Learn About Ground Support Basics for Tunnel Construction

 Ground support

We discussed here the basics of Ground Support for Tunnel Construction.

The dominant factor at all stages of the tunnel system is the degree of support needed to keep the surrounding ground safe. Engineers need to consider the type of support, its strength, and when it will be installed after excavation. An important factor in installing timing support is the so-called stand-up time. That is, it's time to turn the ground upside down, stay safe, and have time to install the support. 

In soft soils, the rise time ranges from a few seconds in soils such as loose sand to a few hours in soils such as sticky clay, where loose sand moves inward into the tunnel and flows under the water table. Then it drops to zero. Rise times on rocks range from minutes on rippling soil (densely broken rocks where fragments gradually loosen and fall) to days on moderately bonded rocks (bond lengths in feet). It can also be measured for centuries on almost intact rocks. Boulders (between joints) are larger than the tunnel opening and do not require support. Miners generally prefer rocks to soft ground, but local cases of major rock defects can actually cause soft ground conditions. Passing through such areas generally requires a radical change in the use of soft ground.

In most cases, tunneling transmits the load to the ground by bending to the sides of the opening. This is the so-called ground arch effect (Figure 1 above). The effect of the head is three-dimensional, locally creating a dome on the ground and bending the luggage back and forth as well as the sides. If the durability of the ground arch is fully guaranteed, the rise time is infinite and no support is required. The strength of the arch on the ground usually decreases over time, increasing the load on the support. Therefore, the total load is distributed between the support and the ground arch in proportion to the relative stiffness by a physical mechanism called the structure-media interaction. When the bedrock loosens due to excessive yielding, which significantly reduces the inherent soil strength, the bearing capacity increases significantly. This can occur if the support is installed too long or can be the result of explosion damage, so quickly maintain the strength of the ground arc as the strongest load-bearing part of the system. Based on the need to do. Proper support will be installed to prevent explosion and movement damage due to the ingress of water that tends to loosen the ground.

As the gap increases, the rise time sharply shortens, so the full-scale propulsion method (center of Fig. 1), which excavates the entire diameter of the tunnel at once, is ideal for strong soil and below. tunnel. The effects of weak ground can be offset by reducing the size of the first acquired and supported openings. For example, high prices and promotion banking methods. In extreme cases of very soft ground, this approach provides a multi-drift method of movement (Figure 2). In this method, individual drifts are reduced to a small size that is safe for drilling, and some of the supports are placed in each. As the anomaly spreads, it drifts and gradually connects. The central core remains unexcavated until the sides and crown are firmly supported, providing convenient central support for strengthening the temporary support for individual drifts. This apparently slow multi-drift method is an ancient technique for very weak soils, but such conditions are still forced to be adopted as a last resort for some modern tunnels. For example, in 1971, at the Straight Creek Freeway Tunnel in Colorado, after a failed operation attempt, multiple drifts were needed to bring this large 42 x 45-foot high horseshoe tunnel to a weak shear zone over 300 feet wide. I found that I needed a complex pattern. The entire surface of the shield.

Early tunnels used wood for initial or temporary support, followed by a permanent surface finish of brick or masonry. Since steel became available, it has been widely used as the first temporary staircase or primary support. In most cases, it is encapsulated in concrete as a second stage or final cover to protect it from corrosion. Steel rib supports with outer wooden blocks are widely used in lock tunnels. The horseshoe shape is common to all rocks except the weakest rocks because it has a flat bottom for easy towing. In contrast, stronger, structurally more efficient circles are generally needed to support larger loads of soft soil. Figure 1 below shows some terms that compare these two shapes and identify different parts of the cross-section of the steel rib type support and adjacent elements. Here, wall plates are generally used only in the top head method. In the top head method, both top heads support arch ribs and span this length to excavate the bench until the post is pushed down. For new types of support, there is a tendency to move from two-stage support to a single support system, some of which are installed early and gradually enhanced to convert to the final full support system. It is described below with a more up-to-date tunneling procedure.

As the gap increases, the rise time sharply shortens, so the full-scale propulsion method (center of Fig. 1), which excavates the entire diameter of the tunnel at once, is ideal for strong soil and below. tunnel. The effects of weak ground can be offset by reducing the size of the first acquired and supported openings. For example, high prices and promotion banking methods. In extreme cases of very soft ground, this approach provides a multi-drift method of movement (Figure 2). In this method, individual drifts are reduced to a small size that is safe for drilling, and some of the supports are placed in each. As the anomaly spreads, it drifts and gradually connects. The central core remains unexcavated until the sides and crown are firmly supported, providing convenient central support for strengthening the temporary support for individual drifts. This apparently slow multi-drift method is an ancient technique for very weak soils, but such conditions are still forced to be adopted as a last resort for some modern tunnels. For example, in 1971, at the Straight Creek Freeway Tunnel in Colorado, after a failed operation attempt, multiple drifts were needed to bring this large 42 x 45 foot high horseshoe tunnel to a weak shear zone over 300 feet wide. I found that I needed a complex pattern. The entire surface of the shield.

Early tunnels used wood for initial or temporary support, followed by a permanent surface finish of brick or masonry. Since steel became available, it has been widely used as the first temporary staircase or primary support. In most cases, it is encapsulated in concrete as a second stage or final cover to protect it from corrosion. Steel rib supports with outer wooden blocks are widely used in lock tunnels. The horseshoe shape is common to all rocks except the weakest rocks because it has a flat bottom for easy towing. In contrast, stronger, structurally more efficient circles are generally needed to support larger loads of soft soil. Figure 1 below shows some terms that compare these two shapes and identify different parts of the cross-section of the steel rib type support and adjacent elements. Here, wall plates are generally used only in the top head method. In the top head method, both top heads support arch ribs and span this length to excavate the bench until the post is pushed down. For new types of support, there is a tendency to move from two-stage support to a single support system, some of which are installed early and gradually enhanced to convert to the final full support system. It is described below with a more up-to-date tunneling procedure.