Download - Engineering Surveyor

BenchmarksEach site should have a primary benchmark, which may be: a convenient nearby Ordnance Survey benchmark (but see note) referenced to an Ordnance Survey benchmark (but see note) referenced to a given level on existing works a height above Ordnance Survey Datum.Note: Traditional Ordnance Survey benchmarks are no longer supported or maintained by the Ordnance Survey andmay be unreliable. For establishing levels relative to Ordnance Datum, see Appendix C: UK National Grid,benchmarks and ground distances.The supervising authority should specify which method applies. If the site levels are to relate to Ordnance Surveybenchmarks but a particular benchmark has not been specified, select at least two local benchmarks and doublelevel between them via at least two TBMs on the site. If height above Ordnance Datum Newlyn (or other OrdnanceSurvey height datum when working in some Scottish islands), the procedure outlined in Appendix C: UK NationalGrid, benchmarks and ground distances, should be followed.Unless all levelling can conveniently be referred to the primary benchmark, it will be necessary to set up secondarybenchmarks. Secondary benchmarks are commonly called temporary benchmarks (TBMs).To ensure accurate primary and temporary benchmarks: establish primary benchmark from the agreed Ordnance Survey benchmark(s)/National Vertical Referencedatum or from existing works and agree the level with the supervising authority in writing plan the locations of TBMs in good time, taking account of temporary and permanent works. All points ofthe works should, where possible, be within 40 m of a TBM and there should be two, independent, TBMsaccessible for all potential instrument set up positions verify the levels of previously established TBMs by levelling from the primary benchmark establish TBMs not more than 80 m apart. Closing error to primary benchmark must not exceed1.5 × √2n mm (where n is the number of set-ups) or whatever is specified in the contract use existing permanent features for establishing benchmarks whenever possible (see illustrations) where no permanent feature is available, establish the benchmark in firm ground and mark as shown inthe illustration protect benchmarks from site traffic and other dangers if an assumed/arbitrary height datum has been used for the scheme, for example as shown on contractdrawings, check with the supervising authority that this datum may be used record position, reference number, level and date last checked of each TBM and the primary benchmarkon the site plan(s) display updated copy of site plan or list of benchmarks (with details) in site office(s) check levels of TBMs at regular intervals † report any apparent disturbance of TBMs and immediately update TBM lists transfer levels from TBMs to permanent works as soon as practicable remove redundant TBMs from the site.BS 5964: Part 2:1996 includes examples of setting-out stations and benchmarks, for short, medium and long-termuse.† Warning: Earthworks and ground settlement, heave, expansion or contraction will affect TBM levels.CIRIA C70939