3.5 What guidance is there?

Transcription

1 3.5 What guidance is there? General A number of specifications and codes of practice for the use of road plates were identified during the literature review. The main points from these documents are summarised below Safety at Street Works and Road Works The Code of Practice: Safety at Street Works and Road Works (DfT, 2001) includes the following: Road Plates may be required to bridge excavations in order to open the carriageway to traffic, e.g. during traffic sensitive periods, at night or at weekends. [Their use must always be authorised by your supervisor who will decide on the size and thickness of the plate to be used. The thickness will depend on the width of excavation to be spanned and the type of traffic expected to use them.] Road Plates must be made of suitable material with an appropriate skid resistant surface. Their installation must not present a hazard to cyclists or motorcyclists. The sides of the excavations must be suitably supported beneath the road plates, and they must be rigidly secured to the road surface. Road Plates must be either sunk into the surface or suitable bitumastic material used to provide a ramp to the plate level. Where ramps are used, appropriate Ramp Warning signs should be used when there is a significant change in the road level. The edges of the Road Plates adjacent to the excavation should be fenced to prevent falls Gas industry requirements A document produced by British Gas (superceded) set out a number of precautions that should be taken when using steel road plates at tranverse trenches. There were no references to linear trenches. The precautions included the following: The trench sides should be stable or suitably supported Nobody should work under a road plate (unless traffic is halted) The plate should be long enough to be firmly supported for at least 600 mm on each side of the trench A subsequent report by National Grid Gas (unpublished) stated that the length of the plate supported on each side of the trench could be relaxed from 600 mm to 300 mm where circumatances allow and the plate is firmly fixed to the carriageway The plate should be wide enough to provide a distance of a least 250 mm from the wheels of vehicles to the edge of the plate The plate should be sufficiently thick to support the range of traffic encountered it was stated that expert advice should be sought if it was deemed necessary Where the plate protrudes above the road surface, ramps should be formed of a suitable material Ramp warning signs should be displayed Interim Report Web Version 31 Mar 2012

2 When the site is not attended, or when subject to frequent use by heavy vehicles, consideration should be given to fixing the plate to the roadway or recessing it into the roadway All road plates on unmanned sites should be inspected (visually) on a regular basis. National Grid Gas has produced a specification for road plates (T/SP/E/42 (National Grid, 2006; link)) which supersedes the British Gas document described above. Neither the British Gas nor the National Grid Gas documents specify a limit to the height that road plates can protrude above the level of the road surface (if ramps are formed and the plates are not fully recessed). According to T/SP/E/42, steel plates of minimum thickness 19 mm can be considered for main carriageways for trenches of maximum width 1.0 m. Road plates should be capable of withstanding an accidental vertical wheel load of 100 kn without deforming more than 2.5 % of the trench width, and withstand repetitive loading for at least 2 years without failure in normal use. The trafficked surface should have appropriate skid resistance. Fixings should be designed to resist a horizontal wheel load of 50 kn British Standards A meeting was held with FACTA (the Fabricated Access Cover Trade Association) to discuss appropriate design standards for covers, including road plates. Plates for openings of width less than 1 m should be designed to BS EN 124 (CEN, 1994) while openings of width greater than 1 m should be designed to BS 9124 (BSI, 2008). BS 9124 specifies design loading in accordance with BS EN 1991 (CEN, various). For small structural members, such as road plates, Load Model 2 (LM2) applies. This comprises a single axle load of 400 kn which is formed from two 200 kn loads applied through wheel contact patch areas of 0.35 m by 0.6 m, although the UK National Annex specifies a 0.4 m by 0.4 m patch area. The centres of the patch areas are 2 m apart and, therefore, for most plates only one patch area load would apply. This loading is considered by TRL to be too onerous for a plate that can be loaded by the wheels from only one vehicle. However, it may be more appropriate for a plate that can be loaded by the wheels of more than one vehicle that are adjacent to each other (i.e. travelling in the same or opposite directions) FACTA FACTA has a specification for fabricated access covers (FACTA, 2007; link). Covers must pass acceptance and strength tests. The test loads are for vehicles up to 44 tonne but under special conditions, including a maximum speed of 20 mph. The load in the non-destructive acceptance test for covers of class equivalent to EN124 Class D400 is kn (= 108 kn wheel load x 1.1 for overload x 1.15 for dynamic effects, or 50 kn x 1.1 x 1.5). The sum of the settlement and residual deflection must not exceed 0.8 % of the test opening. The load in the strength tests is 220 kn (= 137 kn x 1.6). On removal of the test load, the deflection must reduce by at least 20 % Hampshire CC Hampshire CC permits only purpose-built proprietary trench cover systems designed for use in the carriageway and holding certification for the anticipated maximum wheel loads. The requirements state clearly that the use of boiler plate not designed Interim Report Web Version 32 Mar 2012

3 specifically to cover road openings is not permitted on any carriageway. Surfaces of trench cover systems in carriageways must have a minimum skid resistance value of 55 (as measured by the British Pendulum portable skid resistance tester) where laid perpendicular to the direction of traffic, or 65 where laid at an angle. The surface elements must be of a conspicuous colour, preferably yellow, so that they are easily identifiable by road users New South Wales The Roads and Traffic Authority (RTA) New South Wales has prepared a detailed specification covering road plates (New South Wales, 2008; link). Road plates may only be used for what are defined as medium and high risk openings. Low risk openings must be excavated and backfilled to be trafficable within one working day. At the end of each working day, medium and high risk openings must either be fully backfilled to be trafficable or covered with a road plate if not fully backfilled. A site specific structural design must be provided for the excavation, shoring and road plates for medium and high risk openings. Shoring is always required for trenches more than 500 mm deep when plates are to be used. Road plates must not be used on roads with a speed limit of 90 km/h, and only as a last resort on roads with a speed limit between 70 km/h (43 mph) and 90 km/h (56 mph). Plates must be designed for 48 kn wheel loads on a 1.2 m square grid. Live loading factors ranging from 1.8 for major roads to 1.0 for light vehicle property access, and dynamic load allowances ranging from 0.6 to 0.2 apply. Plate dimensions that meet these criteria are given in Table 3.4. The RTA must approve the use of stiffened plates. The live load deflection must not exceed the lesser of 11 mm or 1/150 th of the plate clear span. The skid resistance value, as measured by the British Pendulum Number, must not be less than 55. Table 3.4 Plate dimensions specified in New South Wales (from New South Wales, 2008) Trench width Plate clear span (trench width plus 400 mm) Minimum plate length (trench width plus 800 mm) Minimum plate thickness (Grade 250 plate) Major roads Minor roads Light vehicle property access or less Plate widths between 1.2 and 2.2 m are not permitted where trench widths exceed 1.8 m. For linear trenches, plates must extend past each trench end by a distance not less than the trench depth. A 200 mm wide strip of cold mix bedding material must be placed at least 100 mm from the edges of the trench to support the plates. The bedding layer Interim Report Web Version 33 Mar 2012

4 thickness must be 5 mm to 10 mm thick and prevent contact between the plate and the trench edge under live loading. The plate crossfall must match the adjacent pavement crossfall. Plates must not be recessed into concrete pavements, but for long term installations in flexible pavements or asphalt overlays, plates should be fully recessed to match the adjoining pavement. The finished surface level of plates should be no higher than 70 mm above the pavement. Detailed information is given on how plates should be fixed Calabasas, California The City of Calabasas, California, has a specification for steel road plates (City of Calabasas, 2008; link). They must be designed for HS20-44 truck loading. as in the Caltrans Bridge Design Specification Manual. The minimum coefficient of friction should be 0.35 as determined by California Test Method 342. The minimum required plate thickness for given trench widths are given in Table 3.5. The material type is not specified in the requirements but this may be a requirement of the Bridge Design Specification Manual. Plates must extend a minimum of 300 mm beyond the edges of the trench. The trench must be adequately shored. For speeds more than 70 km/h (43 mph), the pavement is cold planed to a depth equal to the thickness of the plate. For speeds of 70 km/h or less, approach and end plates have to be secured to the pavement by a minimum of two pre-drilled dowels of depth 50 mm at opposite corners. Fine grade asphaltic concrete must be used to form ramps of maximum slope 8.5 % with a minimum 300 mm taper. Table 3.5 Minimum plate thicknesses in Calabasas utility trench plate requirements (from City of Calabasas, 2008) Trench width Minimum plate thickness Thousand Oaks, California This specification states that no trenches in the Public Right of Way can be left open without the permission of the City Engineer. Trenches must be either plated or backfilled and resurfaced with temporary asphalt. All plates subject to traffic must be countersunk unless other configurations are approved prior to the trenching operations. Temporary ramps (300 mm long) must be formed on all sides of plates that are not countersunk. To eliminate lateral movement, plates must be secured to the pavement using 0.5 (12.7 mm) diameter reinforcement or equivalent steel rod. The reinforcement or rod should not restrict vertical movement. Adjacent plates must be tack welded at each Interim Report Web Version 34 Mar 2012

5 corner or as required by the public works inspector. Wedges or non-asphaltic devices must be used for levelling and to eliminate rocking. Compacted temporary asphalt must be used to fill all gaps between the plates and the adjacent pavement surfaces. Plates must be checked twice a day to confirm that they are secure. Plates in streets must be of minimum thickness 1 (25.4 mm), and of minimum thickness 1.25 (31.8 mm) with centre supports for trenches wider that 4 (1.22 m). Plates must extend 12 (305 mm) beyond trenches and have non-skid surfaces. Advance warning signs (i.e. road plates ahead) are required at all installations. 3.6 What products exist? General A number of different plating systems were identified in the review for both narrow and wide trenches. These include plain plates of constant thickness, fabricated plates with stiffeners and bridge deck panel type units. Most products are formed from steel, but some are composite and formed from plastic strengthened with steel. This section provides a brief review of the products found in the review, and identifies those that might be suitable for further consideration Proprietary plates for narrow trenches A number of proprietary plating systems are available for trenches of width 1 m, as described below Trenchlink Trenchlink markets a range of products as shown in Table 3.6. Table 3.6 Trenchlink products Product Name Trench type Max width of trench covered 500Max-Crossing Transverse Max-Crossing Transverse Max-Linear Linear Max-Linear Linear 1000 Box kit * Linear 1000 * Comprises 500Max-Linear plates with a central steel cross beam Trenchlink 500MAX-Crossing plates (Trenchlink, 2009; link) have been widely used for trenches with widths from 300 mm to 500 mm. They are designed for the wheel loading from a vehicle with a gross weight of 44 tonnes. The plates are 750 mm long and so for a 500 mm wide trench they would bear on the road surface within 125 mm of the trench edge, i.e. within the zone of influence. Each plate weighs 24 kg and can therefore be installed manually; four plates are required per metre run (Figure 3.11). Adjusters on the underside of the plates are extended to the trench wall, and are intended to restrict movement of the plates under Interim Report Web Version 35 Mar 2012