Product category:
Ground Engineering
News Release from: Tensar International | Subject: SSLA30 biaxial geogrid
Edited by the Buildingtalk Editorial
Team on 16 March 2004
Trial Use Geogrids To Reduce Aggregate
Requirement
A new Highways Agency road construction specification trial uses Tensar International's SSLA30 biaxial Geogrids to reduce aggregate requirement.
As part of a project to reduce accidents on the A1 at Stannington, near Morpeth, Northumberland, the Highways Agency (HA) has taken the opportunity to undertake a major trial of geogrid construction techniques using Tensar International's SSLA30 biaxial geogrid as a best value solution for highway construction The HA approved the proposed use of Tensar geogrids by Northumberland County Council Highways Department (NCC) and their geotechnical consultants Scott Doherty Associates for a Grade Separated Junction which replaced five At Grade Crossings on a three mile stretch of the dual carriageway
This article was originally published on Buildingtalk on 15 Jul 2003 at 8.00am (UK)
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The stretch of A1 close to Stannington has suffered numerous accidents where traffic attempts to turn right across the lanes of on-coming traffic.
NCC designed a Grade Separated Junction to lead traffic on and off the A1 with four 500 metre long slip roads and minor roads connecting to the other original junctions.
As well as acting as access roads, the new slip roads will also act as a temporary bypass for several months (carrying c.
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16,000 vehicles a day), while the main A1 highway is closed for the construction of a new bridge, roundabout and underpass required for the Grade Separated Junction.
While Tensar geogrids have been widely used in the past for temporary access roads to provide high performance ground stabilisation with reduced aggregate thickness, incorporation of geogrids in a permanent roadway with only a few exceptions, has largely been confined to private roads in the UK, until now.
NCC submitted a request for a departure from the standard specification on cost and environmental grounds, supported by extensive documentation from Tensar.
In its reply to the Departure Application (23/12/02), the HA commented: 'The proposal has been discussed with other members of the geotechnical team who have agreed that this is an ideal opportunity to carry out a trial using geogrids to reduce capping layer quantities used on the trunk road network.' The local ground conditions consist of glacial till deposits with soft areas and occasional hard erractics.
Based on an examination of the site investigation data by Scott Doherty Associates, a California Bearing Ratio (CBR) of 2% was selected for design purposes.
The typical pavement construction specification (HA 25/94 Part 2) would be 150mm of Type 1 sub-base and a 600mm capping material (Class 6F1 or 6F2).
This would be overlain by the black top layers.
In this case, class 6F2 material was selected which meant that the normal specification of Tensar SS30 biaxial geogrid was changed to Tensar SSLA30.
This product has a larger aperture size to accommodate the increase in maximum aggregate particle size, so that full interlock of the granular particles can be maintained.
This also benefited the contractors, as the finer grade of capping aggregate (6F1) is more expensive to obtain.
Using the proprietary TensarPave software, Tensar International's Design and Applications Department was able to propose that, with SSLA30 geogrid laid over the subgrade, the capping layer could be reduced to 300mm in thickness.
This reduced the amount of aggregate required for the capping layer by 50%.
NCC estimated cost savings in aggregate alone in the region of £50,000 - £100,000.
A further environmental and cost benefit of using the Tensar solution results from less spoil being removed from the sub-grade.
The spoil cannot be removed from the site, without incurring additional cost, so on-site storage problems are alleviated with the Tensar solution.
In addition to the Tensar pavement construction, NCC is incorporating a short length of pavement with standard 600mm capping thickness on one of the on/off slip roads.
Following construction, their performance will be monitored using a Falling Weight Deflectometer and compared to the adjacent Tensar reinforced section, so that an objective assessment can be made of the benefits of using Tensar geogrids to reduce capping layer thickness over the lifetime of the pavement.
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