Site Investigation for a Major Wind Farm Project at Farr at Inverness-shire - Case Study Example

Question SITE INVESTIGATION INTRODUCTION This report is on the site investigation for a major Wind Farm project at Farr at Inverness-shire,Scotland in 2004. With increasing growth of urban areas causing depletion of natural resources, there is greater pressure on the surrounding rural landscapes for their resources. Therefore, suitable measures should be employed to ensure sustainable use of the natural reserves and to reduce adverse impacts on the region’s integrity. The investigation of the highly sensitive site of the Farr Wind Farm accomplishes these requirements successfully, using a combination of invasive and non-invasive investigative methodologies. The site investigation extracts the “optimum geotechnical data for the design of a major infrastructure development with the minimum environmental impact in a very short time-frame” (Carpenter et al 2006: 10). THE FARR WIND FARM SITE INVESTIGATION PROJECT Farr was considered as the ideal site for development based on its remoteness from dwellings, the bow-shaped interior of site helping to minimize the visual impact, “and the particularly high average wind speed recorded at the site” (Carpenter et al 2006: 2). However, the same characteristics raised the logistic difficulties in investigating the site. It is one of the most inhospitable areas of the British Isles, with an average elevation of exceeding 500m angle of departure, the peat and moorland vegetation inscribed by several small water courses, and ecological sensitivity requiring a careful approach to the movement of vehicles and drilling plant over the site. Further, the time available for site investigation was limited due to the impending grouse breeding season. The site investigation was optimally conducted, and was in compliance to various legal requirements, safety issues and governmental policies, thus helping to avoid potential problems in the project. The development plan was approved both by the Highland Council and the Scottish Executive, to raise the total energy generated from renewable resources in Scotland to 18%. Forty turbine bases were to generate 92 MW of power sufficient to provide electricity to 53,400 homes, saving 215,900 tonnes of carbon emissions over the expected 20-25 year life of the facility (Carpenter et al 2006). SITE INVESTIGATION TECHNIQUES EMPLOYED For developing a 40 turbine wind farm in the remote Scottish Highlands, a programme of geophysical and geotechnical ground investigation work was carried out, to obtain relevant data and also a conceptual understanding of the “geology, the near-surface geotechnical characteristics, geohazards, hydrogeology and ground conditions” (Carpenter et al 2006: 1) in and around each turbine base and related infrastructure. The geological ground investigation aimed at determining the depth of surface peat cover, thickness of sub-peat glacial deposits, and thickness and strength of the underlying weathered and fresh metamorphic bedrock. Geophysical and geotechnical fieldwork was carried out at the site to obtain adequate data for developing the preliminary design of appropriate foundations for the proposed wind turbines, particularly in relation to the thickness and strength of superficial deposits and the depth and strength of bedrock. Since reinforced concrete pad foundations located in a suitable stratum were traditionally the materials used, the stength of the strata lying below and the depth to suitable foundational stratum were of vital significance. Further, access roads to the site would link the proposed turbine locations, therefore adequate data was required for their design (Carpenter et al 2006: 4). Therefore, the following site investigation programme was undertaken: Desk study and walk-over survey: The desk study consisted of gathering and assessing existing information about the site. The main objectives were to provide the maximum information on probable ground conditions including topography, soil and rock types, ground water, contaminated ground, etc. Further, any previous uses of the site ground, and access to the site were studied. Moreover, environmental and ecological factors that may hamper the execution of future works were also determined. The desk study results were used along with information gathered during a subsequent walk-over survey to identify possible geotechnical hazards (Simons et al 2002). Subsurface exploration by engineering geophysics (Clayton et al 1995): Ground penetrating radar (110MHz antennae), seismic refraction and 2D resistivity imaging were some of the geophysical techniques used at each turbine location to determine the “thickness of superficial deposits, depth and nature of the soil-rock interface, and strength of the weathered bedrock” (Carpenter et al 2006: 4). Trial pits: These were located as close as possible to the proposed turbine locations to be logged according to the stipulations given in BS5930: 1999. Appropriate samples were tested according to the requirements of BS1377: 1991. Rotary drilling: The depth of bedrock was determined using this technique, at particular locations selected across the site. Soft soil probes and shell and auger drilling: This was done as close as possible to proposed turbine locations and additional intermediate locations along proposed road alignments for site access, to guage the thickness of the peat deposits, and calculate an SPT-N value for the sub-peat material (Carpenter et al 2006). The results of the geophysical and geotechnical investigations were found to be consistent at the Farr Wind Farm site. It is essential to avoid any potential problems by selecting the right construction design based on the site investigation studies; and the final design of the construction should be kept in mind, while conducting all the required site investigations. Using only an invasive investigation strategy such as trial pits would be inadequate since the resulting single point data would not provide an accurate picture about the subsurface variations (Simons et al 2002). CONCLUSION This site investigation report has documented the successful ground investigation in an ecologically sensitive environment. The various technical and logistical problems and how they were overcome have been reported. The geophysical, geological and geotechnical data provided information on the required features of engineering design of the wind turbine bases, for obtaining optimal results. The site investigation techniques included a desk study, and non-invasive geophysical surveys: ground penetrating radar, 2D resistivity imaging and seismic refraction in order to create minimum environmental impact on the ecologically sensitive moorland. The results of the surveys were verified using invasive drilling and trial pitting with sampling and laboratory testing. The site investigation was specifically significant because of the logistics involved in transferring the necessary equipment and transporting the field crews to each survey site, over 25 square kilometres of difficult landscape of high elevations, and composed of peat and bogs, while an efficient production rate had to be maintained. Question 2. DEEP SHAFT-SINKING PROJECT INTRODUCTION This report will be based on research related to a case study on Deep Shaft Sinking of the Seymour Shaft at the Seymour-Capilano Filtration Plant. The project began in mid-January 2005, and is the “first large diameter shaft that has been sunk in Western Canada in recent years” (Roach 2008: 806). This deep shaft 11 metres in diameter and 185 metres in depth was excavated by the conventional methods of drill and blast using a mobile crane for hoisting. The Seymour deep shaft-sinking project was the first part of the construction work for establishing the largest water treatment plant in Canada. Twin water transfer tunnels also formed a major part of the treatment scheme. The report will examine the requirement for early site investigation, describe the problems faced in sinking the shaft in the various substrata encountered, report on the techniques used for overcoming the problems, identify methods available for resolving the problems, and explain the safety issues. REPORT ON DEEP SHAFT-SINKING OF THE SEYMOUR SHAFT SITE INVESTIGATION The Seymour Shaft is the main construction access point for both deep shafts on the Capilano side as well as the twin tunnels. It is located about 200 metres west of the treatment plant, at the location of a buried bedrock hump that rises approximately within 30 metres of the surface. Site investigation revealed this location to be most suitable, since prolonged excavation in water-bearing overburden could be avoided (Roach 2008). Thus, the importance of early site investigation cannot be overemphasized. SEYMOUR SHAFT: GEOLOGY Medium to coarse-grained intrusives form the plutonic complex where the deep shaft sinking project as part of the water filtration plant, is located. “The rock types of the intrusive bodies range from quartz diorites to granodiorite, including true granites and diorites” (Roach 2008: 807). The large variety of different rock types has been the outcome of regional faulting, metamorphism, and other different types and degrees of alteration. The bedrock section of the Seymour Shaft and a portion of the twin tunnels, are located in the geological region where the volcanic host rock has been transformed by intruded granitic rock. The metavolcanics were found as discrete rock masses of xenoliths, screens and pendants. Different types and insensities of operation were found among both the metavoltaics as well as granitic rocks. Site investigation revealed the geology of the Seymour Shaft site to include the bedrock to be overlain with 30 metres thick overburden. The top 8 metres of overburden is composed of sands, gravel, and glaciolacustrine silts. “The remaining section to bedrock is within a glacial drift that consists of glacial till-like soils” (Roach 2008: 807). In the overburden section, several boulders measuring less than 1.5 metres were encountered. The largest boulder was around 5 cubic metres in size. In the Seymour Shaft’s Bedrock Section, varying percentages of granodiorite and metavolcanic rock were found to be present. The metavolcanic rock had a higher degree of fracturing than the granitic rock. The unconfined compressive strength (UCS) of both rock types were similar, with an average of 160 Mega Pascal units (MPa). Generally, three-joint sets were found, “with one set flat lying and dipping to the southwest and two sets subvertical and striking northerly and easterly” (Roach 2008: 807). Significantly, however, in the Seymour Shaft, no major faults were encountered while deep shaft sinking. PROBLEMS ENCOUNTERED AND TECHNIQUES USED A chamber at the lower end of the Seymour Shaft served as the launching point for the two open shield hard rock tunnel boring machines (TBMs). This chamber 14 metres wide and around 70 metres long was excavated by drill and blast. The basic design forms and common construction techniques for working underground need to be taken into consideration. For the relatively shallow Seymour Shaft the common techniques of using a head frame with multiple hoists, shaft stations, a Galloway with multiple decks for performing various tasks simultaneously, and equipment decks with shaft muckers and drill jumbos were not used. Instead, the contractor Bilfinger Berger Inc. preferred to use “a mobile crane to hoist waste rock, materials and equipment, two crawler surface drill rigs to drill production holes, and a crawler excavator to muck waste rock and clean the floor” (Roach 2008: 808). A custom-made shotcrete plant with two Aliva 252 shotcrete pumps and hoppers were employed for the shotcrete application. For each of these applications, the plant was lowered to the shaft bottom. The palaeo-weathered bedrock surface inclined downwards towards the east, was encountered at about 27 metres from the surface as predicted by the shaft pilot borehole. The 150-metres long bedrock portion of the shaft with different qualities of quartz diorite and metavolcanic rock, was excavated by drilling and blasting, using fullface, burn cut rounds drilled by two Atlas Copco R3 crawler rigs. The round depths were increased from 1.5 m of 190 holes to 4.0 metres of 225 holes. After blasting, the Caterpillar 308 was used for loading rock into three buckets which were hoisted to surface by a Liebherr 220 t crane. The contractors effectively used a simple dumping frame, thereby significantly improving the efficiency of this arrangement. The bedrock shaft was provided support with rings of 4.0 m x 32 mm fully grouted rockbolts, 25 bolts to a ring. Vertically, one ring was present every 1.5 metres, 150 x 150 x 10 weldmesh and 150 mm minimum thickness of shotcrete. Because of locally close-fractured rock along the east wall of the shaft, a few self-drilling anchors were installed for ensuring safety. The performance of this support system was optimal, resulting in instability being completely eliminated (Brox et al 2006). To awaken the mix for application, S160 Accelerator of MBT Meyco was introduced at the effective 5% dosage to awaken the mix for applicaion. Site investigation and testing revealed that the maximum time at which the batch mix was allowed to “sleep” was approximately four hours, for meeting the minimum strength requirements. Otherwise, Target bagged mix was used for the purpose. The local groundwater table was encountered at 70 metres below the surface, with minor groundwater inflows which increased with increasing shaft depth. The water was collected by ring drains into two 150 mm. PVC drain pipes shotcreted into the shaft wall, and pumped to surface by submersible pumps (Brox et al 2006). CONCLUSION This report has investigated the case study on Seymour deep shaft-sinking project, described the problems encountered in sinking the shaft through various substrata, explained the techniques used for countering the problems, and examined the safety aspects of the project. The importance of early site investigation has been highlighted, and the optimal design forms and construction techniques used have been discussed. REFERENCES Brox, D., Genschel, C., Morrison, T. & Saltis, A. (2006). Seymour-Capilano twin-tunnels project in Vancouver/ Canada. Tunnel: pp.28-33. Retrieved on 20th October, 2010 from: http://www.nodig-construction.com/doks/pdf/PR_tunnel_7-2006_Seymour.pdf Carpenter, D., O’Connor, P. & Donnelly, L. (2006). The Farr Windfarm: Site investigation for an ecologically sensitive infrastructure project. International Association for Engineering Geology Paper No.107. The Geological Society of London. Retrieved on 20th October, 2010 from: http://iaeg2006.geolsoc.org.uk/cd/PAPERS/IAEG_107.PDF Clayton, C.R.I., Matthews, M.C. & Simons, N.E. (1995). Site investigation. Edition 2. London: Blackwell Science. Roach, M.F. (2008). North American tunneling. The United States of America: Society for Mining, Metallurgy and Exploration (SME) Publications. Simons, N.E., Menzies, B.K. & Matthews, M.C. (2002). A short course in geotechnical site investigation. The United Kingdom: Thomas Telford. Read More