their mass and center of gravity. Fortunately, the availability of the input data from rig manufactures has improved with time. For most applications, it has not been U.K. practice to consider dynamic loading in the estimation of track bearing pressures for the use of granular platform design. In routine situations, this may be reasonable due to the low speed of rigs when traveling, low vibration during operation, gradual lifting or release of suspended loads, and that dynamic bearing capacity and stiffness of soils (including the granular platform) are usually greater than static values. Likewise, wind loading may not be significant in routine situations. However, the potential for these additional forces to critically affect the applied track bearing pressures should not be overlooked and should be incorporated into the bearing pressure calculation, when necessary. Design of a Granular Example of piling rig damage due to toppling over At the time BRE 470 was published, the Extensions and add-ons to the standard spreadsheet are possible to accommodate operating conditions outside of the standard limitations (e.g., sloping sites, inclined mast, etc.). Loading Factors Within BRE 470, the margin of safety against the occurrence of bearing capacity failure of the granular platform is covered using load factors applied to the calculated track bearing pressures, with characteristic design parameters for the platform material and subgrade. Two loading cases are considered: 1. Case 1: Where the rig operator is unlikely to be able to aid recovery from an imminent platform failure. Exam- ples include operations such as standing, travelling and handling. 2. Case 2: Where the rig operator can control the load safely to aid recovery from an imminent platform failure. Examples include operations applying to piling rigs such as installing casing, drilling and extracting an auger. BRE 470 includes travelling or slewing with a fixed mast, which has a foot or fixed load (e.g., a pile held in the leader) close to the platform surface within this category. Eurocodes were not in regular use in the U.K., and limit state design was not in routine use for geotechnical structures. Since EC7 does not provide specific guidance for the design of working platforms in general practice, it is not usually used for the design of granular working platforms for piling rigs. Practical Aspects It is relatively straightforward for a compe- tent person to perform the static equilibrium calculations to determine the ground bearing pressures for a given piling rig, but practical and operational factors add complexity to the process. Obtaining relia- ble data on rig component weights and centers of gravity has not always been easy. In 2004, when BRE 470 was published, many rig manufactures did not always supply the information required for the bearing pressure calculations and, for older rigs, the information was simply not avail- able. At times, it was necessary to remove and weigh rig components to determine Loading Condition Case 1 Case 2 Load Factor 1.6 1.2 Loading factor values from BRE 470 DEEP FOUNDATIONS • SEPT/OCT 2018 • 107 Working Platform Provision of Site Information Establishing the subsurface profile and conditions of the near surface and groundwater for the design of the platform’s subgrade is a clear prerequisite to the design process. This step must include adequate topographical information and knowledge of the final reduced level of the working surface, especially on brownfield sites, which typically exhibit high variability laterally and vertically. Experience indicates that the assessment of the subgrade conditions may be the most uncertain step in the design process. Ignoring shallow ground strength where the site investigation is solely designed for deep piled foun- dations must be recognized as a potentially unsafe and costly oversight, especially on sites of large plan area. There is a significant risk associated with isolated spots of weak subgrade, including poorly backfilled excavations. The importance of a thorough proof rolling of the subgrade by the platform installer and then supplemented by inspection by a competent person cannot be over emphasized. Strength testing of the subgrade, usually using plate testing, is sometimes undertaken to inform the design prior to laying the platform.
