Earthworks; Commonhead Land Redevelopment, Swindon
Introduction:
We refined existing information and provided additional detailed assessments across several phases of this ongoing residential and commercial development. We have been commissioned on this project since 2017 to offer supplementary geotechnical and geo-environmental assessments, resulting in better understanding of the site/phase-specific ground models, leading to improved design information and cost savings for the Client.
Details:
We have recently undertaken a detailed investigation for an approximately 15-hectare area of the site allocated for residential development, which included detailed stability analyses and earthworks specification designs for noise attenuation bunds.
Previous 3rd party exploratory investigation works identified a varying ground model across the wider greenfield site, with vary bedrock geology comprising the Lower Greensand Group and the Gault Clay Formation. Further investigations by ESP within specific phases of the development site, allowed a more detailed and directed assessment, to gain a better understanding of the potential hazards relating the proposed structures and site end-users.
This enabled us to work with the Client to improve and modify design proposals, specifically in relation to foundation and floor slab design, road design, protection of buried concrete and re-use of site-won soils.
The recent investigation identified a ground model comprising weathered Gault Clay Formation underlying the surface Topsoil. The Gault Clay is widely known for geo-hazards relating to volume change potential (shrinkage and swelling) and the potential elevated levels of sulphate, which can attack buried concrete (foundations). As well as a generalised investigation, the works were designed to assess these potential geotechnical hazards and provide mitigating measures to ensure the safety of the future development.
As part of the proposed development works, a noise attenuation bund is proposed along the south and north east boundaries, to mitigate noise from the M4 slip road to the south and the A419 (Marlborough Road) to the north east. The development site gently slopes north from the the southern portion (adjacent to the M4 motorway) and therefore in order to construct a suitable development plateau, ground levels in the south of the site will be lowered by up to 2.0m, and the excavated soils are proposed for re-use as site-won fill materials to construct the noise attenuation bunds.
ESP provided detailed recommendations within a formal ‘Specification for Earthworks’ for the proposed earthworks in addition to that provided in the Code of Practice for Earthworks (BS6031), Series 600 of the Specification for Highways Works, ICE Publication “Earthworks: a guide” and other relevant guidance.
Detailed slope stability assessments were also provided utilising the GeoStudio Slope W package, in order to assess the factors of safety (FoS) within the proposed embankment/cut slope in the south of the site, to ensure the long-term stability of the constructed noise attenuation bunds. Based on the preliminary results, adjustments to the bund designs and geometries were made, resulting in adequate Factor of Safety (FoS).