INFLUENCES OF WATER TABLE ELEVATION ON STRUCTURAL PERFORMANCE OF FLEXIBLE PAVEMENT MATRIX WITH DIFFERENT SUBGRADE SOIL TYPES

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1 INFLUENCES OF WATER TABLE ELEVATION ON STRUCTURAL PERFORMANCE OF FLEXIBLE PAVEMENT MATRIX WITH DIFFERENT SUBGRADE SOIL TYPES ALAA M. ALI Ph.D., Assistant Professor, Misr High Institute for Engineering & Technology, Mansoura, Egypt Abstract- Far-reaching research and many test methods have been performed and developed to investigate the effect of water on asphalt mixtures. Most of this work was related to stripping and durability. This paper presents a study on the effect of water table fluctuation on structural performance of flexible pavement matrix. Egypt road network covering north and delta regions are showing severe deterioration due to water intrusion that breaks the bond between aggregates and asphalt film causing raveling and stripping distresses. The main purpose of this paper is to identify the effect of variation in the soil type and hydraulic characteristics besides variations in the ground water table (GWT) and conditions on the water flow through typical flexible pavement systems. It also studying the watering effect on strength and durability of flexible pavement layered system. It was found that the effect of water existence in flexible pavement is very costly in terms of pavement life. The cost increase due to decrease in design-life and increase of maintenance cost by 7% to 84% according to elevation of ground water table and subgrade soil type. It also recommended to keep GWT far by at least 90% of matrix height or 1.0m (which is bigger) below the matrix bottom line Index Terms- Flexible pavement, strength, performance, ground water I. INTRODUCTION Movement of subsurface water in pavement layers is a complex subject not fully understood so far. Several methods are available for estimating the peak flow in pavement layers for drainage design purposes. However, these methods are based on very limited measurements of the infiltration rate through the pavement layers. Additionally, models have been developed over years to predict the total flow from the base layer over a given period of time, but again these models suffer from the limited knowledge of the actual infiltration and flow rates in real roads [1]. Studies have shown that the infiltration rate is affected by a number of variables. These variables include the permeability of the pavement layers, extent and width of cracking, intensity and duration of rainfall events, and the degree of saturation within the base layer. Models of varying complexity have been developed to predict drainage to and from pavement layers. However, there is a need for local experimental and field data on infiltration rates and water flow to validate these models through the different pavement types and layers. An extensive survey has been conducted in conjunction with local roads and traffic departments and GARBLET branches of the West Delta of Egypt governorates (Dakahlia, Gharbia, and Damietta) in addition to the highway-engineering laboratory, in Mansoura University. This survey was carried out to identify the nature of prevailing subgrades as well as base and surface layers mostly used in the Nile-Delta region. Data that covers the past seven years was collected from quality control, follow-up and handing-over reports. The direct simple survey and statistics of Nile-Delta study area indicates that about 84% of the sub-grade soils subsisted in the considered area was classified as A-3, A-4, and A-7-6 as per AASHTO classification system based on location. All base courses used are local aggregate in which about 92% of this aggregate is classified as A-grade according to E.C.P. On the other hand, around 31% of the arterial roads asphalt surfacing was found to be single layered wearing course either 3-D or 4-C class as per E.C.P. while the rest is double layered with 4-C wearing over 3-D base course. II. MATERIALS SELECTION Based on the above, the subgrade soils were selected (or mixed if necessary) in concurrence with the Egyptian code of practice for highways (E.C.P2010.) and were believed to be representative of the Nile-Delta subgrade soils. Three subgrade soil samples were drawn from three different locations in the Nile delta, the first is a sandy soil taken from the north region (Gamasa), while the second is silty soil taken from the west middle delta region (Kalabsho & ziyaan - Belqaas), and the third is clayey soil taken from south delta region (Zefta countryside). The base course of crushed dolomite aggregate is also selected to comply with grade A of the standard Egyptian specifications, which is widely adopted in the Nile-Delta local roads networks. The hot mix asphalt-wearing layer is chosen to be 3-D type for binder and 4-C for wearing courses respectively. 27

2 III. EXPERIMENTAL PROGRAM The purpose of this experimental and testing program is to identify the effect of variation in the soil type, nature and hydraulic characteristics as well as variations in the water table and conditions on the water flow through typical flexible pavement systems. It also aims at studying the watering effect on strength and durability of hot mix asphalt (HMA) layered system. A laboratory-scale test-pit test facility was manufactured to determine strengths and performance of flexible pavement layered materials under different moisture and loading conditions. The test-pit facility constructs and simulates the subgrade, base, and wearing surface layers of a flexible pavement system on a laboratory-scale basis. However, the test-pit testing program has two major concerns; First, find out both deformation and equivalent resilient modulus of the layered system under both static and dynamic loading. Second, evaluate both combined equivalent permeability characteristics, and capillary behavior of the pavement materials with different water table elevations. For evaluation of moisture influence on pavement materials performance, the water table was varied while conducting both static and dynamic loading tests. The research program requires the water table to be changed from drained to flooded conditions. Adopting evaluation of pavement materials using the proposed test-pit serves the following advantages: a) It can be used to simulate the different material components of a pavement system on a laboratory-scale basis. b) It facilitates change of water level to simulate the different moisture conditions in a practical situation. c) Together with a loading system, the test can be used to investigate the deformation characteristics of sub-grade, base and wearing materials due to influence of both static and dynamic loads. The capillary action and resilient deformation of the materials under investigation were evaluated with three different levels of water table to achieve the following conditions: Partially saturated; (No water and Embankment's materials at O.M.C.) Partially submerged; (Water table between wearing and subgrade layers) Flooded; (Water table directly below wearing layers). To compensate the loss due to capillary rise and evaporation, extra water had to be added within the pit to keep the water table constant at each designated elevation prior to the moisture equilibrium and testing. 28 IV. LABORATORY TESTING AND INVESTIGATION Laboratory-scale Test-pit: As shown in schematic Figure-1, the test-pit constructed for the research work is shaped like a rectangular supported steel vessel that is 100cm long, 30cm wide, and 55cm deep, all vessels' sides' surfaces are made from 6mm thickness steel plates to be stiff enough to resist deformations under loading. The front side of the vessel is made from 10mm thickness transparent blex-glass to monitor the water elevation and seepage through the embankment. There are two opposed lateral water tanks to simulate the adjacent roadside waterways. Each lateral side is supplied with three circular controlled orifices to control the water level in the side tanks as needed. Inside the above said vessel, three movable steel skeletons simulate the pavement's layered system The bottom trapezoidal steel Skeleton is 30x100 cm base, 30x70 cm top, and 30 cm height. It is provided to form the typical cross section of the embankment s subgrade inside the steel vessel. Figure-1: Laboratory- scale prototype test pit Loading system Static loading testing: In static loading, a plateloading test was adopted and conducted. It is considered a standard method for non-repetitive Static Plate Load test of soils and flexible pavement components, for use in the evaluation and design of airport and highway pavements. The test was carried out as per BS 1377 Part 9, 1990 Standards. The apparatus as shown in Figure-2 is integrated into the prototype mainframe, which acting as a vertical loading sustain to produce the desired reaction on the test-pit surface under test. The loading effect results from a hydraulic Jack assembly with a spherical bearing attachment, capable of applying and releasing the load increments. The jack has sufficient capacity for applying the maximum load required, and prepared with accurately calibrated gauges that indicate the magnitude of the applied load. The

3 bearing plate is a circular steel plate with 20mm thickness to ensure stiffness and rigidity, and has a diameter of 12.5mm to contact the embankmenttested surface. The dial gauges readings are of 0.02mm units and able of recording accumulated deflection up to 25mm. Dynamic loading testing: In dynamic loading, a repeated cyclic loading was applied; an Electro-Mechanical closed-loop testing system was used. The major components of the system are loading system, time-counter controller, and micrometers for deformation measurements. The loading unit consists of a loading frame and a 5HP electromechanical actuator connected to a 10Ib vertically moving steel rammer with conveyor steel belt that has a removable end to fix the 3"dia circular loading plate. The vertical rammer weight could be modified by adding extra weights on its top. A straight edge bar is supplied at the top edge to measure the surface deflection using back digital vernier. The repetition of the dynamic loading was controlled by adjusting the attached motor speed. In this study, the loading pulse duration and the rest period were set at 0.8 and 1.0 seconds, respectively as per to AASHTO-TP-62. Calibrations were made periodically during the laboratory-testing program. r = horizontal distance from the center of a circularly loaded area in which a change in vertical stress is desired, ft. Figure-3: Vertical stresses induced by uniform load on circular area. (J. E. Bowles 1995) Figure-3 illustrates the computed vertical stress contours that represent the typical bulbs of vertical pressure. The contour labels represent percentages of the applied pressure. Hence, and based on the above, the top embankment s exposed area is chosen to be 25 x 30cm, and the height is 55cm which is 4 times the diameter of the applied load hammer plate (3").(Boussinesq 1885) Semi-Field testing and results As mentioned earlier, three types of soil representing typical Nile-Delta subgrade materials were tested in the test-pit program. For each soil, static and repeated (up to 10,000 cycles for traffic load simulation) load testing is conducted under different levels of groundwater table. Since the resilient behavior of subgrade soil under the dynamic loading was influenced by the soil properties and moisture conditions, a detailed evaluation was made of the moisture profile for various water levels. Figure-2: Schematic of laboratory scale full set testing system Influence of e Loading Depth (Pressure Bulb) According to Boussinesq theory, the change in soil pressure due to a circular applied load may be calculated from the following equation (Boussinesq 1885) P v =[3q/ 2 z 2 ] [1/(1+(r/z) 2 ] -2.5 (1) Where: P v = change in vertical stress at point z below the center of a circularly loaded area, and point r horizontally from the center of the circularly loaded area, lb/ft 2. q = applied stress from structural load, lb/ft 2. z = depth below center of circularly loaded area in which a change in vertical stress is desired, ft. V. ANALYSIS AND DISCUSSION a. Effect of water existence on surface deformation in case of dynamic loading The main objective of this test is to find out the effect of repeated dynamic load on combined embankment section by measure and evaluate the local resulting distresses for different types of subgrade materials with different water levels. A series of repeated dynamic loading tests were conducted for each type of subgrade soil when the moisture equilibrium was achieved after adjusting the groundwater level. The designated test numbers and their corresponding loading conditions for each soil are listed in Table-1 and plotted in Figure 4-a. The relative elevation 0.00h is set at the interface 29

4 between the subgrade and embankment while 0.90h is referred to the top of aggregate base layer. The repetitive dynamic loading continued for ±50 cycles (about five operating hours). This operation time is assigned based on initial operating trials so as not to reach beyond resilience stage or pavement failure. Figure-4.b: Correlation between subgrade modulus (k) reduction factor and relative GWT in R.D.L test Figure 5.a: Correlation between settlement and applying stress in plate bearing test for A-4 soil Figure 4-a: Correlation between surface deformation of pavement matrix and relative GWT in R.D.L test b. Effect of water existence on subgrade modulus (k) in case of static loading This test is carried out to measure and analyze the modulus of subgrade reaction for the combined section with different subgrade materials through the plate-bearing loading test. The test results are plotted in Figures 5.a to 5.c. The calculated modulus of subgrade reaction (k) are tabulated in Table-2 and summarized as shown in Figure 4.b. Figure 5.b: Correlation between settlement and applying stress in plate bearing test for A-3 soil Figure 5.c: Correlation between settlement and applying stress in plate bearing test A-7-6 soil 30 CONCLUSIONS Based upon the experimental work and the rational analysis conducted in this research work, the following conclusions are drawn: 1. Assignment of the groundwater table (GWT) elevation accurately is extremely important. The water table is fluctuating with time as per site and surrounding conditions; hence, for reliable and

5 accurate design, the GWT should be monitored along at least one year to find out the ACTUAL minimum clearance realistically. 2. The GWT should be kept at least 0.9h t or 1.0m (which is bigger) below the pavement section. 3. The effect of water existence in flexible pavement is very costly in terms of pavement life if it considered upon conventional AASHTO-93 design method. 4. The cost increase due to decrease in designlife and increase of maintenance cost due to water consideration were found to be ranged from 7% to 84% according to GWT and subgrade soil type. REFERENCES [1] Charles R. Fitts Groundwater Science, Elsevier, New York. (2002) [2] McAdam, J.L., Report to the London Board of Agriculture, [3] American Association of State Highway and Transportation Officials, AASHTO Guidefor Design of Pavement Structures. Washington, D.C., [4] Philip, J.R., Theory of Infiltration. Advances in Hydroscience, Vol. 5, Academic Press. N Y, AUTHOR BIOGRAPHY Dr. Alaa M. Ali obtained his doctorate degree from Faculty of Engineering, Mansoura University, Egypt. Now he is assistant professor of highways engineering at Misr High Institute for Engineering and Technology, Mansoura, Egypt. He has over seven publications including highways materials evaluation and roads maintenance. His current researches focus on increasing durability of flexible pavements and decreasing pavement construction cost by using alternative materials. 31