Holdfast Training Services Trial Blog

Figure 1 – 45 degree slope with 80T crane surcharge

Whilst wondering around site during my first couple of weeks I couldn’t help but notice the slope in the picture situated in River Terrace Gravels. Having now read the GDR I know the design phi dash of the River Terrace Gravels to be 33^o. Therefore a design angle of Beta at approx 60^o. When I queried a couple of the site/section engineers the response was that they always cut a slope at 45^o and then step it if it is a larger slope. The layer below the River Terrace Gravel is London Clay and I would agree that for the short term in clay this would be sufficient, relying on the undrained shear strength. However is the design of 45^o in the River Terrace Gravels acceptable?

Firstly a slope cut at 45^o would still suggest a safety factor of 1.3, assuming that the pumping of ground water has reduced the ground water regime profile of the water level to below any slip surface. There is a sump reducing the water level to approximately 3m below the toe of the slope, so I will make this assumption.

I modelled this case in Geo5 (Figure 2), producing a result of instability. Geo 5 was showing failure in DC2 but not DC1. This is because DC2 is more conservative where a gamma factor of 1.25 is applied to the tan phi dash, reducing phi dash to 27^o. As this is a temporary load case and we know phi dash will not be as low as 27^o is this suitable for a temporary works solution?

Figure 2 – Slope modelled on Geo 5

Moving on – As you will see in my photos there is a crane (80T) operation at the top of the slope, therefore once you take this load and factor by 1.3 (NA to 1990. NA.A1.2(C)) and re-model in Geo5 you can see it fails with a larger rotational slip. The crane is in fact sitting on a 400mm deep concrete blinding but I cannot model this without Geo 5 ignoring STR failure in the concrete pad. If I ignore the pad then obviously the situation is worse but the principal the same.

Figure 3 – Slope modelled with surcharge

The gamma factors in the Eurocodes are there as guidance and therefore I would argue temporary works is the ideal time to reduce them if considered safe to do so by the engineer (allowing for other factors). Is this an example of just such time when the ground seems to be behaving as expected or is this too great a risk. However currently there is no temporary works design for this slope and the surcharge inflected by the crane; so I would suggest it is too much of a risk!