Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE credits
The different material layers as part of a road construction fill all a function so theroad becomes durable, safe, comfortable and aesthetically pleasing. Recently, anew group of construction materials started to play an important role in roadconstruction – geosynthetics.
This thesis addresses the two most common types ofgeosynthetics used in modern road construction, namely geogrids and geotextiles.The most common use of geogrids is reinforcement of poor subgrade by usinggeogrid soil reinforcement, which occurs when road material particles wedge inthe geogrid’s mesh. Geotextiles act partly as a barrier that prevents the finermaterial in the below ground from being mixed with coarser upper material andalso act as a load spreader.Road contractors often face a choice of whether geogrids and/or geotextiles areappropriate in a particular road project and also how much profit the choice mightbring. This phase in the tender calculation process is the intended scope of thisthesis.
The thesis deals with both the "hard" cost-function aspects and the "softer" values e.g. ecology and social aspects. To facilitate the comparison, a comparative modelwas developed. The comparison is done for two different cases: Case A – roadconstruction on the bank and Case B – road construction in hill cutting. In Case A “with geogrid”, the amount of trenching becomes smaller due to saving ofreinforcement layer thickness.The completed cost comparison indicates an opportunity for significant savings forroad contractors that choose to strengthen the road’s superstructure with geogrid. In Case A “with geotextile”, no trenching saving is likely, but instead, bearingcapacity improvement is a long term financial gain. An estimated cost for Case B “hill cutting road”, is approximately SEK 600 000 which is less than the cheapestcase i.e. Case A “with geogrid”. After the use of geogrids, the function changes are as following:
- Traffic load distribution on the terrace has increased and lateral landmovements have reduced
- Filling material density has increased due to geogrid wedging mechanism
- Frictional resistance has increased due to the fact that pavement materialparticles have been extended due to geogrid’s wedging mechanism
- Superstructure’s total thickness has been reduced due reinforcement layerthickness’s reduction
After the use of geogrids, the function changes are as following:
- The composition and function of the road pavement and terrace materialremains intact. (The words "remains intact" run true to the concept of"functional change" but in this case, it is meant that the materialcomposition and function could have been worse if not properly chosengeotextile was added to the design).
- The scenario "gritty mud" is avoided if the geotextile has been enteredcorrectly with the right overlap.
Results concerning the ecological aspects show that the trenching reduction due touse of geogrids leads to fewer ground motion, lesser soil degradation and fewerenvironmental harmful emissions because the use of road construction equipmentdeclines. Reduced distribution excavation thanks to geotextiles leads to both thesame advantages as in the sentence above and partly to the fact that the amount ofmaterials that need become deposited decreases. In addition, the risk ofgroundwater lowering due to artificial drainage ditch is minimized. The road'stotal life cycle is extended, which contributes to reducing the environmentalimpacts arising from road repair and construction of a new road if the old onestops fulfilling its function. Degradation of geogrids and geotextiles is notenvironmentally harmful, but takes a long time in natural conditions, which meansthat in practice, the use must be documented and taken care of (regarding finalcombustion in a prudent manner).Regarding social sustainability, the following conclusion could be drawn: a roadthat has a higher carrying capacity leads to higher traffic safety due to minimalsubsidence, track formation and cracking. Road safety is seen by citizens not onlyas something that the private motorists are responsible for but also something thatroad authorities should consider when planning for a socially sustainable society.Another conclusion is road maintenance frequency and hereby the taxpayers' longtermeconomic gain. The road extended total life cycle contributes to the reductionof road repairs and new construction of roads. In other words, it is not just “oneroad construction company” that wins economically by minimizing their warrantywork.
The discussion concerns the cases where geosynthetics are not economicallyoptimal bearing capacity choice, such as solid rock cutting or a stretch of roadwhich has weaker parties but for which, a filling material yet compensates for theexcavated. The report concludes with a special discussion of the Swedishgeosynthetics research. The geosynthetics industry is controlled by private actors(developers, manufacturers and others) and contractors who do not like releasinginformation that might reduce their competitiveness. Therefore, the independentresearcher’s role has been quite weak and mostly reduced to “play ‘catch-up’insofar as investigating the nuances of how geosynthetics work "(Koerner, 2005). Another reason for the lack of reports on geosynthetics benefit is the long term asa sharp research project takes to plan, implement, control and evaluate. WhilstTrafikverket’s and local municipalities’ play the leading role in the Swedish roadconstruction industries, it should be in their interest to start taking geosyntheticsmore seriously by implementing credible tests and full scale trials and publishpractically applicable documents based on objective tests of structures containing geosynthetics.
geotextile, geosynthetic, geonet, geogrid, bearing capacity, road deformation