Trenchless technology allows workers to create tunnels underground without requiring a starting trench, exposing the tunnel to the elements above. Tunneling is useful for many projects, including laying large lines such as sewage pipes, without excess destruction of the site. However, the larger the bore, the more it is prone to collapse. There are several areas to consider to avoid tunnel collapse when using a large boring tool.

Geotechnical Investigation of the Project Site

Project engineers address possible concerns about boring large lines during the planning stage. Aspects that may cause tunnel collapse include area geology and groundwater. When surveying the area, geology dictates the overall cost and feasibility of the project.

To determine if a large bore is possible, project planners analyze geological data through a geotechnical investigation to provide the proper information in the final geotechnical report. (Learn more in "Why a Detailed Geotechnical Report Means Success for Your Trenchless Project.")

They also employ ground-penetrating radar, GPR, to help check underground rock properties. (Read on in "The View Underground: Ground Penetrating Radar.")

Groundwater is more difficult to predict and causes the most problems during construction. An overabundance of groundwater causes loss of structural integrity of the surrounding rocks, leading large bore tunnels to collapse. While not entirely accurate, some surveyors use electromagnetic methods to help identify water-bearing strata during the planning phase.

To help determine the geology of an area, planners drill a core sample. However, these examples are not entirely reliable as it only recovers a small sample. It is incapable of predicting discontinuities in geological conditions parallel to the bore site.

Common Reasons for Tunnel Collapse

When a large bore tunnel collapses, it is due to one of three main reasons: geological issues, construction issues or groundwater penetration.

Collapse Due to Geological Issues

Geological issues arise when the rock surrounding the tunnel gives way. These geological problems could be due to the development of a crack in the rock joints due to excessive stress. Groundwater may penetrate the bored shaft and cause a rock slide. Also, the clay between the layers of rock may lose adhesion.

Collapse Due to Construction Faults

Construction causes often come from a misjudgment of the surrounding rock. The neighboring rock strength may not be up to the task of supporting a larger diameter bore, despite the geological survey indications. If the bore is enormous, the tunnel may collapse due to the rockburst phenomenon. A rockburst is the release of a tremendous amount of energy causing the collapse of a wall. The rockburst phenomenon usually only happens in mining sized shafts.

Groundwater is the enemy of tunnel formation. Excessive rain with inadequate draining can cause the soil to lose cohesion and collapse newly bored tunnels. Poor drainage on a worksite causes similar problems.

Tunnel Collapse Prevention Methods

Preventing large-bore tunnel collapse requires some construction intervention and site monitoring. In some cases, the excavation is propped up by new pipeline going into the recently created hole. If that is the case, workers may push the pipe through as the auger drills the way to keep the tunnel from collapsing in on itself. This method enables excavators to proceed even in areas where the ground is less than ideal for underground work.

Workers may need to enhance the supporting strength of the rock itself. They can do so by closing in surrounding rock lending that strength to self-stabilize the newly created tunnel. Constant monitoring of displacement and stress on the walls and ceiling helps them to know when this method is required.

To keep excess ground and rainwater from disturbing the work site, the worker may need to install additional drainage around the area. The site survey and geological analysis tell engineers and planners if drain lines are necessary before beginning construction.

By understanding the worksite, engineers can prevent tunnel collapse from the beginning. It is essential to conduct a thorough geological survey to know what soil and rock conditions lay waiting. Looking at drainage and preparing the site to drain ground and rainwater more efficiently can keep the soil saturation from causing problems during construction. Monitoring the ground conditions during excavation is essential. Finally, using supporting structures may be necessary to ensure the tunnel does not collapse before the lines are entirely placed.