Deformation of pavement layer materials has been the key factor governing design as well as performance of flexible pavements. Most of the surface deformation observed along wheel tracks in flexible pavements over clayey soil subgrades is contributed from subgrade. The placement of reinforcing fabric reduces the load on subgrade and restrains overlying layer material resulting in reduction of associated deformations due to its membrane action. So, it is advantageous if attempts are made to quantify the benefit achievable in terms of reduction of rut using reinforcing fabric at subgrade level. The present paper is intended to assess surface settlements of model unreinforced and reinforced pavement layers from wheel tracking tests and hence to quantify the effect of reinforcement on overlying sub base material and underlying subgrade soil.
KEYWORDS: Flexible Pavements, Clay Subgrades, Rut, Geotextiles, Granular Cushions.
Excessive surface settlement (Rut) of flexible pavements results in uneven pavement surface and seriously affects serviceability of the pavement. Rut affects riding comfort, increases wear and tear of the vehicles and life of the pavement. The excessive rutting along wheel tracks has been reported to result in longitudinal cracking and wavy appearance of pavement surfaces over clay subgrades of high compressibility 1,2,3. Later, the longitudinal cracks become source of moisture entry into cross section of pavement and aggravate subsoil softening and the associated rutting.
Most of the pavement failures and rut observed in flexible pavements over clay subgrades have been reported to be contributed by subgrade 2,4,5. Clayey soils, particularly high compressible clays as subgrades result in higher amount of rut during rainy season below wheel tracks due to their high plasticity and intrusion into structural layers of pavements. Saxena impressed upon road subgrade that the design methodologies are not properly accounting for its safety and hence premature failures are often seen 5. The design methods in use for control of rut in flexible pavements have been developed based on the contention that either rut is minimised by limiting vertical strain at subgrade and controlling quality of base and subbase materials or rut is restricted to a tolerable limit (of about 13 mm) 6,7. The design methods do not deal the aspect of rutting contributed from high plastic clay subgrades due to intermixing of sub base and subgrade due to effects of intrusion of subsoil and penetration of subbase material into softened subgrade. Thus in clay subgrades, the pavement design thickness is proving to be ineffective due to contamination of structural layers of pavements over a period of time.
Efforts have been made to control rut by using subsoil intrusion barriers in the form of sand, moorum and lime stabilised soil layers as blankets at subgrade level in pavement construction8. Sand blankets were observed to be less effective as intrusion barrier due to its granular nature, whereas, moorum and lime stabilised soil blankets resulted in satisfactory functioning as intrusion barriers. Use of deep vertical moisture barriers in the form of synthetic geomembranes has been reported to be used in flexible.
pavement construction over expansive clays in United States to check moisture entry into subgrade and subsequent softening after swelling 3,9. The technique has been reported to be effective in many roads, with little success on some roads. The failure has been attributed to improper compaction of backfill material around the barrier and water table fluctuations.
Geotextile fabrics due to their multi functional behaviour namely separation, reinforcement, drainage and filtration appear promising for use at subgrade to improve performance of flexible pavements over clay subgrades. Though usage of geotextiles usage has been reported as reinforcement in the layers of flexible pavement system to reduce pavement thickness and also in overlay construction to prevent reflection cracking10,11,12, research is still in developmental stage on potential use of the fabrics at subgrade level in flexible pavements over high compressible clays. Based on load deformation characteristics observed from laboratory tests conducted in CBR moulds on geotextile reinforced clays of intermediate to highly compressibility, Shroff and Shah have reported that woven geotextiles give higher percentage improvement against control of deformation 13. The use of reinforcing fabric at subgrade level reduces the vertical stress and also serves as separator (intrusion barrier) between subgrade and sub base. Thus, it may help in reducing the surface settlement of the pavements14. However, based on field studies on reinforced flexible pavements, the relationship between improved trafficability and tensile strength of geosynthetics has been reported to be good in some trials and poor in others11,15,16. Hence, there is still need for establishing ability of geotextile fabrics with respect to different subgrades in controlling rut of flexible pavements. So, in the present paper, the effect of placement of woven and non-woven geotextiles at interface of subgrade and sub base layers to reduce compressibility of subgrade and sub base material has been evaluated by conducting wheel tracking tests on model pavement systems. The tests have been conducted on unreinforced and reinforced model pavement component layers to study the surface settlement (Rut) and compressibility coefficients of the layers. Moorum sub base and Water Bound Macadam (WBM) base layers have been used in the studies.
2. MATERIALS USED IN THE STUDY
2.1. Clay Soil (Subgrade) and Moorum
Clay that has been used in the investigations as subgrade soil has been procured from a site in chintagattu village whereas Moorum has been procured from a quarry near Parkal. The engineering properties of the clay subgrade and moorum have been determined from laboratory testing and are presented in Table 1. The compaction characteristics of clay and moorum have been evaluated from I.S light and heavy compaction tests respectively. The shear Parameters of the materials have been determined from consolidated undrained tests conducted in direct shear tests.
compaction tests respectively. The shear Parameters of the materials have been determined from consolidated undrained tests conducted in direct shear tests.
The grade-III aggregate (MORTH-2001)17 used in the studies was procured from a granite crushing plant, located in Hunter Road, Hanamkonda. The gradation characteristics and engineering properties of aggregate evaluated from laboratory tests are given in Tables 2 (a) and (b).
3. WHEEL TRACKING TEST APPARATUS
Wheel Tracking Test apparatus developed by British National Rail Road Research Institute (Originally developed for measuring resistance to plastic deformation of bituminous mixtures) has been used to assess rut of unreinforced and reinforced model flexible pavement layers. The wheel-tracking test set up used in the tests is shown in Fig. 1. The wheel tracking apparatus simulates the field condition of moving wheels with provision for varying contact pressure. The height of the loading lever has been raised by using the fabricated angle sections controlling the elevation to enable loading onto model pavement layers used in the study.
The apparatus consists of :
i. Test Table: It is fitted with a fabricated mild steel tank of size 300 mm x 300 mm x 400 mm made of 5 mm gauge sheet. The test table is made to reciprocate 42 passes a minute, controlled by autocounter reciprocation of about 230 mm.
ii. Loading Wheel: Total maximum Load of 55 kg, wheel diameter 200 mm, width 50 mm made of solid rubber.
iii. Motor: 75 kW, 400V(200V), Three Phase.
4. SETTLEMENT STUDIES ON UNREINFORCED MODEL PAVEMENT LAYERS
The model pavement layers have been prepared in a mild steel tank of size 300 mm x 300 mm x 400 mm made of 5 mm gauge mild steel sheet. For evaluation of compressibility coefficients of pavement component layers, specimens were prepared by compacting Clay soil, Moorum and Water Bound Macadam (WBM) layers to different thickness. Clay soil and moorum have been compacted under standard and modified proctor compaction conditions respectively. WBM layer has been prepared as per Ministry of Road Transport and Highways (MORTH) specification17 using moorum as screening material. The details of model pavement component layers used in the investigations are given in Table 4. Three types of specimens have been used so that data of rut developed is useful in evaluation of compressibility of component layer material.
A sand cushion of 25 mm thick is provided at bottom of tank below clay to serve as drainage blanket and small diameter vertical sand drains are provided through clay to accelerate saturation of specimen during soaking. The specimens were soaked for a period of 96 hours and tested over wheel tracking apparatus by subjecting them to a contact pressure of 562 kPa. The surface levels have been recorded under wheel track at different repetitions of wheel load and the values of surface settlements have been calculated. The results of tests are presented in Table 5. The rut characteristics of unreinforced model pavement systems are shown in Fig. 2. From Figure, it may be noticed that the rut values initially increased rapidly and later, at a slower rate.
Based on the values of surface settlements (rut) observed under repetitive loading of different specimens, compressibility coefficients of pavement component layers have been determined for the layers using the equation
ax + by + cz = S
S = Surface settlement (rut depth); x, y, z = compressibility coefficients of clay subgrade, moorum and WBM layers; a, b, c = Thicknesses of clay subgrade, moorum and WBM layers
For calculation of compressibility coefficients of model pavement component layers, the zone of influence of wheel load is taken to extend up to 300 mm from surface, considering the wheel load to induce strip loading as rate of loading is quick. The values of compressibility coefficients of subgrade, subbase and base material evaluated at different repetitions of load are presented in Table 6. The variation of compressibility coefficients of pavement component layer materials with log number of repetitions (Log N) has been shown in Fig. 3.
Referring to the Fig. 3, it may be observed that the compressibility coefficient of clay increased significantly with log N. Compressibility coefficients of moorum and WBM layers also increased linearly with increase in log number of load repetitions. However, the compressibility coefficient values of WBM layers are considerably small.
5. SETTLEMENT STUDIES ON REINFORCED MODEL PAVEMENT LAYERS
As the reinforcing fabric placed at subgrade level shares load, it reduces the loading on underlying soil and hence reduces the compressibility of subgrade soil. To assess the effect of reinforcing fabric placed at interface of subgrade and sub base layer on compressibility of clay subgrade, wheel tracking tests have been conducted on reinforced model pavement layers. Woven and non- woven geotextiles have been used in rut control studies as reinforcing fabrics. The fabrics placed at interface of subgrade and sub base serve as reinforcement only if they are held in position (i.e. the fabric does not get pulled in under the applied loading). So, in the model reinforced pavement layer studies, the geotextile fabrics were held in position by tying them with binding wire to square frame as shown in Fig. 4.
Reinforced model pavement layers have been prepared with layer thickness corresponding to Type I and Type II described under unreinforced model pavement sections. The wheel tracking tests have been performed on reinforced specimens prepared after a soaking period of 96 hours. The surface settlement (Rut) along the wheel tracks observed for the reinforced specimens are presented in Table 5. Referring to Table 5, it can be observed that the placement of reinforcing fabrics has resulted in reduction of rut depth and so reinforcing fabrics have taken part in load sharing.
Rut characteristics of reinforced model pavement systems with woven and non-woven geotextiles are shown in comparison to unreinforced model pavement system (Type I) in Fig. 5. It may be seen from the Figure that among the reinforcing fabrics, woven geotextile reinforcement resulted in lower values of rut in comparison to non- woven geotextile. The difference of response of the fabrics to control rut can be attributed to the difference in their stiffnesses and frictional characteristics.
As the reinforcing fabric is placed at the subgrade level, it does not affect the strength and compressibility characteristics of water bound macadam base layer. So, keeping compressibility coefficient of WBM layer unaffected, the compressibility coefficients of moorum and Clay subgrade have been determined using the expression given below:
Xr.a + yr.b +z.c = Sr
Where, Sr = Rut of reinforced flexible pavement specimens; Xr , yr, z = Compressibility coefficients of Clay subgrade, moorum sub base and WBM base material with reinforcement. a,b,c are the thicknesses of clay subgrade, moorum sub base and WBM base layers.
The values of compressibility coefficients of subgrade soil and moorum layers obtained with use of woven and non-woven geotextiles as reinforcement are presented in Table 6. The variations of compressibility coefficients of clay subgrade and moorum subbase material with log number of repetitions of load in unreinforced and reinforced cases are presented in Figs. 6 and 7.
From Fig. 6, it can be observed that the placement of reinforcement fabric has resulted in decreased value of compressibility coefficients of clay subgrade. The reduction in compressibility coefficients of clay subgrade is indicative of reduction of load on it due to reinforcing action of geotextile fabrics. The reduction in compressibility coefficient of clay subgrade is about 10 to 25 percent with use of geotextile fabrics under the study. Fig. 7 infers that the compressibility coefficient of moorum has decreased slightly due to placement of reinforcement at subgrade level. This can be attributed to additional lateral confinement offered to the sub base material by reinforcing fabrics.
Based on the results of studies presented in the paper, the following conclusions are made with regard to use of geotextiles in flexible pavements as reinforcing material.
· Geotextile fabrics successfully function as reinforcement over soft subgrades as geotextile held in position reduced compressibility coefficient of Clay subgrade. However, load sharing ability depends on stiffness of the fabric.
· Placement of geotextile fabric at subgrade level not only reduces load on subgrade soil, but also restrains subbase material and as a result reduces rut in flexible pavements.
· Rutting in flexible pavements can be reduced by about 20 to 30 percent using geotextiles of moderate stiffness at subgrade level.
· Geotextiles are to be held in position to perform its reinforcing action. So, it is necessary to anchor them by burial into longitudinal trenches made in shoulder portion.
1. Gokhale, Y.C. (1969), “Some Highway Engineering Problems in Black Cotton Soil Region”, Proceedings of the Symposium on Characterisation and Construction Techniques in Black Cotton Soil, College of Military Engineering, Poona, India, pp 209-212.
2. Livneh, M and Ishai, I. (1987), “Israeli Experience with Runway Pavements on Expansive Clays”, Proceedings of 6th International Conference on Expansive Soils, New Delhi, India, pp 247-252.
3. Evans, R.P and Mc. Manus, K.J. (1999), “Construction of Vertical Moisture Barriers to Reduce expansive Soil Subgrade Movement”, Transportation Research Record No. 1652, Transportation Research Board, pp 108-112.
4. Reddy, K.C, Chandra Sekhar, V.S and Justo, C.E.G.(1981), “Structural Evaluation of Sub Base Courses”, Jl. of Indian Roads Congress, Vol. 42, No. 2, , pp 215 – 251.
5. Saxena, R.K. (1991), “Can Failures Be Minimised and Pavement Performance Improved by Adequately Designing and Constructing Road Subgrades”, Jl. of Indian Roads Congress, Vol. 52, No. 2, 1991, pp 263-317.
6. Shook, J.F., Finn, N., Witczac,M.W. and Monismith, C.L. (1982), “Thickness Design of Asphalt Pavements – The Asphalt Institute Method”, Proceedings of 5th International Conference on the Structural Design of Asphalt Pavements, Vol.1, pp 17-44.
7. Yang H.Huang ( 1993), “ Pavement Analysis and Design”, Prentice Hall, Englewood Cliffs, New Jersey.
8. Natarajan, T.K and Shanmukha Rao, E.(1979), “Practical Lessons on Road & Construction in Black Cotton Soil Areas”, Jl. of Indian Road Congress, Vol. 40, No. 1, pp 153-185.
9. Steinberg, M.L (1992), “Vertical Moisture Barriers Update”, Transportation Research Record No. 1362, HRB, Washington, pp 111-117.
10. Barksdale, R.D., Brown, S.F and Chan, F (1989), “Potential Benefits of Geosynthetics in Flexible Pavement Systems”, NCHRP Report – 315, TRB.
11. Fannin, R.J. and Sigurdsson, O (1996), “Field Observation on Stabilisation of Unpaved Roads with Geosynthetics”, Journal of Geotechnical Engineering, ASCE, Vol. 122, No. 7, pp 544-552.
12. Perkins, S.W and Lapeyre, J.A (1997), “Instrumentation of a Geosynthetic Reinforced Flexible Pavement System”, Transportation Research Record No. 1596, TRB, pp 31-38.
13. Shroff, A.V and Shah, G.N. (1989), “Load Deformation Characteristics of Fabric Reinforced Weak Soil Subgrades”, Proceedings of International Workshop on Geotexiles, Bangalore, pp 144-147.
14. Koerner, R.M. (1986), “Designing with Geosynthetics”, Prentice–Hall, Englewood Cliffs, New Jersey.
15. Douglas, R.A. (1990), “Anchorage and Modulus in Geotextile Reinforced Unpaved Roads”, Geotextiles and Geomembranes, Elsevier Science Publishers BV (North Holland), Amsterdam, The Netherlands, No.9, pp 261-267.
16. De Gardiel, R and Javor, E. (1986), “Mechanical Reinforcement of Low Volume Roads by Geotextiles”, Proceedings of 3rd International Conference on Geotextiles, Austrian Association of Engineers and Architects, Vienna, Austria, pp 1021-1026.
17. M.O.R.T.H – 2001: Specifications for Roads and Bridge Works, Ministry of Road Transport and Highways, India.