Efficient, economic, and safe excavation of tunnels depends on a detailed understanding of the ground conditions to be encountered at the working face. Unanticipated joints, faults, or shear zones could lead to potentially hazardous conditions that may result in stoppages and resulting claims or disputes. New methods of risk identification during site investigation and tunnel excavation can have a significant impact on the contractors’ competitive advantage or on overall project costs for owners.
The New Tomei-Meishin Expressway, owned by the Japanese Highways Public Corporation, is located in Shizuoka Prefecture, on the flanks of Mt Fuji in Japan. This stretch of divided highway has two major tunnelling projects currently under construction. Kajima Corporation is excavating the Fujikawa and Kanaya tunnels using TBM drives combined with drill and blast and NATM methods to slash out the existing TBM driven tunnels to the required width. The twin bore Fujikawa Tunnel, with a cross-section of 190m2, and a total length of 4160m, passes under the Fuji River in Quaternary andesites, lava, and tuff breccia lenses. The Fujikawa Tunnel drive includes sections of fractures and shears that may carry water.
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The 4454m long, twin bore Kanaya Tunnel, also with a cross-section of 190m2, is being driven through difficult Tertiary sedimentary formations of sandstones, conglomerates, and mudstones. With this in mind Kajima has applied a new method of seismic tomography on the tunnels.
Originally pioneered in Japan, by Kajima and US based NSA Engineering, True Reflection Tomography “TRT™” uses seismic energy from multiple sources, including the TBM cutting action, to create a 3D isometric map of the geological structure some 100m ahead of the tunnel face and up to 30m around the tunnel alignment. Kajima has applied TRT™ on 14 tunnelling projects to date, since its introduction in February 1999. Reflection tomography methods, developed by NSA, process reflections produced by seismic waves, generated from a number of source types and tunnelling machines commonly found in underground construction. An isometric plot of detected reflection anomalies along the tunnel alignment give a 3D image of the geological structure for some distance. This information allows the site engineer and geologist to more accurately assess expected conditions.
Kajima Corporation have used the pilot tunnel as the primary geological exploration tool on the Fujikawa and Kanaya Tunnels, due to the often difficult terrain, but it has also used TRT™ extensively, often in very difficult and varied ground conditions, to manage risk for improved safety and operational management, resulting in major economic benefit to the owner. Data collection is usually governed by the rate of advance for a given tunnel. For the Fujikawa and Kanaya Tunnels, data is typically taken every 80 meters, allowing a 20m overlap between surveys so that there is some correlation of successive images.
Data collection generally takes about four hours, including hardware installation, data collection, and teardown. As the data analysis is complex, it requires special skills to extract accurate and complete geological information. Therefore data is transferred to NSA directly, via the Internet, for processing. Data is input, filtered, picked, and processed for analysis by NSA’s geophysicists and engineers. This process usually requires between 8 to 20 hours depending on the complexity of the geological situation. The resultant analysis is then returned to the site ready for the next morning’s work. The images are correlated against the “as excavated” geological maps and drilling logs, etc. as well as standard rock mass classification systems of Q and RMR1.
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By GlobalDataFujikawa Tunnel
The method has been applied in two sections on the 4520m long Fujikawa Highway Tunnel. The objective of the survey for the first section was to delineate the boundary of a gravel deposit overlying an andesite and tuff breccia formation above the alignment of the TBM tunnel. The survey for the second section was to image ahead of the TBM where ground conditions had deteriorated due to frequent faulting and shear zones where the tunnel passes under the Umuse River flowing into the Fuji River and continues in a 470m section between Sta. 1041+40 and Sta. 1036+70.
Figure 1 shows the location of the arrays and the extent of generated images, combining data from two locations between Sta. 1050+00 and Sta. 1048+00. The first site is in the existing 3.5m diameter pilot tunnel and the second is from the face of the 5m diameter main TBM tunnel. Blue sections along each tunnel mark arrays of accelerometers installed at each site. Red sections mark the range of sledgehammer strikes used as seismic impact sources for each site.
The original assessment of the gravel bottom at the first site was at an elevation of approximately 220m ASL, or 55m above the centreline of the TBM tunnel. Horizontal anomalies were natural for an undisturbed gravel and similar sedimentary deposit. After final adjustment for attenuation of seismic waves, another horizontal reflective boundary was detected at an elevation of 200m ASL, or 35m above the centreline of the TBM tunnel. A nearly identical sequence of reflective anomalies was reconstructed for the same survey in the TBM tunnel. However, the elevation of the lowest boundary was at 202m ASL, or 37m above the TBM tunnel centreline.
Combined images for the pilot tunnel and for the TBM tunnel shown in figure 2 allow a 3D assessment of the shape of the gravel bottom boundary. The horizontal alignment of the boundary appears to correspond well with surface reflection data, but its elevation is higher by approximately 5 to 7m. Also, the boundary appears to dip slightly from the TBM tunnel toward the pilot tunnel. No other explicit horizontal boundaries were identified in the elevation range between 200m ASL and the TBM tunnel.
Imaging ahead of the TBM
A number of anomalies in front of the TBM, possibly associated with interchanging zones of andesite and tuff breccia, were detected. A number of contour reflective anomalies over the range of 125m, ahead of the TBM were found. The image was then extended 20m on each side of the tunnel to show a side view due south through the tunnel centreline over the same distance range and an elevation ranging between 145m and 170m ASL. Figure 3 shows an isometric 3D projection due southwest of both tomograms and the contour reflective anomalies. All reconstructed anomalies appear nearly vertical, or slightly tilted due west. Some of the anomalies appear terminated from the top. The anomalies are most likely boundaries between softer tuff and stronger andesite. The more massive anomalies are possibly associated with more unstable ground conditions.
On a separate occasion earlier on in the project construction, three separate but continuous sections of up to 150m each were imaged. Figure 4 shows the original image between Sta. 1041+38 and Sta. 1039+50 along with forecast comments, geological information, and observations of encountered conditions.
Kanaya Tunnel
A TRT™ survey was also conducted at three sites in the twin bore, 4454m long Kanaya Highway Tunnel. The objective here was to image the lithological boundary between sandstone and mudstone at two sites; to image features associated with a faulting at the third site; and to allow prediction of ground conditions along the alignment of the westbound tunnel. To date, some 1240m between Sta. 413+70 and Sta. 426+10 have been imaged. Figure 5 presents a tomogram (slice) and contour reflection anomalies through the image block at Site 1. This figure combines a plan view of a horizontal tomogram through the tunnel centreline at elevation 138.3m ASL and a side view due north of vertical tomogram along the tunnel alignment with an isometric projection due northwest of contour reflective anomalies above horizontal tomogram.
The image indicates the presence and alignment of the strata boundary by producing a pattern of larger size coloured spots along and on the side of the boundary opposite the sources and accelerometers. The boundary appears to cross the tunnel at approximately STA 414+53, and at an angle of about 105° counter-clockwise from the tunnel alignment. Due to rather small differentiation in rock properties across the strata boundary, the efforts to determine which side of the boundary represented stronger rock were inconclusive at this time. The images also show a zone parallel to the boundary and crossing the tunnel adjacent to the array of accelerometers. The nature of this zone is presently unknown and may be caused by guided, possibly refracted waves travelling along directions parallel to the detected boundary.
Figure 5 presents a tomogram and contour reflective anomalies through the image block at Site 2. This shows a plan view of a horizontal tomogram through the tunnel centreline at elevation 137.5m ASL, combined with a side view due-north of a vertical tomogram along the tunnel alignment and an isometric projection due north of contour reflective anomalies above the horizontal tomogram through the tunnel and in front of vertical tomogram 43m north from the tunnel. The shaded area in this figure indicates general extent and orientation of the fault zone. This zone crosses the tunnel approximately at Sta. 418+40. It appears oriented approximately 75° counter-clockwise from the tunnel alignment and is tilted approximately 10° vertically to the east. Similar to Site 1, small differentiation in rock properties combined with strong attenuation of seismic waves did not allow a reliable distinction between stronger and weaker rock mass associated with the fault. No other explicit structural features were detected at Site 2.
Figure 7 shows a plan view of a horizontal tomogram at Site 3, through the tunnel centreline at elevation 136.5m ASL combined with a side view due-north of a vertical tomogram along the tunnel alignment and an isometric projection due northwest of contour reflective anomalies above the horizontal tomogram through the tunnel and in front of a vertical tomogram 43m north from the tunnel. According to the change in pattern of contour anomalies/colour spots, the sandstone-mudstone boundary crosses the tunnel at Sta. 420+51. Its orientation is at an angle about 105° counter-clockwise from the tunnel alignment, and its tilt appears to match the orientation of the boundary at Site 1. A wide zone crossing the tunnel near the array of accelerometers coincides with a zone of poor rock conditions identified in the tunnel while selecting survey locations.
A major fault zone was detected in front of the TBM in the eastbound Kanaya tunnel between Sta. 417+40 and Sta. 419+6. Figure 8 illustrates an explicit fault plane crossing at the tunnel alignment at Sta. 480+60. This fault plane cuts the westbound tunnel at Sta. 418+50. This result corresponds with the TRT™ survey conducted earlier in the westbound tunnel.
Conclusions
Encountering unforeseen geological conditions while tunnelling is the single biggest contributor to cost overruns and increased risk. TRT™ effectively reduces that risk by providing the operator with a timely and complex analysis of the ground conditions ahead without requiring expensive and highly trained staff on site.
The isometric images of the expected location and relationships of highly sheared and fractured zones, especially water bearing features, allow Kajima engineers to predict logistical requirements i.e. extra steel sets or shotcrete and other supply requirements. In addition, they allow time to plan drilling and/or grouting programs and to carry out targeted drilling on water bearing features, effectively reducing project risk and giving improved safety.
In a number of situations, the image data has been re-evaluated and projected to areas outside the original alignment, to study vent shaft locations and peripheral excavations. Knowing how much more difficult ground to expect has been critical in decisions by Kajima tunnel management and in discussions with the owner. T&TI
