By Jake Swanson and Britt N. Babcock
Tunneling is one of the most in-depth and challenging construction types the design and construction community faces. The challenges of excavating below ground, often within water bearing formations, are incredibly difficult feats to achieve. Hats off to all those working tirelessly to design, construct, and advance the tunneling industry and the processes utilized. This article will highlight the types of grouts commonly used, when and where grouts are generally used, and how grouting is utilized to assist in the development and construction of a tunnel system. This article is not intended to be all encompassing or oversimplifying, but an overview of grout materials, techniques and applications utilized in tunnel construction. This article is a summary of what can be a very complicated process.
Both cement (particulate) and chemical (solution) grouts are used in tunneling applications to fill narrow cavities/cracks, rock fissures and geologic voids, reduce hydrostatic pressures, permeate soils for control of groundwater, solidify flowing soils, create contact between liners and surrounding geology, or control groundwater locally through joints or cracks in the tunnel liner, or more globally with the use of curtain grouting.
Cement grouts can consist of a Portland cement grout or an Ultrafine cement grout. The distinguishing difference between a Portland cement grout and Ultrafine cement grout is particulate size. A Portland cement grout can range from an average of 50 to 100 microns in particle size for a Type I/II, and an average of 15 and 20 microns for a Type III. Ultrafine cement is much smaller with an average particulate size of three microns. For reference, the size of a human hair is approximately 70 microns.
The chemical side of the grout family branches into two different types of grouts: chemically reactive and expansive polyurethane foams. Chemically reactive grouts consist of acrylamide, acrylates, acrylics, and sodium silicates. Acrylamides, acrylates, and acrylics are predominantly used for water control while sodium silicates are used for temporary structural support. Expansive polyurethane grouts consist of two types: hydrophilic and hydrophobic. Hydrophilic grouts are typically single component systems that react with water and cure to an expansive flexible foam or non- expansive gel. Hydrophobic grouts require the use of a catalyst but are also considered single component when pumped. They are typically highly expansive during cure and require very little water to react. Although cement and chemical grouts can differ in composition and application, most often they are complementary products on the same project.
Tunnels vary in size from micro to macro and are installed to convey anything from water to traffic. A tunnel system has components consisting of the tunnel itself, break-in and break-out shafts, and crossover passages between adjacent tunnels where needed. Grouting can be a technology utilized in each component of a tunnel system – before, during, and after construction phases. Whether part of the initial design or due to unforeseen challenges, grouting is often the unsung hero with regards to the successful completion and functionality of any tunneling project. The following are some of the most common purposes for grouting that may be required during tunnel construction. Special situations may occur not listed below:
- Mitigate groundwater intrusion through the formation along the tunnel alignment
- Reduce hydrostatic pressures on tunnel liner
- Stabilize/solidify flowable sands along the tunnel alignment
- Stabilize/solidify soils prone to settlement along the tunnel alignment to mitigate potential subsidence
- Fill geologic voids beyond the tunnel alignment diameter
- Backfill and contact grout to fill voids between the tunnel and geologic formation
- Seal larger areas to control water infiltration. Curtain grouting will be used in the tunnel.
The types of challenges contractors may face will help determine the type of grout used. Grouting of a geologic formation, rock, or soils may occur prior to the advancement of tunnel construction, during the construction, or post-construction. The type of grout chosen is generally dictated by the desired outcome and the geologic conditions. For water infiltration through rock fractures and fissures or to reduce hydrostatic pressures on a tunnel liner, Portland cement is typically the grout of choice due to its performance and economic benefits. When fissures and cracks require a finer material, the contractor may be required to use Ultrafine cement. When encountering flowing sands that can impede tunnel advancement and potentially create subsidence or stability issues for above-grade infrastructure and buildings, Ultrafine cement or solution grouts may be used to permeate the sands and create a stable sand/grout matrix. Geologic voids can occur in some formations that require filling prior to the advancement of tunnel boring excavation machinery. Portland cement grout will often be used, but where applicable, polyurethane foam may be utilized. After a tunnel boring machine excavates the alignment in firm soil or hard rock, a space exists between the excavation limits and the tunnel liner; backfill and contact grouting are performed using a Portland cement grout mixture to achieve intimate contact between the geology and the tunnel liner. In large areas of tunnels or shaft liners, the contractor may perform curtain grouting with either cement or chemical grout to stop water infiltration. Polyurethane grouts are often employed when leaks are more localized to joints or defects.
In summary, the use of cement or chemical grouts often plays a major role in the successful construction of tunnel systems. Accolades to all the tunnel designers and contractors who make it possible.
Jake Swanson is Midwest Regional Manager and Britt N. Babcock, PE, is President of Avanti International, based in Webster, Texas. For information, visit avantigrout.com.