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evolutionwinter2012

Physical Effects The ultimate concerns presented by Rainfall inclement weather are those physical effects that manifest as failure mechanisms: Vegeta�on Flow failure Frost shattering Runoff Prolonged rainfall Granular H Flooding and scour. Cohesive (Impermeable) Flow failure Chalk L Figure 4 Frost Shattering Flow Failure is where the rainfall and run- off towards the crest of a cutting results The usual consequence of frost shattering is in mobilisation of flowing material. An Figure 3 Flow Failure Factors the accumulation of debris at the toe, which example of this type of failure is shown can present a major maintenance problem, in Figure 2. A standard risk assessment of Risk = and when severe affect the track. Likelihood x Consequence was carried out. Less common, but far more severe is To do this, each likelihood factor was the secondary effect of large boulder categorised into scored levels of impact, movement, which could cause an and then a weighting factor applied, in obstruction or damage the rails. order to give a final likelihood value. Figure 5 shows a large flint which was For example, “Previous Failure” had the found protruding from an upper chalk face. options of “Definite” (score 5), “Possible” This required controlled removal during the (score 3) and “None” (score 1). This factor night shift in order to eliminate the risk has a weighting of 1, so for an asset with a of it rolling down slope during normal Figure 2 Flow Failure at Chalfont & documented history of previous instability service hours. Latimer, 1992 this would give a value of 5, which would be included in the total likelihood value. This failure took place in superficial head In contrast, “Slope Angle” also has three material overlying a chalk cutting on the categories – over 35°, 25-35°, under 25° northern extent of the Metropolitan Line, - and a 5-3-1 scoring system, but is between Chorleywood and Chalfont & weighted by 50 in order to reflect the Latimer. This incident has added significance greater relative impact. as it prompted London Underground to commission a review of all “Chalk Cuttings” The consequence severity value was also (Mott MacDonald 1995), as to that point collated using this approach with the the relative composition of the assets consequence factors. between chalk and superficial materials was unknown. The weighting methodology was developed using a combination of historical Factors increasing the likelihood of a observations and engineering judgement. flow failure are: Granular/head material to be mobilised Once all risks were calculated the assets were assigned classifications of “High”, Steep slope angle “Medium”, “Low” or “Negligible” based on Previous instability set score ranges, and all assets were given a Adjacent land sloping towards the relative ranking. cutting crest Impermeable surface next to crest, Frost Shattering eg a car park On the London Underground network Figure 5 Large Flint this mode of failure is confined to the Lack of binding vegetation. Metropolitan line as only this route cuts Another secondary effect of frost shattering through a chalk outcrop. is the loss of root embedment for trees, Factors increasing the severity of which in turn break free and fall, thus consequences are: Where the chalk face becomes exposed, creating a new exposure and increasing the Proximity of the asset to the track, (perhaps due to sudden tree falls or primary risk. (and lineside services where present) over-steepened cuts), rainfall can infiltrate Curved track preventing adequate into the chalk structure. On freezing the Considering the basic frost shattering sighting time for braking expansion causes shattering on a varying mechanism as progressive rather than Speed of train scale from surface fragments to block or sudden, (such as flow failure), the flint extrusions. The chalk cuttings are methodology for its risk assessment was Frequency of train service generally steep, so giving added momentum determined by what could be observed Trees or other large vegetation being to rolling debris. Figure 4 shows a face to be present or developing in terms of uprooted. exposed by tree fall, giving an increased risk extent of debris and exposure, as shown Figure 3 shows some of the above factors. of frost shattering. in Figure 6. 7


evolutionwinter2012
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