From straightforward void filling to the complex and challenging protection of historic or sensitive structures during subsurface work, grouting plays an important role in many below-grade construction projects. This most diverse of all specialty geotechnical disciplines encompasses a range of individual techniques to cater to a broad spectrum of ground improvement and groundwater control needs.
Ground improvement techniques such as permeation, compensation, jet and compaction grouting have direct pre-construction applications to tunneling or pipejacking projects. These techniques may also be geared specifically toward groundwater control, a less direct but nevertheless vital consideration in the overall scope of a new or remedial underground projects.
Grouting for Ground Improvement
Selection of the optimum approach to ground improvement prior to or during underground work is a function of the geotechnical conditions and the project objective. Potentially difficult ground conditions revealed during the geotechnical investigation should be thoroughly evaluated during the preliminary design stage to determine the type of grouting that best meets the needs of the project. The experience of the grouting contractor is paramount in developing the grout design, grouting plan, delivery method and quality control measures that will result in a technically successful project completed on time and within budget.
The range of planned ground improvement objectives pertinent to tunneling and pipejacking operations includes increased bearing capacity, settlement control, reduced permeability and increased stand-up time.
Permeation grouting involves the injection of low-viscosity grout (sodium silicate, acrylate or microfine cements) into the pores of granular soils without displacing or changing the soil structure. The characteristics of the ground are modified with the hardening or gelling of the grout. Permeation grouting is used to either increase the strength and cohesion of the soil or to decrease its hydraulic conductivity (watertightening). For soil stabilization during tunneling, sodium silicate is the most commonly used grout.
This technique has been applied on a number of tunneling projects to increase the strength and cohesion of soils along the tunnel alignment. Similarly, it is also an effective technique to improve the canopy soils for shallow-cover pipejacking operations and to seal windows in “bathtub” excavations and shafts.
Compensation grouting has the specific design intent of precisely injecting grout to compensate for potential structural settlement or to control/reverse ongoing settlement. During compensation grouting, ground fracturing techniques are typically used to intentionally fracture the ground by the high-pressure injection of a cement-based grout, creating intertwined lenses or veins of grout. This results in very localized heave and even some consolidation of the soil matrix. This fracturing technique also provides reinforcement of the soil in situations where stability or stand-up time of the material is needed. Grouting is performed through sleeve port pipes in several phases, with repeat, limited-volume injections at each port to provide control over the operation. Compensation grouting is typically coupled with real time monitoring of the structure such that the grouting can be performed concurrent with the tunneling or as the settlement is realized. This technique can be successful across a wide range of soil types and has been successfully applied for tunneling operations beneath historic or sensitive structures bearing on shallow foundations.
Jet grouting is a technique that uses high-pressure, high-velocity jets located on a drill-mounted monitor to hydraulically erode, mix and partially replace the in situ soil or weak rock with cementitious grout. The process creates an engineered soil-cement product of high-strength and low-hydraulic conductivity. Jet grouting can be performed above or below the water table and in most subsurface soils varying from sands and gravels to clays.
Jet grouting can be used to homogenize mixed face conditions to be encountered within the horizon of the tunnel or pipeline bore. The technique has also been used to underpin existing structures during adjacent excavation for new construction.
Compaction grouting is the pressurized injection of the in situ soil with a relatively stiff, “low-mobility” grout. The grout does not permeate the soil matrix but rather forms a bulbous mass around the point of injection, displacing and thus densifying the surrounding soils. The technique can be accomplished in low-headroom or restricted access conditions, often with minimal disruption to normal facility activities.
During tunneling operations, minor ground losses often occur which, when disregarded, will result in surface settlements. Compaction grouting, when initiated through pre-placed grout pipes immediately after the tunnel shield has passed, will protect vulnerable structures by re-densifying the loosened soils above the tunnel crown.
Grouting for Groundwater Control
Permeation and jet grouting are widely used for groundwater control during underground work, either in conjunction with or in support of dewatering operations. Jet grouting can be used to create hydraulic barriers for containment, as well as groundwater control to facilitate access shaft excavation, to provide a bottom seal for a watertight “bathtub” excavation or to bridge gaps in excavation support systems where overhead or subsurface obstructions prevent continuity of the cut-off. Permeation grouting, when applied to increase the stand-up time of the soils, can also provide a measure of groundwater exclusion. This technique is also becoming more widely prescribed in “bathtub” excavations designed to minimize off site groundwater impacts or to lower the quantity of water to be pumped in urban areas.
When the Unexpected Happens
When an unanticipated problem occurs during the course of a tunneling or pipejacking project, the cost of scheduling delays or even shut-down, can be onerous. If a problem impacts adjacent completed residential, commercial or industrial buildings, damage to the structure may result. And in the public sector, when infrastructure elements such as highways, bridges or lifelines are affected, the need for speedy and effective resolutions becomes even more critical.
Various remedial grouting methods are available to cater to unexpected problems. Depending on the geologic conditions, compensation grouting with ground fracturing, compaction grouting and permeation grouting may be used to stabilize soils, increase standup time or help control groundwater inflow. These measures can alleviate issues with the tunneling or jacking process, which in turn will limit structural settlement caused by adjacent or underlying tunneling operations.
Compaction grouting can be used to re-densify surficial soils that had been poorly placed or loosened during tunneling. Permeation grouting may be used to stabilize running sands ahead of the tunneling operation, and jet grouting may be applied to underpin sensitive structures which can not be protected by other means.
Cooperation Is the Key
When the unexpected happens, good geotechnical information, cooperation between all parties, swift response and experienced engineering to determine the best site-specific approach is the key to a successful resolution.
It must also be noted that, while grouting techniques can, and do, successfully fulfill many tunneling and pipejacking applications, there are always circumstances where other approaches should be considered. Ground freezing may be warranted where both groundwater exclusion and support of excavation is required. This technique has also been successful in stabilizing running ground to allow retrieval of a mired TBM. If a problem is due to the presence of groundwater, dewatering, possibly in conjunction with grouting, may be the answer. Every case is different and consulting a contractor that can evaluate the feasibility of all potential techniques should always be the first step.
David Lim is a senior engineer for New Jersey-based specialty geotechnical contractor Moretrench.