Technical Manual for Design and Construction of Road Tunnels - Civil Elements

Chapter 3 - Geotechnical Investigations

3.5.4 Field Testing Techniques (Pre-Construction)

Field testing for subsurface investigations includes two general categories of tests:

• a) In situ tests
• b) Geophysical testing

In situ tests are used to directly obtain field measurements of useful soil and rock engineering properties. Geophysical tests, the second general category of field tests, are indirect methods of exploration in which changes in certain physical characteristics such as magnetism, density, electrical resistivity, elasticity, or a combination of these are used as an aid in developing subsurface information. There are times that two testing methods can be performed from a same apparatus, such as using seismic CPT

3.5.4.1 In situ Testing

In situ tests are used to directly obtain field measurements of useful soil and rock engineering properties. In soil, in situ testing include both index type tests, such as the Standard Penetration Test (SPT) and tests that determine the physical properties of the ground, such as shear strength from cone penetration Tests (CPT) and ground deformation properties from pressure meter tests (PMT). In situ test methods in soil commonly used in the U.S. and their applications and limitations are summarized in Table 3-5.

Common in situ tests used in rock for tunnel applications are listed in Table 3-6. One significant property of interest in rock is its in situ stress condition. Horizontal stresses of geological origin are often locked within the rock masses, resulted in a stress ratio (K) often higher than the number predicted by elastic theory. Depending on the size and orientation of the tunneling, high horizontal stresses may produce favorable compression in support and confinement, or induce popping or failure during and after excavation. Principally, two different general methods are common to be employed to measure the in situ stress condition: hydraulic fracturing and overcoring. Note that in situ stress can only be measured accurately within a fair or better rock condition. However, since weak rocks are unable to support large deviatoric stress differences, the lateral and vertical stresses tend to equalize over geologic time.

3.5.4.2 Geophysical Testing

Geophysical tests are indirect methods of exploration in which changes in certain physical characteristics such as magnetism, density, electrical resistivity, elasticity, or a combination of these are used as an aid in developing subsurface information. Geophysical methods provide an expeditious and economical means of supplementing information obtained by direct exploratory methods, such as borings, test pits and in situ testing; identifying local anomalies that might not be identified by other methods of exploration; and defining strata boundaries between widely spaced borings for more realistic prediction of subsurface profiles. Typical uses of geophysical tests include determination of the top of bedrock, the ripability of rock, the depth to groundwater, the limits of organic deposits, the presence of voids, the location and depth of utilities, the location and depth of existing foundations, and the location and depth of other obstruction, to note just a few. In addition, geophysical testing can also obtain stiffness and dynamic properties which are required for numerical analysis.

Geophysical testing can be performed on the surface, in boreholes (down or cross hole), or in front of the TBM during construction. Typical applications for geophysical tests are presented in Table 3-7.

Table 3-8 briefly summarizes the procedures used to perform these geophysical tests, and notes their limitations.

Symbols used in Table 3-5:

It is important to note that the data from geophysical exploration must always be correlated with information from direct methods of exploration that allow visual examination of the subsurface materials, direct measurement of groundwater levels, and testing of physical samples of soil and rock. Direct methods of exploration provide valuable information that can assist not only in the interpretation of the geophysical data, but also for extrapolating the inferred ground conditions to areas not investigated by borings. Conversely, the geophysical data can help determine appropriate locations for borings and test pits to further investigate any anomalies that are found. Readers are also referred to FHWA publication "Application of Geophysical Methods to Highway Related Problems" for more detailed information.

3.5.5 Laboratory Testing

Detailed laboratory testing is required to obtain accurate information for design and modeling purposes.

Soil Testing Detailed soil laboratory testing is required to obtain accurate information including classification, characteristics, stiffness, strength, etc. for design and modeling purposes. Testing are performed on selected representative samples (disturbed and undisturbed) in accordance with ASTM standards. Table 3-9 shows common soil laboratory testing for tunnel design purposes.

Rock Testing Standard rock testing evaluate physical properties of the rock included density and mineralogy (thin-section analysis). The mechanical properties of the intact rock core included uniaxial compressive strength, tensile strength, static and dynamic elastic constants, hardness, and abrasitivity indices.

In addition, specialized tests for assessing TBM performance rates are required including three drillability and boreability testing, namely, Drilling Rate Index (DRI), Bit Wear Index (BWI), and Cutter Life Index (CLI). Table 3-9 summarizes common rock laboratory testing for tunnel design purposes.

It is desirable to preserve the rock cores retrieved from the field properly for years until the construction is completed and disputes/claims are settled. Common practice is to photograph the rock cores in core boxes and possibly scan the core samples for review by designers and contractors. Figure 3-8 shows a roc core scanning equipment and result.

Figure 3-8 Rock Core Scanning Equipment and Result

3.5.6 Groundwater Investigation

Groundwater is a major factor for all types of projects, but for tunnels groundwater is a particularly critical issue since it may not only represent a large percentage of the loading on the final tunnel lining, but also it largely determines ground behavior and stability for soft ground tunnels; the inflow into rock tunnels; the method and equipment selected for tunnel construction; and the long-term performance of the completed structure. Accordingly, for tunnel projects, special attention must be given to defining the groundwater regime, aquifers, and sources of water, any perched or artesian conditions, water quality and temperature, depth to groundwater, and the permeability of the various materials that may be encountered during tunneling. Related considerations include the potential impact of groundwater lowering on settlement of overlying and nearby structures, utilities and other facilities; other influences of dewatering on existing structures (e.g., accelerated deterioration of exposed timber piles); pumping volumes during construction; decontamination/treatment measures for water discharged from pumping; migration of existing soil and groundwater contaminants due to dewatering; the potential impact on water supply aquifers; and seepage into the completed tunnel; to note just a few:

Groundwater investigations typically include most or all of the following elements:

• Observation of groundwater levels in boreholes
• Assessment of soil moisture changes in the boreholes
• Groundwater sampling for environmental testing
• Installation of groundwater observation wells and piezometers
• Borehole permeability tests (rising, falling and constant head tests; packer tests, etc.)
• Geophysical testing (see Section 3.5.4)
• Pumping tests

During subsurface investigation drilling and coring, it is particularly important for the inspector to note and document any groundwater related observations made during drilling or during interruptions to the work when the borehole has been left undisturbed. Even seemingly minor observations may have an important influence on tunnel design and ground behavior during construction.

Groundwater observation wells are used to more accurately determine and monitor the static water table. Since observation wells are generally not isolated within an individual zone or stratum they provide only a general indication of the groundwater table, and are therefore more suitable for sites with generally uniform subsurface conditions. In stratified soils with two or more aquifers, water pressures may vary considerably with depth. For such variable conditions, it is generally more appropriate to use piezometers. Piezometers have seals that isolate the screens or sensors within a specific zone or layer within the soil profile, providing a measurement of the water pressure within that zone. Readers are referred to Chapter 15 Geotechnical and Structural Instrumentation for detailed illustrations and descriptions about the wells and piezometers.

Observation wells and piezometers should be monitored periodically over a prolonged period of time to provide information on seasonal variations in groundwater levels. Monitoring during construction provides important information on the influence of tunneling on groundwater levels, forming an essential component of construction control and any protection program for existing structures and facilities. Local and state jurisdictions may impose specific requirements for permanent observation wells and piezometers, for documenting both temporary and permanent installations, and for closure of these installations.

3.5.6.1 Borehole Permeability Testing

Borehole permeability tests provide a low cost means for assessing the permeability of soil and rock. The principal types of tests include falling head, rising head and constant head tests in soil, and packer tests in rock, as described below. Additional information regarding the details and procedures used for performing and interpreting these borehole permeability tests are presented by FHWA (2002b). Borehole tests are particularly beneficial in sands and gravels since samples of such materials would be too disturbed to use for laboratory permeability tests. A major limitation of these tests, however, is that they assess soil conditions only in the immediate vicinity of the borehole, and the results do not reflect the influence of water recharge sources or soil stratification over a larger area.

Borehole permeability tests are performed intermittently as the borehole is advanced. Holes in which permeability tests will be performed should be drilled with water to avoid the formation of a filter cake on the sides of the borehole from drilling slurry. Also, prior to performing the permeability test the hole should be flushed with clear water until all sediments are removed from the hole (but not so much as would be done to establish a water well).

In soil, either rising head or falling head tests would be appropriate if the permeability is low enough to permit accurate determination of water level versus time. In the falling head test, where the flow is from the hole to the surrounding soil, there is risk of clogging of the soil pores by sediments in the test water. In the rising head tests, where water flows from the surrounding soil into the hole, there is a risk of the soil along the test length becoming loosened or quick if the seepage gradient is too large. If a rising head test is used, the hole should be sounded at the end of the test to determine if the hole has collapsed or heaved. Generally, the rising head test is the preferred test method. However, in cases where the permeability is so high as to preclude accurate measurement of the rising or falling water level, the constant head test should be used.

Pressure, or "Packer," tests are performed in rock by forcing water under pressure into the rock surrounding the borehole. Packer tests determine the apparent permeability of the rock mass, and also provide a qualitative assessment of rock quality. These tests can also be used before and after grouting to assess the effectiveness of grouting on rock permeability and the strength of the rock mass. The test is performed by selecting a length of borehole for testing, then inflating a cylindrical rubber sleeve ("packer") at the top of the test zone to isolate the section of borehole being tested. Packer testing can thus be performed intermittently as the borehole is advanced. Alternatively, testing can be performed at multiple levels in a completed borehole by using a double packer system in which packers are positioned and inflated at both the top and bottom of the zone being tested, as illustrated in Figure 3-9. Once the packer is inflated to seal off the test section, water is pumped under pressure to the test zone, while the time and volume of water pumped at different pressures are recorded. Guidelines for performing and evaluating packer tests are presented by Mayne et al. (FHWA, 2002b), and by Lowe and Zaccheo (1991).

3.5.6.2 Pumping Tests

Continuous pumping tests are used to determine the water yield of individual wells and the permeability of subsurface materials in situ over an extended area. These data provide useful information for predicting inflows during tunneling; the quantity of water that may need to be pumped to lower groundwater levels; and the radius of influence for pumping operations; among others. The test consists of pumping water from a well or borehole and observing the effect on the water table with distance and time by measuring the water levels in the hole being pumped as well as in an array of observation wells at various distances around the pumping well. The depth of the test well will depend on the depth and thickness of the strata being investigated, and the number, location and depth of the observation wells or piezometers will depend on the anticipated shape of the groundwater surface after drawdown. Guidelines for performing and evaluating pumping tests are presented by Mayne et al. (FHWA, 2002b).

Figure 3-9 Packer Pressure Test Apparatus for Determining the Permeability of Rock (a) Schematic Diagram; (b) Detail of Packer Unit (Lowe and Zaccheo, 1991)

3.6 Environmental issues

Although tunnels are generally considered environmentally-friendly structures, certain short-term environmental impacts during construction are unavoidable. Long-term impacts from the tunnel itself, and from portals, vent shafts and approaches on local communities, historic sites, wetlands, and other aesthetically, environmentally, and ecologically sensitive areas must be identified and investigated thoroughly during the project planning and feasibility stages, and appropriately addressed in environmental studies and design. Early investigation and resolution of environmental issues is an essential objective for any underground project since unanticipated conditions discovered later during design or construction could potentially jeopardize the project.

The specific environmental data needed for a particular underground project very much depend on the geologic and geographic environment and the functional requirement of the underground facility. Some common issues can be stated, however, and are identified below in the form of a checklist:

• Existing infrastructure, and obstacles underground and above
• Surface structures within area of influence
• Land ownership and uses (public and private)
• Ecosystem habitat impacts
• Contaminated ground or groundwater
• Long-term impacts to groundwater levels, aquifers and water quality
• Control of runoff and erosion during construction
• Naturally gassy ground, or groundwater with deleterious chemistry
• Access constraints for potential work sites and transport routes
• Sites for muck transport and disposal
• Noise and vibrations from construction operations, and from future traffic at approaches to the completed tunnel
• Air quality during construction, and at portals, vent shafts and approaches of the completed tunnel
• Maintenance of vehicular traffic and transit lines during construction
• Maintenance of utilities and other existing facilities during construction
• Access to residential and commercial properties
• Pest control during construction
• Long-term community impacts
• Long-term traffic impacts
• Temporary and permanent easements
• Tunnel fire life safety and security
• Legal and environmental constraints, enumerated in environmental statements or reports, or elsewhere

3.7 Seismicity

The release of energy from earthquakes sends seismic acceleration waves traveling through the ground. Such transient dynamic loading instantaneously increases the shear stresses in the ground and decreases the volume of voids within the material which leads to an increase in the pressure of fluids (water) in pores and fractures. Thus, shear forces increase and the frictional forces that resist them decrease. Other factors also can affect the response of the ground during earthquakes.

• Distance of the seismic source from the project site.
• Magnitude of the seismic accelerations.
• Earthquake duration.
• Subsurface profile.
• Dynamic characteristics and strengths of the materials affected.

In addition to the distance of the seismic source to the project site, and the design (anticipated) time history, duration and magnitude of the bedrock earthquake, the subsurface soil profile can have a profound effect on earthquake ground motions including the intensity, frequency content, and duration of earthquake shaking. Amplification of peak bedrock acceleration by a factor of four or more has been attributed to the response of the local soil profile to the bedrock ground motions (Kavazanjian et.al, 1998).

Chapter 13 discusses the seismic considerations for the design of underground structures and the parameters required.

The ground accelerations associated with seismic events can induce significant inertial forces that may lead to instability and permanent deformations (both vertically and laterally) of tunnels and portal slopes. In addition, during strong earthquake shaking, saturated cohesionless soils may experience a sudden loss of strength and stiffness, sometime resulting in loss of bearing capacity, large permanent lateral displacements, landslides, and/or seismic settlement of the ground. Liquefaction beneath and in the vicinity of a portal slope can have severe consequences since global instability in forms of excessive lateral displacement or lateral spreading failure may occur as a result. Readers are referred to FHWA publication "Geotechnical Earthquake Engineering" by Kavazanjian, et al. (1998) for a detailed discussion of this topic.

3.8 Additional Investigations During Construction

3.8.1 General

For tunneling projects it is generally essential to perform additional subsurface investigations and ground characterization during construction. Such construction phase investigations serve a number of important functions, providing information for:

• Contractor design and installation of temporary works
• Further defining anomalies and unanticipated conditions identified after the start of construction
• Documenting existing ground conditions for comparison with established baseline conditions, thereby forming the basis of any cost adjustments due to differing site conditions
• Assessing ground and groundwater conditions in advance of the tunnel heading to reduce risks and improve the efficiency of tunneling operations
• Determining the initial support system to be installed, and the locations where the support system can be changed
• Assessing the response of the ground and existing structures and utilities to tunneling operations
• Assessing the groundwater table response to dewatering and tunneling operations
• Determining the location and depth of existing utilities and other underground facilities

A typical construction phase investigation program would likely include some or all of the following elements:

• Subsurface investigation borings and probings from the ground surface
• Test pits
• Additional groundwater observation wells and/or piezometers
• Additional laboratory testing of soil and rock samples
• Geologic mapping of the exposed tunnel face
• Geotechnical instrumentation
• Probing in advance of the tunnel heading from the face of the tunnel
• Pilot Tunnels
• Environmental testing of soil and groundwater samples suspected to be contaminated or otherwise harmful

Some of the above investigation elements, such as geotechnical instrumentation, may be identified as requirements of the contract documents, while others, such as additional exploratory borings, may be left to the discretion of the contractor for their benefit and convenience. Tunnel face mapping and groundwater monitoring should be required elements for any tunnel project since the information obtained from these records will form the basis for evaluating the merits of potential differing site condition claims.

3.8.2 Geologic Face Mapping

With open-face tunneling methods, including the sequential excavation method (SEM), open-face tunneling shield in soil, and the drill-and-blast method in rock, all or a large portion of the tunnel face will be exposed, allowing a visual assessment of the existing ground and groundwater conditions. In such cases, the exposed face conditions are documented in cross-section sketches (face mapping) drawn at frequent intervals as the tunnel advances. Information typically included in these face maps include the station location for the cross-section; the date and time the face mapping was prepared; the name of the individual who prepared the face map; classification of each type of material observed; the location of interface boundaries between these materials; rock jointing including orientation of principal joints and joint descriptions; shear zones; observed seepage conditions and their approximate locations on the face; observed ground behavior noting particularly the location of any instability or squeezing material at the face; the location of any boulders, piling or other obstructions; the location of any grouted or cemented material; and any other significant observations. In rock tunnels where the perimeter rock is left exposed, sketches presenting similar information can be prepared for the tunnel walls and roof. All mapping should be prepared by a geologist or geological engineer knowledgeable of tunneling and with soil and rock classification.

The face maps can be used to accurately document conditions exposed during tunneling, and to develop a detailed profile of subsurface conditions along the tunnel horizon. However, there are limitations and considerable uncertainty in any extrapolation of the observed conditions beyond the perimeter of the tunnel.

When used in conjunction with nearby subsurface investigation data and geotechnical instrumentation records, the face maps may be used to develop general correlations between ground displacement, geological conditions and other factors (depth of tunnel, groundwater conditions, etc.).

3.8.3 Geotechnical Instrumentation

Geotechnical instrumentation is used during construction to monitor ground and structure displacements, surface settlement above and near the tunnel, deformation of the initial tunnel supports and final lining, groundwater levels, loads in structural elements of the excavation support systems, and ground and structure vibrations, among others. Such instrumentation is a key element of any program for maintenance and protection of existing structures and facilities. In addition, it provides quantitative information for assessing tunneling procedures during the course of construction, and can be used to trigger modifications to tunneling procedures in a timely manner to reduce the impacts of construction. Instrumentation is also used to monitor the deformation and stability of the tunnel opening, to assess the adequacy of the initial tunnel support systems and the methods and sequencing of tunneling, particularly for tunnels constructed by the Sequential Excavation Method (SEM) and tunnels in shear zones or squeezing ground. Chapter 16 provides a further discussion of geotechnical instrumentation for tunnel projects.

3.8.4 Probing

If applicable, such as for SEM and hard rock tunneling projects, probing ahead of the tunnel face is used to determine general ground conditions in advance of excavation, and to identify and relieve water pressures in any localized zones of water-bearing soils or rock joints. For tunnels constructed by SEM, probing also provides an early indication of the type of ground supports that may be needed as the excavation progresses. Advantages of probing are a) it reduces the risks and hazards associated with tunneling, b) it provides continuous site investigation data directly along the path of the tunnel, c) it provides information directly ahead of the tunnel excavation, allowing focus on ground conditions of most immediate concern to tunneling operations, and d) it can be performed quickly, at relatively low cost. However, disadvantages include a) the risk of missing important features by drilling only a limited number of probe holes from the face, and b) the interruption to tunneling operations during probing. Probing from within the tunnel must be considered as a supplementary investigation method, to be used in conjunction with subsurface investigation data obtained during other phases of the project.

Probing typically consists of drilling horizontally from the tunnel heading by percussion drilling or rotary drilling methods. Coring can be used for probing in rock, but is uncommon due to the greater time needed for coring. Cuttings from the probe holes are visually examined and classified, and assessed for potential impacts to tunnel excavation and support procedures. In rock, borehole cameras can be used to better assess rock quality, orientation of discontinuities, and the presence of shear zones and other important features..

The length of the probe holes can vary considerably, ranging from just 3 or 4 times the length of each excavation stage (round), to hundreds of feet. Shorter holes can be drilled more quickly, allowing them to be performed as part of the normal excavation cycle. However, longer holes, performed less frequently, may result is fewer interruptions to tunneling operations.

3.8.5 Pilot Tunnels

Pilot tunnels (and shorter exploratory adits) are small size tunnels (typically at least 6.5 ft by 6.5 ft in size) that are occasionally used for large size rock tunnels in complex geological conditions. Pilot tunnels, when used, are typically performed in a separate contract in advance of the main tunnel contract to provide prospective bidders a clearer understanding of the ground conditions that will be encountered. Although pilot tunnels are a very costly method of exploration, they may result in considerable financial benefits to the client by a) producing bids for the main tunnel work that have much lower contingency fees, and b) reducing the number and magnitude of differing site condition claims during construction.

In addition to providing bidders the opportunity to directly observe and assess existing rock conditions, pilot tunnels also offer other significant advantages, including a) more complete and reliable information for design of both initial tunnel supports and final lining, if any, b) access for performing in situ testing of the rock along the proposed tunnel, c) information for specifying and selecting appropriate methods of construction and tunneling equipment, d) an effective means of pre-draining groundwater, and more confidently determining short-term and long-term groundwater control measures, e) an effective means for identifying and venting gassy ground conditions, e) a means for testing and evaluating potential tunneling methods and equipment, and f) access for installation of some of the initial supports (typically in the crown area of the tunnel) in advance of the main tunnel excavation. Consideration can also be given to locating the pilot tunnel adjacent to the proposed tunnel, using the pilot tunnel for emergency egress, tunnel drainage, tunnel ventilation, or other purposes for the completed project.

3.9 Geospatial Data Management System

Geographic Information System (GIS) is designed for managing a large quantity of data in a complex environment, and is a capable tool used for decision making, planning, design, construction and program management. It can accept all types of data, such as digital, text, graphic, tabular, imagery, etc., and organize these data in a series of interrelated layers for ready recovery. Information stored in the system can be selectively retrieved, compared, overlain on other data, composite with several other data layers, updated, removed, revised, plotted, transmitted, etc.

GIS can provide a means to enter and quickly retrieve a wide range of utility information, including their location, elevation, type, size, date of construction and repair, ownership, right-of-way, etc. This information is stored in dedicated data layers, and can be readily accessed to display or plot both technical and demographic information.

Typical information that could be input to a GIS database for a tunnel project may include street grids; topographic data; property lines; right-of-way limits; existing building locations, type of construction, heights, basement elevations, building condition, etc.; proposed tunnel alignment and profile information; buried abandoned foundations and other underground obstructions; alignment and elevations for existing tunnels; proposed structures, including portals, shafts, ramps, buildings, etc.; utility line layout and elevations, vault locations and depths; boring logs and other subsurface investigation information; geophysical data; inferred surfaces for various soil and rock layers; estimated groundwater surface; areas of identified soil and groundwater contamination; and any other physical elements of jurisdictional boundaries within the vicinity of the project.