Holdfast Training Services Trial Blog

This is only my second post since being on Phase 3. The reasoning for it has been this bloody Cat 3, external check, on a pile design. The deadline for it was the 14th, and then I took some time writing up my TMR 4, which happens to be about piling.

The pile design was completed by Skanska, for 3 x 30 story buildings. The buildings were sandwiched between the River Thames and a 4 track main route into Waterloo.

The first considerations that I looked into where the geological conditions. As London is located on, what use to be a sedimentary basin some 66 million years ago, much of the immediate ground is made up of clay. The clay is formed of both London Clay and Lambeth Group. What does that mean for the design? After some detailed revision of John’s notes and a little research I released that there is the potential for two events: Overconsolidation and Artesian pressure. The next question was, how can these effect the design.

Overconsolidation in the clay meant that there was a high probability of anisotropic properties in the soil, this translated into a Overconsolidation Ration (OCR) of potentially 5-6. Hence I could have differing values for soil properties such as strength and stiffness, depending on the plane in which I was analysing. This also meant that as we excavated the soil, we could expect much more swelling than would otherwise be expected. This would cause excessive swelling, much more than would be expected from elastic swelling. Hence there would be heave on the piles, however the piles would experience differing amounts of heave as the stiffness of the soil would be greater, the deeper the pile was. This would cause a stretching effect on the pile, which would need tension reinforcement.

Secondly if Artesian pressure was present in the lower aquifer, which is contained in the Thanet Sand and chalk, there might be a need for dewatering to reduce heave on the piles. Clearly this would only be necessary if the piles where penetrating the lower aquifer, hence they would have to be long piles. As a side note, the Shard has 200ft piles which did penetrate the lower aquifer, however there was no Artesian pressure in that area (due to historical welling taking place).

On top of both of these geological considerations, I was dealing with Clay. Therefore I had a long term and short term analysis to consider. In the short term (undrained) I would be using total stress parameters, such as Shear Strength (Su). In the long term (Undrained) I would be using effective stress analysis and properties such as Effective Cohesion (C’) and Effective friction Angle.

I knew that the pile resistance was calculated using the Shaft Resistance and the Toe bearing capacity. Hence I needed to use:

Shaft resistance = Cu N

Toe bearing = alpha.Cu or a more accurate effective stress analysis equation which I can’t write down as I haven’t got time (John you will see it in my TMR)

So the first step was to conduct the same stages as we did on Ex Cofferdam. Look at the boreholes, compose a worst case scenario of the cross-section the piles would be penetrating. From there, I created a spread sheet. The columns included, depth, depth of strata, soil properties, end bearing resistance, shaft resistance etc. The next step was to apply the factors from EuroCode 7. This gave me the ULS 1, ULS 2 and SLS combinations. It turned into a mammoth spreadsheet, I might get it framed.

I then looked at the actions on the piles, from the column loads. Applied the factors giving me SLS,ULS1, ULS2 and compared these to the previous results. If the actions where smaller than the resistance from the soil, the piles where OK.

The last check was the structural capacity of the pile. I needed to check the reinforcement in compression, bending shear and tension (as previously described). This required the use of the Broms equation, to turn the horizontal loads (given in the piling schedule) into moments. For each check, I used Eurocode 2 and Rich Farmers notes on column design to guide me through. I designed the piles as short stocky piles, as the effective length was minimal as the soil supported the column/pile the entire length. As John Moran suggested, as long as post holing does not occur, this design method seemed appropriate.

The next stage was to analyse the retaining wall, which was supported by buttress’. I have got time to do this justice, so I will explain this in the next blog next week. It was surprising interesting, so I will produce a few sketches.

Burton out!!!