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High cost of conservatism


Over the last 12 months I have encountered increasing resistance from clients on recommendations in the BRE guidance: Working platforms for tracked plant (2004).

The document recommends non-uniform loading distributions be transformed into equivalent uniform loads over a reduced area using the method after Meyerhof (B'=B-2e and L'=L-2e). So far I agree.

In Appendix A the adopted failure mechanism (punching failure) for a two layer system is shown in Figure A1 and the text does indeed say 'it is considered that this simpli'ed approach has within it a degree of conservatism'.

The punching failure approach (after Meyerhof and Hanna) in this particular application is too simplistic. As any young geotechnical engineer knows, the applied pressure at a depth z is somewhat less than at the underside of a foundation (in this case the machine tracks).

It can be shown that the diminishing stresses are a function of the foundation (track) width, such that for a typical 600mm wide track there is likely to be only 20% to 55% of the applied stresses at the base of a 600mm thick mat.

I have used the Newmark approach to adjust the track pressures to more realistic values. It is these reduced stresses that should be considered at the subgrade as those that are causing the punching failure. After all, punching of the mat alone is unlikely to occur first where the mat strength is stronger than the underlying subgrade, which in the majority of cases it is.

Furthermore, the punch mechanism ignores any internal (and beneficial) shear stresses in the mat. My own studies using the finite element method suggest that the above hypothesis is valid.

Using the Newmark approach one can arrive at a mat thickness using an iterative procedure by first estimating the mat thickness and calculating the stresses at the subgrade; these stresses are then used to check whether the mat thickness is sufficient.

This is a simple, yet valid, adjustment worthy of consideration for inclusion in the BRE 'routine' design approach.

I have been able to validate my approach against finite element studies which clearly show high stresses within the mat and low stresses in the subgrade below.

Given there are a limited number of types of piling rig in the UK fleet, it would not be a particularly onerous task to look at the load cases pertaining to each of these against a series of typical ground conditions using finite element analysis.

These ground conditions might be a range of undrained shear strengths and a range of drained soil parameters, typical of UK sites. A series of charts could be produced from which the suitably qualified user could read a platform thickness against a particular piling rig type and ground model.

Turning to the effects of the conservative assumptions that are a feature of the method: BRE qualifies its approach in that it should not be used where undrained shear strengths are less than 20kPa or greater than 80kPa.

I was recently asked to design a mat for a Junttan PM20LC driven piling rig, which has a high Case 1 loading pressure due to lifting and pitching of piles into the leader.

The subgrade was firm clay with an undrained shear strength around 40kPa, falling neatly within the range that BRE says is acceptable for its punching approach.

Adopting the BRE method the mat thickness was about 1100mm 1 compared with 400mm adopting my own reduced stresses approach.

This particular project required a mat some 30m by 100m, hence the total required extra material of the thicker mat was about 3500t.

This project was fairly typical of a medium sized piling contract common in the UK today.

I believe there is scope for rationalisation of the current approach.

For those who have been involved with mat design for some time, as I have, publication of this document must have raised a few eyebrows, particularly in piling contractors' design offices.

It is clear that the potential additional costs of the BRE approach are significant both economically and environmentally.

I would like to invite readers to debate this issue, particularly those who have found similar results and believe the current approach is far too conservative, and those who can comment with greater authority as regards to the cost to the economy and the environment. I also invite the BRE to comment.

Stuart Bradshaw, managing director, Terrain Geotechnical Consultants 1

BRE declares in Section A1.4 that the method is inappropriate where D/W>1.5, in this cited case D/W was 1.47 for a 1100 mm thick mat and 750 mm wide tracks.

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