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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.

Table 3-5 In-situ Testing Methods Used in Soil (After FHWA, 2002a)
MethodProcedureApplicable Soil TypesApplicable Soil PropertiesLimitations / Remarks
Electric Cone Penetrometer (CPT)A cylindrical probe is hydraulically pushed vertically through the soil measuring the resistance at the conical tip of the probe and along the steel shaft; measurements typically recorded at 2 to 5 cm intervals Silts, sands, clays, and peatEstimation of soil type and detailed stratigraphy Sand: φ', Dr, σho'; Clay: Su, σp' No soil sample is obtained; The probe may become damaged if testing in gravelly soils is attempted; Test results not particularly good for estimating deformation characteristics
Piezocone Penetrometer (CPTu)Same as CPT; additionally, penetration porewater pressures are measured using a transducer and porous filter elementSilts, sands, clays, and peatSame as CPT, with additionally: Sand: uo / water table elevation Clay: σp', ch, kh OCRIf the filter element and ports are not completely saturated, the pore pressure response may be misleading; Compression and wear of a mid-face (u1) element will effect readings; Test results not particularly good for estimating deformation characteristics
Seismic CPTu (SCPTu)Same as CPTu; additionally, shear waves generated at the surface are recorded by a geophone at 1-m intervals throughout the profile for calculation of shear wave velocitySilts, sands, clays, and peatSame as CPTu, with additionally: Vs, Gmax, Emax, ρtot, eo First arrival times should be used for calculation of shear wave velocity (if first crossover times are used, the error in shear wave velocity will increase with depth)
Flat Plate Dilatometer (DMT)A flat plate is hydraulically pushed or driven through the soil to a desired depth; at approximately 20 to 30 cm intervals, the pressure required to expand a thin membrane is recorded; Two to three measurements are typically recorded at each depth.Silts, sands, clays, and peatEstimation of soil type and stratigraphy Total unit weight Sand: φ', E, Dr, mv Clays: σp', Ko, su, mv, E, ch, kh Membranes may become deformed if over-inflated; Deformed membranes will not provide accurate readings; Leaks in tubing or connections will lead to high readings; Good test for estimating deformation characteristics at small strains
Pre-bored Pressure meter (PMT)A borehole is drilled and the bottom is carefully prepared for insertion of the equipment; The pressure required to expand the cylindrical membrane to a certain volume or radial strain is recorded.Clays, silts, and peat; marginal response in some sands and gravelsE, G, mv, su Preparation of the borehole most important step to obtain good results; Good test for calculation of lateral deformation characteristics
Full Displacement Pressure meter (PMT) A cylindrical probe with a pressure meter attached behind a conical tip is hydraulically pushed through the soil and paused at select intervals for testing; The pressure required to expand the cylindrical membrane to a certain volume or radial strain is recorded Clays, silts, and peat E, G, mv, su Disturbance during advancement of the probe will lead to stiffer initial modulus and mask liftoff pressure (po); Good test for calculation of lateral deformation characteristics
Vane Shear Test (VST)A 4 blade vane is hydraulically pushed below the bottom of a borehole, then slowly rotated while the torque required to rotate the vane is recorded for calculation of peak undrained shear strength; The vane is rapidly rotated for 10 turns, and the torque required to fail the soil is recorded for calculation of remolded undrained shear strengthClays, Some silts and peats if undrained conditions can be assumed; not for use in granular soilssu, St, σp' Disturbance may occur in soft sensitive clays, reducing measured shear strength; Partial drainage may occur in fissured clays and silty materials, leading to errors in calculated strength; Rod friction needs to be accounted for in calculation of strength; Vane diameter and torque wrench capacity need to be properly sized for adequate measurements in various clay deposits

Symbols used in Table 3-5:

φ':Effective stress friction angleGmax:Small-strain shear modulus
Dr:Relative densityG:Shear modulus
σho':In-situ horizontal effective stressEmax:Small-strain Young's modulus
su:Undrained shear strengthE:Young's modulus
σp':Preconsolidation stressρtot:Total density
ch:Horizontal coefficient of consolidationeo:In-situ void ratio
kh:Horizontal hydraulic conductivitymv:Volumetric compressibility coefficient
OCR:Overconsolidation ratioKo:Coefficient of at-rest earth pressure
Vs:Shear wave velocitySt:Sensitivity
Table 3-6 Common in situ Test Methods for Rock (after USACE, 1997)
ParameterTest MethodProcedure / Limitations / Remarks
In situ StressHydraulic FracturingTypically conducted in vertical boreholes. A short segment of the hole is sealed off using a straddle packer. This is followed by the pressurization by pumping in water. The pressure is raised until the rock surrounding the hole fails in tension at a critical pressure. Following breakdown, the shut-in pressure, the lowest test-interval pressure at which the hydrofrac closes completely under the action of the stress acting normal to the hydrofractures. In a vertical test hole the hydrofractures are expected to be formed in vertical and perpendicular to the minimum horizontal stress.
OvercoringDrills a small diameter borehole and sets into it an instrument to respond to changes in diameter. Rock stresses are determined indirectly from measurements of the dimensional changes of a borehole, occurring when the rock volume surrounding the hole is isolated from the stresses in the host rock
Flat Jack TestThis method involves the use of flat hydraulic jacks, consisting of two plates of steel welded around their edges and a nipple for introducing oil into the intervening space. Flat jack is inserted into the slot, cemented in place, and pressurized. When the pins have been returned to the initial separation, the pressure in jack approximates the initial stress normal to the jack.
Modulus of DeformationPlate Bearing TestA relatively flat rock surface us sculptured and level with mortar to receive circular bearing plates 20 to 40 inches in diameter. Loading a rock surface and monitoring the resulting displacement. This is easily arranged in the underground gallery. The site may be selected carefully to exclude loose, highly fractured rock.
Borehole Dilatometer TestA borehole expansion experiment conducted with a rubber sleeve. The expansion of borehole is measured by the oil or gas flow into the sleeve as the pressure raised, or by potentiometers or linear variable differential transformers built inside the sleeve. One problem with borehole deformability test is that it affect a relatively small volume of rock and therefore contains an incomplete sample of the fracture system.
Flat Jack TestThis method involves the use of flat hydraulic jacks, consisting of two plates of steel welded around their edges and a nipple for introducing oil into the intervening space. Provide measurement points on the face of the rock and deep slot (reference points). Modulus of deformation could be calculated from the measured pin displacements.
Radial jacking testLoads are applied to the circumference of a tunnel by a series of jacks reacting against circular steel ring members. This test allows the direction of load to be varied according to the plan for pressuring the jacks.
Pressuremeter The pressure required to expand the cylindrical membrane to a certain volume or radial strain is recorded in a borehole. It is applicable for soft rocks.
Dynamic MeasurementThe velocity of stress waves is measured in the field. The wave velocity can be measured by swinging a sledgehammer against an outcrop and observing the travel time to a geophone standing on the rock at a distance of up to about 150 ft. The stress loadings sent through the rock by this method are small and transient. Most rock mass departs significantly from the ideal materials, consequently, elastic properties calculated from these equations are often considerably larger than elastic properties calculated from static loading tests, particularly in the case of fractured rocks.
Imaging and DiscontinuitiesAcoustic TeleviewingAcoustic Televiewers (ATV) produce images of the borehole wall based on the amplitude and travel time of acoustic signals reflected from the borehole wall. A portion of the reflected energy is lost in voids or fractures, producing dark bands on the amplitude log. Travel time measurements allow reconstruction of the borehole shape, making it possible to generate a 3-D representation of a borehole.
Borehole Video TeleviewingThe Borehole Video System (BVS) is lowered down boreholes to inspect the geology and structural integrity. The camera view of fractures and voids in boreholes provides information.
Permeability (Section 3.5.6)Slug TestSlug tests are applicable to a wide range of geologic settings as well as small-diameter piezometers or observation wells, and in areas of low permeability where it would be difficult to conduct a pumping test. A slug test is performed by injecting or withdrawing a known volume of water or air from a well and measuring the aquifer’s response by the rate at which the water level returns to equilibrium. Permeability values derived relate primarily to the horizontal conductivity. Slug tests have a much smaller zone of infiltration than pumping tests, and thus are only reliable at a much smaller scale.
Packer TestIt is conducted by pumping water at a constant pressure into a test section of a borehole and measuring the flow rate. Borehole test sections are sealed off by packers, with the use of one or two packers being the most widely used techniques. The test is rapid and simple to conduct, and by performing tests within intervals along the entire length of a borehole, a permeability profile can be obtained. The limitation of the test is to affect a relatively small volume of the surrounding medium, because frictional losses in the immediate vicinity of the test section are normally extremely large.
Pumping TestsIn a pumping test, water is pumped from a well normally at a constant rate over a certain time period, and the drawdown of the water table or piezometric head is measured in the well and in piezometers or observation wells in the vicinity. Since pumping tests involve large volumes of the rock mass, they have the advantage of averaging the effects of the inherent discontinuities. Most classical solutions for pump test data are based on the assumptions that the aquifers are homogeneous and isotropic, and that the flow is governed by Darcy's law. The major disadvantage is the period of time required to perform a test. Test durations of one week or longer are not unusual when attempting to approach steady-state flow conditions. Additionally, large diameter boreholes or wells are required since the majority of the conditions encountered require the use of a downhole pump.
Table 3-7 Applications for Geophysical Testing Methods (after AASHTO, 1988)
Geological Conditions to be InvestigatedUseful Geophysical Techniques
SurfaceSubsurface
Stratified rock and soil units (depth and thickness of layers)Seismic RefractionSeismic Wave Propagation
Depth to BedrockSeismic Refraction Electrical Resistivity Ground Penetrating RadarSeismic Wave Propagation
Depth to Groundwater TableSeismic Refraction Electrical Resistivity Ground Penetrating Radar 
Location of Highly Fractured Rock and/or Fault ZoneElectrical ResistivityBorehole TV Camera
Bedrock Topography (troughs, pinnacles, fault scarp)Seismic Refraction Gravity 
Location of Planar Igneous IntrusionsGravity, Magnetics Seismic Refraction 
Solution CavitiesElectrical Resistivity Ground Penetrating Radar GravityBorehole TV Camera
Isolated Pods of Sand, Gravel, or Organic MaterialElectrical ResistivitySeismic Wave Propagation
Permeable Rock and Soil UnitsElectrical ResistivitySeismic Wave Propagation
Topography of Lake, Bay or River BottomsSeismic Reflection (acoustic sounding) 
Stratigraphy of Lake, Bay or River Bottom SedimentsSeismic Reflection (acoustic sounding) 
Lateral Changes in Lithology of Rock and Soil UnitsSeismic Refraction Electrical Resistivity 
Table 3-8 Geophysical Testing Methods
MethodProcedureLimitations / Remarks
Seismic RefractionDetectors (geophones) are positioned on the ground surface at increasing distance from a seismic impulse source, also at the ground surface. The time required for the seismic impulse to reach each geophone is recorded.Distance between closest and furthest geophone must be 3 to 4 times the depth to be investigated. Reflection from hard layer may prevent identification of deeper layers. Other conditions affecting interpretation: insufficient density contrast between layers; presence of low-density layer; irregular surface topography.
Seismic ReflectionPerformed for offshore applications from a boat using an energy source and receiver at the water surface. The travel time for the seismic wave to reach the receiver is recorded and analyzed.The position and direction of the boat must be accurately determined by GPS or other suitable method. Reflection from hard layer may prevent identification of deeper layers.
Electrical Resistivity /ConductivityWenner Four Electrode Method is type most commonly used test in the U.S. Four electrodes are placed partially in the soil, in line and equidistant from each other. A low magnitude current is passed between the outer electrodes, and the resulting potential drop is measured at the inner electrodes. A number of traverses are used, and electrode spacing is varied to better define changes in deposits and layering.Results may be influenced by presence of underground obstructions, such as pipelines, tanks, etc.
Seismic Wave Propagation:
Cross-HoleAt least 2 boreholes are required: a source borehole within which a seismic pulse is generated, and a receiver borehole in which a geophone records generated compression and shear waves. For increased accuracy additional receiver boreholes are used.Receivers must be properly oriented and securely in contact with the side of the borehole. Boreholes deeper than about 30 ft should be surveyed using an inclinometer or other device to determine the travel distance between holes.
Up-Hole or Down-HolePerformed in a single borehole. In up-hole method, a sensor is placed at the ground surface and shear waves are generated at various depths in the borehole. In down-hole method, seismic wave is generated at the surface and one or more sensors are placed at different depths within the hole.Data limited to area in immediate vicinity of the borehole.
Parallel SeismicUsed to determine the depth of existing foundations, an impulse wave is generated at the top of the foundation, and a sensor in an adjacent borehole records arrival of the stress wave at set depth increments.Requires access to top of foundation.
Ground Penetrating RadarRepetitive electromagnetic impulses are generated at the ground surface and the travel time of the reflected pulses to return to the transmitter are recorded.The presence of a clay layer may mask features below that layer.
GravityA sensitive gravimeter is used at the ground surface to measure variations in the local gravitational field in the earth caused by changes in material density or cavities.May not identify small changes in density. May be influenced by nearby surface or subsurface features, such as mountains, solution cavities, buried valleys, etc. not directly in area of interest.
MagneticsMagnetic surveys can be performed using either ground-based or airborne magnetometers. With ground equipment, measurements of changes in the earth's magnetic field are taken along an established survey line.Monitoring locations should not be located near man-made objects that can change the magnitude of the earth's magnetic field (pipelines, buildings, etc.). Corrections need to be made for diurnal variations in the earth's magnetic field.

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.

Table 3-9 Common Laboratory Tests for Rock (after USACE 1997)
ParameterTest Method
Index properties
  • Density
  • Porosity
  • Moisture Content
  • Slake Durability
  • Swelling Index
  • Point Load Index
  • Hardness
  • Abrasivity
Strength
  • Uniaxial compressive strength
  • Triaxial compressive strength
  • Tensile strength (Brazilian) Shear strength of joints
Deformability
  • Young's modulus Poisson's ratio
Time dependence
  • Creep characteristics
Permeability
  • Coefficient of permeability
Mineralogy and grain sizes
  • Thin-sections analysis
  • Differential thermal analysis
  • X-ray diffraction

Rock 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).

Packer Pressure Test Apparatus for Determining the Permeability of Rock (a) Schematic Diagram; (b) Detail of Packer Unit

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.

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Updated: 06/19/2013
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