provided engineering and design services for major water/wastewater as well as transit and transportation clients throughout North America. Irwan’s expertise includes design and construction of mined and cut-and-cover tunnels in both soft ground and hard rock; TBM-driven, drill-and-blast, and SEM tunneling; soil/rock-structure interaction analyses; and foundation engineering.