Ground Stabilization for Tunneling
By Paul C. Schmall, P. E., & Lucian P. Spiteri, P.E.
Soft ground tunneling and related works can, and often do, present a number of soil-related challenges. In the great majority of cases, areas of “geotechnical vulnerability” are realized at the design stage and provisions are made for ground modification. Sometimes, though, the unexpected can happen when excavation or mining is under way, resulting in a significant impact on what is almost certainly critical path work. In either situation – pre-planning or emergency response – the specialized expertise of geotechnical contractors is vital in determining which of the available soil stabilization options available is the best for the geotechnical condition at hand.
Choosing the Optimal Method
When faced with the potential need for soil stabilization, owners and their engineers must bear in mind that that there is no “cook book” solution. More than one soil stabilization method can theoretically be used to achieve the same result, and the experience of the geotechnical contractor comes into play in evaluating both the surface and subsurface conditions, including groundwater, to arrive at the optimum approach to the work, with the appropriate risk tolerance. The condition of the ground is, of course, the governing criterion, followed by the underground excavation methodology and the purpose of the soil stabilization program. Is it to facilitate TBM break-in or break-out? Is a launch box or access shaft to be excavated through water-bearing soils? Are mixed-face conditions or shallow cover present along the tunnel alignment? Are sensitive structures present?
Other factors will also affect the final selection. Is the soil stabilization method required to be watertight, structural, or both? Is settlement or heave of overhead structures the concern or could settlement or heave during the soil stabilization itself be a consideration? Is vertical access to the target ground restricted/not possible? Is high volume spoil generation a consideration?
The intent of this article is to broaden the understanding of the complexities involved in tunnel-related soil stabilization and to offer an experienced-based guideline to the geotechnical methods most frequently used to resolve commonly encountered problems. Every project, however, must be thoroughly evaluated on a case by case basis before determining the method, or methods, best suited to meeting the purpose of work.
Launches and Retrievals
Launching or retrieving a TBM can be a dicey operation. The support of excavation for the shaft (typically steel sheeting) must be cut to permit the sending or receiving of the TBM, requiring improvement of the ground behind so that the exposed “eye” is stable. Cutting the eye typically occurs well below the water table and the manner in which the excavation support is removed permits little advance confirmation of the adequacy of the ground stabilization. The improvement must be structural as well as watertight. Until the eye has been cut through the shaft lining and the machine completely inserted into that space, there is a danger of rapid soil and groundwater run-in into the launch box that can be difficult to control and then correct. Disturbed ground and the resultant ground loss behind the excavation support can cause settlement and possibly alignment problems for the TBM.
A retrieval is more risky and complicated than a launch. The improved ground for a launch allows the cutting of the eye and stable conditions for mining through the full thickness of the block. Upon retrieval, however, the overcut of the cutterhead and the open annular space between the skin of the TBM and the improved ground block provides a flowpath for water and soil until the TBM reaches the seal. Where a segmental lining is installed, the retrieval block can be extended beyond the length of the TBM so that the annular space can be grouted. The problem is not so easily addressed with microtunneling where the pipe itself moves.
Dewatering and/or ground improvement should always therefore be tailored for the soil conditions. Where groundwater is the culprit, dewatering should always be considered as a primary means of ground stabilization, in many cases to be supplemented with other ground improvement methods. The ground improvement methods most often used are permeation and jet grouting. Selection of which method is a function of groundwater head, the diameter of the opening, the structural requirement of the stabilized soil block, and the length of time that the eye will be exposed.
Permeation grouting is more cost-effective per unit than jet grouting, but relies on clean granular soils to achieve thorough penetration. The permeation grouted soils will still be lower strength though (depending on the grout used). Where there is concern about ungrouted inclusions of siltier soil within the target zone, partial dewatering or pressure relief is an excellent supplement. This is a given when the shaft itself is dewatered. Jet grouting, since it is an erosion and replacement technique, is not as soil-dependent as permeation grouting, but the presence of boulders and other obstructions can cause shadowing and incomplete coverage. The soil strength that can be achieved by jet grouting is significantly higher than that achievable by permeation grouting. Thus, while permeation is the ideal approach under the right conditions, jet grouting should be considered as the default option.
Ground freezing may be the method of choice when ground conditions are not amenable to either permeation or jet grouting. It is more cost-effective for microtunnel eyes that are exposed for only a matter of hours and can be quickly frozen with a single shot of liquid nitrogen.
Mixed Ground Conditions
The intent of soil stabilization for mixed-face conditions is to provide a homogeneous ground condition so that the TBM (which is built for a specific range of ground conditions) can proceed with business as usual. The typical mixed-face condition is where there is a small section of overburden or weathered rock within a hard rock alignment, although a mix of very soft and very dense soils can be problematic as well. Ground improvement is often employed in these cases to create a more uniform face condition for the tunnel and/or to assist in groundwater control.
In evaluating which technique to utilize, the first question asked is usually “What are the consequences of this ground improvement being only partially effective?” The subsequent questions are probably “How far below the water table is this zone?” and “What is the permeability of the ground and the likelihood of having an uncontrollable running ground condition, and if that occurred, what would be the consequences?” Somewhere in the evaluation process, the per-day cost of the tunneling operation is considered. In the final analysis, the decision-making process comes down to this – what degree of assurance must be afforded?
Similar to launches and retrievals, the tools of choice are dewatering, permeation grouting, jet grouting, and ground freezing, in order of ascending cost but also degree of assurance. Unlike launches and retrievals, the mixed-face condition, by definition, involves a range of ground types and this favors ground freezing, the least soil-dependent methodology of the three.
On the Second Avenue Subway Project in New York City, ground freezing was performed at a mixed-face zone for several reasons:
The greatest factor in favor of ground freezing, however, was that this method offered the highest degree of assurance. With tunneling being performed directly beneath a busy city thoroughfare, a catastrophic failure would have untold consequences.
Tunneling Through Very Soft Soils
Tunneling through very soft soils is an issue that plagues microtunneling projects more than others. Usually the end product, a partially full pipe in the ground, is lighter than the soil that was removed, so from a long-term standpoint no ground improvement is required provided the soils are at least normally consolidated. This concept, however typically does not sit well with designers and owners, and often an improved ground “cradle” is desired beneath the final pipe. The greater concern to the contractor is generally to prevent the heavy MTBM from diving in the very soft soils. A solution to both scenarios would be the pre-construction of a series of improved ground piers through which the MTBM can mine and which will act as temporary support for the MTBM and permanent support of the pipe.
Improvement can be accomplished with jet grout columns or soil mixed columns. The columns themselves can be tangent, overlapping, or with gaps between, depending on the loads and the dimensions of the TBM or pipe sections, and extend fully or partially through the tunnel face. The piers must extend down to a better load-bearing stratum.
Soil mixing will provide a consistent column diameter but will create the most surface disturbance, an issue when working in urban areas cluttered with utilities and shallow obstructions. Jet grouting is effective in the widest range of soil conditions, but column geometry can be somewhat difficult to predict in very soft or fibrous soils.
A minimum improved ground strength is required to achieve the design intent and this is typically going to be driven by the in-situ soil type. However, too much strength can be a hindrance to the subsequent tunneling operation. Achieving the desired minimum and maximum strengths can therefore be tricky, especially in variable ground conditions. The resulting ground (groundwater) chemistry should be considered so as to avoid detrimental effects on slurry performance.
Tunneling in urban environments often brings with it the need to protect overlying aging or sensitive structures from settlement arising from tunneling or shaft excavation. Settlement control falls into three categories: preventative measures; pre-meditated, but concurrent measures; and unanticipated emergency response measures.
Preventative measures should be performed where reasonable predictions indicate settlements above the tolerable limits and there is access to physically perform the work. Preventative measures may include single event grouting programs designed to prevent ground losses from propagating up to the structure(s) of concern, or they may be structural, such as conventional or micropile underpinning. When grouting is utilized as a preventative measure, it will typically be performed with permeation or jet methods that do not inherently result in ground movements or displacements so as not to disturb the structure to be protected.
Pre-meditated, concurrent measures would include active foundation jacking as movements occur or compensation grouting when there is restricted access to the structure itself and/or the amount of settlement is uncertain. The intent of compensation grouting is to replace lost ground as quickly as possible so that ground “relaxation” does not occur. Compensation grouting is as complex and highly engineered as grouting can be; there are numerous considerations pertaining to ground behavior and responsiveness, grout consistency for injectability and long-term behavior, ground/structure interactions, etc.
Unanticipated settlements will occur with any kind of uncontrolled ground flow into a TBM, exposed windows in a “bathtub” excavation support, and sometimes even with the installation of the support of excavation. Such occurrences warrant immediate response to stabilize the conditions and prevent further disturbance. Compaction and jet grouting are usually relied upon because they can be put into place quickly.
Classic Methods and Classic Challenges
Until the 1990s, the open-face shield was the predominant tunneling method. Open-face tunneling was marked by high productivity and an ability to cope with a wide range of geologic conditions. These advantages were offset to some degree by the requirement for a significant dewatering effort and limited built-in control over flowing ground conditions from the heading, which typically necessitated installation of a sodium silicate grout canopy above the tunneling alignment. In recent years, pressurized-face TBMs have largely replaced the use of open-face shield tunneling.
However, there always seems to be an abundance of 4- to 8-ft diameter, liner plated, shallow, railroad crossings. These are often accomplished by less costly open-face methods because the run is typically relatively short, mobilization cost is less, and pressurized face tunneling requires greater cover.
Open-faced tunneling does involve challenges. Railroad crossings tend to be in low lying areas with shallow groundwater, and thus dewatering must be a primary component of the work. Shallow cover also means greater settlement potential. And when permeation grouting is utilized for soil stabilization above the tunnel alignment, as is often the case, the grout will function as designed and follow the path of least resistance, commonly disappearing into the openwork rail ballast or daylighting to the surface.
Where there is more cover and loss of grout to rail ballast is less of a concern, the shallow cover still limits the grout pressures that can be used, and therefore limits the injection effectiveness.
Overcoming this situation in a manner that addresses the need for soil stabilization coupled with a solution that does not impact rail service requires careful evaluation of the site-specific conditions, followed by development of a customized grouting plan involving multiple rounds of grouting to address the “path of least resistance” issue.
Water Control for Shafts
Conventional dewatering in conjunction with the installation of beams and lagging or perimeter sheeting is the most common method used for shaft excavation through water bearing soils. Other cut-off methods that may be used include secant piles, soil mixing or structural slurry walls.
In sensitive situations, the pumping of groundwater may be strictly limited. Such circumstances would include the presence of contaminated groundwater, compressible soils, or wooden pile foundations within the groundwater drawdown zone. If groundwater cannot be lowered and there is no continuous aquitard for the excavation support to key into, a horizontal bottom seal must be created.
Bottom seals are typically created using permeation grouting or jet grouting techniques. For permeation grouting to be properly effective, the ground must be relatively homogeneous and readily groutable. In marginally groutable soils, jet grouting is the better option. The bottom seal itself is generally a thin, non-structural zone of treated soil constructed at sufficient depth to counteract the uplift forces placed on it by the static groundwater head.
Alternatively, a slab of improved ground, thick enough so that its weight offsets the hydrostatic pressure, may be constructed beneath subgrade.
Irrespective of the method of construction or location, creation of a bottom seal is the most risk-prone application of ground improvement in terms of quality control. There are no practical or cost-effective methods or instruments available to confirm continuity of the seal, and once the work is completed, there is little that can be practically or cost-effectively done to repair leakage, which may be at numerous points, evenly distributed over the seal, or concentrated in one or more locations. This high-risk aspect of the bottom seal installation underscores the need for such work to be undertaken only by geotechnical contractors with a history of relevant experience, who will approach the job with due caution, conduct sufficient test sections, and implement good grouting practices to maintain the high standards required for a successful project.
Urban Fill and Disturbed Ground Conditions
In areas of older urban construction, the condition of building foundations is often uncertain or unknown and the buildings themselves are frequently deteriorating and extremely sensitive. In addition, a thick layer of uncontrolled, non-uniform, rubble-strewn urban fill is usually present at these sites. The combination of sensitive structures and poor, unpredictable soils creates the potential for differential settlements should adjacent construction activities precipitate foundation soil disturbance. Such settlements can be sudden and severe, sometimes occurring over a matter of hours.
Prior to tunneling activities in these vulnerable areas, it is good practice to identify these sensitive structures and implement preventative ground improvement measures to mitigate ground loss and subsequent settlement once construction begins. Several grouting methods, including compaction, permeation, fracture and compensation grouting, alone or in combination, can be used to mitigate settlement potential as well as to remediate disturbed ground conditions and alleviate actual structural settlement.
The ground stabilization for the construction of cross passages is influenced by two factors, access and ground consistency, but the degree of assurance required is often the decision maker. Cross passages can sometimes be dewatered, but only where they are relatively shallow and the resulting conditions will be bone dry. The preferred method of dewatering is with wells from the surface, but lances can be installed from inside a tunnel. Permeation grouting can be highly effective, performed from either inside the tunnel or the surface, but only when the ground is amenable to grouting without the presence of ungrouted inclusions which could be eroded by water flow. In less favorable ground conditions, jet grouting may be warranted, but can only be implemented from the surface. Excessive borehole deviation with depth may be a limitation for jet grouting. When dewatering, permeation grouting, and jet grouting are limited in providing the necessary degree of assurance, ground freezing is the only alternative. Ground freezing can be implemented from inside the tunnel, and through the widest range of ground conditions.
A large-diameter TBM that becomes damaged and inoperable during a tunnel drive presents a particular set of problems, not the least of which is the considerable contract delay and resulting cost incurred while the problem is evaluated. Accessing the TBM with an emergency shaft from the surface is cost and logistically prohibitive, particularly if the tunnel is being driven at depth and under significant hydrostatic pressure.
When the TBM was damaged during tunneling of the Brightwater Conveyance System’s Tunnel No. 3, it was decided to abandon the TBM in place. Gwildis et al reported on the use of ground freezing to permit the TBM to be dismantled and salvaged and the tunnel connected with another TBM drive (“Use of Ground Freezing for Connecting Two Tunnel Boring Machine-Driven Tunnels 300 Feet Underground,” Proceedings of North American Tunneling 2012).
Ground freezing, in fact, has a history of success in TBM and MTBM rescue. The frozen ground can achieve the critical perfect connection to the TBM itself, often under considerable water head and within native ground, disturbed ground around the face, and through overcut of the cutterhead.
While the geotechnical challenges that can impact tunneling operations are many and complex, experienced geotechnical contractors have the expertise and the tools to address them as they need to be addressed – on a site-specific basis, taking into account the desired result and the degree of assurance required. Involving the geotechnical contractor early in the process has proved time and again to be integral to successful and speedy problem resolution and its importance cannot be overestimated.
Paul Schmall is a Vice President and Chief Engineer and Lucian Spiteri is a Senior Project Manager for specialty geotechnical contractor Moretrench.