The working platform debate continues (GE March 2006). Stuart Bradshaw, David Corke and John Gannon all offer their opinions on a topic that is still generating reader response.
Stuart Bradshaw, managing director, Terrain Geotechnical Consultants
Further to my letter published in the March 2006 issue of Ground Engineering relating to the BRE guidance: Working platforms for tracked plant (2004) and John Gannon's and David Corke's contributions (GE April and June 2006), I have a few more points to consider.
It appears design charts were discounted due to the wide range of rigs. But any variations would change the applied bearing pressures, so surely they are as applicable to the current numerical approach as they would be to a graphical one?
The re-distribution of a triangular applied bearing pressure to a rectangular pressure enables calculation of a single factor of safety against bearing capacity failure for a foundation loaded eccentrically.
It is clearly an over-simpli. cation of the true insitu stresses, yet has stood the test of time.
A bearing capacity failure of a foundation loaded eccentrically can, using this approach, be related to a more sensible average applied pressure rather than the peak pressure, which could be considered overly conservative.
Stress distribution and high mat stresses David Corke's apparent reticence towards my suggestion that applied stresses diminish with increasing depth below the tracks and his concern that platform material has the strength to distribute the load in this way is surprising.
The reduction of applied stresses with depth is common practice and in many ways is analogous to the stress distribution through a road pavement.
Piling mat materials have angles of friction over 40°. It would be expected that well compacted angular hard demolition material with these shear strengths should be capable of distributing applied bearing pressures through to a weaker subgrade.
John Gannon refers to the internal stress equations in BRE 470 which include a passive term generated by the platform.
Punch mechanism ignores any internal or bene. cial stresses because the mat (in general) is the stronger of the two materials and can generate very high internal stresses.
Below the mat applied stresses drop off rapidly.
This arching effect appears to be present even in relatively thin platforms (300mm), my own (albeit limited) FE studies show the failure mechanism to be typically Prandtl.
This suggests the failure surfaces should move up to the underside of the tracks, as even for a 300mm thick platform, the mechanism at failure appears to be dominated by the bearing capacity of the platform material itself. It appears the stronger platform material is attracting the applied load rather like in frame analysis, where the stiffer sections attract bending moments in proportion to their stiffness.
While the BRE approach considers a passive contribution from the platform, it cannot take account of stress reduction at the subgrade, unless another mechanism is introduced, such as Newmark.
It would be impractical for FE to be adopted in everyday use but it can be used to measure the effectiveness of more simple approaches.
Platform material strength
The WRAP document quotes some surprisingly high angles of friction (up to 64°), but is it realistic to expect engineers to select such high parameters, particularly when test results are sparse or non-existent?
The observed angle of repose of stockpiled recycled aggregates suggests an angle of about 45° is more realistic.
The BRE document refers only to angles as high as 45° with only Kptand values up to this.
The BRE document, which makes no reference to the WRAP document, will tend to produce thick platforms, primarily because it does not recognise a reduction in the applied stresses at the subgrade due to the presence of the platform, which surely is the purpose of it being there.
David Corke suggested higher angles of friction are applicable for the platform material and that these should be bene. cial, but how does is that controlled?
Shear box test results would be linked to a grading curve and to ensure replication the material would have to be tightly controlled and this is unfeasible in practice.
Lower angles will be used because engineers have no experience with materials displaying the angles in the WRAP report.
I hope the City University research project does not recommend shear box testing take a pivotal role in working platform design.
In reality such testing will not happen.
The black box I was surprised that the 'loading factors' are not load factors or factors of safety but are 'empirical'. This makes the method appear to be a black box approach. The rationale behind these factors should be there for all to see.
BRE 470 goes a long way from rudimentary approaches but the broadly adopted 300mm (or 1 foot) thick stone mat has served the industry reasonably well.
However, the BRE method creates mats two to four times thicker than what was once taken for granted.
This is a cause for a re-think, particularly with today's focus on environmental costs.
David Corke, managing director, DCProjectsolutions
The difficulty with producing charts for platform design is the number of variables involved. A one page spreadsheet provides a simple means to calculate platform thickness and carry out checks.
Platform material friction angle is a sensitive issue. During the design method development it became clear that if the platform material angle of friction was assumed to be within 'normal' values, then uneconomical platforms thicknesses would be calculated.
Loading factors could have been adjusted so that 'normal' angles of friction produced 'normal' platform thicknesses, but this was potentially dangerous. Using a high, but realistic, angle of friction, would result in a dangerously thin platform. The only safe course of action was to maintain the use of realistic angles of friction, but recognise that an ongoing education process would be needed to ensure realistic values would come to be acknowledged and accepted.
The BRE method was conceived to be simple and robust, and the report makes it quite clear that the analysis is semi-empirical. By the still continuing substantial sales of BR470, it has fulfilled its intended purpose well in excess of expectations.
Some while ago, someone said to me 'surely the rig manufacturers design the rigs to have similar bearing pressures.' Nothing could be further from the truth. Track bearing pressures range from less than 100kN/m 2 to in excess of 600kN/m 2.Stuart Bradshaw refers to 'the broadly adopted 300mm (or 1 foot) thick stone mat served the industry reasonably well'.
Bearing in mind the range of loadings, track sizes and underlying ground conditions that can be encountered, such a simplistic approach cannot be safe and could lead to serious underestimation of platform thickness.
The BRE design method is accused of being a 'mechanism that derives mat thicknesses of two to four times what industry once took for granted'.
This is both alarmist and incorrect. Realistic input into the BRE design method will produce realistic platform thicknesses. The output from the BRE design method was subjected to a lengthy benchmarking process, with a wide range of rig and crane types, to ensure that with realistic input parameters, safe but economical platform thicknesses would be produced.
There is a particular problem to be addressed and resolved; how best to determine realistic and reliable design parameters, in particular, platform material angle of friction.
Existing testing is not ideal, because of the British Standard's size limitation, but it has to be better than guesswork, although a new or modified test needs to be developed. I think we should be seeking to improve reliable working platform safety. Just because something is difficult, and differs from an historical approach, we should not be dissuaded from its pursuit.
John Gannon, director, Byland Engineering Design charts
Design charts are, in principle, attractive. However, the calculation procedure of BRE 470 uses 14 input variables for a cohesive subgrade and 17 for a granular subgrade, each beneath an unreinforced mat, which may make charts overcomplicated.
The dimensioning of an equivalent rectangular stress block is a tried, tested and satisfactory means of dealing with eccentric loading from a track.
Stress distribution and high mat stresses
A pavement type approach to platform design would not necessarily yield a more economic platform thickness. For permanent pavement design (which adopts an empirical chart based or an analytical stiffness based approach), the contact stresses will be lower and the loaded areas will generally be smaller than piling platforms. Also, the combined thickness of bound and unbound pavement layers is seldom less than 450mm and on weak subgrades is often considerably more. During construction the thickness of capping used beneath the sub-base to protect the formation from overstress and damage by construction plant is satisfi ed by the requirements imposed by the permanent works design.
Where the platform designer can reasonably assume that punching shear will not be the critical failure mechanism, an alternative approach to that of BRE 470 is warranted.
This might consider the 'angle of load spread' through the platform material and a bearing capacity check on the subgrade using the diminished stresses. The extent to which load spread and arching are developed will depend, inter alia, on mat thickness and its shear strength.
Platform material shear strength
Designers will be reluctant to adopt friction angles for platform material much above 45infinity. Site investigations often largely ignore the upper 1m to 2m of ground from where piles are usually installed.
During construction, there is often no time or budget to take samples of platform material or to undertake shear box tests. Insitu plate bearing tests are often carried out on subgrade and the platform as a quality control device to check the construction standard is met. Where the loading plate is of adequate size, tests can yield layer stiffness values which would be useful in a stiffness-based design method, if one were available.
The black box
The BRE 470 design method is not a black box approach. It is a fairly transparent calculation procedure with known assumptions, inputs, outputs and limitations. It is perhaps conservative in ignoring the ability of arching in the platform to reduce stresses in the subgrade arising from surface applied loads, so it should be reviewed to take advantage of the potential benefits of arching.
If a calculation procedure considered shear strength, stiffness and stress state in the platform and subgrade, then more economic designs could emerge. It is impracticable to propose a continuum analysis approach for routine design. Therefore, a chart based procedure, validated by model tests and instrumentation of actual platform behaviour, using as few easily determined parameters as possible, should be developed.