However, this does not remove the risk that isolated weak spots may still exist. It is the responsibility of the platform installers to exercise rigorous inspection and control to ensure the strength requirements for the platform subgrade are achieved at all points across the platform area. The analysis and design calculations presented in BRE 470 are based on the principle of a relatively shallow thickness of platform placed atop a weak subgrade. Experience has indicated that this has been appropriate for most of the piling sites; however, the designer should be aware of the limitations of this method in other design situations. The bearing resistance is the sum of the shear required to punch through a vertical plane in the granular platform material and the bearing capacity of the subgrade. Applicability of the method in BRE 470 is limited to undrained shear strength of the subgrade between 415 and 1,670 psf (20 and 80 kPa) and where the platform thickness is less than 1.5 times the track width (W). Where poor subgrade strength prevails and the BRE 470 method is not appropriate, complex design methods are required (e.g., propriety methods by geosynthetic sup- pliers, and in CIRIA SP123 [Jewell, 1996]). The specialist capability of geosynthetic suppliers can lead to efficient granular platform design particularly for very weak subgrades, but their proprietary design methods can be opaque and may be very difficult to verify independently. Mode of punching and bearing capacity failure W Platform Subgrade Bearing Capacity Failure The BRE 470 method has generally proved to provide a straightforward design method for many of the piling sites in the U.K. and has gained wide acceptance. The complexi- ties of other methods (e.g., CIRIA SP123) can be a barrier to their use by all but geo- synthetic design specialists and are not widely used. On the relatively small propor- 108 • DEEP FOUNDATIONS • SEPT/OCT 2018 Punch tion of sites where poor subgrade strength prevails, many contractors rely on the design provided by geosynthetic suppliers. Management and Driving Change in the Industry At the launch of BRE 470 in 2004, FPS member companies decided not to work onsite unless a signed FPS-developed Working Platform Certificate was in place. The certificate provides control documen- tation to confirm that a working platform has been designed by a competent person, constructed and, where specified, tested in accordance with the design, and will be regularly inspected and adequately maintained to ensure its integrity over its design life by a named organization (usually, the principal contractor in the U.K.). The FPS continues to make the Working Platform Certificate freely available on at https://www.fps.org.uk/content/uploads/ 2017/01/FPS- WPC4d-June-2015.pdf. Acceptance of the Working Platform Certificate in 2004 was not universal, although it was generally welcomed by the construction industry as a step forward in the safety culture of piling operations. Two common complaints were that granular working platforms had suddenly become thicker and more expensive, and that additional program time was required to design, construct and test platforms. On the positive side, the incidences of toppling piling rigs were demonstrably reduced and, where problems have been encountered, a better understanding of the causes of failures and the development of improved piling practice has been facilitated. At the current time, due to the concerted efforts of the FPS, the use of the working practice is now universal in the U.K. on construction sites where piling and ground improvement is undertaken. Dr. Derek Egan, BEng, CEng, FICE, is director and founder of Remedy Geotechnics Ltd. He has more than 25 years of experience in the construction industry, geotechnical contracting, consulting and academic fields. Egan has numerous published technical papers covering a wide range of geotechnical and construction topics. Loaded Area L D
