Topic 7

Pavement Drainage

Instructor: Shih-Hsien Yang

N674300 Pavement Analysis & Design

Objectives

vImpact of moisture on pavement distress

vDefine drainage factors

vList properties that influence drainability

vDescribe principle behind drainage timev DRIP

How Much Water Gets into PavementvRain: dry-wet; freeze-thawvPavement design, age, and conditionvSurface drainage: longitudinal and transverse

slopes, ditches, and storm drain.

Sources of Moisture

SeepageThrough Permeable Surface

Water Table

Capillary Vapor

Water Table

Pavement Infiltration

Water in Pavements

vStripping in HMAvLoss of subgrade supportvReduction of granular layer

stiffnessvErosion of cement-treated

base layersvReduction in the pavement

service life if base is saturated for sometime

vDebond between layers

How to Address the Problem?

v Pavement geometry (slopes and ditches)v Crack sealingv Treated Layerv Thicker Layersv Full Widthv Subsurface Drainage

0.02 m/m0.04 m/m

Subgrade

Treated base

HMA

Aggregate base

Pavement Slopes

Sx SRS

W LR

( )S S SR x= +2 2 0 5.

LR W SSX

= +12 0.5

Tan A SSX

( ) =

( )

)(

Pavement Slopes

vSurface and subsurface slopes

vAlways positive SR

vRecommended slopes:v 0.01 – 0.025 m/m (spec range)v 0.02 m/m (normal conditions)v 0.025 m/m (high rainfall)

Drain Requirements

vSufficient stiffness to support traffic without significant permanent deformation under dynamic loading

vSufficient transmissivity to rapidly drain the pavement section and prevent saturation of the base

vSufficient air void to provide a capillary break

Permeable Base System

Rigid outlet pipe

Longitudinalpipe edgedrain

10-yearflow

Ditch

PavementPermeable baseBase layer

Shoulder

Shoulder

Permeable base

Base layer

Pavement

SubgradeDitch

Fabric

Embankment

Geocomposite Edgedrains

Sand Backfill

Prefabricated Geocomposite Edge Drain (PGED)

Aggregate base

Subbase/Subgrade

25 mm

100 mm

ShoulderPavement

Subsurface Drainage

OGDL EDGE DRAIN

PGED

Pavement Design Equations (AASHTO 1993)

SN = a1D1 + a2D2m2 + a3D3m3

log10W18 = (ZR)(S0) + (9.36)(log10(SN+1) - 0.20 + log10[ΔPSI/(4.2-1.5)]/[0.40 + (1.094/(SN+1)5.19)] + (2.32)(log10MR) - 8.07

Drainage - mi

Adjust layer coefficients to account for effects of certain levels of drainage on untreated materials

Quality of Drainage Water Removed WithinExcellent 2 hours

Good 1 day

Fair 1 week

Poor 1 month

Very poor Water will not drain

Quality of Drainage

Percent of Time Pavement Structure is Exposed to Moisture Levels Approaching

SaturationLess Than

1% 1-5% 5-15% Greater Than 25%

Excellent 1.40-1.35 1.35-1.30 1.30-1.20 1.20

Good 1.35-1.25 1.25-1.15 1.15-1.00 1.00

Fair 1.25-1.15 1.15-1.05 1.00-0.80 0.80

Poor 1.15-1.05 1.05-0.80 0.80-0.60 0.60

Very Poor 1.05-0.95 0.95-0.75 0.75-0.40 0.40

* Quality of drainage is a function of permeable base quality of drainage and effectiveness of drainage system

Permeabilityv Ability of materials to

carry waterv Coefficient of permeability

(k)v Measure of permeability v Rate of flow through a unit area

with a unit hydraulic gradientv Affected by

v Porosityv Effective grain size, D10v Percent passing #200 sieve

v Measured in the lab:v Falling head testv Constant head test

v Field testing:v Field permeability testing devicev Tipping buckets

Air

Water

Solid

Wt of waterWw=

Weight—Volume Relationship

Ws= Wt of solids

WT= Total wt

VA= Air vol.

VW= Water vol.

VS= Solid vol.

VV= Voids vol.

VT Total vol.

Vv = Volume of voids

VT = Total volume

= Unit dry weight, kN/m3

Gsb = Bulk specific gravity

Porosity

v Porosity: indicates aggregate’s ability to store or give up water

γd

𝑁 =𝑉$𝑉%

= 1 −Υ)

9.81𝐺./

Effective Porosity

where, N = PorosityWL = Water loss, %

v Effective porosity indicates the amount of drainable water (how strong the soil will hold water)

𝑁0 =𝑉𝑜𝑙𝑢𝑚𝑒𝑜𝑓𝑤𝑎𝑡𝑒𝑟𝑑𝑟𝑎𝑖𝑛𝑒𝑑

𝑇𝑜𝑡𝑎𝑙𝑉𝑜𝑙𝑢𝑚𝑒= 𝑁×𝑊𝐿

Saturation

where,

Drained water = Ne x U = Vv - Vw

Ne = Effective porosity

U = Percentage drained

v Saturation: Measure of the amount of water in the soil; Vw = Vv% Saturation

𝑆 =𝑉D𝑉$

×100

MOVEMENT OF WATER

vGravity, capillary action, vapor pressuresvFine-grained soil ⇒ capillaryvGranular materials ⇒ gravity

v = ki (Darcy’s law)v = discharge velocity (not actual seepage velocity)k = coefficient of permeabilityi = hydraulic gradient

Q = vA (Discharge = volume of flow per unit time):A = cross-sectional area normal to the direction of flow

Darcy’s Law Assumptions

vSteady-state flowvSoil is porous and homogenousvLaminar flow

Surface Inflow

vTwo ApproachesvSteady-State Flow

v Rainfallv Percentage of rainfall that enters the

pavement

vTime-to-Drain

Time-to-Drain Design Assumptions

vDuring a rainfall event water infiltrates into the permeable base until it is saturated

vExcess rainwater runs off to a side ditchvTime required to drain certain amount of water

from the drainage layer after a rain event

Time-to-Drain

PavementPermeable baseBase layer

Shoulder

qi

qd

Q

Pavement Discharge Rate, qd

vqd is based on time-to-drain analysis

where:qd = pavement discharge, m3/day/mW = width of the permeable base, mH = base thickness, mNe = effective porosity of base materialU = percent drained in decimaltd = time to drain, hrqi = pavement infiltration, m3/day/m2

HMA pavements: 0.10 to 0.15 m3/day/m2

PCC pavements: 0.15 to 0.20 m3/day/m2

𝑞) = 24𝑊𝐻𝑁0𝑈1𝑡)= 𝑞K𝑊

Excellent 2 hoursGood 1 dayFair 7 daysPoor 1 monthVery Poor Does not drain

For Pavement: Time required to drain 50% of the drainable water

Design Standards

For Freeways: 50% drained in 2 hrsIf heavy traffic, in 1 hr

Excellent < 2 hoursGood 2 to 5 hoursFair 5 to 10 hoursPoor > 10 hoursVery Poor >>> 10 hours

For pavement rehabilitation, 85% saturation

Design Standards

Effect of Degree Saturation on Deformation for Granular Material

t = T x m x 24

Time-to-Drain Calculation

m-factor (days)Time factor

Time to drain (hrs)

How to Estimate Time to Drain (t)

vInput: v S and Sx

v W, H, k, γd, Gsb, WL (for permeable base)vInterim Output:

v SR, LR, S1 {S1 = (LR x SR)/H}v T for a desirable degree of drainage (U)v N and Ne

v N = [1- {γd / (9.81 x Gsb)}]v Ne = N x WL

v “m” factor: m = (Ne x LR2) / (k x H)

vOutput: t = T x m x 24

Transmissivity

U-T Relationship

Deg

ree

of D

rain

age

-U

.01 .05 .2 1 5 201

.7

.3

0

Time Factor - T

10 8 6 4 2 1 .6.8S1 = .4 0

1

2

3

5

7

0.01 .03 .10 .30 .60

Slop

e Fa

ctor

(S1)

Time Factor (T50)

T for U = 50% Drained

Time-to-Drain and Permeability Relationship

Time to Drain, hrs

Perc

ent D

rain

ed

0 1 3 5 7100

20

0

k = 305 m/day

k = 610 m/dayk = 915 m/day

40

60

80

k = 153 m/day H = 0.15 mNe = 0.25LR = 7.6 mSR = 0.02 m/m

Base DrainabilitiesA

AcceptableM

MarginalU

Unacceptable

Subg

rade

Soi

l D

urab

ility

Fair

Poor

EXC G F to P

F to P P to VP VP

G F P to VP

EXC - ExcellentG - GoodF - FairP - PoorVP - Very Poor

Combining Base and Subgrade Drainage

PERFORMANCE

Good

Time to drain

For two lane road - Lane width = 24 ft, Slope = 0.01

Base k (ft/day) time to drain Quality

OGB 1000 2 hrs to drain Excellent

DGAB 1 1 week Fair

DGAB w/ fines 0.1 1 month Poor

Reality no drains does not drain Very Poor

Aggregate Gradations

1 0

Percent Passing

Sieve Sizes, mm

20

40

60

80

100

12.5 25.4 37.50 9.54.752.075

AASHTO No. 67 AASHTO No. 57

Pea gravel

Dense Agg. base Sand

Fine Sand MediumSand

CoarseSand Gravel

010

50

7060

403020

8090

100

.075 .180 .300 .600.850

1.182.00

2.36 4.759.525.012.5

19.0 37.5 63.0

K (ft. / day)

Sieve Sizes

Perc

ent P

assi

ngInternal Drainage Factors - Permeability

Aggregate Gradation Importance

vEffective grain size, D10v Sieve size corresponding to 10% passing (by wt)v Affects permeability

vCoefficient of uniformity (indicates stability),

vCoefficient of gradation/curvature,

Cu =D60

D10D60 = Sieve size corresponding to 60% passing (by wt) D10 = Sieve size corresponding to 10% passing (by wt)

Cz = D230

D10 * D60D30 = Sieve size corresponding to 30% passing (by wt)

* <4 may be unstable

Types of Current Permeable Bases

v Unstabilized permeable basevDevelops stability through aggregate interlock and

internal frictionvFiner gradation than stabilized bases

v Stabilized permeable basevStability is achieved by interlock and

cementation/binder (AC or PC)vMore open-graded than unstabilized layers

Construction of Permeable Bases

v Good and stable supporting layerv Stablev Avoid contaminationv No discontinuities in the flow pathv Placement is very important (consider a control

strip)v Over compaction can be a problem v Resultant permeability should meet or exceed the

design one

Factors Affecting The Design of Drainage Layer

vPavement slopesvAggregate gradationvPorosity and effective porosityvLayer saturationvPermeability

The Need for a Separator Layers

vDense-graded aggregatevDense-graded HMAvCement-treated base vGeotextilesvAsphalt chip sealsvTreated subgrade

Design of Aggregate Separator Layer: Gradation

vSatisfy filtration and uniformity requirements:v Subgrade/separator layer interface

v D15(separator layer) < 5*D85(subgrade)v D50(separator layer) < 25*D50(subgrade)v Practice:

v D15(separator layer) < 3.50 mm (filtration)v D50(separator layer) < 3.25 mm (uniformity)

v Permeable base/separator layer interfacev D15(permeable base) < 5*D85(separator layer) v D50(permeable base) < 25*D50(separator layer) v Practice:

v D85(separator layer) > 0.44mm (filtration)v D50(separator layer) > 0.24mm (uniformity)

v Percent passing #200 sieve should be <12%v CU > 20, preferably in the vicinity of 40

Physical Properties of the Aggregate Separator Layer

vMinimum CBR of 50% vLow permeability (< 5 m/day)vGood interlock through durable and angular crushed

stonevMinimum thickness of 100mm

Gradation of a Separator Layer (DGAB)

D60

D10

Dense-graded aggregate base

D10= 0.98 mm

Cu= 45.97Perc

ent P

assi

ng

Sieve Sizes, mm

20

40

60

80

100

12.5 25.4 37.50 9.54.752.075Max. 12% fines

Example of Separator Layer Gradation

Grain Size - mm

Separator layer gradation

20

40

60

80

100

01 5 100.50.10.050.01 50 100

Design EnvelopePerc

ent P

assi

ng

GeotextileSeparator Layer Design

vCheck forv Separation criteria (soil retention)v Survivability and endurance criteria v Clogging potential criteria* v Permeability criteria*

Time Domain Reflectometry

0

5

10

15

20

3/3/00 3/17/00 3/31/00 4/14/00 4/28/00 5/12/00 5/26/00

Date

Moi

stur

e C

onte

nt (%

)

0

5

10

15

20

25

30

35

40

45

50

Prec

ipita

tion

(mm

)

Precipitation

Moisture Content

0

5

10

15

20

3/14/00 3/28/00 4/11/00 4/25/00 5/9/00 5/23/00

Date

Moi

stur

e C

onte

nt (%

)

0

5

10

15

20

25

30

35

40

45

50

Prec

ipita

tion

(mm

)

Precipitation

Moisture Content

SECTION B

SECTION J

Degree of Saturation (%)

Def

lect

ion

(mm

)

60 70 80 90 100

2.5

5.0

7.5

10.1

12.6

15.1

6.2 % Fines9.1 % Fines

11.5 % FinesCrushed stoneGravel

Deflection vs. Saturation of Base

Monitoring Techniques

vPerformance monitoring can identify potential problems sooner and save moneyv Visual condition surveysv FWD testing (structural integrity)v Profile (ride quality)

vIf sub-drainage is present, can the outlets be found and are they clear of debris?

vAre inlets clear and functioning?vAfter a rain, is water flowingvAre the joints or cracks properly sealed?vAre ditches clear of standing water and/or

grass/weeds?vAre typical signs of pumping evident?

Visual Evaluation

Moisture Related Distresses-HMA

Moisture Related Distresses-PCC

Click to edit Master title style

PAVEMENT PERFORMANCE IS A TRUE REFLECTION OF DRAINAGE SYSTEM

OGDL Problems

OGDL Failure

Guide for Selecting m2

Quality of Drainage*

Excellent

Good

Fair

Climate Condition A

1.25 - 1.20

1.25 - 1.20

1.25 - 1.20

Climate Condition F

1.25 - 1.15

1.25 - 1.15

1.15 - 1.05

v Recommended drainage coefficient, m2

* Quality of drainage is a function of permeable base quality of drainage and effectiveness of drainage system

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