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Evaluation and Modelling the Performance of Pavement Drainage Systems

Prof. S.S. Jain, Civil Engineering & Associated Faculty; Yogesh U.Shah, Research Scholar, Centre for Transportation Systems (CTRANS), Indian Institute of Technology Roorkee; Dr. Devesh Tiwari, Scientist E II, Central Road Research Institute (CRRI), New Delhi, and Dr. M.K. Jain, Assistant Professor, Dept. of Hydrology, Indian Institute of Technology, Roorkee.

Introduction

Urban road drainage is one of the key components which needs to be considered along with other parameters for urban pavement management system. Excessive water content in the pavement base, sub-base, and sub-grade soils can cause early distress and lead to a structural or functional failure of pavement, if counter measures are not undertaken. In spite of this, adequate priority for drainage system is rarely accorded, whether it is in the matter of planning, organization, fund allocation or monitoring. Water is also responsible for a large number of non-load related distresses such as: D-cracking in concrete pavements, and accelerated aging and oxidation in asphalt pavements. Therefore, pavement drainage design should be at the forefront of pavement design and not an afterthought.

The inclusion of pavement drainage issues in pavement maintenance management system caters to deal with various aspects like studying the water ingress and egress from pavement structure, water related distresses, methods of controlling water entry in pavements, quantifying the water penetrated the pavement layers, design aspects of surface and sub-surface drainage, modelling & simulation of water movements in pavements, analyzing the effects of drainage on pavement performance and life cycle costing and much more. This article mainly focus on two aspects viz. measurement techniques of water in pavements and drainage modelling and design software’s.

Measurement Techniques for Water Movement in Pavements

Water entering a pavement structure migrates as moisture through the structure. The amount of water penetrating the pavement is dependent on precipitation, drainage, design of the road structure, type and condition of the surface layer (cracks, joints) and shoulders and the materials in the pavement, subgrade and subsoil. The most important parameters related to the movement of water through pavement structures are the quantity and spatial distributions of moisture inside the pavement, the coefficient of permeability of individual layers and their matric suctions (pore water pressure). These parameters can be determined experimentally, either in the field or in the laboratory, by various techniques. The most common of these methods used in road engineering are briefly reviewed in this section.

Measurement of Moisture Content

A fundamental parameter that characterises the water movement in pavements is the water content. This provides information on the condition of the road layers regarding the moisture saturation stage, which controls the main parameters in the governing equations for water flow. The measurement methods can be divided into destructive methods (gravimetric methods) and non-destructive methods that provide indirect measurements of the water content. (Apul, D. et al. 2002 & Dawson, A. 2008).

Gravimetric method

The gravimetric method is based on weighing a moist soil sample and drying it in an oven (105°C) until a constant weight is achieved. The moisture content is calculated from the ratio of the weight loss during drying to the dry weight of the sample. If the volume of the sample and density of the water is known, volumetric water content can also be calculated. This method has two major drawbacks: the sampling is destructive for the road and the method cannot be used to make in-situ measurements in real time. However, it is an accurate method and is often used to calibrate other measurement techniques.

Non-destructive methods

A number of methods exist for estimating the soil water content of road materials in a non-destructive way assuming that the instruments are placed in the road during the construction phase. They are all indirect methods as they involve measurements of some property of the material affected by the water content or they measure a property of some object placed in the material. Some of the more common indirect methods used in the highway environment are briefly described here.

Neutron Scattering Method

In the neutron scattering or neutron thermalization method, a probe acts as a radioactive source and detector (Fig. 3a). High energy (fast) neutrons are emitted into the soil, which bounce off of soil and soil moisture reducing the energy level of the neutrons. Hydrogen atoms are much more effective at reducing the energy level of neutrons making this technique more sensitive to moisture content. The proportion of neutrons returning to the probe and reduction in neutron energy is related to the water content. This method is non-destructive and can do profiling as well as continuous measurements.

Time Domain Reflectometry Technique

Moisture content determination with TDR is based on the measurement of the velocity of an electromagnetic signal through a probe inserted in the soil. A schema of a TDR probe is shown in Fig. 3b. The measured velocity of electromagnetic energy is used to calculate the apparent dielectric constant, which in turn is used to calculate the moisture content.

Evaluation and Modelling the Performance of Pavement Drainage Systems
Figure 3: Non-destructive Methods for Measurement of Moisture Content

Ground Penetrating Radar

In Ground Penetrating Radar (GPR) (Fig. 3c), electromagnetic waves are sent out from a transmitter on or above the ground surface and picked up by a receiver after penetrating and returning from the soil. The velocity of the electromagnetic wave propagation in soils is dependent on the soil bulk permittivity modulus. Thus the underlying principles of the GPR soil moisture measurements are the same as those of Time Domain Reflectometry except that in TDR the electromagnetic waves travel along a waveguide whereas with GPR the propagated electromagnetic waves are unconstrained. GPR therefore has the potential to cover a much larger soil volume than does TDR. GPR can be air launched or surface launched or used in boreholes and is completely non-invasive, whereas TDR requires the penetration of rods (waveguides) into the pavement structure.

Capacitance Measurements

Capacitive sensors measure the resonant frequency of an inductance-capacitance (LC) tuned circuit where the soil located in between two flat waveguides is the dielectric material. The inductance is kept constant and the resonant frequency f measured and therefore the capacitance can be calculated from equation 1.

f = ½π√LeCe. . . . Eqt. (1)

where Le is the inductance and Ce is the capacitance. The capacitance Ce is a measure of the relative bulk dielectric constant of the soil and is a function of the water content of the soil.

Other Methods

Near infrared reflectance spectroscopy (NIRS) (Fig. 3d), seismic methods and thermal properties are all methods that can be used for estimation of soil water content. Although they are in many respects good and accurate methods, they all have some drawbacks making them non-suitable as routine methods to be used in the pavement environment.

Measurement of Permeability

The permeability of soils is a material parameter that relates the rate of water flow to the hydraulic gradient in the soil and, therefore, determines the material’s suitability for drainage layers. For road construction layers, water movements below the ground water table are almost entirely horizontal and thus it is the horizontal permeability that should be measured. The permeability of soils can either be estimated in saturated conditions or for partially saturated conditions.

Permeability Tests of Saturated Soils and Aggregates

Traditionally in geotechnical engineering, the saturated permeability is estimated in the laboratory in a constant head test (Fig. 4a) for coarse grained soils whereas a falling head test (Fig. 4b) is used for fine grained soils. An oedometer test (Fig. 4c) can also provide a measure of the saturated permeability for fine grained soils in the laboratory. Field tests which provide a measure of the saturated permeability are usually a kind of pumping well test (Fig. 4d), injection test or tracer test.

Evaluation and Modelling the Performance of Pavement Drainage Systems
Figure 4: Permeability Measurement Techniques

Permeability Tests of Unsaturated Soils

The flow of water in saturated soils is commonly described using Darcy’s law which relates the rate of water flow to the hydraulic gradient. The methods to measure the unsaturated permeability of soils can be classified into steady or unsteady methods. In the laboratory, the steady state method (Fig. 4e) is recommended as it is relatively simple and has few ambiguities. However, the method can be quite time consuming as the flow rate is very low, especially under conditions of high matric suction and it can be difficult to measure the low flow rate accurately due to air diffusion. The unsteady laboratory methods, such as the thermal method, instantaneous profile method and the multi-step outflow method are usually much quicker than the traditional steady state method but are usually not as accurate. In the field, the tension infiltrometer, instantaneous profile method and the cone penetrometer methods can be used.

Measurement of Suction

Soil suction or capillary pressure head can be measured either in the laboratory in an undisturbed sample of soil or directly in the field. Soil suction or total suction consists of the matric suction and the osmotic suction. As osmotic suction is not taken into account, only the main in-situ methods for measuring matric suction are referred to. Many different approaches exist for measuring pore water pressure or soil suction, some of these are briefly explained below (Dawson, A. 2008).

Tensiometers

A tensiometer consists of a fine porous ceramic cup connected by a tube to a vacuum gauge (Fig. 5a). The entire device is filled with de-aired water. The porous tip is placed in intimate contact with the soil and the water flows through the porous cup (in or out) until the pressure inside the ceramic cup is in equilibrium with the pore water in the soil. The reading on the pressure measuring device, once corrected for the water column in the device, is the matric suction.

Thermal Conductivity Sensors

Thermal conductivity sensors (TCS) are used to indirectly relate matric suction to the thermal conductivity of a porous medium embedded in a mass of unsaturated soil (Fig. 5b). Any change in the soil suction results in a corresponding change in the water content of the porous medium (governed by its characteristic curve). The thermal conductivity of a rigid porous medium is a direct function of the water content. Therefore, if the thermal conductivity of the porous medium is measured, the matric suction of the soil may be indirectly determined by correlation with a predetermined calibration curve.

Evaluation and Modelling the Performance of Pavement Drainage Systems
Figure 5: Suction Measurement Techniques

Suction Plate

A simple laboratory variant of the tensiometer method for measuring matric suction of fine-grained soils uses a semi-pervious sintered glass plate. Fig. 5c shows the laboratory procedure for measuring suction using plate.

Contact Filter Paper Techniques

The filter paper method is used as an indirect means of measuring soil suction. Fig. 5d shows the filter paper setup and installation to put it in direct contact with the soil specimen. In the laboratory, the filter paper is placed in contact with the soil specimen in an airtight container for seven days and thereafter the water content of the filter paper is determined and the matric suction of the soil specimen is inferred from a calibration curve.

Drainage Modelling And Design Softwares

The effect of moisture movement into and through pavement structure can be studied by simulating the field conditions including pavement layers and its material properties, rainfall and temperature conditions, road geometry, etc. Different software’s for modelling the movement of moisture into and through the sub-surface of pavement is available. Also, software for designing the surface and sub-surface drainage systems for pavement structure can be readily used. A brief review of some software’s is discussed in following subsection (Voller, V. 2003).

Software’s for Modelling Moisture Movement in Pavements

PURDRAIN: is a research tool and software that solves the two-dimensional pressure-head form of the Richards equation. Finite difference methods are used to achieve this. A simple geometric cross-section of a pavement drainage system is used. The program runs under DOS and has a menu based user interface for the detailed specification of geometry, boundary conditions, and materials types.

The Integrated Climate Model: This is a finite-difference model that takes into account the effect of a range of climate factors on the pavement structure. Three separate models are combined: (1) the Climatic-Materials-Structures Model (CMS) developed at the University of Illinois (2) the Infiltration and Drainage Model (ID) developed by the Texas Transportation Institute at Texas A&M University, and (3) the CRREL Frost Heave and Thaw Settlement Model developed at the United States Army Cold Regions Research and Engineering Laboratory (CRREL). The model runs under DOS with menu based user input.

Hydrus2D: The HYDRUS-2D software package is distributed by the International Groundwater Modelling Centre (IGWMC) located at the Colorado School of Mines. This two-dimensional program, runs in the windows operating system, and performs a coupled analysis of subsurface moisture and solute flow. Hydrus2D has been used to model drainage.

SEEP/W: This program is one of a suite of programs from Geo-Slope International. It is a finite element program solely dedicated to the transient two-dimensional (plane strain and axis symmetric analysis) of saturated/unsaturated flow. The program uses linear and quadratic triangular elements and bi-linear and quadratic quadrilateral elements. The orders of the elements determine the degree of the interpolation functions used. SEEP/W can and has been used for modeling pavement drainage.

Software for Design of Sub-Surface Drainage

DRIP: This is a windows based design program available from the FHWA. It is a module-based program with the ability to combine various design modules into a comprehensive design and analysis exercise. In terms of modelling, the flow of moisture in the saturated/unsaturated region, a main focus of this work, the analysis is based on initial work by Moulton. In essence, approximate analytical methods for the design of subsurface drainage systems based on fundamental seepage theory are used to arrive at graphical design aids that can be readily applied by the highway designer. The main design outputs are estimates of the drainage performance under saturated conditions and the time for the system to drain.

Software for Design of Surface Drainage

PAVDRN is intended for use by highway design engineers to determine the likelihood of hydroplaning on various highway pavement sections. It does this by computing the longest flow path length over the design pavement section and determining the water film thickness (depth of water above the asperities of the pavement surface) at points along the path. The water film thickness is used to estimate the speed at which hydroplaning will occur (if at all) along the longest flow path (critical path) in the section. The predicted hydroplaning speed along this path is then compared to the design speed of the facility, a parameter selected by the designer. PAVDRN runs under Windows 3.1 and above. The user interface was programmed in Visual Basic. The computational algorithms were programmed in FORTRAN 77 (NCHRP 1998).

Summary

This article discusses two aspects related to study of pavement drainage system. In order to have an efficient highway drainage system, all the aspects should be studied in detail and specifications shall be developed by conducting field and laboratory studies. Funds required for a drainage system are small as compared to the development of infrastructure and the recurring losses which the society and the government have to suffer from year to year. It is necessary to give due priority to this area of development and satisfactory arrangements ensured by way of proper design and planning.

References

  • AASHTO (1993). Guide for Design of Pavement Structures, American Association of State Highway and Transportation Officials, Washington, D.C.
  • Apul, D., Gardner, K., Eighmy, T. and Brannaka, L. (2002). A Review of Water Movement in Highway Environment, Durham.
  • Dawson, A. (2008). Water in Road Structures – Movement, Drainage and Effects, Springer Publications.
  • NCHRP (1998). A web document 16, ‘Improved Surface Drainage of Pavements’, Final Report.
  • Voller, V. (2003). Designing Pavement Drainage Systems: The Mndrain Software, Final Report, Minnesota Department of Transportation, Minnesota.

NBMCW March 2012


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