Tunnel construction stands as a monumental feat, often involving intricate planning and precise execution. One critical aspect that underpins the success of tunnel projects is the comprehensive battery of tests employed throughout the construction process. These tests serve as the guardians of structural integrity, ensuring the safety and longevity of tunnels in diverse geological conditions. In this article, we will explore the realm of tunnel construction, unraveling the significance of various tests that play a pivotal role in guaranteeing the resilience and reliability of these subterranean structures. Join us as we explore the indispensable tests required to navigate the complexities of tunnel construction with confidence and precision.
Important Tests Required in Tunnel Construction
A tunnel is a corridor that is underground or under the sea. Excavated through surrounding soil, rock, or earth, or submerged in water, it is enclosed except for an entrance and exit, usually located at each end. Many tunnels are considered technological masterpieces and governments have honored tunnel engineers as heroes. Tunneling design which is an interdisciplinary subject, involving geotechnical engineering, geology, geochemistry, and hydrogeology, and its construction are very challenging, however, they are becoming more popular because of the attractive solutions they provide for railways, roadways, public utilities, and telecommunications.
There are many different ways to excavate a tunnel, including manual labour, the use of explosives, rapid heating and cooling, tunneling machinery, or a combination of these methods. Earlier, drilling and blasting in which holes are drilled into the rock, packed with explosives, and blasted was the commonest method of tunnel construction. In recent times, tunnels are becoming increasingly common, due to newly available equipment such as tunnel boring machines, road headers, relatively safe blasting techniques, and so on. Tunnel boring machines (TBMs) are widely used today due to their ability to construct tunnels at record speeds. The cutter head is the most crucial component of a tunnel boring machine. A group of disc cutters forced into the rock make up the cutter head. The rock is eventually broken into fragments by the disc cutters while the machine rotates. After being cut, rock fragments are gathered in a bucket called a muck bucket. Through the use of conveyor belts or trains, muck is removed from the tunnel. As its name suggests, the gripper holds onto the tunnel walls to enable the boring machine to be forcefully driven into the rock.
As noted earlier, tunnels are commonly driven into rocks and certain numbers of tests are required before and during this process. These tests are grouped into field tests and laboratory tests.
Tests Required in Tunnel Construction
1. Field Tests for Tunnel Construction
Field tests are conducted to investigate the suitability of the sites for tunnel construction. The necessary tests include:
Borings should be done about 15 m (50 feet) below the intended tunnel bottom, according to AASHTO (American Association of State Highway and Transportation Officials) recommendations. The limit of 15 m is used because drilling beyond the tunnel bottom value is frequently unnecessary. Many boring rounds would be required for most tunnel designs. The requirements for this information should determine the boring spacing.
To collect data on groundwater, piezometers ought to be deployed. The tunnel cross-section should be used to screen the piezometers. To get data on overburden aquifers, overburden piezometers ought to be put as well. This is necessary because, during construction, there are numerous instances where water leaks into the tunnel from overburdened aquifers.
iii. Geophysical Survey
On a few chosen borings, geophysical surveys are usually performed. Geophysical surveys can provide information on weak zones, density, porosity, elastic modulus, and Poisson’s ratio of rock. To conduct a geophysical survey, a seismic signal is sent, and the return signal is measured. If a large percentage of the signal returns, then the rock is hard and free of fractures. However, fractures and cracks in the rocks are most likely the cause if most of the signal is lost. There are single-hole and double-hole geophysical method variants available.
iv. Packer Tests
Packer tests are conducted to measure the permeability of rock layers. An inflatable object made of rubber is inserted into the rock and pressurized. The pressure drop during the process is measured. If the rock is highly fractured, a large pressure drop will be noted. Packer tests should be conducted at the tunnel roof elevation, the tunnel cross-section, and the tunnel bottom. Usually, during the boring program, engineers will be able to locate fractured rock zones. This can be achieved by analyzing the rock core logs and RQD values. Groundwater plays a major role during the construction phase of the tunnel. By conducting packer tests at different locations, engineers can assess the impact of groundwater.
2. Laboratory Test for Tunnel Construction
Laboratory tests are conducted to investigate the strength, hardness, swelling properties, tensile strength, and other properties of rock that may influence the design process. The laboratory tests include:
i. Unconfined Compressive Strength Test
The unconfined strength test is performed by placing a rock sample in the testing apparatus and crushing it to failure. This test provides the cohesion of intact rock. Unconfined compressive strength is a very important test since many correlations have been developed using unconfined compressive strength test values.
ii. Mineral Identification
Mineral identification becomes critical while burrowing through soft rock. Minerals like pyrite are signs of the presence of acidic or caustic groundwater. In this instance, the acidic groundwater may cause chemical deterioration of the tunnel concrete. Minerals that show signs of swelling in the presence of moisture include montmorillonite, chlorite, attapulgite, and illite.
iii. Petrographic Analysis
Petrography is the description and systematic classification of rocks by examination of thin sections. The petrographic analysis would include determining such properties as grain size, texture, color, fractures, and abnormalities.
3. Tri-Axial Tests
Tri-axial tests are conducted to investigate the shear strength of rock under confining pressure. Usually, tri-axial tests are done on soft rocks.
4. Tensile Strength Test
Rocks just above the tunnel roof will be subjected to tensile forces. Hence, it is important to investigate the tensile strength of the rock stratum, especially above tunnel roofs.
5. Hardness Tests
The Schmidt hammer test is the most popular method to investigate the hardness of rocks. The hardness of rock is important for tunnel boring machine selection.
6. Consolidation Tests
Consolidation tests are done to investigate the consolidation characteristics of soft rocks. Usually clay-shale, mudstone, and argillaceous soft rocks—can consolidate under pressure. Consolidation tests can be conducted to investigate the settlement properties of soft rock.
7. Swell Tests
Many soft rocks swell when exposed to moisture. Swelling is very common in rocks with clay minerals. Water tends to flow from areas of high stress to areas of low stress. When a tunnel is excavated, soil pressure in adjacent rocks is relieved. Rock units with relatively low-stress concentrations will absorb water and swell.
The construction of tunnels demands a meticulous approach to testing, safeguarding against potential challenges and ensuring the durability of these underground marvels. From geotechnical assessments to structural analyses, each test plays a crucial role in fortifying tunnels against the forces of nature and the test of time. As the demand for efficient transportation and infrastructure continues to rise, the importance of rigorous testing protocols in tunnel construction cannot be overstated. Embracing a holistic perspective on testing not only ensures the safety of the tunnels but also contributes to the overall success of the projects, marking them as enduring testaments to engineering prowess. With a steadfast commitment to these essential tests, the future of tunnel construction holds the promise of robust, resilient, and sustainable subterranean passages.
Rajapakse, R. Geotechnical Engineering Calculations and Rules of Thumb.