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evolutionwinter2012

INfrastructure Orientation of flow path Presence of lining Remediation against deep-seated failure. Where there is crest or mid-slope remediation such as a bored pile wall, then some resistance is given to scour, but this is still not desirable. Figure 6 Frost Shattering Risk Assessment and Evaporation Calculation System). This allows an ongoing review of status for the Summary of Risks Lower frequency severe events such as High risk assets. Figure 8 indicates the current risk profile boulder or tree movement can be for London Underground with respect to deduced to be more likely on a slope with Flooding and Scour inclement weather effects. high risk of frost shattering, but require A risk of instability from flooding mainly site specific review as a direct correlation affects the embankments where inundation cannot be assumed. of the toe by flooding rivers or streams can lead to increased pore pressures Prolonged Rainfall within the embankment, which may in turn Prolonged rainfall can saturate the soil to compromise the slope stability. the extent that low or even zero SMD values develop, especially during winter as The locations of watercourses and their rainfall increases and moisture uptake from floodplains on the London Underground vegetation decreases. A corresponding network were identified, and the probable Figure 8 Risks by Type and Frequency increase of pore water pressures and effect on each relevant asset reviewed. decrease in effective shear strength then Environment Agency data was also essential Flow failure can be seen to be the most presents a risk of deep-seated failure. to the risk assessment as showed historical prevalent risk by far, although the other Figure 1 shows an extreme example of flooding elevations. risk types must not be discounted as the failure mode. even one incident can have a serious Not all embankments near to watercourses effect on the railway. In geographical This type of instability is more likely to were assessed as being at risk, where: terms, the chalk cuttings stretch of the occur in cohesive embankments or cuttings, Embankments lay outside flood extent Metropolitan line is the most at risk, as and for London Underground the London subject to frost shattering and flow failure. Clay assets are particularly vulnerable. Risk Across the rest of the network the geologyEmbankment has had physical remedial levels for such assets have been assessed and topography influence which failureworks such as bored pile wall, offering with respect to: types are most probable.increased resistance to flooding Embankments with flood-level pore Composition – whether the London water pressures still yielding acceptable Mitigation Measures Clay proportion is significant factors of safety. With the effects of inclement weather Factor of Safety against failure under translated into risks, this allowed London London Underground Standards with The remaining assets were categorised as: Underground to improve in the following either a directly established or assumed areas:Medium risk-factors of safety following groundwater regime flooding are expected to be between 1 Raising awareness and understanding Sensitivity to changes in pore water pressure. Rainfall measurement and trigger levelsand 1.15 High risk – Factors of safety following Advance warning and planning flooding are expected to be less than 1. The assets were assigned to the following Vegetation management categories: Scour is where a watercourse is flowing Strategic Risk Assessment & works at the toe of an embankment, fast enough Low risk-assets were not considered prioritisation. to undercut or detach sections of the further toe, as indicated in Figure 7. This can Medium risk-assets which, based on the lead to progressive failure, and eventual Conclusions current factors of safety, are considered undermining of the track. Overall, the London Underground review to be sensitive to pore water pressure of inclement weather and its effects on changes but would still have a margin of earth structures found that: stability when saturated Inclement weather is here to stay, with High risk-assets which, based on the a likely increase in uncertainty as to current factors of safety, are considered severity and duration of incidents to be sensitive to pore water pressure Risks should be assessed both individually changes and would potentially become and strategically unstable or have marginal stability when Management plans should be robust, and saturated. regularly reviewed Rainfall measurements are available Figure 7 Scour causing detachment Sound engineering judgement informed remotely from dedicated weather stations by monitoring data should be factored across the London Underground network, Risk was assessed through considering: into asset intervention decision-making. and SMD data is obtained on a monthly Distance to river/ stream from the basis from MORECS (Met Office Rainfall embankment toe 8


evolutionwinter2012
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