STUDY ON ROADWAY SUBSURFACE DRAINAGE …iaeme.com/MasterAdmin/UploadFolder/IJCIET_06_09_012/IJCIET_06_0… · Volume 6, Issue 9, Sep 2015, ... The AASHTO Guide for Design of Pavement

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International Journal of Civil Engineering and Technology (IJCIET)

Volume 6, Issue 9, Sep 2015, pp. 128-138, Article ID: IJCIET_06_09_012

Available online at

http://www.iaeme.com/IJCIET/issues.asp?JTypeIJCIET&VType=6&IType=9

ISSN Print: 0976-6308 and ISSN Online: 0976-6316

© IAEME Publication

STUDY ON ROADWAY SUBSURFACE

DRAINAGE SYSTEM AND RELATED

PERFORMANCE USING FEM

Thair Jabbar Mizhir Alfatlawi

Lec. Doctor, Civil Engineering Department, Babylon University, Iraq

ABSTRACT

Poor subsurface drainage is one of the factors that causes pavement

distress and reduces pavement service life. The evaluation of roadway

subsurface drainage system required good knowledge in groundwater flow

especially the unsaturated water flow through pavement layers and related

properties that affect the ability of drainage system to remove moisture helds

in base, subbase and subgrade layers. This paper uses the finite element

method (FEM) to solve the well-known unsaturated flow equation called

'Richards Equation' that applied to simulate water flow in roadway subsurface

drainage layers with the presence of edge drain. Three granular bases, five

subgrade soils and one type of trench have been used to study the effect of

pavement layers properties on the performance of subsurface drainage system.

Current study reveals that the performance of drainage system not only

depends on edge drain design but it also heavily depends on subgrade layer

properties. It is found that residual degree of saturation after long time not

highly affected by pressure heads at air entry but this suction pressure affects

the time required to reach the FHWA indicator and the total moisture that

drained by edge drain decreases with the increase of hydraulic conductivity of

subgrade soil.

Key words: Edge Drain, Subsurface Drainage, Unsaturated Flow, Subgrade

Soil, Granular Base, Hydraulic conductivity.

Cite this Article: Thair Jabbar Mizhir Alfatlawi. Study on Roadway

Subsurface Drainage System and Related Performance Using Fem.

International Journal of Civil Engineering and Technology, 6(9), 2015, pp.

128-137.

http://www.iaeme.com/IJCIET/issues.asp?JTypeIJCIET&VType=6&IType=9

1. INTRODUCTION

Highway pavements are very susceptible to the damaging effects of water

which enters the pavement as surface water through cracks, joints, pavement

infiltration and as groundwater from an intercepted aquifer. Edge drains systems that

allow water to filter down through shoulder gravel into trenches and pipes that carry

the water away have been a preferred method for managing subsurface drainage.

Study on Roadway Subsurface Drainage System and Related Performance Using Fem


These systems keep moisture from softening subgrades and reducing the structural

strength of pavement. Longitudinal drains are placed parallel to the pavement

centerline in both the horizontal and vertical alignments. This type of drain consists of

a trench with a perforated collector pipe surrounded by a protective filter. These

drains are usually placed under the pavement edge joints or under shoulders where

most water infiltrates the pavement; but, on wide pavements such as runways they

might also be placed at the center and intermediate points to draw down the water

table. There are some general rules for selecting and placing the perforated or slotted

pipe used in longitudinal and transverse subsurface drainage systems including soil

conditions, load requirements, durability, and environmental conditions. Figure (1)

shows the component of roadway subsurface drainage using edge drain.

2. LITERATURE REVIEW

The problem of draining excess water from beneath highway pavements has

received much attention from pavement designers and researchers for at least 50

years. One of the first sets of design recommendations regarding subsurface drainage

for roadways was published by the Federal Highway Administration (FHWA) in the

early 1950’s. Since that time there have been numerous attempts to develop highway

subsurface drainage and since that time new drainage products and materials have

become available to the designer/contractor for implementation of subsurface

drainage into roadway systems (Canelon and Nieber, 2009).

Cedergreen et al. (1972) evaluated field tests that concerns with both drained

and undrained pavement sections and concluded that to achieve proper drainage,

drains (or ditches) a long side of road are essential to collect water from road surface

and surrounding areas and lead it to an exit point where it can be safely discharged.

The AASHTO Guide for Design of Pavement Structures (1986) included drainage as

an essential element of pavement design, the methods of drainage include: free

draining bases, French drains, pipe and aggregate subdrains and most recently

prefabricated "geocomposite" edge drains. Forsyth (1987) presented the economic

impact of subsurface drainage system taking in account a number of case studies

related to pavement drainage, the researcher confirmed that the use of edge drains

usually improve the durability of pavements. Tart and Rupter (2000) investigated the

influence of rain event on pavement performance. Mallela et al. (2000) considered the

drainage system design and construction. Statistical and numerical modeling of

pavement drainage systems is dealt by Lytton et .al, (1989). Ceylan et al. (2013)

conducted an extensive performance review of primary interstate pavement subdrains

Edge drain

Subgrade

Pavement

Granular base (Subbase)

Shoulder

Water table

Gravel trench

Figure 1 Roadway subsurface drainage system using edge drain



in Iowa and investigated the effect of poor subdrain performance due to improper

design, construction, and maintenance on pavement surface distresses. Zumrawi

(2013) studied the various impacts of inadequate drainage on road pavement

condition and found that most roads in Khartoum State suffered from poor drainage

which causes severe distresses and damages of pavement, design guidelines were

suggested to assist the highway engineers in designing proper drainage to increase

service life of pavement.

3. THEORY

The unsaturated zone is located above the water table. Within this zone, the

pore spaces are usually only partially filled with water. Hence, porosity of soil is

larger than volumetric water content. Due to the fact that water in this zone is held in

the soil pores under surface-tension forces, negative pressures or suction pressures are

developed (Ariza, 2002). Flow through unsaturated soils is complicated to understand

where the permeability coefficient is a function of water content and negative pore

water pressure and proportioned directly with them (Freeze and Cherry, 1979).

Rainfall on pavement runs off and infiltrates through the base, subbase, and subgrade

soils. Infiltration of any layer depends upon its porosity, thickness and quantity of

water present. Water that has entered into the pavement can be carried to edge drains

through permeable layers and then drained out of pavement through outlet pipes.

Under the pavement surface, a certain thickness of treated or untreated aggregates is

placed as base or subbase to endure the compressive load transferred from the surface.

The soil below the subbase works as a bed for those upper layers. Thus, the materials

used in a pavement range from fine clay soil to coarse as gravel, to cohesive as

asphalt and portland cement concrete.

Water flow in the unsaturated zone is solved using a form of the Richards

Equation. Richards Equation is derived from substituting Darcy’s Law into the

unsaturated continuity equation. The unsaturated continuity equation is an expression

for conservation of mass and states that the mass entering a specific volume of interest

less the mass leaving the volume is equal to the change in mass storage with time. It is

expressed as:

where:

θ = volumetric water content, [-],

t = time [T],

x,y,z = dimensional coordinates [L],

ρ = density of fluid [ML –3

], and

qx ,qy ,qz = Darcy’s velocity in three dimensions [LT -1

].

For constant density, the ρ term cancels out on both sides:

Or, can be written using vector calculus form as follows:

In saturated groundwater flow, Darcy's velocity is:



Where H= total head [L] and Ksat= saturated hydraulic conductivity (LT-1

).

In the unsaturated states, the movement of the moisture is governed by a

modification of the Darcy Equation suggested by Richards:

In which, the hydraulic conductivity K(h) is defined as function of pressure head

(h) by Richards. Richards Equation is obtained by substituting Richards definition of

hydraulic conductivity for Darcy’s velocity (q) in the continuity equation and

arranging equation (3) to be:

Equation (6) called the mixed form Richards equation.

To complete the model of variably saturated flow, auxiliary relationships that

relate the hydraulic conductivity and moisture content to the pressure head are

needed. Ariza and Birgisson, 2002 gave a detailed literature for these relationships,

especially those suitable for pavement engineering. Recent study adopted Brooks-

Corey model where it verified by many previous studies (Apul et .al, 2002). Brooks-

Corey model suggested the following relationships:

where:

θsat = saturated water content, [-],

θres = residual water content, [-],

hd= pressure head at air entry ,(negative), [L], and

's = materials properties constants [-].

Finite element technique have been used to simulate roadway subsurface

drainage system with the presence of edge drain shown in figure 1, groundwater

modeling software, Mn-Drain, applied to represent both of saturated and unsaturated

flow through the studied system.

The efficiency of edge drain to remove infiltrated water that entered the granular base

classified by FHWA, 1994 depending on the time required to reduce the moisture

content in the granular base from 100% to a moisture level of 85%. Table 1 shows the

pavement rehabilitation guidance manual for time required to lower moisture from

100% to 85%



Table 1 Quality of drainage by edge drain depending on time required to lower moisture

content from 100% to 85% (FHWA, 1994).

Quality Material

Excellent Less than 2 hrs Good 2 to 5 hrs Fair 5 to 10 hrs Poor Greater than 10 hrs

Very poor Much greater than 10 hrs

4. CASE STUDY

4.1. Dimensions and Meshing

Road drainage system concerned in recent study consisted of pavement layer

placed on granular base layer (subbase) of 30cm thickness, assumed to be initially

saturated, laid above a subgrade layer. The water table assumed to be 0.5m below the

granular base. The subsurface drainage system consisted of 10cm radius edge drain,

edge drain laid below road shoulder and along road length through 40cm x 40cm high

permeability gravel trench. The finite element mesh optimized and discretized using

the auto-run mesh routine EasyMesh. A schematic shown in figure 2 explains the

dimensions and meshing geometry of the studied subsurface drainage system.

4.2. Materials

Three granular base types (subbase A, subbase B and subbase C), five

subgrade soils (loamy sand, loam, silt loam, clay and impermeable) and one type of

edge drain trench (gravel) used to study the performance of subsurface drainage

system under variable subbase and subgrade properties. Table 2 shows current

material data, the subgrade data is based on descriptions and values provide with the

HYDRUS2D software, gravel and granular values are selected to provide the range of

observed behaviors. Data in Table 2, are calculated corresponding to Brooks- Corey

model referred to equations 7 and 8.

Figure 2 Case study dimensions (in centimeters) and geometry discretization.

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

210

220

230

0 50 100 150 200 250 300 350 400 450 500

This is the mesh that will be used in the calculation

Check out dimensions and regions

30cm granular base

50cm subgrade layer above W.T.

40cm*50cm gravel trench

Edge drain of 20cm diameter



Table 2 Current study material data.

NO Material 1

(-)

()

θsat

(-)

θres

(-)

hd

(cm)

2

(-)

Ksat

(m/day)

1 Gravel 0.2 0.45 0.03 -0.5 13 0.1

2 Subbase A 0.2 0.45 0.03 -1 13 5x10-4

3 Subbase B 0.5 0.45 0.03 -2 7 2x10-4

4 Subbase C 1 0.45 0.03 -5 5 1x10-3

5 Loamy sand 0.474 0.401 0.035 -8.7 7.22 1.7x10-3

6 Loam 0.22 0.434 0.027 -11.15 12.1 3.67x10-4

7 Silt loam 0.211 0.486 0.015

-20.75 12.48 1.89x10

-4

8 Clay 0.131 0.385 0.09 -37.31 18.267 1.67x10-5

9 Impermeable 1 0.4 0.1 -5 5 1x10-20

4.2. Initial and Boundary Conditions

Initially, the granular base assumed to be saturated, so that the initial condition

taken as saturated granular base with zero pressure head. Water table under the

granular base considered as the end of domain and the boundary condition and

pressure head fixed to zero. Nodes lies on the edge drain have zero pressure head

unless this condition caused an influx of moisture into the trench; in this case,

boundary condition replaced by a no flow condition at that node.

5. RESULTS AND DISCUSSTION

A simplified finite element simulation of roadway cross-section, shown in figure 2,

used to illustrate the saturated and unsaturated flow through pavement layers. Figures

3 to 5 show the variation of the degree of saturation with time for different

combinations of granular bases and subgrade soils. It can be seen that with using

impermeable and clayey subgrades, the edge drain no longer able to drain roadway

cross-section under any granular base condition. Other subgrade soils are of

"excellent" drainage according to FHWA, 1994 classification shown in table 1 where

they satisfy the degree of saturation of 85% (S85%) at running time less than 2hrs. On

the other hand, effect of granular base (subbase) properties on drainage system

behavior is clearly appeared at later running time, subbase C results more reduction in

degree of saturation after 10hrs compared with other granular bases under all

subgrade conditions (using of subbase C resulted a degree of saturation less than 45%

for silty loam, loam and sandy loam subgrade soils).

Figure 3 Degree of saturation variation with time (using subbase-A).



Figure 4 Degree of saturation variation with time (using subbase-B).

Figure 5 Degree of saturation variation with time (using subbase-C)

Figure 6 show the effect of subgrade soil hydraulic conductivity on FHWA

indicator (S85%), FHWA indicator (S85%) decreases with the increase of saturated

hydraulic conductivity of subgrade soil. Clayey soils and soils of lower hydraulic

conductivity disappeared in figure 6 because these soils don't match the degree of

saturation of 85% during the running interval of 10hrs which reveals that these soils

are of poor drainage and cannot satisfy the FHWA requirement for granular base

drainage.

Figure 6 Ksat versus S85% relationship



In vadose zone flow, the hydraulic conductivity is a nonlinear function of

volumetric water content as stated in equation 8 and the soil pressure head is negative.

Figures 7 and 8 show the effect of saturated content and pressure head at air entry,

respectively on the time required to reach S85%. The time required to reach S85%

increases as the saturated water content increasing according to figure 7, while the

most negative pressure head at air entry soils requires larger time to reach FHWA

indicator (S85%)

Figure 7 Effect of saturated water content on time required to reach S85%.

Figure 8 Effect of pressure head at air entry on time required to reach S85%.

The variation of total moisture drained by the edge drain with subgrade soil

hydraulic conductivity is shown in figure 9, the relation clarified that the higher the

hydraulic conductivity, the lower the amount of drained by the edge drain. This

behavior may be belonged to the increase of subgrade soil ability to drain granular

base water with the increasing of hydraulic conductivity. The incompatible behavior

of impermeable subgrade layer is expounded where the efficiency of edge drain in

this case is highly reduced because of laying half of the edge drain and the

surrounding gravel trench into an impermeable layer (see figures 1 & 2).

Water content of soils in unsaturated flow is less than porosity, so water will held

between soil grains under the act of surface tension forces. This suction pressure is

affected both water content and hydraulic conductivity. Figure 10 shows that the

increase in pressure head at air entry will cause an increase in the total moisture that



drained by the edge drain, an incompatible behavior appeared again for the

impermeable layer. Figure 11shows that the residual degree of saturation after 10hrs

running time not highly affected by pressure heads at air entry, but it highly sensitive

to the hydraulic conductivity as it appears in impermeable and nearly impermeable

(clayey soil) subgrades behavior.

Figure 9 Total moisture drained by edge drain

Figure 10 Variation of total moisture drained by edge drain with pressure head at air entry.

Figure 11 Variation of residual degree of saturation after 10hrs with pressure head at air

entry.



5. CONCLUSIONS

The quality of subsurface drainage is one of the most important factors because poor

drainage leads to distresses and deterioration of road pavement. Numerical study

shows that the performance of drainage system not only depends on edge drain design

but it also heavily depends on subgrade layer properties. The saturated hydraulic

conductivity (Ksat) has a significant effect on drainage efficiency where higher

permeability layer causes an abrupt reduction in the moisture content of road layers.

Impermeable and low permeability (such as clay) subgrade layers not suitable to use

with edge drain for roadway subsurface drainage because it reduces edge drain

efficiency as it fouled a part of edge drain and prevent water to enter easily into the

surrounding gravel trench. The time required to reach FHWA indicator (S85%)

increases as the saturated water content increasing and/or saturated hydraulic

conductivity decreasing. As the pressure head at air entry is a measure to how much a

soil able to retain water, the residual degree of saturation after long time not highly

affected by pressure heads at air entry but it affects the time required to reach the

FHWA indicator. Total moisture that drained by edge drain decreases with the

increase of hydraulic conductivity of subgrade soil. Finally, recent study reveals that

using the unsaturated flow- finite element software (Mn-Drain) is an efficient tool for

subsurface drainage system design and evaluation.

REFERENCES

[1] American Association of State Highway and Transportation Officials, AASHTO

Guide for Design of Pavement Structures. Washington, D.C., 1998.

[2] Apul, D., Gardner, K., Eighmy, T., Benoit, J. and Brannaka, L. A Review of

Water Movement in the Highway Environment: Implications for Recycled

Materials Use, Recycled Materials Resource Center, University of New

Hampshire, Durham, New Hampshire, 2002.

[3] Ariza, P. and Birgisson, B. Evaluation of Water Flow through Pavement Systems,

Minnesota Department of Transportation, Report No. MN/RC - 2002-30, 2002.

[4] Ariza, P. Evaluation of Water Flow through Pavement Systems, M.Sc. Thesis,

University of Florida, Minnesota Department of Transportation, 2002.

[5] Canelon, J. D. and Nieber, J. L. Evaluating Roadway Subsurface Drainage

Practices. Minnesota Department of Transportation, Research Services Section,

2009.

[6] Cedergreen, H.R., O’Brien K.H., and Arman J.A. Guidelines for the Design of

Subsurface Drainage Systems for Highway Structural Sections. Publication No.

FHWA-RD-72-30, Washington, D.C., 1972.

[7] Ceylan, H., Gopalakrishnan, K., Kim, S., and Steffes, R. F. Evaluating Roadway

Subsurface Drainage Practices. InTrans Project Report 12-428. Ames, Iowa:

Institute for Transportation, Iowa State University, 2013.

[8] FHWA, Drainable Pavement Systems - Instructor's Guide, Demonstration Project

87, Publication No. FHWA-SA-94-062, Office of Technology Applications and

Office of Engineering, Federal Highway Administration, Washington, D.C.,

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[9] Mustafa Hamid Abdulwahid and Kadhim Naief Kadhim. Application of Inverse

Routing Methods to Euphrates River (IRAQ). International Journal of Civil

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[10] Forsyth, R.A. the Economic Impact of the Pavement Subsurface Drainage.

Transportation Research Record 1121, Transportation Research Board, National

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[11] Freeze, R.A. and Cherry, J.A. Groundwater. Prentice-Hall, Inc. Englewood

Cliffs, New Jersey, 1979.

[12] Lytton, R.L., Pufahl, D.E., Michalak, C.H., Liang, H.S., and Dempsey, B.J. An

Integrated Model of the Climatic Effects on Pavements. FHWA-RD-90-033,

Federal Highway Administration, 1989.

[13] Mallela, J., Leslie T.J., and Micheal I.D. Considerations for Providing Subsurface

Drainage in Jointed Concrete Pavements. Transportation Research Record 1709,

Transportation Research Board, National Research Council. Washington, D.C.,

2000.

[14] Kadhim Naief Kadhim. Feasibility of Blending Drainage water with River Water

for Irrigation in Samawa (IRAQ). International Journal of Civil Engineering and

Technology, 4(5), 2013, pp. 22 - 32.

[15] Tart, Jr. and Rupter, G. Pavement Distress and Roadway Damage Caused by

Subsurface Moisture and Freezing Temperatures. Transportation Research

Record 1709, Transportation Research Board, National Research Council.

Washington, D.C., 2000.

[16] Kadhim Naief Kadhim Al-Taee, Thair Jabbar Mizhir Al-Fatlawi and Zainab

Falah Hussein Al-Barrak. Lowering Groundwater in the Archaelogical Babylon

City Using Underground Dams. International Journal of Civil Engineering and

Technology, 6(4), 2015, pp. 107 - 119.

[17] Zumrawi, M. Pavement Design for Roads on Expansive Clay subgrades.

University of Khartoum Engineering Journal (UOFKEJ), 3(1), 2013, pp. 52-58.

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STUDY ON ROADWAY SUBSURFACE DRAINAGE …iaeme.com/MasterAdmin/UploadFolder/IJCIET_06_09_012/IJCIET_06_0… · Volume 6, Issue 9, Sep 2015, ... The AASHTO Guide for Design of Pavement