Mechanically stabilized layers in road construction

Transcription

1 Mechanically stabilized layers in road construction Zikmund Rakowski, Jacek Kawalec Tensar International, UK, Technical University of Silesia, Poland Abstract: Effective and economical technologies are more and more import ant for road construction. A concept of mechanically stabilized layer is presented in the paper. The technology for the most efficient distribution of stress under top asphalt layers is applied and quantified. The Technology allows to save reasonable amount of granular material not reducing but even increasing road service life. INTRODUCTION: The use of geogrids in road construction, introduced in 80-ties of XX century by Netlon, becomes a standard for many projects around the world. Since that time rapid increase of different type of materials manufactured world-wide for road applications is observed as well as number of road projects with geosynthetics in use. The problem faced up by designers is how to correctly select proper geogrid from all available materials for real improvement of the structure. Quite often there is no link between physical parameters of the geogrid and their efficiency. Commonly used specifications are describing parameters which are not relevant with mechanism of interaction between geogrid and aggregate. Unfortunately, clear classification based on differential performance of different geogrids quite often is not considered as the most useful one. As final result many geogrid applications in roads do not shows any improvement to the structure with time. To avoid that situation paper discuss the most important parameters for effective mechanical stabilization of grains provided by geogrid. GENERAL CLASSIFICATION OF MATERIALS There is several classifications of geosynthetics where the simplest one describes different type of products: Nonwoven geotextiles Woven geotextiles Geonets Low height geocells Geogrids

2 Geomembranes Geomats Geocomposities From all groups listed above material usable for mechanical stabilization are geogrids. The other materials has different function and should not be used for this application. An exception from this are geocomposities but only those where the main active component is geogrid. Looking closer on geogrids we can find that there is few different ways of manufacturing them with different impact on reinforcing mechanism. CROW 157 from the Netherlands [5] has taken a first step to try to characterize the benefit expected from a material and has produced a chart (Fig. 1) which indicates the expected benefits based on the manufacturing process. This chart shows that there is no simply equal status between different materials from performance point of view. Understanding of differences is a key to economical design and satisfactory use of geosynthetics. Figure 1. Chart to quantify the benefits of geosynthetics in unbound layers in terms of reduced base thickness Next, very important issue is proper identification of function of reinforcement according to application. As mentioned earlier, the most suitable materials for reinforcement are geogrids and further discussion will concentrate on them. Table presented below shows two completely different groups of application and as result different conditions for geogrid in the construction.

3 Table 1. Application differentiation according to timing and load direction A. Load applied temporary at reinforcement depth, mainly perpendicular to geogrid in plane Mechanically stabilized layers under roads, railways, runways, etc., Subgrade reinforcement under industrial floors Embankment foundations on weak soil Asphalt concrete reinforcement Protection layers for linear structures constructed over mining activity areas Reinforcement of stone-pillows under shallow foundations B. Load applied permanent to at reinforcement depth, parallel to geogrid in plane Slope reinforcement Retaining walls Bridge abutments designed as earth structures As there are different applications presented above, the circumstances for reinforcing material are also quite opposite. Generally for applications listed under column A good interlocking is a critical parameter whilst for applications listed under column B the most important parameter is tensile strength of the reinforcement. Applications listed under column A requires maximum interaction between aggregate and geogrid at the very beginning of deformation. For those applications the most important feature is stabilization of grain within grid aperture. Due to short time of single load impulse on geogrid, even if counted in millions cycles, naturally reduced with time during subsoil consolidation, the problem of polymer creeping for those applications is irrelevant. The stability of the aperture and general stiffness of the grid at very low strains has a critical impact on successful or unsuccessful grain stabilization. Long term strength with creep influence should be considered only for those application where load parallel to geogrid in plane applies with the same value during whole life of the structure. This mentioned diversification is very important for proper understanding of the mechanism of grain stabilization and should be analyzed at one of first steps during design work. Unfortunately analysis of many projects and work specifications shows that this phenomena is very rarely considered by designers. As the result there are many projects observed where improper reinforcemet are specified for mechanically stabilized layers what causes application failures in time. Good understanding of differentiation of mechanisms helps to avoid mistakes in specification. MECHANISMS The consideration of how geosynthetics could provide a benefit in terms of reinforcement to the granular pavement layer began with work by Giroud et al [1] with geotextiles and then moved on to the additional benefits provided by geogrids in 1984, again by Giroud et al [2]. The paper by Jenner et al. [3] describes the lessons learnt from research and full

4 scale projects from 1981 through to 2000 with particular reference to the identification of the mechanismss that provide the reinforcement function. In most pavement construction the efficient reinforcement mechanism is one which can be mobilised without undue deformation of the surface. This mechanism is one of constraint of the aggregate particles in the apertures of the geogrids and is termed interlock. (Fig. 2) [4]. The other mechanism which has been considered is the tensioned membrane where the geosynthetic material is deformed by channelised traffic so that it develops its tensile strength to act as a membrane supporting the aggregate layer. If the traffic is channelised and the material is anchored to each side of the loaded path then this mechanism could be mobilized but it requires large deformations to develop the necessary strength and resistance. Fig 3. Hence, interlock is the critical mechanism for efficient reinforcement [4]. Figure 2. Grain interlock mechanism Figure 3. Tensioned membrane mechanism PRINCIPLES OF MECHANICALLY STABILIZED LAYERS (MSL) The Mechanically Stabilized Layer (MSL) in road construction is based on perfect interaction between well graded aggregate interlocked in appropriate in size and shape, stiff aperture of the geogrid. For this reason MSL can be design only with grids for which an nterlocking mechanismm is appropriate to describe their function. It s important to recognize that also for some of geogrids the tensioned membrane is the real mechanism. For those grids, due to weak aperturee stability under load there are no odds for effective

5 MSL. One of reasons is lack of aperture stiffness in any direction different than MD and CMD as shown on figure 4 [6]. Figure 4. Example of apperture deformation which makes material unsuitable for MSL There is a strong relationship between multi-directional stiffness of the aperture and MSL efficiency. The most appropriate geogrid for MSL seems to be triaxial one where naturally load distribution is in true with mechanics more optimal (see figure 5). Biaxial geogrids have tensile stiffness predominantly in two directions. Triaxial geogrids have three principal directions of stiffness, which is further enhanced by their rigid triangular geometry. This produces a significantly different structure than any other geogrid and provides high stiffness through 360 degrees what has strong impact for MSL conditions. Figure 5. Radial load distribution under wheel is typical condition for Mechanically Stabilized Layer (MSL) CONFINEMENT EFFECT For economical design of Mechanically Stabilized Layer (MSL) very important is to determine optimal thickness of aggregate acting with geogrid. The best interaction occurs at the bottom of aggregate layer over geogrid where grains are interlocked in grid apertures. This layer is defined as the Fully Confined Zone (see figure 6). With distance increase from geogrid aggregate confinement effect is reduced and this layer is defined as Transition Zone with Partial Confinement. Above Transition Zone next layer of aggregate

6 is free from geogrid influence and defined as Unconfined Zone. Aggregate in Unconfined Zone for properly designed MSL should be reinforced again at bottom of this layer. Thickness of Fully Confined and Transition Zones is an individual parameter determined for individual type of geogrid for action with individual aggregate. There is no single similarity between different groups of geosynthetics classified in accordance to manufacturing process and as consequence every design of MSL must be done according to full knowledge about geogrid and aggregate for use in application. Any change in material both in geogrid and aggregate require redesign of MSL. Figure 6. The confinement effect within aggregate layer interlocked with geogrid CONCLUSIONS The use of geogrid for Mechanically Stabilized Layer (MSL) could rapidly improve circumstances for aggregate used for both base and sub-base in road construction. Proper selection of geogrid and aggregate is the key to optimal design of effective thickness what has a strong impact for road lifecycle extension. Not every geosynthetic may be used in MSL, due to relatively small thickness of the structure and limitation in deformation at depth of geogrid any material acting as tension membrane is not relevant for Mechanically Stabilized Layer. There is strong economical need for using MSL solutions in roads due to cost reduction and road life increase. REFERENCES [1] Giroud, J. P., Noiray, L., (1981), Geotextile-Reinforced Unpaved Road Design. Journal of the Geotechnical division, ASCE, Vol. 107, No. GT9, Proc. Paper [2] Giroud, J.P., Ah-Line, C., Bonaparte, R., (1984), Design of Unpaved Road and Trafficked Areas with Geogrids. Polymer Grid Reinforcement, Thomas Telford.

7 [3] Jenner, C. G., Paul, J., (2000), Lessons learned from 20 years experience of geosynthetic reinforcement on pavement foundations. Second European Geosynthetics Conference, Bologna. pp [4] Jenner, C, G. (2007) The Reinforcement Of The Granular Layers Of Roads And Railways, Railway Engineering Conference, London [5] CROW Publicatie 157 (2002), Dunne asfaltverhardingen: dimensionering en herontwerp [6] Report NB-209/RB-7/2006 (2007) Tests of geo-grids reinforcement effectiveness in geo-mattresses integrated in the foundation of embankment on soft subsoil, Technical University of Silesia,