Scope and terminology
The mechanisms that govern unpaved road performance are complex. Therefore, it is legitimate to try to get quantitative data from laboratory and field tests, but test interpretation can be correct only if there is a good understanding of the mechanisms (which was the purpose of the Part 1 article), rigorous planning of the tests (which is the purpose of this Part 2 article), and appropriate implementation of the tests (which will be the purpose of the Part 3 article).
A field test may include one or more test sections. In accordance with the terminology used in the Part 1 article (Giroud and Han, 2016), unpaved road test sections incorporating geosynthetics can be referred to as mechanically-stabilized test sections while unpaved road test sections without geosynthetic can be referred to as non-stabilized test sections. More specifically, a mechanically-stabilized test section may be referred to as geosynthetic-stabilized test section to indicate that stabilization results from the use of a geosynthetic; furthermore, the type of geosynthetic may be indicated, as in geogrid-stabilized test section. Non-stabilized test sections are often used as control sections for benefit evaluation of mechanically-stabilized test sections.
Unpaved road performance
Unpaved roads include haul roads, working platforms, and aggregate-surfaced roads, on which fewer vehicles travel at slower speed than those on paved roads. AASHTO (1993) allows aggregate-surfaced roads to be designed for up to 100,000 equivalent single axle loads (ESALs). The most relevant deformation related to unpaved road performance is rutting. Rutting is permanent deformation that accumulates as the number of axle loads increases. Large rutting may cause discomfort to drivers, damage to vehicles, and instability of the vehicles; therefore, excessive rutting should be avoided.
In the literature, rut depth is defined in two ways: apparent rut depth and elevation rut depth (Cuelho et al., 2014). The apparent rut depth is defined as the maximum vertical distance between the peak and the valley of a wheel path cross section. The elevation rut depth is defined as the maximum vertical distance between the original elevation of the road surface and the valley of the wheel path cross section. When the subgrade soil is saturated or nearly saturated, it is incompressible or nearly incompressible; in this case, the subgrade soil moves down under the wheel and moves up around the wheel. As a result, the apparent rut depth is larger than the elevation rut depth. In field tests reported by Cuelho et al. (2014), the apparent rut depth was about 1.5 to 2.0 times the elevation rut depth. Most agencies or projects adopt the apparent rut depth as a way to quantify road deformation and they allow its magnitude to be up to 50-100mm (2-4in.) for unpaved roads. Giroud and Han (2004) used an apparent rut depth of 75mm (3in.) as the serviceability limit for their unpaved road design method. AASHTO (1993) limits apparent rut depths to a typical value of 25-50mm (1-2in.) for aggregate surfaced roads. Since the rut depth generally used is the apparent rut depth, it is recommended to use this parameter for the evaluation of trafficking tests.
Mechanisms that improve unpaved road performance
Giroud and Han (2016) indicate that the mechanisms through which geosynthetics improve the performance of unpaved roads include separation between base and subgrade, lateral restraint of the base material, vertical restraint of the subgrade soil, and tensioned membrane effect. Among the mechanisms other than separation, the dominant mechanisms improving unpaved road performance within tolerable deformations (i.e., apparent rut depth smaller than 100mm [4in.]) are lateral restraint of the base material and vertical restraint of the subgrade soil. The tensioned membrane effect becomes important only when large deformations occur in soft subgrade (i.e., subgrade deformation that results in apparent rut depth larger than 100mm [4in.]). Measured data reported by Cuelho et al. (2014) show that: (i) the geosynthetic at the edge of the wheel first moved outward due to lateral spreading of the base course and then inward due to the accumulated rutting; and (ii) this displacement transition happened at the elevation rut depth of 50mm (2in.). Cuelho et al. (2014) attributed this transition to the start of the tensioned membrane effect. Indeed, an elevation rut depth of 50mm (2in.) may be equivalent to an apparent rut depth of 75 to 100mm (3 to 4in.), at which the tensioned membrane effect starts to become important, as shown by Giroud and Noiray (1981). This example illustrates that using appropriate parameters (such as apparent rut depth) is necessary to make correct interpretation of the mechanism involved.
The tensioned membrane effect becomes important only when large deformations occur in soft subgrade.
Objectives of field evaluation
Field evaluation of unpaved road performance may be conducted with different objectives: (1) quality assurance, (2) benefit evaluation, and (3) comparative study.
Field evaluation is often performed as part of road construction quality assurance. Such field evaluation, being done for actual projects, is often performed using non-destructive methods in a fast manner.
The evaluation of the benefit provided by mechanical stabilization of unpaved roads is often done by constructing mechanically-stabilized test sections and comparing their performance to that of a control section, which consists of a non-stabilized test section. For easy evaluation, mechanically-stabilized test sections and control sections should be constructed on the same subgrade soil at the same moisture content and state of compaction, with a base layer of the same thickness, grading and moisture content, and using the same construction method.
A comparative study may be used to evaluate the relative performance of geosynthetic-stabilized unpaved roads with different base thicknesses or different geosynthetic products. For a fair comparison, the parameter being evaluated should vary from one section to another, while other parameters should be kept constant.
Test sections for benefit evaluation and comparative study may be evaluated by non-destructive methods and/or destructive methods.
Test methods used to evaluate soil properties
In addition to density tests (e.g., nuclear gauge test, sand cone test, etc.), vane shear and dynamic cone penetrometer tests can be used to evaluate soil properties. The vane shear test consists in applying a torque to a metal vane inserted in soil to generate shear failure of the soil. As a result, the undrained shear strength of the soil is estimated. The dynamic cone penetrometer (DCP) test uses a falling weight to apply an impact load that forces a steel rod with a cone tip to penetrate into the soil. The amount of penetration under each blow can be used to estimate strength and modulus of the soil. The vane shear test is mainly used to evaluate the subgrade soil, while the DCP test can be used to evaluate subgrade soil and base course material. The vane shear test and the DCP test evaluate soil properties at specific depth. They have been mostly used for site investigation, quality control before construction, and quality assurance after construction, but they cannot be used to evaluate the performance of a test section.
Test methods used to evaluate road performance
Deflectometers (falling weight deflectometer [FWD] or light weight deflectometer [LWD]) generate a load pulse by dropping a weight on a circular plate, which induces a deflection basin at the road surface. Based on the load pulse and the road surface deflection with known layer thicknesses, moduli of layers under the plate can be back-calculated. There are several types of FWD devices, which have falling weights ranging from 445 to 6675 N (100 to 1,500 lbs). The LWD is a portable falling weight deflectometer that has a typical falling weight of 100 N (22 lbs). Since the LWD test has a light falling weight, it is mainly used to evaluate subgrade and base; since the FWD test has a heavy falling weight, it can be used to evaluate subgrade, base, and asphalt layer. Both FWD and LWD tests are considered non-destructive tests because they induce small deformations. Research showed that the FWD and LWD tests are not effective in detecting the improved performance immediately after the construction of test sections incorporating geosynthetics because their induced deformations are too small to mobilize the contribution of geosynthetics. However, after test sections are trafficked by wheels, geosynthetics can minimize the deterioration of granular bases so that the modulus of the base is retained for a longer performance period. Then, the FWD test can detect the higher retained composite modulus of the test section with geosynthetic compared to the composite modulus of the test section without geosynthetic, as demonstrated by Jersey et al. (2012).
Both falling weight deflectometer (FWD) and light weight deflectometer (LWD) tests are considered non-destructive tests because they induce small deformations.
The plate loading test consists in applying a load on a loading plate seated on a road surface. The road surface deforms with an increase of load magnitude, with time under a constant load, and/or with the number of load repetitions. Static and repetitive plate loading tests are used, as discussed below.
In the case of the static plate loading test, the load is maintained after each load increment and the deformation increase with time is measured. The initial deformation within the elastic limit is used to calculate the composite elastic modulus of the test section while the additional deformation close to failure is used to estimate its ultimate bearing capacity. Static plate loading tests can be used to evaluate the benefits of geosynthetics in stabilizing base courses over soft subgrade, which include increased section composite modulus and bearing capacity.
In the case of the repetitive plate loading test, the load on the loading plate is repeatedly increased and reduced, and the total deformation and the rebound (or “recovery deformation”) are measured for each loading cycle. The difference between the total deformation and the total rebound is the permanent deformation, which is often related to the rut depth of a road. During this loading process, the load intensity may be increased. The cyclic plate loading test is a special repetitive plate loading test, where a cyclic load is automatically and continuously applied at a fixed frequency by an actuator or air cylinder. White (2015) conducted cyclic plate loading tests including a sensor kit to measure ground deflections at selected radial distances from the plate center. Repetitive plate loading tests can induce elastic rebound and permanent deformations; therefore, these tests are effective in evaluating the benefit of geosynthetics in stabilizing base courses over subgrade under repeated loading, which includes increased section composite resilient modulus (related to the rebound). These tests can be conducted to large deformations, even up to failure of test sections.
The trafficking test consists in repeatedly applying axle loads on a road surface via moving wheel(s) and measuring rut depths as a function of the number of vehicle or axle passes. This is typically achieved by driving a loaded truck on the road. To reduce the time needed for evaluation, an accelerated pavement test (APT) facility can be used to run loaded wheels on the road surface in the field or laboratory in an accelerated manner. The APT is more commonly done in a laboratory than in the field. The main advantage of an APT done in the laboratory is to have better control of moisture, temperature, and wind. APT sections in the laboratory closely simulate road sections in the field; therefore, they can be considered equivalent to field evaluation. The trafficking can be conducted with reciprocating wheel action or single direction wheel travel. This detail should be documented. The APT method can generate small to large deformations and even failure of a road.
The FWD and LWD tests are the fastest and least expensive among all the tests discussed above, while the trafficking test is the slowest and most expensive, and the plate loading test is in the middle between FWD/LWD and trafficking tests. All of these test methods have been successfully used to evaluate the performance of unpaved roads without geosynthetics. The effectiveness of these test methods to evaluate the performance of geosynthetic-stabilized unpaved roads will be discussed in a later section.
Selection of test methods for field evaluation
Depending on the objective of field evaluation, different test methods and their procedures may be adopted.
For quality assurance, field tests are performed to evaluate whether a geosynthetic-stabilized unpaved road meets the design requirements. FWD, LWD, and static and repetitive plate loading tests may be performed. Static plate loading tests can assess the composite elastic modulus increase of a test section by geosynthetic while repetitive plate loading tests can evaluate the composite resilient modulus increase of a test section by geosynthetic.
Accelerated pavement test sections in the laboratory closely simulate road sections in the field.
To verify the benefit of geosynthetic stabilization, a control section should be constructed on the same subgrade soil with the same granular layer of the same thickness using the same construction method as for the geosynthetic-stabilized section. FWD and LWD tests are not able to evaluate the benefit of geosynthetic immediately after the construction of the road because, then, the geosynthetics are not mobilized; but FWD and LWD tests can detect the improved performance after the road has been trafficked for a certain time period because, then, geosynthetics are mobilized as a result of accumulated deformation of the road structure. In particular, FWD and LWD tests may be used to assess the benefit of geosynthetic on the retained composite modulus of the test section over time as shown by Jersey et al. (2012). Static and cyclic plate loading tests can be performed to evaluate the benefit of geosynthetic in increasing the composite elastic modulus and composite resilient modulus of the test section, respectively. The benefit of geosynthetic in increasing the road life can be evaluated by trafficking tests. If test sections allow for tests to be run to failure, the increased bearing capacity can be evaluated by static plate loading tests or the prolonged road life can be evaluated by trafficking tests to large rut depths.
Comparing the performance of two or more test sections with different geosynthetics or different base thicknesses in a comparative study is difficult, and it may be misleading to generalize the results obtained from a comparative study done by performing field tests under specific conditions because the performance of a road incorporating a geosynthetic depends on many factors, such as those related to the soil material and its variability, the construction method, the geosynthetic, and the soil/geosynthetic interaction.
Here, the discussion is limited to factors related to the geosynthetics.
Factors related to geosynthetics include, for example, type of geosynthetic, type of polymer, type of manufacturing process, and geometry and mechanical properties of the geosynthetics. Even if a comparative study is limited to a certain type of geosynthetic, the number of parameters can be large. For example, for geogrids, the relevant properties include, but are not limited to, aperture stability modulus, junction strength, aperture shape, aperture size and aspect ratio, rib thickness and profile, and tensile stiffness. It is hard to identify which parameters have the most important effect on the performance of unpaved road sections stabilized with different geosynthetics. The field study conducted by Cuelho et al. (2014) confirmed such difficulties. A comparative study is more feasible and reliable if the number of variables is limited, such as a study conducted for different geosynthetic products made with the same polymer, the same manufacturing process, and even the same manufacturer. In this case, the different geosynthetics of the same group are described using the term grade, which depends on basic parameters such as thickness and stiffness. The geosynthetic with a large thickness and high stiffness is considered as a high-grade product. For example, Qian et al. (2013) used three punched-drawn triangular aperture polypropylene geogrids of different grades and equivalent aperture size in cyclic plate loading tests in a large box and clearly demonstrated the effect of the geogrid grade on the performance of geogrid-stabilized bases over soft subgrade.
When different types of geosynthetics are used for a comparative study, an effort should be made to investigate and quantify the mechanisms that govern performance to ensure better interpretation and possible generalization. To that end, instrumentation of the field test sections should be undertaken. For example, White et al. (2010) installed earth pressure cells vertically in the base and the subgrade to measure horizontal stresses and evaluate the lateral restraint mechanism for woven geotextile, biaxial geogrid, and triangular aperture geogrid. They found that the triangular aperture geogrid was most effective in increasing the horizontal stress in the base as well as reducing the horizontal stress in the subgrade. To evaluate the benefit of geosynthetics for the tensioned membrane mechanism, a large rut depth (e.g., apparent rut depth greater than 100mm [4in.]) must be allowed to develop. As a general rule, a comparative study should be conducted by varying one influence factor of interest and fixing other influence factors. For example, to investigate the effect of base thickness on performance of geosynthetic-stabilized unpaved roads, the base thickness should be varied for a specific subgrade condition with a specific geosynthetic product.
When a thick granular base without any geosynthetic is compared with a thin granular base with a geosynthetic in a comparative study for equivalent performance of two unpaved test sections, it involves two variables: base thickness and geosynthetic. The equivalent performance is contributed by the combined effect of these two variables. Different base thicknesses of two test sections with equivalent performance may be designed using available design methods and evaluated by plate loading tests and/or trafficking tests. To ensure true equivalency in the mechanisms, mechanical action on the subgrade should be the same in the two test sections. Instrumentation is then needed to check that the surface deformation and the vertical stress on top of the subgrade under the same loading condition are equivalent.
Comparing the performance of two or more test sections with different geosynthetics or different base thicknesses in a comparative study is difficult.
Design of test sections
Selection of test sections
The design of test sections depends on the objective of field evaluation:
- When testing for quality assurance purposes, test sections should be randomly selected along the road.
- To evaluate the benefit of a geosynthetic, at least two test sections should be designed with same base material and thickness on the same subgrade, which include one control section without any geosynthetic and another section with a geosynthetic.
- For a comparative study, the number of test sections to be designed depends on the number of geosynthetics, subgrade conditions, and/or base thicknesses to be evaluated (at least two geosynthetic products or two base thicknesses should be used).
The size and the number of test sections are an important consideration. They will be addressed in the Part 3 article.
Design methods available in the literature may be used for the design of unpaved test sections, for example, those included in the FHWA “Geosynthetic Design and Construction Guidelines” (Holtz et al., 2008). The outcome of the design of test sections is the thickness of base course.
To achieve meaningful results, field tests should be designed in accordance with the mechanisms that govern the performance of unpaved roads (see the Part 1 article, Giroud and Han, 2016). Since the performance of unpaved roads with or without a geosynthetic depends on several influence factors, these influence factors should be considered during the design of test sections. These factors, which include performance criteria, loading, and parameters related to the materials used in the tested unpaved roads, are discussed below.
Performance criteria include rut depth and number of vehicle passes. An apparent rut depth of 75mm (3in.) has been commonly used as a serviceability limit for design of unpaved roads, which may also be used for the design of test sections. For benefit evaluation, the number of vehicle passes should be limited by a tolerable apparent rut depth (typically smaller than 25mm [1in.]) if test sections will be used as a service road or be paved later, or limited by time and/or budget. For a comparative study, the number of vehicle passes should also be limited due to time and cost considerations. The typical number of vehicle passes used in field trafficking tests of unpaved roads is 1,000. If an accelerated pavement testing facility is used, a large number of axle passes may be adopted, typically 5,000 passes or more.
The typical number of vehicle passes used in field trafficking tests of unpaved roads is 1,000.
Loading and tire pressure
For most unpaved road applications, tire pressure ranges from 400 to 700 kPa (approximately 60 to 100 psi) and wheel load ranges from 20 to 90 kN (5 to 20 kips) for a single axle or 35 to 180 kN (8 to 40 kips) for a tandem axle. The most commonly used tire pressure and wheel load for trucks in the United States are 550 kPa and 40 kN (80 psi and 9 kips), respectively. High tire pressure necessitates a high-quality granular material for the base in an unpaved road but does not necessarily require a thick base.
Subgrade strength is a key parameter for the design of unpaved roads with or without geosynthetic. Subgrade shear strength is often quantified using undrained shear strength, which can be measured by the vane shear test in the field or unconfined compression test in the laboratory; also, it can be estimated using available correlation with the California Bearing Ratio (CBR). However, for a specific subgrade soil, it is preferable to develop a site-specific correlation. There is also a common correlation between CBR and DCP penetration index (e.g., Webster et al., 1994). If rainfall is expected during field tests, the soaked subgrade shear strength should be used for design.
A sensitive subgrade, the strength of which decreases after disturbance by trafficking, should be avoided because it will introduce complexities in interpretation of test results. When a sensitive subgrade cannot be avoided in test sections, a remolded subgrade strength should be used for design.
Variability in subgrade strength exists in the field. Examples of variability characterized by the coefficient of variation (COV) are as follows:
- White et al. (2005) reported that the COV values for DCP penetration indices of base and natural subgrade ranged from 14.3% to 47.0%.
- Phoon (2007) indicated that: (i) COVs for geotechnical properties ranging from 10% to 30% are considered low; and (ii) typical COVs for undrained shear strengths of clays obtained from unconfined undrained (UU) tests and vane shear tests are 10% to 30% and 10% to 40%, respectively.
Avoiding variability in subgrade strength is key to achieving meaningful results. The variability in subgrade strength should be checked across individual sections as well as across all sections. The number of tests thus required depends on the variability in subgrade strength and will be further discussed in the section “Representativeness of test sections” in the Part 3 article.
Since almost all the design methods for unpaved roads have been developed based on 50% reliability (i.e., average performance), it is appropriate to use average subgrade strength for the design of test sections. However, if there is variability, using averaging over the whole test area may not be representative and could lead to premature failure of some test sections. In this case, averaging over each individual test section should be used rather than averaging over the whole test area.
Base material properties
The base in an unpaved road is in direct contact with wheels; therefore, the base material should have sufficient strength, modulus, and abrasion resistance to withstand trafficking effects for the service life of the road. Granular material is generally used as base material. For the selection of the base granular material, the following can be considered: (i) rounded or subrounded particles are not suitable for a granular layer used as a base because granular layers constructed with such particles have low strength and modulus; and (ii) single-sized angular particles are difficult to compact and tend to break under wheel loading. As a result, the most suitable granular material is well-graded crushed aggregate.
The strength and modulus of well-graded aggregate are often quantified by CBR tests and/or DCP tests. It should be pointed out that the CBR value of a granular layer in the field is often lower than that determined by standard CBR tests in the laboratory because the granular layer in the field is less confined and more difficult to compact than in the laboratory, especially when the subgrade is soft.
Geosynthetics, commonly used to improve the performance of unpaved roads, are nonwoven geotextile, woven geotextile, geogrid, and geocell.
Nonwoven and woven geotextiles can serve a function of separation between granular base and subgrade soil and the key geotextile parameter is then the apparent opening size, which should be selected based on the gradation of the subgrade soil. Geosynthetics with high tensile strength and low interlock capabilities (such as some woven geotextiles and some geogrids with apertures too small to interlock with aggregate) may serve as a tensioned membrane providing additional force to support wheel loads if a large rut depth (> 100mm [4in.]) is allowed.
If the aggregate is open graded, a nonwoven geotextile may be placed under the geogrid.
A stiff geogrid, able to restrict lateral displacement of aggregate by interlock, can contribute to separation between well-graded aggregate and fine-grained subgrade by maintaining the integrity of the aggregate layer. However, if the aggregate is open graded, a nonwoven geotextile may be placed under the geogrid. Geogrid properties considered to be important for lateral restraint of the granular material are rib shape, rib thickness, aperture size, initial tensile modulus, in-plane flexural stiffness of the ribs, and junction efficiency (Webster, 1992; Giroud, 2009). In addition, aperture shape plays an important role in geogrid-particle interlocking. To ensure effective interlocking between geogrid and granular material, the particle size and gradation should be controlled and a geogrid with compatible aperture size should be selected. Holtz et al. (2008) suggested that the geogrid aperture size should be larger than the mean particle size and smaller than twice the particle size corresponding to 85% finer. Giroud and Han (2015) concluded that the optimum aperture size for geogrid interlocking with granular material is approximately twice the mean particle size.
Geocells can provide closed confinement to granular material and their effectiveness depends on geocell height, pocket diameter, welding strength, and degree of compaction of the granular material.
Recommendations and conclusion
Field evaluation of unpaved roads incorporating geosynthetics can have different objectives.
The following recommendations can be made from the above discussions.
The objective of field evaluation should be clearly defined. The methods for evaluation may be different for different objectives (i.e., quality assurance, benefit evaluation, comparative study).
Test sections for benefit evaluation and comparative study should be planned in a way that ensures they will be performed under well-controlled conditions. In particular, uniformity of subgrade is essential and should be required.
Appropriate design of the base and appropriate test methods are key to a successful field evaluation.
Field evaluation of unpaved roads incorporating geosynthetics can have different objectives: quality assurance, benefit evaluation, and comparative study. Design of test sections and selection of test methods depend on the objective of field evaluation.
Representative test sections should be carefully designed. Falling weight deflectometer (FWD), lightweight deflectometer (LWD), static, and repetitive plate loading tests may be considered for quality assurance and benefit evaluation. Trafficking tests can be planned for benefit evaluation and comparative study.
In conclusion, this article provides guidance for properly planning field tests for quality assurance, benefit evaluation, and comparative study. Proper planning of field tests requires a good understanding of the mechanisms that govern the performance of unpaved roads (which was the purpose of the Part 1 article published in the February/March issue of Geosynthetics), while adequate implementation of the field tests is essential (which will be addressed in the Part 3 article to be published in the June/July issue of Geosynthetics).
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Jie Han, Ph.D., is a professor at the
University of Kansas and has academic and industrial experience in geosynthetic research and applications. In 2014, he received an IGS award for his research on design of unpaved and paved roads using geosynthetics.
J.P. Giroud, Ph.D., is a consulting engineer, a past president of the International Geosynthetics Society (IGS), and a member
of the U.S. National Academy of Engineering. He has published unpaved road design methods since 1980.