Grouting in Tunneling

By Joel Joaquin

Grouting in tunneling is primarily used to manage unexpected ground conditions, such as groundwater ingress or instability. The most common reason for grouting is to control water entering the tunnel, which can be handled through pre- and post-excavation grouting. Additionally, grouting can address unstable rock or soil conditions, improving stability by filling cracks and discontinuities.

There are two main types of tunnel grouting:

1. Pre-excavation grouting: Boreholes are drilled from the tunnel face into untouched rock, with grout pumped in to seal the area before tunnel advancement.
   

Typically, Portland cement, fast-setting microfine cement, and mineral grouts are the most common materials for this type of grouting.

2. Post-excavation grouting: Grouting is done after tunnel excavation, typically to address water ingress in already excavated or lined areas.

In addition to cementitious materials, polyurethanes, acrylics, and other chemical grouts are typically used for post-injection grouting.

The overall cost of post-injection grouting is much higher than pre-excavation grouting.
Cement-based grouts are most commonly used, though chemical and mineral grouts are also effective. The pressure grouting technique has been used for over 70 years and has evolved significantly, especially in Scandinavia. Successful pre-grouting can prevent significant water ingress, making tunnels virtually drip-free or reducing water entry to manageable levels.

Purpose for Pre injection grouting:
The primary goal of pre-injection grouting is to ensure the stability and safety of tunnel excavation by addressing potential challenges before tunneling begins. This process involves several key objectives:

1. Control Water Inflow: Pre-injection grouting prevents water from entering the tunnel by sealing cracks in the rock mass before excavation. It’s essential in areas with high-pressure groundwater to avoid delays and risks.

2. Improve Rock Mass Quality: Grouting strengthens and stabilizes the surrounding rock by filling voids and fractures, reducing the risk of instability and ensuring consistent tunneling progress.

3. Limit Groundwater Drawdown: Pre-injection grouting prevents groundwater table lowering, avoiding ground settlement and protecting nearby structures by sealing pathways that could allow water migration.

By addressing these key issues, pre-injection grouting helps create a safer, more controlled environment for tunneling, improving both the efficiency and longevity of the tunnel while minimizing environmental and structural risks.

Pre-injection versus post-injection:
Experience demonstrates the importance of pre-and post-injection grouting techniques in addressing groundwater-related challenges in tunneling.

1. Post-injection is costly and challenging: Relying solely on post-injection in tunnels with high groundwater inflows is expensive and complex. It can be time-consuming, and its effectiveness is limited when water inflows are significant.

2. Pre-injection solves most problems: Pre-injection grouting seals cracks before excavation, reducing water ingress risks and making tunneling more efficient and cost-effective.

3. 100% sealing with pre-injection is unrealistic: Achieving complete sealing with pre-injection is difficult due to variable rock conditions, but even partial sealing can significantly improve stability and minimize delays.

4. Post-injection as a supplement is effective: Post-injection, when used after excavation, can address remaining water ingress issues and enhance pre-injection sealing for a more comprehensive solution.

5. Optimizing pre- and post-injection is recommended: Combining pre- and post-injection methods is the best approach, addressing water inflow before and after excavation and reducing risks and costs.

6. Planning for injection is crucial: Planning for either pre- or post-injection before tunneling begins to avoid costly delays and disruptions.

7. Pre-injection is lower risk than post-injection: Pre-injection addresses water issues upfront, reducing the likelihood of costly, complex problems during tunneling, making it a more reliable solution than post-injection.

Drill patterns in pre-excavation grouting
The number of pre-excavation grouting holes in TBM tunneling depends on tunnel size (typically 1-1.5 m distance between holes), rock mass permeability (the more permeability, the more holes are needed), and geological conditions (joint and fractures).

The drilling length typically goes from 18 to 24 meters with an overlap of at least one tunnel diameter and drilling at an angle of 5 to 10 degrees of the tunnel axis.

Cement grout characteristics, particle size, and fineness:
Microfine cements are typically used for pre-injection grouting. Fast-setting microfine/ultrafine cement, such as the MasterRoc MP 650 and MP 900, has low viscosity with a w/c ratio 1.0, balancing workability and strength. Its Marsh cone time is 32–36 seconds (water = 29 sec), indicating good fluidity. Bleeding is under 2% in the first hour, ensuring minimal segregation. The final set time is 120–150 minutes, offering enough flexibility for application. It has about 1 hour of open time when agitated, allowing ample time for adjustments. The material also shows good stability under pressure, making it ideal for demanding conditions.

Cement for injection varies in type, with coarser cement suited for larger openings. Permeation capability depends on maximum particle size and distribution. The Blaine value reflects particle size, which increases with finer grinding, and the d95 value shows the size below which 95% of particles pass. A smaller maximum particle size prevents blockage in fine openings.

Project examples:
In the U.S., there is experience using Type III cement, microfine cement, and colloidal silica for pre-injection grouting; projects like the Atlanta Water Tunnel (GA), Rondout Bypass Tunnel (NY), White River Lower Pogues (IN), the Hartford Tunnel (CT) and Lake Mead tunnel (NV).

The Hartford tunnel is an example of the successful use of Microfine cement, where over 1,500 tons of MasterRoc MP 650 and MasterRoc MP 900 were used during construction to stop water inflows in specific areas.

In the Arrowhead tunnel in California, microfine cement and over 1,400 tons of colloidal silica MasterRoc MP 320 T were used. While Portland and microfine cement alone would typically suffice to meet water ingress limits in hard rock, the Arrowhead East tunnel faced challenges. Extensive use of ultrafine cement struggled to penetrate and block groundwater flow effectively. Boreholes showed unpredictable results, sometimes accepting grout and other times almost none, regardless of initial water yield from a few gallons to 200 GPM.

Joel Joaquin is Tunneling Manager – North America for Master Builders Solutions.