Factors affecting repeated loading properties

Many factors simultaneously affect both the resilient modulus and permanent deformation properties of granular materials. However, their influence on the resilient modulus was not the same as on permanent deformation properties. In this section a brief overview of the factors influencing both the resilient modulus and permanent deformation is presented. The variation in the influence of these properties is also described.

- Aggregate type and particle shape

Previous research has shown that gravel has a higher resilient modulus than crushed limestone (Lekarp, Isacsson et al. 2000). However, many researchers have reported that crushed aggregate, having angular to subangular shaped particles, provides better load spreading properties and a higher resilient modulus than uncrushed gravel with subrounded or rounded particles. A rough particle is also said to result in a higher resilient modulus. Allen argued that angular materials, such as crushed stone undergo smaller plastic deformations compared to materials with rounded particles (Allen 1973). This behaviour was said to be the result of a higher angle of shear resistance in angular materials due to better particle interlock. Barksdale and Itani investigated the influence of aggregate shape and surface characteristics on aggregate rutting (Barksdale and Itani 1989).

They concluded that a blade shaped crushed aggregate is slightly more susceptible to rutting than other types of crushed aggregate. Moreover, cube-shaped, rounded river gravel with smooth surfaces is much more susceptible to rutting than crushed aggregates (Lekarp, Isacsson et al. 2000).

- Compaction Method and density

Generally, two compaction methods are recommended for the preparation of test specimens: kneading or impact and static. The resilient modulus is directly related to the stiffness, which increases with an increase in compactive effort. This increase in stiffness varies with different materials and depends on the water content of which there was an increase in stiffness when going from standard Proctor energy up to a modified Proctor compaction energy.

Hicks and Monismith found the effect of density to be greater for partially crushed than for fully crushed aggregates (Hick and Monosmith 1971). They found that the resilient modulus increased with relative density for the partially crushed aggregate tested, whereas it remained almost unchanged when the aggregate was fully crushed. They further reported that the significance of changes in density decreased as the fine grains content of the granular material increased. Barksdale and Itani reported that the resilient modulus increased markedly with increasing density only at low values of mean normal stress (Barksdale and Itani 1989). At high stress levels, the effect of density was found to be less distinct. Vuong reported test results showing that at densities above the optimum value, the resilient modulus is not very sensitive to density (Voung and Brimble 2000). Resistance to permanent deformation in granular materials under repetitive loading appears to be highly improved as a result of increased density.

Barksdale studied the behaviour of several granular materials and observed an average of 185% more permanent axial strain when the material was compacted at 95% instead of 100% of maximum compaction density. Allen reported an 80%

reduction in total plastic strain in crushed limestone and a 22% reduction in gravel as the specimen density was increased from Proctor to modified Proctor density (Allen 1973). For rounded aggregates, this decrease in strain with increasing

density is not considered to be significant, as these aggregates are initially of a higher relative density than angular aggregates for the same compactive effort (2).

Figure 2.28 The effect of density on permanent strain (Barksdale and Itani 1989)

- Fine particle content

Studies demonstrating the variation in response of granular materials subjected to repeated axial stresses indicate that the fines content (percent passing No.200 sieve) can also affect the resilient behaviour (Hick and Monosmith 1971). Hicks and Monismith observed some reduction in the resilient modulus with increasing fines content for the partially crushed aggregates tested, whereas the effect was reported to be the opposite when the aggregates were fully crushed. The variation of fines content in the range of 2-10% was reported by Hicks to have a minor

influence on resilient modulus. Yet, a dramatic drop of about 60% in resilient modulus was noted by Barksdale and Itani when the amount of fines increased from 0 to 10%. An initially increasing stiffness and then a considerable reduction as clayey fines were added to a crushed aggregate. The effect of the fine content was investigated and it was noted permanent deformation resistance deformation in granular materials is reduced as the fines content increases as shown in Figure 2.29.

Figure 2.29 Effect of grading and compaction on plastic strain (Barksdale and Itani 1989)

- Gradation and physical properties of grain

Kolisoja showed that for aggregates with similar grain size distribution and the same fines content, the resilient modulus increased with increasing maximum

particle size (Kolisoja 1997). As the size of the particle increases, the particle to particle contact decreases resulting in less total deformation and consequently higher stiffness. Thom and Brown concluded that uniformly graded aggregates were only slightly stiffer than well-graded aggregates (Thom and Brown 1988).

They further indicated that the influence of gradation on the permanent deformation depends on the level of compaction. The effect of gradation on permanent deformation was more significant then the degree of compaction, with the highest plastic strain resistance for the densest mix.

If the grading is changed in such a way that relative density increases, then resistance to permanent deformation will rise. Significantly higher permanent strains may be expected for aggregates containing extremely high fines content (d

< 0.074 mm > 15 %) or at a low content of fine as shown in Figure 2.30. This could be explained by the assumption that the entire fines fraction does not necessarily fit into the pore spaces between the large particles. Therefore a skeleton of larger particles in full contact does not exist. As a result the resistance against permanent deformation and stiffness decreases.

Figure 2.30 The effect of grain size distribution on the aggregate’s susceptibility to permanent deformations (Kolisoja 1997)

A significant factor influencing the grain shape of coarse-grained aggregates is the mineralogical composition of the aggregate particles. The crushing technique used to produce the aggregates affects the grain shape of the crushed materials. With respect to grain shape, two general groups can be formed; the first group is composed of natural sands and gravels and the second group of crushed materials.

In this group, particle contact is between two smooth surfaces, whilst in the second group (i.e. crushed aggregates) the particle edges can be very sharp. This difference between the natural aggregate having rounded grains and crushed aggregate having sharp-edged grains is of most significance in long term and permanent deformation behaviour. The crushed material is likely to have more grain abrasion than the natural aggregate, especially at high stresses.

Many investigations have addressed the effect of macro and micro roughness of the particles on the deformation behaviour (Thom and Brown 1988; Kolisoja 1997). The surface friction angle can be measured using a sliding type test performed by pulling a representative specimen of aggregate loaded on a rough test surface. The surface friction at the contact points of particles can be assumed to affect the resilient deformation behaviour, especially when the external load reaches the value, which makes the particles slide.

- Moisture content

In practice there is always water within an UGM later. The water film on the surface of the grains influences the shear resistance. The occurrence of a moderate amount of moisture benefits the strength and the stress and strain behaviour of UGMs. Having achieved total saturation, repeated load applications may lead to the development of positive pore water pressure with any further increases in the water content. Excessive pore water pressure reduces the effective stress, resulting in diminishing permanent deformation resistance of the material. Thus a high

water content within an UGL causes a reduction in stiffness and hence deformation resistance of the layer. Figure 2.31 shows RLT test results whereby both samples started at the same moisture content but one was allowed to drain, like a real UGM layer in pavements, (so it became dryer) while the other one stayed at the same moisture content and experienced a much larger amount of permanent deformation.

Figure 2.31 Influence of drainage on permanent deformation development (Dawson A R and F 1999)

The moisture content of most untreated granular materials has been found to affect the resilient response characteristics of the material in both laboratory and in situ conditions. Many researchers who studied the behaviour of granular materials at high degrees of saturation have all reported a notable dependence of resilient modulus on moisture content, with the modulus decreasing with growing saturation level (Lekarp, Isacsson et al. 2000). Research has shown that the effect of moisture also depends on the analysis. Hicks stated that a decrease in the resilient modulus due to saturation is obtained only if the analysis is based on total

stresses (Hick and Monosmith 1971). At below the optimum moisture content stiffness tends to increase with increasing moisture level, apparently due to development of suction. Beyond the optimum moisture content, as the material becomes more saturated and excess pore water pressure is developed, the effect changes to the opposite and stiffness starts to decline fairly rapidly. As moisture content increases and saturation is approached, positive pore pressure may develop under rapid applied loads. Excessive pore pressure reduces the effective stress, resulting in diminishing permanent deformation resistance of the material.

The literature available suggests that the combination of a high degree of saturation and low permeability due to poor drainage leads to high pore pressure, low effective stress, and consequently, low stiffness and low deformation resistance. In a study conducted by Haynes and Yoder (Haynes and Yoder 1963), the total permanent axial strain rose by more than 100% as the degree of saturation increased from 60 to 80%. Barksdale observed up to 68% greater permanent axial strain in soaked samples compared with those tested in a partially soaked condition (Barksdale 1972). Thompson reported results of repeated load triaxial tests on the crushed stone from the AASHTO Road Test at varying degrees of saturation (Thompson M. R. and Naumann 1993). In all cases, the samples experienced a substantial increase in permanent deformation after soaking. It was suggested that one reason for the observed increase was the development of transient pore pressures in the soaked samples.

Holubec performed RLT tests and studied the deformation behaviour of drained crushed aggregates at a range of water contents (Holubec 1969). The test results showed that increasing the water content led to higher permanent deformations.

At 1,000 load cycles, the total permanent axial strain of a waterbound macadam pavement rose by about 300 % as the water content increased from 3.1% to 5.7%.

Similarly, the total permanent axial strain of gravel sand grew up to 200 % as the

water content increased from 3 % to 6.6 %. Thom and Brown studied the impact of the water content on the permanent deformation behaviour of Dolomite-material (Thom and Brown 1988). The outcome of the investigation showed a serious increase of permanent deformations resulting from the rise in water content of the specimens. Furthermore, it became clear that a relatively small increment of water content had a disproportionate effect on the increase in permanent deformation. This tendency was also observed without the creation of pore water pressure. It was stated that this behaviour could be attributed to the fact that the existence of water within granular assemblies partly lubricated the particles and consequently resilient as well as permanent deformations rose.

- Specimen size

Austroad and AASHTO specifies that the diameter of the specimen is a function of the maximum size of the aggregate used in the base material. Further, it specifies that the diameter to height ratio is 1:2 (Voung and Brimble 2000;

AASHTO 2002). Thus, according to this for a maximum aggregate size of 19 mm, the size of the specimen is 100 mm in diameter with a height of 200 mm. Reliable results could be obtained with soil specimens having regular ends provided the slenderness (height to diameter ratio, l/d) is in the range of 1.5 to 3.0. Since then, many researchers have studied end restraint effects on the shear strength of soils and concluded that sample slenderness can be reduced to 1.0 if frictionless platens are used (Adu-Osei 2000). Adu-Osei et al. changed the specimen size from a l/d ratio of 2:1 to 1:1. They found that specimens with l/d ratio of 2:1 gave reliable results when the end platens were lubricated. These specimens were also more stable and practical.

- Stress state

Previous investigations and studies show that stress state has the most significant impact on the resilient and permanent properties of granular materials and that duration and frequency have very little effect. Hicks conducted tests at stress durations of 0.1, 0.15, and 0.25 s and found no change in the resilient modulus (Hick and Monosmith 1971). However, the resilient modulus increases considerably with an increase in confining pressure and sum of principal stresses.

Monismith et al. reported an increase as great as 500% in the resilient modulus for a change in confining pressure from 20 kPa to 29 200 kPa (Monismith, H. B. Seed et al. 1967). An increase of about 50% in the resilient modulus was observed by Smith and Nair when the sum of principal stresses increased from 70 kPa to 140 kPa (Smith and Nair 1973). Allen and Thompson compared the test results obtained from both constant confining pressure tests (CCP) and variable confining pressure tests (VCP) (Allen 1973) as shown in Figure 2.32. They reported higher values of the resilient modulus computed from the CCP test data. They showed that the CCP tests resulted in larger lateral deformations. Brown and Hyde suggested later that VCP and CCP tests yield the same values of resilient modulus provided that the confining pressure in the CCP test is equal to the mean value of the pressure used in the VCP test (Brown and Hyde 1975).

Figure 2.32 Triaxial test results with CCP and VCP (Allen 1973)

Many studies have indicated a high degree of dependence on confining pressure and the first stress invariant (sum of principal stresses) for the resilient modulus of untreated granular materials. The resilient modulus is said to increase considerably with an increase in confining pressure and the sum of principal stresses, while permanent deformation decreases with an increase in confining pressure. Compared to confining pressure, deviator or shear stress is said to be much less influential on the resilient modulus of the material. In laboratory triaxial testing, both constant confining pressure and variable confining pressure are used.

Brown and Hyde suggested that variable confining pressure and constant confining pressure tests yield the same values of resilient modulus, provided that the confining pressure in the constant confining pressure test is equal to the mean value of the pressure used in the variable confining pressure test (Brown and Hyde 1975). The accumulation of axial permanent strain is directly related to deviator stress and inversely related to confining pressure. Several researchers have reported that permanent deformation in granular materials is principally governed by some form of stress ratio consisting of both deviator and confining stresses. Lekarp and Dawson argued that failure in granular materials under repeated loading is a gradual process and not a sudden collapse as in static failure tests (Lekarp, Isacsson et al. 2000). Therefore, ultimate shear strength and stress levels that cause sudden failure are of no great interest for analysis of material behaviour when the increase in permanent strain is incremental. The magnitude of permanent deformations developed strongly depends on the stress level and increases with rising deviator stress and decreasing confining stress. Morgan studied the behaviour of sand under repeated loading with an increasing number of load cycles and observed the impact of deviator stress and confining stress on the cumulative permanent deformations (Morgan J R 1966). A direct dependency

between the sum of permanent strains, number of load cycles applied and deviator stress was found at a particular level of confining stress. By maintaining the deviator stress at a steady level, the permanent axial strains were inversely proportional to confining stress levels. Barksdale conducted numerous RLT tests on UGMs at constant confining pressures and up to 100,000 load cycles (Barksdale 1972). He drew the conclusion that permanent deformations were highly dependent on the applied load and increased when confining pressure decreased and deviator stress rose. Pappin studied a limestone of good grading with RLT tests (Pappin J W 1979). He recognised the permanent strains to be a function of the length of the stress path and the stress ratio (deviator stress/

confining stress). The resistance to permanent deformation decreased when the applied stress approached the failure curve, i.e. the accumulated permanent strains increased at rising deviator stress.

- Stress history

Studies have indicated that stress history may have some impact on the resilient behaviour of granular materials. Repeated load triaxial tests on samples of a well-graded crushed limestone were studied, all compacted to the same density in a dry state (Brown and Hyde 1975). The results showed that the material was subjected to stress history effects, but these could be reduced by preloading with a few cycles of the current loading regime and avoiding high stress ratios in tests for resilient response. Hicks reported that the effect of stress history is almost eliminated and a steady and stable resilient response is achieved after the application of approximately 100 cycles of the same stress amplitude (Hick and Monosmith 1971). Allen suggested that specimens should be conditioned for approximately 1,000 cycles prior to repeated load resilient tests (Allen 1973). The resilient characteristics of unbound granular materials are basically insensitive to stress history, provided the applied stresses are kept low enough to prevent

substantial permanent deformation in the material. Therefore, large numbers of resilient tests can be carried out sequentially on the specimen to determine the resilient parameters of the material. Permanent deformation behaviour of granular materials is directly related to the stress history. Brown and Hyde showed that the higher the stress level, the higher is the permanent deformation as shown in Figure 2.33 (Brown and Hyde 1975). They also indicated that permanent strain resulting from a successive increase in the stress level is considerably smaller than the strain that occurs when the highest stress is applied immediately.

Figure 2.33 The effect of stress history on permanent strain (Brown and Hyde 1975)

The number of load repetitions does not only affect the permanent strain response but always has to be considered as a combination of the number of load repetitions and the stress condition. If the intensity of the applied loading is not too high, the accumulation of permanent deformations on a certain stress path is normally assumed to stabilise as the number of load repetitions increases. The

curve representing the accumulated permanent deformation asymptotically approaching a limiting value, i.e. the permanent deformation rate per load cycle tends towards zero. Increasing stress ratios lead to a progressive rise of the accumulating permanent deformations. The number of load repetitions required while investigating the permanent deformation behaviour in RLT tests is of great importance from a practical point of view (time required for completing the tests and hence overall experimental costs). Sometimes this number of load repetitions is not adequate. Kolisoja found that specimens can apparently stabilize after 80,000 load cycles (degressive curve linearity) (Kolisoja 1997) as shown in Figure 2.34.

Morgan studied the behaviour of two types of sand under a repeated vertical load at both constant and varying confining pressures using a RLT apparatus (Morgan J R 1966). He applied up to 2 million load cycles and recognized that even towards the end of the tests the permanent deformations still increased. Barksdale investigated the behaviour of UGMs using RLT tests at constant confining

Morgan studied the behaviour of two types of sand under a repeated vertical load at both constant and varying confining pressures using a RLT apparatus (Morgan J R 1966). He applied up to 2 million load cycles and recognized that even towards the end of the tests the permanent deformations still increased. Barksdale investigated the behaviour of UGMs using RLT tests at constant confining