Report on Geotechnical Assessment Land Capability Assessment – Wilton Junction Hume Highway and Picton Road, Wilton
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Report on Geotechnical Assessment Land Capability Assessment – Wilton Junction Hume Highway and Picton Road Wilton Prepared for Wilton Junction Landowners Group c/o Elton Consulting Pty Ltd Project 73467.00 June 2014 Page 1 of 3 Executive Summary This report, including Sections 1 to 18, is written in response to the NSW Department of Planning and Environment Draft Study Requirements (DSR), specifically DSRs 7 and 15, which state: x DSR 6 – Mine Subsidence (Part 2 only) - Geotechnical analysis to ensure identified development areas and design parameters for housing and infrastructure are appropriate to manage impacts from mine subsidence and other geotechnical risks. x DSR 7 – Topography, soils and geology - Analysis on the suitability of the proposed lakes and on-site water recycling facilities to ensure that water quality is managed tom minimise impacts to natural water sources and to the water regime / water availability for habitats. - Analysis on maintaining environmental flows into the natural system from the urban areas and management of sediment, chemical and nutrient build up during low flow (drought) periods. - Salinity assessment for building, water management and open space. - Consideration of future land use zones to reflect appropriate levels of protection for ridges, waterways and other natural features. x DSR 15 – Agricultural land suitability (refer Appendix H) - Undertake a high level study of the agricultural potential of the land. This report presents the results of a geotechnical assessment undertaken by Douglas Partners Pty Ltd (DP) as part of a greater land capability assessment for a 2741 hectare site, which forms the land parcel known as Wilton Junction, situated about the existing intersection of Picton Road and the Hume Highway in the suburb of Wilton. The work was commissioned by Mr Brian Elton of Elton Consulting Pty Ltd, the client’s representative and proponent, on 3 May 2013, on behalf of the client, the Wilton Junction Landowners’ Group (WJLG). It is understood that the site is subject to a rezoning application ultimately for the staged urban development in general accordance with the proposed master plan (prepared by Connor Holmes Property Services, Project No. 64127, dated 19 May 2014). The Master plan is shown as Figure 1 and indicates staged developments comprising approximately 11 000 to 13 000 residential dwellings, five schools, five shopping centres, commercial and retail facilities, community facilities, open spaces and associated roads and infrastructure, to be constructed over a period of approximately 30 years. To assist the rezoning application, DP has undertaken a high-level geotechnical assessment of the site to determine its suitability for urban development, with specific consideration given to the draft study requirements (DSRs) outlined by the NSW Department of Planning and Environment (refer DSRs 6, 7 and 15). The geotechnical investigation addressed the site’s geotechnical surface and subsurface conditions, slope instability risk potential, soil erosion risks, soil salinity, agricultural potential (undertaken by a sub-consultant) and geotechnical developmental constraints. The Wilton Junction Study Area covers 2741 hectares of land situated about the intersection of the Hume Highway and Picton Road, Wilton (refer Figure 2). The land is held by many title holders. Report on Geotechnical Assessment – Wilton Junction Project 73467.00 – Rev 6 Hume Highway and Picton Road, Wilton June 2014 Page 2 of 3 Individual parcels range from large holdings held by significant land developers to individually owned rural and residential allotments. The current rezoning application is being undertaken as a collaborative approach essentially progressed by a consortium of four land developers, Lend Lease Communities (LLC), Bradcorp Wilton Park Pty Ltd (Bradcorp), Governor’s Hill (GH) and Walker Corporation Pty Ltd (WC), collectively known as the Wilton Junction Landowners’ Group (WJLG). Together, the WJLG own approximately 70% to 75% of the Wilton Junction site, with the remaining 25% to 30% owned by other small landholders. The land ownership can be seen on Figure 3. The 455 hectares currently controlled by LLC has already been rezoned and has progressed to staged construction (commenced in late 2006), with several development stages either complete or nearing completion. Accordingly, this part of Wilton Junction does not require consideration under the current assessment and thus Lend Lease is not a proponent for rezoning. The existing Wilton village has also been excluded from the assessment, as rezoning of this area is not required. In addition, smaller landholders are to be considered from a desktop perspective only due to site access constraints. A review of available geology maps for the site indicated that the central part of the Site is underlain by Ashfield Shale with some higher elevations possibly underlain by Bringelly Shale. The formations typical comprise laminite, siltstone, shale, carbonaceous claystone and coal in parts. Lower elevations and riparian zones within the site are underlain by Hawkesbury Sandstone, comprising medium to coarse-grained quartz sandstone, very minor shale and laminite lenses. The Mittagong Sandstone transitional member underlies areas of the site affected by the interface of the Ashfield Shale and Hawkesbury Sandstone formations. Soil landscape maps indicate that the site includes four soil landscape groups, including Blacktown, Lucas Heights, Hawkesbury and Luddenham soil landscapes. Mapping indicates that most of the site comprises soils of the Blacktown soil landscape, which is a residual soil landscape that comprises soils that are of low fertility, moderately reactive, highly plastic and generally low wet strength. Riparian areas of the site are within the Lucas Heights soil landscape (mapping unit lh), which is a residual soil landscape that comprises soils that are of low fertility, stony and slightly reactive and have low available water capacity. Areas surrounding the Nepean River, Cordeaux River and Allen Creek are within the Hawkesbury soil landscape. This is a colluvial soil landscape that comprises soils that are exhibit a high erosion hazard, are generally shallow, stony, highly permeable and of low fertility. Higher elevations at localised peaks are within the Luddenham soil landscape, which is an erosional soil landscape that comprises soils that are associated with a high soil erosion hazard, localised impermeable highly plastic subsoil and are generally moderately reactive. Subsurface conditions encountered during the geotechnical investigation confirmed the presence of the mapped soil types and rock formations. Soil types are generally clayey, with rock encountered mostly within 2 m of the existing ground surface. Laboratory test results undertaken on soil samples collected from the site indicated the soils to be general of medium to high plasticity, to have a predisposition to erosion and a moderate level of sodicity. Soils were found to be non-saline and generally non-aggressive to concrete or steel. Based on the results of the assessment thus far, the following summary points are noted: x No evidence of hillside/slope instability was observed within the proposed net developable area. It is considered that such instability does not impose significant constraints on the proposed site development under the current Master plan. Report on Geotechnical Assessment – Wilton Junction Project 73467.00 – Rev 6 Hume Highway and Picton Road, Wilton June 2014 Page 3 of 3 x The presence of erosive soils on site should not present significant constraints to development provided they are well managed during earthworks and site preparation stages. x No significant evidence of saline soil was identified within the site. Although further salinity testing is considered necessary, at this stage salinity levels are sufficiently low for this site to be deemed free of significant salinity constraints. x Although mild aggressivity to concrete is regularly encountered across the site, aggressivity levels are considered to be manageable, subject to appropriate design and construction consideration. x Highly sodic and sodic soils appear widespread and will require management to reduce dispersion, erosion and to improve drainage. x The results of the land capability assessment have not identified any issue that would preclude the rezoning of the Wilton Junction site for urban development. The results of the land capability assessment to date have not identified any issue that would preclude the rezoning of the Wilton Junction site for urban development. Further investigation will, however, be required as the project progresses to development application and during project construction. Investigations would include (but not necessarily be limited to): x Additional salinity investigations for site soils and surface waters (i.e. dams) to increase the density of the data obtained to date. The investigation programme should be increased to compliment the current study and augment the findings to a frequency of testing satisfying one test pit location per one to two hectares, including additional full depth profile sampling and laboratory analysis. A cost effective way of conducting the salinity assessment would be to measure site conductivity using an electro-magnetic (EM) transceiver mounted to an all-terrain vehicle (ATV or quad-bike), thus reducing the number of test pits required for the assessment. This method would also significantly increase the number