Appendix 6 Geotechnical report

Transcription

1 Page 56 Appendix 6 Geotechnical report 1. Introduction The following provides an initial and preliminary description/assessment of the overall geology, the likely ground conditions and preliminary geotechnical recommendations for the possible construction of a duplication tunnel adjacent to the existing Mt. Victoria Tunnel. Initial recommendations in relation to site investigation requirements are included also. All the following information is preliminary only and it should be understood that limited information is available. The information used for this geotechnical assessment is limited to historical pilot tunnel logs, site walkovers, the general geomorphology and geological maps. No site investigation was carried out to date to confirm the assumptions. 2. Geology The following provides an initial assessment and summary of the geology in the vicinity of the existing Mount Victoria Tunnel. The assessment is based on the following information only: The Geology of the Wellington Area, 1:50,000 scale map; and Pilot Tunnel Logs from A review of information regarding the topographical and geological setting of the Mount Victoria tunnel has been undertaken and is presented below. 3. Topographical setting The tunnel is located to the south east of Wellington city centre and strikes from NWW to SEE through Mount Victoria connecting the suburbs of Mt Victoria, to the west, and Hataitai, to the east. 4. Geological setting The geological map relevant to the project is the 1:50,000 scale Geology of the Wellington Area map sheet R27, R28 and part Q27, dated The Geological Survey Map indicates the area to be located within a geologically complex area (Figure 1), belonging to the Rakaia Terrane.

2 Page 57 Figure 1: Wellington regional geology (1:50000) Characteristically this terrane comprises late Carboniferous to late Triassic, quartzofeldspathic, metamorphosed sandstone and mudstone sequences together with poorly bedded sandstone with minor coloured mudstone of marginal marine to submarine origin. The Mt Victoria Tunnel has been driven through Greywake sandstone (Figure 2), comprising monotonous, complexly folded and steeply dipping sequences of uniformly low-grade metamorphosed turbidites consisting of cyclical sedimentary units of sand grading up to mud. The Wellington region is seismically active and is cut by several major active NE to SW trending faults, and numerous other faults with a NNE to SSW orientation (Figure 1). The closest major fault to the tunnel is the Wellington Fault, located approximately 2.70km to the west. An un-named fault running north-south from Oriental Bay to Lyall Bay passes the eastern portal of the Mt Victoria tunnel. Figure 2: Mount Victoria Geology (1:25000) 5. Ground conditions A practicable simplification of the likely rock and rock mass conditions leads initially to two different rock types and three different rock mass conditions to be considered. Apart from localised faults (eastern portal, see Section 2.2), a major or active fault is not known to exist in direct proximity to the proposed tunnel alignment. It is assumed that the -estimated and simplified ground conditions described below will not significantly vary in the area of the existing Mt. Victoria tunnel.

3 Page 58 Site investigations are required to determine/confirm the estimated ground conditions and to determine parameter values for tunnel design. 6. Rock types A review of the Pilot Tunnel Logs for the Mt Victoria Tunnel Duplication, dated 1973, has been undertaken. Based on this review the ground conditions can be summarised into two main rock types: Siltstone/Sandstone The Siltstone/Sandstone can initially be described as slightly to moderately weathered, weak to moderately strong rock. Bedding thickness is approximately 2 10 cm, and joint spacing is in a range of 1 to 20 cm and joints are mostly tight. Particularly in or near faults the grade of weathering increases to highly to completely weathered and the strength can decrease to weak. Clay stains were observed on the bedding planes. The UCS is estimated to be in the range of 35 to 120 MPa Greywacke The Greywacke can be described as unweathered to slightly weathered, moderately strong to strong rock. Joint spacing is in a range of 5-20 cm. The joints are tight to slightly open. Localised intact areas with a joint spacing of 0.6 to more than 1m may exist. Argillite beds exist within the rock. The UCS is estimated to be in the range of 140 to 220 MPa.

4 Page Rock mass conditions Based on the information from the pilot tunnel logs the rock mass is divided into three different categories which should provide a simplified description of the overall situation on site: Thinly interbedded Siltstone/Sandstone/Greywacke with Siltstone dominating Strong fractured Greywacke Greywacke with wider spaced beds of Siltstone/Sandstone with Greywacke dominating. The preliminary assumed and simplified distribution of these rock mass types is as following: Rock Mass Type Assumed Distribution (preliminary) 1 1/3 towards the West 2 1/3 in Centre (may be less) 3 1/3 towards East Rock mass properties were estimated by using different evaluation criteria as: Hoek-Brown RMR (Bieniawski) Rock Quality Tunnelling Index Q (Barton) Again, it should be noted that currently the determination of the rock mass parameters is based on vague assumptions and can only be seen indicatively. Re-evaluation is required when more site investigation data is available. Considerable variations to the currently estimated parameters are possible. Hoek-Brown The computer program Roclab (Rocscience) was used to determine the Hoek-Brown parameters. Table 1 below summarises the initially estimated rock mass parameters. Table 1 Rock Mass Type 1) Thinly bedded siltstone/ sandstone/ greywacke 2) Strong fractured Greywacke 3) Greywacke with Siltstone / Sandstone interbeds Estimated GSI Estimated UCS (MPa) RM UCS (MPa) RM Global Strength (MPa) RM Deformation Modulus (MPa) Cohesion (MPa) Tunnel Phi ( ) Tunnel Cohesion (MPa) General Phi ( ) General

5 Page 60 RMR (Bieniawski) Table 2 below presents the outcome of the initially estimated RMR rating. Table 2 Rock Mass Type 1) Thinly bedded siltstone/ sandstone/ greywacke 2) Strong fractured Greywacke 3) Greywacke with Siltstone / Sandstone interbeds RMR Rating Description Cohesion (KPa) Phi ( ) Very Poor to Poor Rock 59 Fair to Good Rock Poor Rock Rock Tunnelling Quality Index Q (Barton) The following formula is used for the determination of Q: Q RQD Jr Jw Jn Ja SRF Due to the uncertainties in the input parameters a range of values was considered appropriate. Table 3 below summarises the assumptions and the result: Table 3 Q determination Rock Mass RQD Jn Jr Ja Jw SRF Q Type 1) Thinly bedded siltstone/ sandstone/ greywacke (nominal) 2) Strong fractured Greywacke 3) Greywacke with Siltstone / Sandstone interbeds 10 (nominal) Ground conditions at portal locations For likely portal locations it may be assumed that either Rock Mass Type 1 or 3 are to be expected with a higher degree of weathering. The rock may be overlain by colluvium deposits which may consist of rock fragments in a clay/silt matrix.

6 Page Hydrogeological situation Due to the lack of drillholes at this stage, there is almost no information about the groundwater level available at this stage. Some vague information about the water inflow can be obtained by the review of the pilot tunnel logs and from observations in the existing tunnel. A note from a pilot tunnel inspection in 2004 indicates the presence of water in that tunnel. It is not clear whether the tunnel is completely or partly flooded. Some drainage apparently exists. It is also known that sinkholes appeared in the past above the alignment of the pilot tunnel. The pilot tunnel logs indicate damp to dripping conditions with a tendency of increased inflow towards the centre of the tunnel, which is described as seepage. The inflow is mostly related to faults and joints. In the existing Mount Victoria tunnel, particularly towards the centre, there is some continuous inflow through the construction joints. The combined inflow through the joints on the south side of the tunnel is estimated to be in the range of 3 to 15 l/s for the entire tunnel length. This does not include any base drainage and discharge there might be significantly higher. The overall bulk rockmass permeability is estimated to be approximately in a range of 10-6 to 10-7 m/s. 9. Preliminary geotechnical recommendations Tunnel It is currently expected that conventional tunnelling methods can be used for the tunnel excavation. In the highly fractured sandstone/siltstone rock mass to the west and the fractured greywacke/siltstone rock mass to the east (rock mass types 1 and 3), a roadheader in combination with rock breakers and conventional excavation may be suitable. It should be assessed whether the use of a roadheader is economic - although the intact compressive strength of some rock mass types is above the desired maximum strength for road headers, the fracturing of the rock will contribute to breaking. Where intact greywacke rock is expected close to the centre of the tunnel, it may be required to apply blasting techniques. This is due to the high compressive strength and the lesser degree of fracturing. Road header wear may become unacceptable and rock breaking may not deliver the desired progress. For rock mass types 1 and 3 it is initially assumed that medium sets at 1.0 to 1.5m spacing with 100mm to 150mm shotcrete support (crown and sides) and systematic bolting is required for excavation support. For rock mass type 2, sets may not be required with systematic bolting in place and mm shotcrete to the crown. A composite liner system incorporating a full area drainage liner (e.g. Enkadrain / Enkamat) is recommended to reduce the water pressure to the permanent tunnel liner. The permanent liner may be an RC Liner with a thickness between 450 and 600mm, depending on the tunnel geometry, span and excavation support. Portals Significant cuts are likely to be required to establish the invert levels at the proposed tunnel portals in the foot of the hillsides of the Mt Victoria. The likely portal locations are assumed to be in developed urban areas. In order to reduce the land take it is therefore desired to establish steep cut slopes at the portal areas. As described in section 3.3, colluvium soils are expected above weathered rock. Retaining measures are required in the upper colluvium soils, while slope stabilisation measures may be sufficient in the weathered and in the fractured rocks. It is initially assumed that an overall cut angle of can be achieved with reasonably economic retaining and stabilisation measures. These may consist of RC

7 Page 62 retaining walls in the upper collovium soils and highly weathered rocks and rock slope stabilisation (e.g. bolts, nets and localised shotcrete) in the lower part. If steeper slope angles are required, full height retaining walls should be considered. 10. Initial thoughts for site investigation requirements Extensive site investigations are required in the proposed alignment of the duplicate tunnel and in the portal areas. The site investigations will consist of mainly intrusive investigations (boreholes) and associated in-hole testing and in-hole installations. Groundwater and settlement monitoring is required before, during and after the construction of the tunnel. Laboratory testing is required to be carried out on retrieved samples. Vibration monitoring stations are recommended to be set up prior to commencement of the tunnelling works. A dilapidation survey of the properties above the tunnel alignment and adjacent to the portals is highly recommended. Due to access limitations across Mt Victoria (Town Belt), it is desired to keep the site investigation locations to a minimum. It is therefore recommended to carry out a few high quality boreholes with an extensive regime of in-hole testing. High quality (HQ) drilling means to employ drilling techniques which are able to recover undisturbed samples in weathered and highly fractured rocks. For example, this drilling method could be Geobor S with polymer modified drilling fluid. This is a triple tube wire line drilling method. The correct selection of the drill bit in combination with the drill rpm will also significantly contribute to the successful winning of undisturbed samples. In-hole testing should include permeability testing with packers, acoustic and/or optical televiewers (HiRat/OPTV), axial extensiometers and high pressure dilatometer testing. In addition to the HQ boreholes, a series of groundwater monitoring wells should be installed in the greater surrounding area, accompanied by a grid of settlement monitoring stations. The groundwater monitoring installations would require standard coreholes only and the location for the HQ-holes should be determined depending on the initial logs of the groundwater wells. Standard drilling (e.g. rotary coring) in combination with trial pits can be carried out at the portal areas, although at least one HQ borehole at each portal with in-hole testing may be beneficial for a more economic and less conservative design. Depending on the desired design safety, construction flexibility and cost/benefit relationship, horizontal (directional) drilling axial to the proposed alignment is considered to be beneficial, as it would provide continuous information about variations of the rock quality (in combination with data from the HQ boreholes). This would allow for a significantly lower degree of conservatism in the design and in the pricing of the tunnelling works. It will also provide a higher level of safety in design and, together with the above mentioned site investigations, it should be considered as part of a standard level site investigation for a tunnelling project of this size. Cost estimates for the horizontal drilling should be obtained and it should be decided if a partial penetration into the Mt. Victoria from both sides, or a complete perforation is preferable. In general, a phased site investigation is recommended to allow for flexibility in scope in the following phases. The scope of the following phases can be reviewed and optimised after evaluation of the results from the previous phase. This will guarantee a high degree of efficiency and will avoid doubling of information. A first initial/indicative schedule for the investigation works is provided in Table 4 below. Table 4

8 Page 63 SI Phases Phase Location Description Total Qantity 1 All Topographical Survey NA 1 East and West Portal Trial Pits to investigate shallow strata to 4.0m 6 1 Tunnel Alignment 1 East and West Portal Standard rotary coring holes including installation of Piezometers Standard rotary coring holes including installation of Piezometers All Associated Laboratory Testing NAa 2 East and West Portal Trial Pits to investigate shallow strata to 4.0m 6 2 Tunnel Alignment 2 East and West Portal Standard rotary coring holes including installation of Piezometers Standard rotary coring holes including installation of Piezometers All Istallation of Settlement Stations All Associated Laboratory Testing NA 3 Tunnel Alignment Horizontal drilling in tunnel axis including in-hole geophysics (e.g.hirat) 1 4 Tunnel Alignment HQ Coreholes with in-hole testing regime Tunnel Alignment Associated Laboratory Testing NA

9 Page 64 Appendix 7 New Tunnel Long Section

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