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TENSARTECH® PLATEAU™
load transfer platform systemover piled foundations
Constructing Load Transfer Platforms over Weak Ground with Piled Foundations
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Engineers often need to construct embankments in areas where minimal settlement can be tolerated. This is especially true when the serviceability of adjacent structures would be compromised by anything other than minimal settlement. The engineer, therefore, needs to design a foundation that will provide a firm support without the consequential settlements and consolidation of underlying soils.
Using a load transfer platform (LTP) over a piled foundation avoids the delay required for consolidation settlement as a result of placing embankment fill over compressible soils. The Tensar load transfer platform distributes embankment loads efficiently onto a series of piles or vibro concrete columns (VCCs) which bear on firmer strata below. This avoids the cost of using a concrete raft. In situations where the foundation soils can be assured to provide some permanent support to the area between the piles, then a TensarTech® Plateau™ LTP system of good quality granular fill reinforced with multiple layers of Tensar geogrids can be used. In other installations an alternative design using Tensar Basetex can be employed and may accommodate poorer quality fill if necessary.
TensarTech Plateau LTP systems have also been successfully used below concrete ground floor slabs in order to avoid designing the slabs to span between piles. However, LTPs are not suitable for use with some ground treatment techniques such as vibro stone columns.
When settlement restrictions dictate that a deep foundation solution is required to support an embankment or ground-bearing slab, a TensarTech Plateau LTP system:
• Avoids the delay required for consolidation settlement
• Avoids the need for concrete rafts, or ground beams
• Can be used under ground-bearing slabs to avoid or minimise bending
• Is rapid and economical to install
• Is a proven, reliable solution
Cut-away view of embankment supported over a load transfer platform.
Installing pile caps. Geogrid installation. Placing a fill layer.
Tensar® Technology – proven, practical solutions
and the know-how to get them designed and built.
Tensar Technology is widely adopted for Pavement Optimisation and Subgrade Stabilisation to improve the structural performance of paved roads and unbound roads and platforms. Tensar Technology is also adopted for Earth Retaining Systems for cost effectiveness and versatility over other traditional methods. By delivering real savings in cost and time, Tensar Technology can help you improve the bottom line on your project as well as preserving the invested capital.
Load Transfer Mechanisms
Figure 1: TensarTech Plateau Enhanced Arch mechanism.
ENHANCED ARCH LOAD TRANSFER PLATFORM
The inclusion of Tensar geogrids within granular layers has been shown to increase the angle of load distribution considerably in comparison with the unreinforced condition. The geogrids interlock with the fill to mobilise the maximum shear strength of the granular layer and enhance the load spreading mechanism.
A TensarTech Plateau LTP utilises the enhanced arch mechanism and is designed such that the majority of the overlying embankment load is transferred directly to the pile heads via a reinforced granular layer. The Tensar geogrid confines the granular material, thereby resisting dilation and effectively increasing its shear strength - see figure 1.
The granular fill material below the theoretical arch must be permanently supported by the geogrids and the subgrade in combination, while the arch itself must be loaded by either additional fill or a concrete slab to ensure that the arch is permanently locked in compression.
TENSIONED MEMBRANELOAD TRANSFER PLATFORM (LTP)
Where conditions do not suit the TensarTech Plateau LTP system, an alternative Tensioned Membrane solution using Tensar Basetex high strength geotextile may be used.
A Tensioned Membrane LTP is designed such that the embankment fill is supported directly by the Basetex geotextile spanning between the piles. This load is transferred into the piles by tension in the geotextile. Arching within the fill results in some reduction in vertical stress applied to the geotextile compared with the full embankment height. There is no special requirement for fill properties - see figure 2.
Permanent supportfrom subgradebetween piles
Tensar geogrids interact with granular fill to enable enhanced arch to develop
Granular fill
Figure 2: Alternative Tensioned Membrane mechanism with Tensar Basetex.
Embankment fillReduced verticalstress carried byreinforcement dueto arching in fill
Tensar Basetex reinforcement
Tensioned membranetransfer load to piles
Tensar Design ServiceTensar’s experienced civil engineers are able to help take your project onto the next stage. Our Design service is on-hand to provide standard Application Suggestions to establish viability of Tensar’s products and systems and enable planning costs, right through to preparing certified detailed design and construction drawings for using Tensar products and systems on your project. Upon request, we can provide all necessary design certification and working calculations in a form ready for checking, with drawings issued for construction as well as all the crucial specification and installation details.
Embankment fill
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Examples of Load Transfer Platforms
TENSARTECH PLATEAU ENHANCED ARCH LOAD TRANSFER PLATFORM
Embankment support over soft ground where only minimal settlements can be tolerated.
Highway construction adjacent to a rigid railway structure.
New highway
Piles
Existinglight
railways
Tensargeogrid
Embankment support over soft ground where time for consolidation is either not available or can not be reliably determined.
Motorway toll booth plaza constructed on a TensarTech Plateau LTP - the alternative to surcharge avoiding uncertainty of predicting consolidation period.
Bulk fillembankment
Well-gradedgranular fill
Granular fillworking carpet
Estuarine clay& peat
Two layers ofTensar geogrid
Varies
Loadtransferplatform
Originalground level
Support of concrete ground floor slabs over soft ground. Building of a supermarket using a TensarTech Plateau LTP is a proven and reliable solution.
Reduced span Reinforcedconcrete slab
VCCs
Three layersof Tensargeogrid
TENSIONED MEMBRANE LOAD TRANSFER PLATFORM
Basetex high-strength geotextiles provide the necessary load/strain properties when the basal reinforcement is designed to act as a Tensioned Membrane spanning between
the piles. This approach is used, for example, when there is insufficient granular fill available to form the LTP layer.
Reinforced soil embankment constructed over Tensioned Membrane LTP using Tensar Basetex.
Tensar Basetex Tension Membrane below High Speed 1 railway embankment.
Vibroconcretecolumn
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Once the supporting piles/VCCs have been installed the simple construction process involves the inclusion of 2 or 3 layers of Tensar geogrids in the granular layers above the supports. Dead weight compaction of the cushion layer of fill under the
bottom grid, and the layer of fill between the bottom two grids, is the only deviation from standard embankment placement and compaction operations. Overlapping, or tying, of adjacent grids provides the continuity required.
Construction of a TensarTech Plateau Enhanced Arch Load Transfer Platform System
A grid layer is placed. The design thickness of granular fill is placed, spread and compacted.
Small pile caps being cast prior to constructing the TensarTech Plateau load transfer platform system.
The Toll Plaza area for the Second Severn Crossing in the UK was built over compressible soils supported on VCCs. Load from the embankment and pavement above was transmitted to the VCCs using a TensarTech Plateau enhanced arch load transfer platform (LTP). The graphs summarise the performance from monitoring this LTP. Settlement above the VCCs and mid-way between them is almost the same,
whereas stress measured at subgrade level between the VCCs is almost zero. These results show how well the design assumptions have been realised in practice. The TensarTech Plateau Enhanced Arch LTP has performed as a stiff raft, transmitting loads from the pavement and embankment directly to the VCCs using the excellent load spreading capability of a granular layer reinforced with Tensar geogrids.
Case Study - Second Severn Crossing, UK
Second Severn Crossing TensarTech Plateau load transfer platform performance.
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Stre
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ttle
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Emba
nkm
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OD) Final earthworks
Roadconstruction
Traffic loading
Above VCCs (range)Mid span vcc’s (range)Construction history
0 28 56 84 112 140 168 195 224Days
Details of TensarTech Plateau load transfer platform at Second Severn Crossing.
Tensar geogridsat 150mm
vertical spacing
75mm downgranular LTP600mm thick
Rockfill
500mmdiameter
VCCs
2.2 to 2.7m spacing with triangular layout
Contact Tensar or your local distributor to receive further literature covering Tensar products and applications.
Also available on request are product specifications, installation guides and specification notes.
The complete range of Tensar literature consists of:
• Tensar® Geosynthetics in Civil Engineering A guide to products, systems and services
• Subgrade Stabilisation Stabilising unbound layers in roads and trafficked areas with a Tensar MSL
• Spectra® Pavement Optimisation System Improving the structural performance of whole pavements with a Tensar MSL
• Asphalt Pavements Reinforcing asphalt layers in roads and trafficked areas
• TensarTech® Earth Retaining Systems Bridge abutments, retaining walls and steep slopes
• Railways Mechanical stabilisation of track and sub-ballast
• TensarTech® Plateau™ Load transfer platform system over piled foundations
• Basal Reinforcement Basetex high-strength geotextiles
• TensarTech® Stratum™ Cellular foundation mattress system for foundations with controlled settlement
• Tensar® Erosion Control A guide to products and systems
Your local distributor is:
Tensar International LimitedUnits 2-4 Cunningham CourtShadsworth Business ParkBlackburn BB1 2QXUnited Kingdom
Tel: +44 (0)1254 262431Fax: +44 (0)1254 266867e-mail: [email protected]
Copyright ©Tensar International Limited 2013
Printed September 2013, Issue 13, 485/09-2013
The copyright in this brochure (including without limitation all text, photographs and diagrams) and all other intellectual property rights and proprietary rights herein belongs to Tensar International Limited and/or its associated group companies and all rights are reserved. This brochure, whether in whole or in part, may not be copied or redistributed or reproduced or incorporated in any other work or publication in any form whatsoever without the permission of Tensar International Limited. The information in this brochure supersedes any and all prior information for the products referred to in previous versions of this brochure, is of an illustrative nature and supplied by Tensar International Limited free of charge for general information purposes only. This brochure is notintended to constitute, or be a substitute for obtaining, project specific engineering, design, construction and/or other professional advice given by someone with full knowledge of a particular project. It is your sole responsibility and you must assume all risk and liability for the final determination as to the suitability of any Tensar International Limited product and/or design for the use and in the manner contemplated by you in connection with a particular project. The contents of this brochure do not form part of any contract or intended contract with you. Any contract for the provision of a Tensar International Limited product and/or design service will be on Tensar International Limited’s Standard Conditions in force at the time of entering into the contract. Whilst every effort is made to ensure the accuracy of the information contained in this brochure at the time of printing, Tensar International Limited makes no representations about the suitability, reliability, comprehensiveness and accuracy of the information, services and other content of this brochure. Save in respect of Tensar International Limited’s liability for death or personal injury arising out of negligence or for fraudulent misrepresentation (if any), Tensar International Limited shall not be liable to you directly or indirectly in contract, tort (including negligence), equity or otherwise for any loss or damage whatsoever or howsoever arising in connection with the use of and/or any reliance placed upon the contents of this brochure including any direct, indirect, special, incidental or consequential loss or damage (including but not limited to loss of profits, interest, business revenue, anticipated savings, business or goodwill). Tensar, TensarTech, TriAx and Spectra are registered trademarks.In case of legal disputes between the parties, the original English version of this disclaimer shall prevail.
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EMS 86463ISO 14001:2004