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Arno Hefer Administrator Posts: 18
10/29/2024
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The Rubicon Toolbox suite of Layered Elastic Theory (LET) Tools can be used effectively to incorporate design with geosynthetics, using concepts universally accepted in this industry. This overview mainly deals with the use of geosynthetics in granular and/or subgrade layers. The term stabilisation goes beyond just chemical stabilisation or the mechanical blending of different natural materials – it also includes the use of geosynthetics to enhance the structural performance of pavements.
The background presented is largely obtained from Hefer et al (2023).
Asphalt reinforcement is not included here; for more information, consult other sources such as the South African Sabita Technical Guideline on Asphalt Reinforcement (TG3).
Mechanisms
For geogrids, the stabilisation effect occurs through internal confinement – interlock of the aggregate with the grid enhances both shear strength and stiffness which propagates into the layer. This is achieved by introducing tensile stiffness at the base of the layer using a suitable planar geosynthetic system. Two key mechanisms contribute to this effect: interface friction and interlocking. While friction can occur between geotextiles and aggregate, interlocking is unique to the interaction between geogrids and aggregate, as the grid structure physically engages with the aggregate particles, preventing their lateral movement and improving the overall stability of the layer. Both the properties of the geogrid and characteristics of the aggregate influence the level of confinement achieved.
Key geogrid factors that affect the interaction between geogrid and aggregate include:
- Aperture size in relation to aggregate size and grading
- Aperture shape
- Rib shape and stiffness
- Stiffness of the junctions between ribs
Beyond these geogrid-related factors, the shear properties of the aggregate also play a role in determining the effective thickness of the confinement zone within the layer. To select the optimum aperture size, designers often refer to the maximum aggregate size or a specific sieve size. Research indicates that the maximum geogrid-aggregate interaction is achieved when the aperture size (S) is approximately two to three times the median aggregate size (D50) as shown in Figure 2. Table 1 can be used as a guide for selection of geogrid aperture sizes compatible with different gradations.
Research based on a maximum aggregate size of 31.5 mm shows that the following assumptions can be made regarding the height of the full confinement (Vega et al, 2018):
- Flexible geogrids: Two to three times the maximum aggregate size
- Rigid geogrids: Four to six times the maximum aggregate size
- Dissipation of confinement: Over a zone of four to five times the maximum aggregate size
For stiff geocells, the stabilisation effect is achieved through external confinement – lateral and shear restraint is facilitated by a three-dimensional matrix of interconnected cells, which imparts tensile stiffness to the unbound layer. The external confinement of the granular infill within the geocell structure further enhances the stiffness of the layer and effectively limits or prevents deformation of the stabilised material.
The properties of geocell material directly influence both the immediate confinement achieved and the long-term performance of the system. The key material properties that ensure effective confinement include:
- High dynamic tensile stiffness of the cell walls
- Resistance to permanent deformation over time
- High tensile strength
The confinement effect can be assumed to be equal to the cell height as a minimum – due to the spread of the confinement effect, an additional 20 mm of full confinement is generally assumed above the cell (and below, depending on placement). Beyond the fully confined zone, the interlocking effect diminishes over a distance of six to eight times the maximum aggregate size from the top of the geocell.
While the quality of the infill material affects the level of modulus improvement, the 3D confinement provided by the geocell allows for the use of a broad range of granular materials, from marginal to high-quality
Concepts of Improvement Factors
The use of improvement factors is widely accepted and caters for implementation using mechanistic-empirical pavement design methods. These factors quantify the improvement of a material or layer quality facilitated by the inclusion of geosynthetics – and is typically expressed in terms of a ratio of the material property (such as modulus) before and after improvement.
Suppliers have developed graphs such as the one shown in Figure 4 to determine improvement factors for different geosynthetic products. Relationships typically include the original material quality, geosynthetic properties, and supporting conditions. The generalised example relationships shown below were produced from interpolation between data published for various biaxial polypropylene extruded grids of different strengths, assuming good interlock through aggregate-geogrid aperture/opening size compatibility.
Due to the popularity of the AASHTO (1986/1993) empirical pavement design method in the latter half of the 20th century many efforts to develop these factors focused on parameters compatible with this method, notably the Layer Coefficient Ratio (LCR) – defined as the ratio between the layer coefficient of the geosynthetic stabilised layer to the layer coefficient of the original/unstabilised layer. Functions are available to relate the layer modulus with the layer coefficient, and can be used for conversion.
Figure 5 shows an example relationship developed for stiff geocells manufactured with novel polymer alloys (NPA). The concept is the same as for LCR but defined in terms of improvement of the material/layer modulus – the Modulus Improvement Factor (MIF). The MIF is defined as the ratio of the modulus of the layer incorporating the geosynthetic, to the modulus of the layer without the geosynthetic.
As shown in Figure 5, the MIF in the geocell example is a function of the modulus of the original/infill material and the modulus of the support. The MIF can be implemented for any layer in the pavement structure, but since it implicitly depends on the modulus of the support, this modulus should represent a modulus equivalent to the modulus of a subgrade, i.e. a semi-infinite half-space.
While the improvement factors LCR and MIF implicitly consider the support, the resulting modular ratio still needs to be checked for reasonableness. The Improved Modulus Ratio (IMR) is based on the full semi-infinite support.
IMR = Es/Esup
Where IMR is the Improvement Modulus Ratio, Es the elastic modulus of the geosynthetic stabilised layer, and Esup the semi-infinite modulus (or equivalent) of the supporting layers. A maximum ratio of 5 for geogrid stabilised layers is proposed based on the maximum value known for granular materials (Giroud and Han, 2004). For novel polymer alloy (NPA) stiff geocells, a maximum ratio of 7.6 is proposed (Pokharel, 2010).
Design Implementation
See the post How To Use The LET Standard Axle Design Tool for mechanistic-empirical design using Rubicon Toolbox. The recommended procedure for incorporating geosynthetics into the design process is outlined in the following figure. This method applies to both granular base and subbase materials, as well as selected gravel and subgrade layers. It is essential to carefully select the geosynthetic based on its intended function and appropriate specifications.
For effective design, it is crucial that geogrids or geocomposites are compatible with the natural materials being used to ensure aggregate interlock. Additionally, the confinement thickness or effective thickness must be established as an input for the model, as discussed above.
If the final layer thickness surpasses the effective confinement height, two layers may be considered where practical, or the equivalent modulus concept can be applied to combine them into one layer. The latter approach is often necessary when the maximum number of layers is exceeded.
The recommended approach for designing geosynthetic-stabilised pavements is to first conduct a full failure analysis without the inclusion of geosynthetics, which serves as baseline/reference design.
Once an appropriate geosynthetic product has been selected, considering all relevant specifications and design factors, it can be reasonably assumed that stabilisation will enhance the shear properties within the confinement zone as discussed above, and the effective height of confinement can be determined. In setting up the pavement structural model, therefore, only the increase in stiffness needs to be accounted for in the final design and current practice suggests that failure analysis for the stabilised layer can be excluded.
A description of the type of geosynthetic and key properties can be included when setting up the pavement structure in Rubicon Toolbox by adding a User Defined Material – See the post Adding Materials.
Design Example
In this example, the South African Mechanistic Design Method (SAMDM) and associated material classes are used.
1. Benchmark Design
2. Design with Geosynthetics
- In this analysis the cement stabilised subbase is replaced with a stiff NPA geocell stabilised subbase.
- Assume that G7 quality material is available as an infill material. An elastic modulus of 120 MPa is typically associated with this material type.
- An equivalent semi-infinite elastic modulus of 82 MPa is calculated for the two layers supporting the cemented layer in the original design. Equivalent modulus estimates can be obtained using classic techniques such as Palmer and Barber (1940), or simply selecting an equivalence criterion (such as deflection) and calculating the equivalent/combined modulus through an iterative process using the LET Standard Axle Design Tool or the LET Stress Strain Calculator available in your Rubicon Toolbox.
- A valid modulus improvement factor (MIF) and maximum improved modulus ratio (IMR) is obtained from the supplier related to the product material specifications. In this example Figure 5 was used; having established the unstabilised modulus Eu of 120 MPa and modulus of the support (Esup) of 82, the resulting MIF ≈ 3.5.
- The modulus of the geocell stabilised material, Es = MIFˑ Eu = 420 MPa
- The improved modular ratio (IMR) is Es/Esup = 5.8 which is less than the maximum value of 7.6 associated with stiff geocells. An improved modulus of 420 MPa is therefore accepted.
- It should be noted that, while an improvement in the granular base modulus can be expected and incorporated due to the support provided by the stiff geocell-stabilised subbase layer (with an improved modulus of 420 kPa), an increase in the shear strength of the granular base material should also be considered. This involves selecting appropriate shear parameters as part of the granular failure criterion inputs.
- The analysis using the LET Standard Axle Analysis Tool including a stiff NPA geocell is shown below.
- An NPA geocell with 150 mm thickness and 30mm G7 overfill is recommended resulting in a subbase layer with total thickness of 180 mm.
References
Cook, J. and Horvat, F. (2014). Assessment of particle confinement within a mechanically stabilised layer. 10th Int. Conference on Geosynthetics (ICG), 21-25 September 2014, Berlin, Germany.
Giroud, J.P. and Han, J. (2004) Design method for geogrid-reinforced unpaved roads – Part II: Calibration and verification. ASCE Journal of Geotechnical and Geo-environmental Engineering, 130(8), 787-797.
Hefer, A.W., Zannoni, E. and Du Preez, H. (2023). Pavement Design with Geosynthetics. Proc. 13th Conf. on Asphalt Pavements for Southern Africa, 15-18 October 2023, Champagne Sports Resort, KZN, South Africa.
Palmer, L.A. and Barber, E.S. (1940) Soil displacement under a circular loaded area. Proceedings, Highway Research Board, Vol 20.Pokharel, S.K. (2010) Experimental Study on Geocell-reinforced Bases under Static and Dynamic Loading. PhD Dissertation. Civil Environmental and Architectural Engineering and Graduate Faculty of the University of Kansas, USA.
Mulabdić, M. and Minažek, K. (2012) Nature of friction between geogrids and soil. 5th European Geosynthetic Congress, Valencia, Vol 5, pp. 435 – 440.
Mulabdić, M., Minažek, K. and Kaluder, J. (2018) Geogrids – what is important. Certa 2018, 5th Int. Conference on Road and Rail Infrastructure, 17-19 May 2018, Zadar, Croatia.
Vega, E., van Gurp, C. and Kwast, E. (2018) Geokunststoffen als funderingswapening in ongebonden funderingslagen. SBRCURnet (CROW), Delft, Netherlands.
Zannoni, E. (2016) Geosynthetics – from product to technology in road rehabilitation. Civil Engineering, April 2016, pp. 31-35.
Palmer, L.A. and Barber, E.S. (1940) Soil displacement under a circular loaded area. Proceedings, Highway Research Board, Vol 20.
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