Domain

Explanation

Speaker

Dr S A Tan, Harry is an Associate Professor and Director of Centre for Soft Ground Engineering in the Department of Civil Engineering at the National University of Singapore. Professor Harry Tan is currently a Core Member of the International Society for Soil Mechanics and Geotechnical Engineering Technical Committee TC9 on Earth Reinforcement, and a Board member of "Geotextiles and Geomembranes" and SEA "Geotechnical Engineering". He has published many technical papers in international journals and conferences covering many topics including geosynthetics reinforcements and soil nails, highway pavement materials, land reclamation and excavation problems.

Professor Harry Tan is a registered professional engineer in Singapore. He has served as a geotechnical and pavement consultant for many government and private organisations involving projects in Singapore and the region on paved and unpaved roads, geosynthetics applications, soil nails, slope stability, land reclamation, excavations effects of soil movement on adjacent structures, seepage studies for Pulau Semakau landfill and Dolphin Lagoon, railway track embankment and sub grade studies for Malaysian DRB-Hicom.

• Professor Harry Tan is also actively involved in conducting courses on geotechnical softwares like Plaxis, Geoslope Office: Slope/W, Sigma/W, Seep/W, and CADS Piled Wall Suite. These courses have been conducted through NUS, and more recently for the region through IEM, Malaysia, and SNUT in South Korea.

Abstract

• Unpaved roads are usually temporary roads built over soft, weak subgrade with base aggregates placed directly above it. No permanent surfacing like concrete or asphalt is placed on the base layer. In cosmopolitan Singapore, unpaved roads are generally used as site access roads and training roads in the military training area. Un-reinforced unpaved roads are prone to rutting and mud-pumping problem especially when the groundwater table is close to the subgrade surface, leading to high cost in track maintenance. Geotextiles when placed at the subgrade and base layer interface provide separation, reinforcement, filtration and drainage functions. Results from laboratory static and tracking model tests provided better understanding to the mechanism of geotextiles in improving the performance of unpaved roads. The importance of geotextiles tensile modulus and anchoring in mobilising the reinforcement contribution is confirmed by the results of these model tests. The innovative use of pre-tensioning to enhance the reinforcement contribution of geotextiles can be successful with careful considerations of the durability and survivability aspect of the geotextiles when pre-tensioned. These research findings have been applied to the road tracks of SAFTI Training Area. Performance over the last two years showed the value of geotextiles separator in extending the life of tracks so as to minimise maintenance costs for the users.

TOC

• Issues
• Unpaved road R&D
• Lab model testing
• Results & findings
• Singapore field trials

Unpaved roads

• Paved roads:
• CBR>5%: well-compacted, well-drained, strong
• Permanent surfacing
• Unsuitable for tracked vehicles: high maintenance
• Low camouflage during rains: road network from aerial view
• Unpaved roads:
• Temporary roads over weak subgrades (CBR<3%): any higher, geotextiles not much use
• No permanent surfacing
• Used in mining areas, site access, forest & low access roads (over vast lands like Canada & Australia)
• Singapore: military area (¬30km), construction sites (short stretches)
• Conventional cross-section:
• Base layer at top: stones & gravels (expensive, must not lose, else maintenance): crusher run
• Separation layer: with or without geotextiles separation
• Subgrade layers: CBR<3%: well-compacted, stiff, but in suction, thus absorbs water à rutting occurs

Rutting

• Surface dents: low speed of tonners & tanks (moves zig-zag ways)
• Causes of rutting:
• High water table
• Wheel loads weigh down on unpaved road base layer
• Pushes crusher run stones & gravels into soft subgrades
• Fines move upwards to replace stones
• Hence, loss of base layer stones & need to be replaced (maintenance)
• More ponding: maintenance cost high
• Top up base layer, yet continuously losing stones
• Need to stop loss of stones by separation
• Need to strengthen unpaved road capacity by reinforcement
• Solution: geosynthetics

Geosynthetics

• Artificial planar products from petrochemical polymers
• Used with soil, rock, earth, geotechnical material
• Families:
• Geotextiles: fabrics
• Geogrades: junction by exclusion of polymer
• Geonet: same polyethylene with 3-D light structure for high drainage
• Geomembrane: high density, seal off or cut off water: landfill, land reclamation, slope stability, geomembrane liner
• GCL: geomembrane + bentonite: mixed layers, able to self-heal, squeeze together when punctured by nails
• Geocomposite: combinations of geosynthetics: plastic vertical drains
• Advantages for unpaved roads application of geosynthetics:
• High cost-effectiveness
• Easy to install, low maintenance
• Effective for weak subgrades only: since need strain deformation mobilization for reinforcement or pre-tensioning
• Geotextiles: separation, segregate stones & subgrades; prevent loss of stones; polypropylene
• Geogrid: reinforcement; polyester; mobilized at higher strains, thus effective for weak subgrades only (CBR<3%)

Separation

• Base layer
• Geotextile separation layer
• Subgrade layers

Drainage-infiltration

• High porosity: ¬80% air à allows water drainage flows easily
• Water flows into & gets trapped by geotextiles

Tension membrane effects

• Wheel loading
• Unpaved road deforms à induced geotextile into tension, yet closely adhering to layers à membrane effects
• Tension membrane effects

Lateral restraints

• Base restraint:
• Bearing capacity restraint, stress trajectory
• Subgrade restraint:
• Geotextile pulls inwards soil that is moving outwards due to wheel loading on unpaved roads

R&D focus

• How to make geotextile work
• Resolve controversy from separation & reinforcement
• Dispute between tension modulus (E, stiffness) & anchorage (strength, length)
• High E, high K: reinforcement mobilized at lower strains
• High anchorage, longer length: reinforcement mobilized at lower strains
• Derive reinforcement effects from service deformations
• SLS limits
• Varying:
• Anchorage
• Reinforcement
• Pre-tensioning
• Measurements:
• Subgrade stress
• Geotextile strain response: use Improved NUS strain gauging system
• Aim: quantify the effect of transfer (stress/strength & strain/deformation) from base to geotextile to subgrade layer

Geotextiles used

• Machine-fabricated direction (stiffer, less strains) & cross-machine direction (weaker, more strains)
• Non-woven: mixed, no real direction, low tensile strength, high failure strain
• Woven: dual direction, high tensile strength, low failure strain (brittle)
• Composite: mixture of both

Anchorage system

• Geotextile have to be pulled across subgrade layer
• Tie one edge along one road shoulder
• Pull across
• Anchor at the opposite road shoulder: either by soil nails or just dead weight into side drain
• Pre-tension to appropriate strain: mobilize reinforcement through tension membrane effects

Results

• At same working pressure by wheels on roads:
• Deformation from more to less: non-woven à composite à woven
• More anchorage + more pre-tensioning: more tension membrane effect, more loads taken by geotextile (assuming no slippage), less subgrade stresses, less subgrade deformations & rutting
• Overall deformation from worst to best:
• Conventional compacted unpaved roads without geotextile separation & reinforcement
• Separation effect: keep base layer & subgrade layers apart
• Anchored separation & reinforcement effect
• Pretensioned (anchored already) effect: best
• Issues:
• Minimum requirement: separation effects
• For low CBR <3%

Conclusions

• Not use: surface settlement as judgement of effectives, due to influence of densification effect of base layer (from new to seasoned one)Not use: surface settlement as judgement of effectives, due to influence of densification effect of base layer (from new to seasoned one)
• Subgrade settlement
• Strain movement effect: tension membrane effect
• Reinforcement only: low for non-woven, since at high strains only for mobilization of required stiffness
• More anchorage, high E, high K, high durability, more reinforcement
• Pre-tensioning effect: high strains induced to mobilize reinforcement effect