Basement Impact and Ground Movement Assessment
As urban environments become increasingly congested, designers, councils and other stakeholders increasingly need ground movement assessments to demonstrate that construction activities are well planned and controlled, and will not result in unreasonable impact to buildings, infrastructure or the natural environment.
CGL is an industry leader in providing ground movement assessments to aid the geotechnical design process, reduce risk and gain regulatory and stakeholder approval for construction projects. CGL’s work includes deep basements, steep retained slopes and construction over and around existing sensitive structures.
CGL’s experienced geotechnical engineers use a broad range of design software packages. These include industry standard packages such as PDISP (soil displacement), WALLAP (embedded retaining walls), GWALL (gravity retaining walls) and SLOPE (slope stability) to assess ground movements and develop impact assessments.
Where geometry, construction sequences or ground conditions are complex, CGL uses finite element/finite difference techniques for sophisticated continuum analysis, using software packages including FLAC (2D) and PLAXIS (3D) to model the soil structure movement.
Basement impact assessments (BIAs)
CGL has been an early provider of BIAs following the original guidance document by the London Borough of Camden which has since been mirrored by other local authorities. CGL have completed over 500 BIAs and Ground Movement Reports since 2011 and its extensive experience within the London area means CGL can address queries and provide an holistic analysis to determine the potential risks accurately and support the planning process.
The BIA process requires a comprehensive review of surface and groundwater conditions, and incorporates geotechnical analysis, where appropriate, to establish ground movements arising from the basement development and therefore its potential impact on neighbouring properties. This is particularly relevant where party wall foundations are to be supported.
CGL increasingly supports its clients with complex soil and structure interaction modelling, using finite element/difference analysis to quantify and control the risks to adjacent or buried infrastructure in support of modern UK and European standards.
Finite element/difference analysis represents the state of the art in geotechnical analysis and, correctly applied, can generate significant savings in analytical time and effort and in construction costs. CGL’s specialist geotechnical engineers get the most out of the highly versatile FLAC and PLAXIS software packages.
Typical engineering challenges include:
- Assessment of the impact of a proposed new build over historic brick lined sewers in North London: the analysis calculated stress changes developed in the sewers and tunnel moments and ‘ovalisation’ throughout the construction sequence.
- Modelling of the flow of soil gases from a landfill to assess the effectiveness of proposed remedial measures: the model was calibrated to comply with gas monitoring data available for local properties.
- Determination of the form of slope failure for a site involving ‘varved’ clays, using a FLAC model: the analysis was used to support a litigation case regarding significant movements on a historic culvert.
- Slope stability and loads generated in soil nails for a railway cutting in chalk: the model predicted a significant reduction in the number of soil nails required.
- Predictions of ground movements from a new basement construction immediately adjacent to existing properties: the analysis combined the effect of soil heave, basement wall deflections, determined prop loads and retaining wall deflections, settlement beneath party walls, and informed the construction monitoring plan.
- Assessment of the impact of laterally loaded piles installed close to a railway piled structure in very soft alluvial soils: the analysis was used to show both the structural engineers and other parties that the impact of the new piles would be minimal.
Slope stability assessments
The key to a successful slope stability assessment is a strong knowledge of soil behaviour coupled with an understanding of the analytical approaches necessary to balance safety with design efficiency. Successful infrastructure design can require the geotechnical engineer to push cost effectiveness limits. Maximising slope angles often allows increased developable areas and the reduction of construction materials.
CGL has a long established track record in the assessment of slope stability from works on road and rail embankments/cuttings as well as design of reinforced earth slopes and platforms. CGL designs remedial works on failed slopes or emergency stabilisation measures and provides expert opinions in respect of the causes of failures.
CGL’s investment in analytical software allows the company to make detailed assessments of complex ground movements or use predictive modelling and implement the Observational Method to control structures during construction with significant cost savings.
Ground Engineering magazine awarded CGL ‘Best UK Project with a Geotechnical Value Between £1-£3m’ for our work using these techniques at the Subsea 7 headquarters in Sutton, Surrey. By using targeted site investigation to understand realistic geotechnical design parameters and hydrology, CGL modelled the predicted ground movements in structured chalk. This enabled a pragmatic approach using observational methods with a cost saving combination of open cut excavation and King Posts with robust monitoring controls, avoiding a contiguous pile wall solution for a 20,000 cubic metre basement.