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

Chapter 3 - Geotechnical Investigations

3.4 Geologic Mapping

After collecting and reviewing existing geologic maps, aerial photos, references, and the results of a preliminary site reconnaissance, surface geologic mapping of available rock outcrops should be performed by an experienced engineering geologist to obtain detailed, site-specific information on rock quality and structure. Geologic mapping collects local, detailed geologic data systematically, and is used to characterize and document the condition of rock mass or outcrop for rock mass classification (Chapter 6) such as:

• Discontinuity type
• Discontinuity orientation
• Discontinuity infilling
• Discontinuity spacing
• Discontinuity persistence
• Weathering

The International Society of Rock Mechanics (ISRM) (www.isrm.net) has suggested quantitative measures for describing discontinuities (ISRM 1981). It provides standard descriptions for factors such as persistence, roughness, wall strength, aperture, filling, seepage, and block size. Where necessary, it gives suggested methods for measuring these parameters so that the discontinuity can be characterized in a constant manner that allows comparison.

By interpreting and extrapolating all these data, the geologist should have a better understanding of the rock conditions likely to be present along the proposed tunnel and at the proposed portal and shaft excavations. The collected mapping data can be used in stereographic projections for statistical analysis using appropriate computer software (e.g., DIPS), in addition to the data obtained from the subsurface investigations.

In addition, the following surface features should also be observed and documented during the geologic mapping program:

• Slides, new or old, particularly in proposed portal and shaft areas
• Faults
• Rock weathering
• Sinkholes and karstic terrain
• Groundwater springs
• Volcanic activity
• Anhydrite, gypsum, pyrite, or swelling shales
• Stress relief cracks
• Presence of talus or boulders
• Thermal water (heat) and gas

The mapping data will also help in targeting subsurface investigation borings and in situ testing in areas of observed variability and anomalies. Section 4 of AASHTO Subsurface Investigations Manual (1988) provides details of commonly used field geologic mapping techniques and procedures.

Geologic mapping during and after tunnel excavation is briefly discussed in Section 3.8. For details of in-tunnel peripheral geologic mapping refer to US Army Corps of Engineers - Engineering Manual EM-1110-1-1804 for Geotechnical Investigations (USACE, Latest).

3.5 Subsurface Investigations

3.5.1 General

Ground conditions including geological, geotechnical, and hydrological conditions, have a major impact on the planning, design, construction and cost of a road tunnel, and often determine its feasibility and final route. Fundamentally, subsurface investigation is the most important type of investigations to obtain ground conditions, as it is the principal means for:

• Defining the subsurface profile (i.e. stratigraphy, structure, and principal soil and rock types)(Figure 3-4)
• Determining soil and rock material properties and mass characteristics;
• Identify geological anomalies, fault zones and other hazards (squeezing soils, methane gas, etc.)
• Defining hydrogeological conditions (groundwater levels, aquifers, hydrostatic pressures, etc.); and
• Identifying potential construction risks (boulders, etc.).

Click image to enlarge.

Figure 3-4 Cumberland Gap Tunnel Geological Profile

Subsurface investigations typically consist of borings, sampling, in situ testing, geophysical investigations, and laboratory material testing. The principal purposes of these investigation techniques are summarized below:

• Borings are used to identify the subsurface stratigraphy, and to obtain disturbed and undisturbed samples for visual classification and laboratory testing;
• In situ tests are commonly used to obtain useful engineering and index properties by testing the material in place to avoid the disturbance inevitably caused by sampling, transportation and handling of samples retrieved from boreholes; in situ tests can also aid in defining stratigraphy;
• Geophysical tests quickly and economically obtain subsurface information (stratigraphy and general engineering characteristics) over a large area to help define stratigraphy and to identify appropriate locations for performing borings; and
• Laboratory testing provides a wide variety of engineering properties and index properties from representative soil samples and rock core retrieved from the borings.

Unlike other highway structures, the ground surrounding a tunnel can act as a supporting mechanism, or loading mechanism, or both, depending on the nature of the ground, the tunnel size, and the method and sequence of constructing the tunnel. Thus, for tunnel designers and contractors, the rock or soil surrounding a tunnel is a construction material, just as important as the concrete and steel used on the job.

Therefore, although the above subsurface investigative techniques are similar (or identical) to the ones used for foundation design as specified in Section 10 of AASHTO 2006 Interim and in accordance with appropriate ASTM or AASHTO standards, the geological and geotechnical focuses for underground designs and constructions can be vastly different.

In addition to typical geotechnical, geological, and geo-hydrological data, subsurface investigation for a tunnel project must consider the unique needs for different tunneling methods, i.e. cut-and-cover, drill-and-blast, bored, sequential excavation, and immersed. Table 3-2 shows other special considerations for various tunneling methods.

As discussed in Section 3.1.1, subsurface investigations must be performed in phases to better economize the program. Nonetheless, they are primarily performed during the design stage of the project, with much of the work typically concentrated in the preliminary design phase of a project. These investigations provide factual information about the distribution and engineering characteristics of soil, rock and groundwater at a site, allowing an understanding of the existing conditions sufficient for developing an economical design, determining a reliable construction cost estimate, and reducing the risks of construction. The specific scope and extent of the investigation must be appropriate for the size of the project and the complexity of the existing geologic conditions; must consider budgetary constraints; and must be consistent with the level of risk considered acceptable to the client. To ensure the collected data can be analyzed correctly throughout the project, the project coordinate system and vertical datum should be established early on and the boring and testing locations must be surveyed, at least by hand-held GPS equipment. Photographs of the locations should be maintained as well.

Since unanticipated ground conditions are most often the reason for costly delays, claims and disputes during tunnel construction, a project with a more thorough subsurface investigation program would likely have fewer problems and lower final cost. Therefore, ideally, the extent of an exploration program should be based on specific project requirements and complexity, rather than strict budget limits. However, for most road tunnels, especially tunnels in mountainous areas or for water crossings, the cost for a comprehensive subsurface investigation may be prohibitive. The challenge to geotechnical professionals is to develop an adequate and diligent subsurface investigation program that can improve the predictability of ground conditions within a reasonable budget and acceptable level of risk. It is important that the involved parties have a common understanding of the limitations of geotechnical investigations, and be aware of the inevitable risk of not being able to completely define existing geological conditions. Special considerations for various geological conditions are summarized in Table 3-3 (Bickel, et al., 1996).

A general approach to control the cost of subsurface investigations while obtaining the information necessary for design and construction would include a) phasing the investigation, as discussed in Section 3.1.1, to better match and limit the scope of the investigation to the specific needs for each phase of the project, and b) utilizing existing information and the results of geologic mapping and geophysical testing to more effectively select locations for investigation. Emphasis can be placed first on defining the local geology, and then on increasingly greater detailed characterization of the subsurface conditions and predicted ground behavior. Also, subsurface investigation programs need to be flexible and should include an appropriate level of contingency funds to further assess unexpected conditions and issues that may be exposed during the planned program. Failure to resolve these issues early may lead to costly redesign or delays, claims and disputes during construction.

Unless site constraints dictate a particular alignment, such as within a confined urban setting, few projects are constructed precisely along the alignment established at the time the initial boring program is laid out. This should be taken into account when developing and budgeting for geotechnical investigations, and further illustrates the need for a phased subsurface investigation program.

3.5.2 Test Borings and Sampling

3.5.2.1 Vertical and Inclined Test Borings

Vertical and slightly inclined test borings (Figure 3-5) and soil/rock sampling are key elements of any subsurface investigations for underground projects. The location, depth, sample types and sampling intervals for each test boring must be selected to match specific project requirements, topographic setting and anticipated geological conditions. Various field testing techniques can be performed in conjunction with the test borings as well. Refer to FHWA Reference Manual for Subsurface Investigations (FHWA, 2002b) and GEC 5 (FHWA, 2002a) for guidance regarding the planning and conduct of subsurface exploration programs.

Figure 3-5 Vertical Test Boring/Rock Coring on a Steep Slope

Table 3-4 presents general guidelines from AASHTO (1988) for determining the spacing of boreholes for tunnel projects:

The above guideline can be used as a starting point for determining the number and locations of borings. However, especially for a long tunnel through a mountainous area, under a deep water body, or within a populated urban area, it may not be economically feasible or the time sufficient to perform borings accordingly. Therefore, engineering judgment will need to be applied by a licensed and experienced geotechnical professional to adapt the investigation program.

In general, borings should be extended to at least 1.5 tunnel diameters below the proposed tunnel invert. However, if there is uncertainty regarding the final profile of the tunnel, the borings should extend at least two or three times the tunnel diameter below the preliminary tunnel invert level. Borings at shafts should extend at least 1.5 times the depth of the shaft for design of the shoring system and shaft foundation, especially in soft soils.

3.5.2.2 Horizontal and Directional Boring/Coring

Horizontal boreholes along tunnel alignments provide a continuous record of ground conditions and information which is directly relevant to the tunnel alignment. Although the horizontal drilling and coring cost per linear feet may be much higher than the conventional vertical/inclined borings, a horizontal borings can be more economical, especially for investigating a deep mountainous alignment, since one horizontal boring can replace many deep vertical conventional boreholes and avoid unnecessary drilling of overburden materials and disruption to the ground surface activities, local community and industries.

A deep horizontal boring will need some distance of inclined drilling through the overburden and upper materials to reach to the depth of the tunnel alignment. Typically the inclined section is stabilized using drilling fluid and casing and no samples are obtained. Once the bore hole reached a horizontal alignment, coring can be obtained using HQ triple tube core barrels.

Figure 3-6 Horizontal Borehole Drilling in Upstate New York

3.5.2.3 Sampling - Overburden Soil

Standard split spoon (disturbed) soil samples (ASTM D-1586) are typically obtained at intervals not greater than 5 feet and at changes in strata. Continuous sampling from one diameter above the tunnel crown to one diameter below the tunnel invert is advised to better define the stratification and materials within this zone if within soil or intermediate geomaterial. In addition, undisturbed tube samples should be obtained in each cohesive soil stratum encountered in the borings; where a thick stratum of cohesive soil is present, undisturbed samples should be obtained at intervals not exceeding 15 ft. Large diameter borings or rotosonic type borings (Figure 3-7) can be considered to obtain special samples for classification and testing.

Figure 3-7 Rotosonic Sampling for a CSO Tunnel Project at Portland, Oregon.

3.5.2.4 Sampling – Rock Core

In rock, continuous rock core should be obtained below the surface of rock, with a minimum NX-size core (diameter of 2.16 inch or 54.7 mm). Double and triple tube core barrels should be used to obtain higher quality core more representative of the in situ rock. For deeper holes, coring should be performed with the use of wire-line drilling equipment to further reduce potential degradation of the recovered core samples. Core runs should be limited to a maximum length of 10 ft in moderate to good quality rock, and 5 ft in poor quality rock.

The rock should be logged soon after it was extracted from the core barrel. Definitions and terminologies used in logging rock cores are presented in Appendix B. Primarily, the following information is recommended to be noted for each core run on the rock coring logs:

• Depth of core run
• Core recovery in inches and percent
• Rock Quality Designation (RQD) percent
• Rock type, including color texture, degree of weathering and hardness
• Character of discontinuities, joint spacing, orientation, roughness and alteration
• Nature of joint infilling materials

In addition, drilling parameters, such as type of drilling equipment, core barrel and casing size, drilling rate, and groundwater level logged in the field can be useful in the future. Typical rock coring logs for tunnel design purpose are included in Appendix B.

3.5.2.5 Borehole Sealing

All borings should be properly sealed at the completion of the field exploration, if not intended to be used as monitoring wells. This is typically required for safety considerations and to prevent cross contamination of soil strata and groundwater. However, boring sealing is particularly important for tunnel projects since an open borehole exposed during tunneling may lead to uncontrolled inflow of water or escape of slurry from a slurry shield TBM or air from a compressed air tunnel.

In many parts of the country, methods used for sealing of boreholes are regulated by state agencies. FHWA-NHI-035 "Workbook for Subsurface Investigation Inspection Qualification" (FHWA, 2006a) offers general guidelines for borehole sealing. National Cooperative Highway Research Program Report No. 378 (Lutenegger et al., 1995), titled "Recommended Guidelines for Sealing Geotechnical Holes," contains extensive information on sealing and grouting boreholes.

Backfilling of boreholes is generally accomplished using a grout mixture by pumping the grout mix through drill rods or other pipes inserted into the borehole. In boreholes where groundwater or drilling fluid is present, grout should be tremied from the bottom of the borehole. Provision should be made to collect and dispose of all drill fluid and waste grout. Holes in pavement and slabs should be patched with concrete or asphaltic concrete, as appropriate.

3.5.2.6 Test Pits

Test pits are often used to investigate the shallow presence, location and depth of existing utilities, structure foundations, top of bedrock and other underground features that may interfere or be impacted by the construction of shafts, portals and cut-and-cover tunnels. The depth and size of test pits will be dictated by the depth and extent of the feature being exposed. Except for very shallow excavations, test pits will typically require sheeting and shoring to provide positive ground support and ensure the safety of individuals entering the excavation in compliance with OSHA and other regulatory requirements.

The conditions exposed in test pits, including the existing soil and rock materials, groundwater observations, and utility and structure elements are documented by written records and photographs, and representative materials are sampled for future visual examination and laboratory testing. The excavation pits are then generally backfilled with excavation spoil, and the backfill is compacted to avoid excessive future settlement. Tampers and rollers may be used to facilitate compaction of the backfill. The ground surface or pavement is then typically restored using materials and thickness dimension matching the adjoining areas.

3.5.3 Soil and Rock Identification and Classification

3.5.3.1 Soil Identification and Classification

It is important to distinguish between visual identification and classification to minimize conflicts between general visual identification of soil samples in the field versus a more precise laboratory evaluation supported by index tests. Visual descriptions in the field are often subjected to outdoor elements, which may influence results. It is important to send the soil samples to a laboratory for accurate visual identification by a geologist or technician experienced in soils work, as this single operation will provide the basis for later testing and soil profile development.

During progression of a boring, the field personnel should describe the sample encountered in accordance with the ASTM D 2488, Practice for Description and Identification of Soils (Visual-Manual Procedure), which is the modified Unified Soil Classification System (USCS). For detailed field identification procedures for soil samples readers are referred to FHWA-NHI-035 "Workbook for Subsurface Investigation Inspection Qualification".

For the most part, field classification of soil for a tunnel project is similar to that for other geotechnical applications except that special attention must be given to accurately defining and documenting soil grain size characteristics and stratification features since these properties may have greater influence on the ground and groundwater behavior during tunneling than they may have on other types of construction, such as for foundations, embankments and cuts. Items of particular importance to tunnel projects are listed below:

• Groundwater levels (general and perched levels), evidence of ground permeability (loss of drilling fluid; rise or drop in borehole water level; etc.), and evidence of artesian conditions
• Consistency and strength of cohesive soils
• Composition, gradation and density of cohesionless soils
• Presence of lenses and layers of higher permeability soils
• Presence of gravel, cobbles and boulders, and potential for nested boulders
• Maximum cobble/boulder size from coring and/or large diameter borings (and also based on understanding of local geology), and the unconfined compressive strength of cobbles/boulders (from field index tests and laboratory testing of recovered samples)
• Presence of cemented soils
• Presence of contaminated soil or groundwater

All of the above issues will greatly influence ground behavior and groundwater inflow during construction, and the selection of the tunneling equipment and methods.

3.5.3.2 Rock Identification and Classification

In rock, rock mass characteristics and discontinuities typically have a much greater influence on ground behavior during tunneling and on tunnel loading than the intact rock properties. Therefore, rock classification needs to be focused on rock mass characteristics, as well as its origin and intact properties for typical highway foundation application. Special intact properties are important for tunneling application particularly for selecting rock cutters for tunnel boring machines and other types of rock excavators, and to predict cutter wear.

Typical items included in describing general rock lithology include:

• General rock type
• Color
• Grain size and shape
• Texture (stratification, foliation, etc.)
• Mineral composition
• Hardness
• Abrasivity
• Strength
• Weathering and alteration

Rock discontinuity descriptions typically noted in rock classification include:

• Predominant joint sets (with strike and dip orientations)
• Joint roughness
• Joint persistence
• Joint spacing
• Joint weathering and infilling

Other information typically noted during subsurface rock investigations include:

• Presence of faults or shear zones
• Presence of intrusive material (volcanic dikes and sills)
• Presence of voids (solution cavities, lava tubes, etc.)
• Groundwater levels, and evidence of rock mass permeability (loss of drilling fluid; rise or drop in borehole water level; etc.)

Method of describing discontinuities of rock masses is in accordance with International Society of Rock Mechanics (ISRM)’s "Suggested Method of Quantitative Description of Discontinuities of Rock Masses" (ISRM 1981) as shown in Appendix B. Chapter 6 presents the J values assigned to each condition of rock discontinuities for Q System (Barton 2001).

Index properties obtained from inspection of the recovered rock core include core recovery (i.e., the recovered core length expressed as a percentage of the total core run length), and Rock Quality Designation or RQD (the combined length of all sound and intact core segments equal to or greater than 4 inches in length, expressed as a percentage of the total core run length).

For detailed discussions of rock identification and classification readers are referred to Mayne et al. (2001) and the AASHTO "Manual on Subsurface Investigations" (1988). Another useful reference for rock classification is "Suggested Methods for the Quantitative Description of Discontinuities in Rock Masses" from the International Society of Rock Mechanics (1977). For detailed field identification procedures readers are referred to FHWA-NHI-035 "Workbook for Subsurface Investigation Inspection Qualification" and "Rock and Mineral Identification for Engineer Guide."

Often, materials encountered during subsurface investigations represent a transitional (intermediate) material formed by the in place weathering of rock. Such conditions may sometimes present a complex condition with no clear boundaries between the different materials encountered. Tunneling through the intermediate geomaterial (IGM), in some cases referred as mixed-face condition, can be extremely difficult especially when groundwater is present. In the areas where tunnel alignment must cross this transition zone, the subsurface investigation is conducted much as for rock, and when possible cores are retrieved and classified, and representative intact pieces of rock should be tested. More discussions are included in Chapter 8.