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Technical Manual for Design and Construction of Road Tunnels - Civil Elements
Chapter 3 - Geotechnical Investigations
To successfully plan, design and construct a road tunnel project requires various types of investigative techniques to obtain a broad spectrum of pertinent topographic, geologic, subsurface, geo-hydrological, and structure information and data. Although most of the techniques and procedures are similar to those applied for roadway and bridge projects, the specific scope, objectives and focuses of the investigations are considerably different for tunnel and underground projects, and can vary significantly with subsurface conditions and tunneling methods.
A geotechnical investigation program for a tunnel project must use appropriate means and methods to obtain necessary characteristics and properties as basis for planning, design and construction of the tunnel and related underground facilities, to identify the potential construction risks, and to establish realistic cost estimate and schedule. The extent of the investigation should be consistent with the project scope (i.e., location, size, and budget), the project objectives (i.e., risk tolerance, long-term performance), and the project constraints (i.e., geometry, constructability, third-party impacts, aesthetics, and environmental impact). It is important that the involved parties have a common understanding of the geotechnical basis for design, and that they are aware of the inevitable risk of not being able to completely define existing subsurface conditions or to fully predict ground behavior during construction.
Generally, an investigation program for planning and design of a road tunnel project may include the following components:
It is beyond the scope of this manual to discuss each of the above components in details. The readers are encouraged to review the FHWA and AASHTO references provided in this Chapter for more details. Similar investigations and monitoring are often needed during and after the construction to ensure the problems that occurred during construction are rectified or compensated, and short term impacts are reversed. Geotechnical investigations after construction are not discussed specifically in this Chapter.
3.1.1 Phasing of Geotechnical Investigations
Amid the higher cost of a complete geotechnical investigation program for a road tunnel projects (typically about 3% to 5% of construction cost), it is more efficient to perform geotechnical investigations in phases to focus the effort in the areas and depths that matter. Especially for a road tunnel through mountainous terrain or below water body (Figure 3-1), the high cost, lengthy duration, limited access, and limited coverage of field investigations may demand that investigations be carried out in several phases to obtain the information necessary at each stage of the project in a more cost-efficient manner.
Figure 3-1 Water Boring Investigation from a Barge for the Port of Miami Tunnel, Miami, FL
Furthermore, it is not uncommon to take several decades for a road tunnel project to be conceptualized, developed, designed, and eventually constructed. As discussed in Chapter 1, typical stages of a road tunnel project from conception to completion are:
Throughout the project development, the final alignment and profile may often deviate from those originally anticipated. Phasing of the geotechnical investigations provides an economical and rational approach for adjusting to these anticipated changes to the project.
The early investigations for planning and feasibility studies can be confined to information studies and preliminary reconnaissance. Geological mapping and minimum subsurface investigations are typically required for EIS, alternative studies and conceptual design. EIS studies may also include limited topographical and environmental investigations to identify potential "fatal flaws" that might stop the project at a later date. A substantial portion of the geotechnical investigation effort should go into the Preliminary Design Phase to refine the tunnel alignment and profile once the general corridor is selected, and to provide the detailed information needed for design. As the final design progresses, additional test borings might be required for fuller coverage of the final alignment and for selected shaft and portal locations. Lastly, depending on the tunneling method selected, additional investigations may be required to confirm design assumptions, or to provide information for contractor design of temporary works. Figure 3-2 illustrates the flow process of the phases of investigations.
Figure 3-2 Phased Geotechnical Investigations with Project Development Process
This Chapter discusses the subsurface investigation techniques typically used for planning, design and construction of road tunnels. Additional information on this subject is available from FHWA Geotechnical Engineering Circular No. 5 (FHWA, 2002a), FHWA Reference Manual for Subsurface Investigations – Geotechnical Site Characterization (FHWA, 2002b), FHWA Reference Manual for Rock Slopes (FHWA, 1999), and AASHTO Manual on Subsurface Investigations (AASHTO, 1988).
3.2 Information Study
3.2.1 Collection and Review of Available Information
The first phase of an investigation program for a road tunnel project starts with collection and review of available information to develop an overall understanding of the site conditions and constraints at little cost. Existing data can help identify existing conditions and features that may impact the design and construction of the proposed tunnel, and can guide in planning the scope and details of the subsurface investigation program to address these issues.
Published topographical, hydrological, geological, geotechnical, environmental, zoning, and other information should be collected, organized and evaluated. In areas where seismic condition may govern or influence the project, historical seismic records are used to assess earthquake hazards. Records of landslides caused by earthquakes, documented by the USGS and some State Transportation Departments, can be useful to avoid locating tunnel portals and shafts at these potentially unstable areas.
In addition, case histories of underground works in the region are sometimes available from existing highway, railroad and water tunnels. Other local sources of information may include nearby quarries, mines, or water wells. University publications may also provide useful information.
Table 3-1 presents a summary list of potential information sources and the type of information typically available.
Today, existing data are often available electronically, making them easier to access and manage. Most of the existing information such as aerial photos, topographical maps, etc. can be obtained in GIS format at low or no cost. Several state agencies are developing geotechnical management systems (GMS) to store historical drilling, sampling, and laboratory test data for locations in their states. An integrated project geo-referenced (geospatial) data management system will soon become essential from the initiation of the project through construction to store and manage these extensive data instead of paper records. Such an electronic data management system after the project completion will continue to be beneficial for operation and maintenance purposes. Geospatial data management is discussed in Section 3.9.
3.2.2 Topographical Data
Topographic maps and aerial photographs that today can be easily and economically obtained, are useful in showing terrain and geologic features (i.e., faults, drainage channels, sinkholes, etc.). When overlapped with published geological maps they can often, by interpretation, show geologic structures. Aerial photographs taken on different dates may reveal the site history in terms of earthwork, erosion and scouring, past construction, etc.
U.S. Geological Survey (USGS) topographic maps (1:24,000 series with 10 ft or 20 ft contours) may be used for preliminary route selection. However, when the project corridor has been defined, new aerial photography should be obtained and photogrammetric maps should be prepared to facilitate portal and shaft design, site access, right-of-way, drainage, depth of cover, geologic interpretation and other studies.
3.3 Surveys and Site Reconnaissance
3.3.1 Site Reconnaissance and Preliminary Surveys
As discussed previously, existing lower-resolution contour maps published by USGS or developed from photogrammetric mapping, are sufficient only for planning purposes. However, a preliminary survey will be needed for concept development and preliminary design to expand existing topographical data and include data from field surveys and an initial site reconnaissance. Initial on-site studies should start with a careful reconnaissance over the tunnel alignment, paying particular attention to the potential portal and shaft locations. Features identified on maps and air photos should be verified. Rock outcrops, often exposed in highway and railroad cuts, provide a source for information about rock mass fracturing and bedding and the location of rock type boundaries, faults, dikes, and other geologic features. Features identified during the site reconnaissance should be photographed, documented and if feasible located by hand-held GPS equipment.
The reconnaissance should cover the immediate project vicinity, as well as a larger regional area so that regional geologic, hydrologic and seismic influences can be accounted for.
A preliminary horizontal and vertical control survey may be required to obtain general site data for route selection and for design. This survey should be expanded from existing records and monuments that are based on the same horizontal and vertical datum that will be used for final design of the structures. Additional temporary monuments and benchmarks can be established, as needed, to support field investigations, mapping, and environmental studies.
3.3.2 Topographic Surveys
As alternatives are eliminated, detailed topographic maps, plans and profiles must be developed to establish primary control for final design and construction based on a high order horizontal and vertical control field survey. On a road tunnel system, centerline of the roadway and centerline of tunnel are normally not identical because of clearance requirements for walkways and emergency passages as discussed in Chapter 2. A tunnel centerline developed during design should be composed of tangent, circular, and transition spiral sections that approximate the complex theoretical tunnel centerline within a specified tolerance (0.25 in.). This centerline should be incorporated into the contract drawings of the tunnel contract, and all tunnel control should be based on this centerline. During construction, survey work is necessary for transfer of line and grade from surface to tunnel monuments, tunnel alignment control, locating and monitoring geotechnical instrumentation (particularly in urban areas), as-built surveys, etc. Accurate topographic mapping is also required to support surface geology mapping and the layout of exploratory borings, whether existing or performed for the project. The principal survey techniques include:
The state-of-art surveying techniques are discussed briefly below. Note that the accuracies and operation procedures of these techniques improve with time so the readers should seek out up-to-date information when applying these techniques for underground projects.
Global Positioning System (GPS) utilizes the signal transit time from ground station to satellites to determine the relative position of monuments in a control network. GPS surveying is able to coordinate widely spaced control monuments for long range surveys, as well as shorter range surveys. The accuracy of GPS measurement is dependent upon the number of satellites observed, configuration of the satellite group observed, elapsed time of observation, quality of transmission, type of GPS receiver, and other factors including network design and techniques used to process data. The drawback for GPS survey is its limitation in areas where the GPS antenna cannot establish contact with the satellites via direct line of sight, such as within tunnels, downtown locations, forested areas, etc.
Electronic Distance Measuring (EDM) utilizes a digital theodolite with electronic microprocessors, called a "total station" instrument, which determines the distance to a remote prism target by measuring the time required for a laser or infrared light to be reflected back from the target. EDM can be used for accurate surveys of distant surfaces that would be difficult or impractical to monitor by conventional survey techniques. EDM can be used for common surveying applications, but is particularly useful for economically monitoring displacement and settlement with time, such as monitoring the displacement and settlement of an existing structure during tunneling operations.
Remote Sensing can effectively identify terrain conditions, geologic formations, escarpments and surface reflection of faults, buried stream beds, site access conditions and general soil and rock formations. Remote sensing data can be easily obtained from satellites (i.e. LANDSAT images from NASA), and aerial photographs, including infrared and radar imagery, from the USGS or state geologists, U.S. Corps of Engineers, and commercial aerial mapping service organizations. State DOT aerial photographs, used for right-of-way surveys and road and bridge alignments, may also be available.
Laser Scanning utilizes laser technology to create 3D digital images of surfaces. Laser scanning equipment can establish x, y and z coordinates of more than one thousand points per second, at a resolution of about 0.25 inch over a distance of more than 150 feet. Laser scanning can be used to quickly scan and digitally record existing slopes to determine the geometry of visible features, and any changes with time. These data may be useful in interpreting geologic mapping data, for assessing stability of existing slopes, or obtaining as-built geometry for portal excavations. In tunnels, laser scanning can efficiently create cross sections at very close spacing to document conditions within existing tunnels (Figure 3-3), verify geometry and provide as-built sections for newly constructed tunnels, and to monitor tunnel deformations with time.
Figure 3-3- 3D Laser Scanning Tunnel Survey Results in Actual Scanned Points
3.3.3 Hydrographical Surveys
Hydrographic surveys are required for subaqueous tunnels including immersed tunnel (Chapter 11), shallow bored tunnel, jacked box tunnel, and cofferdam cut-and-cover river crossings to determine bottom topography of the water body, together with water flow direction and velocity, range in water level, and potential scour depth. In planning the hydrographic survey, an investigation should be made to determine the existence and location of submarine pipelines, cables, natural and sunken obstructions, rip rap, etc. that may impact design or construction of the immersed tunnel or cofferdam cut-and-cover tunnel. Additional surveys such as magnetometer, seismic sub-bottom scanning, electromagnetic survey, side scan sonar, etc., may be required to detect and locate these features. These additional surveys may be done simultaneously or sequentially with the basic hydrographic survey. Data generated from the hydrographic survey should be based on the same horizontal coordinate system as the project control surveys, and should be compatible with the project GIS database. The vertical datum selected for the hydrographic survey should be based on the primary monument elevations, expressed in terms of National Geodetic Vertical Datum of 1929 (NGVD), Mean Lower Low Water Datum, or other established project datum.
3.3.4 Utility Surveys
Utility information is required, especially in the urban areas, to determine the type and extent of utility protection, relocation or reconstruction needed. This information is obtained from surveys commissioned for the project, and from existing utility maps normally available from the owners of the utilities (utility companies, municipalities, utility districts, etc.). Utility surveys are performed to collect new data, corroborate existing data, and composite all data in maps and reports that will be provided to the tunnel designer. The requirement for utility information varies with tunneling methods and site conditions. Cut-and-cover tunnel and shallow soft ground tunnel constructions, particularly in urban areas, extensively impacts overlying and adjacent utilities. Gas, steam, water, sewerage, storm water, electrical, telephone, fiber optic and other utility mains and distribution systems may require excavation, rerouting, strengthening, reconstruction and/or temporary support, and may also require monitoring during construction.
The existing utility maps are mostly for informational purposes, and generally do not contain any warranty that the utility features shown actually exist, that they are in the specific location shown on the map, or that there may be additional features that are not shown. In general, surface features such as manholes and vaults tend to be reasonably well positioned on utility maps, but underground connections (pipes, conduits, cables, etc.) are usually shown as straight lines connecting the surface features. During original construction of such utilities, trenching may have been designed as a series of straight lines, but, in actuality, buried obstructions such as boulders, unstable soil or unmapped existing utilities necessitated deviation from the designed trench alignment. In many instances, as-built surveys were never done after construction, and the design map, without any notation of as-constructed alignment changes, became the only map recording the location of the constructed utilities.
In well-developed areas, it may not be realistic to attempt to locate all utilities during the design phase of a project without a prohibitive amount of investigation, which is beyond the time and cost limitations of the designer's budget. However, the designer must perform a diligent effort to minimize surprises during excavation and construction. Again, the level of due diligence depends on the method of excavation (cut-and-cover, or mined tunnel), the depth of the tunnel, and the number, size and location of proposed shafts.
3.3.5 Identification of Underground Structures and Other Obstacles
Often, particularly in dense urban areas, other underground structures may exist that may impact the alignment and profile of the proposed road tunnel, and will dictate the need for structure protection measures during construction. These existing underground structures may include transit and railroad tunnels, other road tunnels, underground pedestrian passageways, building vaults, existing or abandoned marine structures (bulkheads, piers, etc.), and existing or abandoned structure foundations. Other underground obstructions may include abandoned temporary shoring systems, soil treatment areas, and soil or rock anchors that were used for temporary or permanent support of earth retaining structures. Initial surveys for the project should therefore include a survey of existing and past structures using documents from city and state agencies, and building owners. In addition, historical maps and records should be reviewed to assess the potential for buried abandoned structures.
3.3.6 Structure Preconstruction Survey
Structures located within the zone of potential influence may experience a certain amount of vertical and lateral movement as a result of soil movement caused by tunnel excavation and construction in close proximity (e.g. cut-and-cover excavation, shallow soft ground tunneling, etc.). If the anticipated movement may induce potential damage to a structure, some protection measures will be required, and a detailed preconstruction survey of the structure should be performed. Preconstruction survey should ascertain all pertinent facts of pre-existing conditions, and identify features and locations for further monitoring. Refer to Chapter 15 for detailed discussions of structural instrumentation and monitoring.