Analysis of the Underground Tunneling Construction Safety and Possible Dangers

Background

The first tunneling was likely done by prehistoric people for the purposes of enlarging their caves (“Tunnels and Underground Excavation”, 2018). It wasn’t until the growth of Europe’s transportation needs during the 18th century, which resulted in major railroad expansions, was increase in tunneling that lasted for roughly 100 years experienced (“Tunnels and Underground Excavation, 2018).

Tunneling beneath rivers was thought to be impossible until the development of the protective shield in England (“Tunnels and Underground Excavation”, 2018). The new technology was first used of the shield in 1825 during the excavation of the Wapping-Rotherhithe Tunnel under the Thames River (“Tunnels and Underground Excavation”, 2018).

Underground tunneling next major breakthrough was the development of the first mechanical rotary excavator in 1954. It was called the “Mittry Mole.” (“Tunnels and Underground Excavation”, 2018).

Construction Practices and their impact on Health and Safety

The construction industry shows a tendency to rely on the experience and expertise of the contractor tasked with the job (“Underground Construction”, 2008). There are however standard practices which have been put into place to ensure the safety of workers underground. Practices differ across the globe. The following practices are common in America:

• The responsibility of an employer to notify employees of hazardous conditions upon receipt of such knowledge.
• The responsibility of the employer to maintain a check-in/check-out procedure so as to ensure the accuracy in numbers of workers underground and the prevention of unauthorized personnel from obtaining access to the site.
• The prohibition of open flames in subterrain construction areas except where allowed for cutting, welding or additional hot work services.
• The provision of fresh air to all subterrain work areas so as to prevent dangerous levels of fumes
• Should there be 25 or more underground workers at one point in time it is the the employer’s responsibility to provide no less than rescue teams consisting of 5 persons per team.
• The aforementioned practices were obtained from (“Underground Construction”, 2008)

Case Study: Lake Mead Intake No. 3 Tunnel

The Lake Mead intake tunnel was constructed in order to maintain the water supply to the Las Vegas greater metropolitan area (Anagnostou; Schouerch & Perazzelli, 2018). Before the project began, a set of analyses were performed that included an overview of the general features of the lake such as its depth and rock cover. Geological analyses consisted of the rock make up of the area, tectonic activity and reports on possible faults (Anagnostou; Schouerch & Perazzelli, 2018).

Pre-Construction Considerations

Flooding and collapse were the main concerns of the project’s engineers. These concerns were exacerbated by the presence of faults in the area. A big factor in avoiding issues was the choice of tunnel boring machine (Anagnostou; Schouerch & Perazzelli, 2018). It was decided that the machine should be convertible, meaning that it could quickly switch between open and closed mode (Anagnostou; Schouerch & Perazzelli, 2018). The closed mode would prevent collapse by swiftly extracting excavated rock, while the open mode would prevent flooding by countering hydrostatic pressure (Anagnostou; Schouerch & Perazzelli, 2018). Even with these measures in place, hydrostatic pressure made inspection and maintenance of the TBM machine extremely difficult. The proposed resolution was an advance grouting and drainage system to be used in the case of an emergency (Anagnostou; Schouerch & Perazzelli, 2018).

Face stability was another major pre-construction consideration. This refers to the stability on the internal working face of a tunnel during excavation (Anagnostou; Schouerch & Perazzelli, 2018). The most important consideration however was the stand up time of the face. It was predicted that Lake Mead would experience ever changing stand up times in important parts of the tunnel (Anagnostou; Schouerch & Perazzelli, 2018). The variations could fluctuate between a few hours and a few days depending on the stage of drilling (Anagnostou; Schouerch & Perazzelli, 2018). Stable stand up times over a few days would be good for open mode drilling whilst hourly fluctuations would call for either open or closed mode drilling with decreased slurry pressure (Anagnostou; Schouerch & Perazzelli, 2018).

A squeeze assessment was performed in order to determine the risk for rockfall. In order to avoid this, the tunnel face would require a thrust force (Anagnostou; Schouerch & Perazzelli, 2018).

Construction Experience

Tunneling began in May 2010. Soon after starting an unexpected normal fault was experienced (Anagnostou; Schouerch & Perazzelli, 2018). The fault progressively entered the tunnel cross section causing slow disintegration at the face and gradual water intake into the working chamber (Anagnostou; Schouerch & Perazzelli, 2018). In June of 2010 a significant instability resulted in the flooding of the tunnel and the shaft (Anagnostou; Schouerch & Perazzelli, 2018). In order to address this a combination of sub-horizontal and vertical boreholes were used to examine the extent and geometry of the fault (Anagnostou; Schouerch & Perazzelli, 2018). It was determined that the material of the fault lacked cohesion. This coupled with high water pressure resulted in the decision to rotate the tunnel axis in an easterly direction by 23 degrees. Following this intervention the team was able to complete the starter tunnel without further problems (Anagnostou; Schouerch & Perazzelli, 2018).

Construction Constraints

Various constraints were experienced in the excavation of the sedimentary and metamorphic rocks. In the presence of metamorphic rocks the TBM’s successful advance must be credited to a trade-off between the forced needed to bore and address shield skin friction and the force needed to restrict the inflow of water and maintain face stability (Anagnostou; Schouerch & Perazzelli, 2018).

Atmospheric and hydrostatic pressures in the excavation chamber created a dangerous situation for the maintenance team. As a result, the TBM components associated with closed mode operation experienced substantial wear and tear (Anagnostou; Schouerch & Perazzelli, 2018). These conditions proved the necessity for water inflow estimations.

Drilling of the final portion of metamorphic rocks exposed a severely fractured rock mass creating dangerous geological conditions. In addition to this, drilling in the basal rock portion of the final part of the alignment resulted in unexpected pressure oscillations. This was a clear indication of the occurrence of cave-ins (Anagnostou; Schouerch & Perazzelli, 2018).

Dangers

Rockbursts are is one of the most dangerous problems found during underground projects.. They are often responsible serious damage to equipment, points of entry and work station facilities. Injuries resulting from rockbursts can also be fatal (Mazaira & Konicek, 2015).In order to predict possible rockbursts engineers are required to perform comprehensive geological modeling tests (Mazaira & Konicek, 2015). On going evaluation throughout the duration of the project is also required (Mazaira & Konicek, 2015). Should the data obtained from these evaluations indicate areas that are prone to rockburst activity, three prevention approaches can be carried out:

• Optimizing the project’s layout scheme
• Rock mass pre-conditioning
• Reinforcement and support measures
• Environmental Impact

There are several environmental problems that can be attributed to the construction of underground project. These include:

• The collapse of structures due to ongoing triggering of surface settlements. This issue also causes slope instabilities.
• Groundwater alterations and springs are prone to drying up during subterranean works.
• A lack of consideration for the storage and reuse of excavated materials
• Noise pollution
• On going vibrations
• Groundwater pollution due to foreign material injection such as concrete
• Increase dust and air pollutant emissions.
• The effects of these issues often outlast the construction term (Gattinoni; Pizzarotti; & Scesi, 2014)

Regulations and limitations

In America, the Department of Energy has resolved to using contract reform in order to address underground construction issues. The department anticipates that this measure will aid in better definitions of expectations, give clear measures of performance, give better incentives, and assist with the management of departmental contracts (“Underground Construction”, 2008) Significant issues such as Health and Safety considerations can also be better addressed. (“Underground Construction”, 2008).The construction of underground tunnels, chambers, shafts and passageways in America is governed by the Occupational Health and Safety Administration underground construction regulation (“Underground Construction”, 2008).