Chapter VI HIGHWAY DRAINAGE Tewodros N. www.tnigatu.wordpress.com tedynihe@gmail.com
I. INTRODUCTION Provision o of adequate drainage is an essential part of pavement design. Protection of pavement structure Improves road safety Can be categorically studied in three parts: 1. Surface Drainage Drainage on the adjoining land and roadway surface Side Drainage and Cross Drainage 2. Sub-surface Drainage
I. INTRODUCTION Effects of water on the pavement structure t Presence of moisture causes: o reduction in the stability of the soil mass. o considerable variation in volume of subgrade in clayey soils. o Waves and corrugations failure in flexible pavements. o Stripping failure in flexible pavements. o Md Mud pumping failure in rigid pavements.
II. DESIGN OF SURFACE DRAINAGE SYSTEMS Can be divided into three phases: i. Estimation of the quantity of water that can reach any element of the system. ii. Hydraulic design of each element of the system. iii. Comparison of alternative systems and materials Criteria-Lowest annual cost alternative
II. DESIGN OF SURFACE DRAINAGE 1. Rainfall Intensity SYSTEMS Runoff is obtained by considering expected sever storm. Return period of 5, 10, 20, 25, 50, and 100 years Quantity of runoff depends on intensity and duration. Duration= Time of Concentration The time required for water from the remotest place to reach a specific point on the drainage system. =T 1 +T 2 T 1 = over land flow time T 2 = time of flow in the longitudinal drain
II. DESIGN OF SURFACE DRAINAGE SYSTEMS Source: ERA Manual, 2002
II. DESIGN OF SURFACE DRAINAGE SYSTEMS Source: ERA Manual, 2002
II. DESIGN OF SURFACE DRAINAGE SYSTEMS 2. Computation of Runoff Rain water expelled from the road surface i. Infiltration ii. iii. Runoff Evaporation- insignificant Infiltration depends on: Type and gradation of soil Soil covers, moisture content of the soil Presence of impervious layers near the surface.
II. DESIGN OF SURFACE DRAINAGE Infiltration contd. SYSTEMS Rate of infiltration on bare soil is less than on a turfed soil. Frozen soil is impervious Rate of infiltration is assumed to be constant during any specific design storm. Runoff depends on: Nature of the ground, degree of saturation, and slope of the surface Rate of runoff greater on smooth surfaces.
II. DESIGN OF SURFACE DRAINAGE SYSTEMS Rational Formula- accurate way of estimating runoff up to areas of 0.5 km 2 Q 0. 00278 CIA C A1 C A 1 A A 1 2 2 2 Q= runoff (m3/sec) C If the water shade is made up of different surfaces C=coefficient, representing ratio of runoff to rainfall I= intensity of rainfall (mm/hr) for a duration equal to the time of concentration A= catchment area tributary to the design location, ha
II. DESIGN OF SURFACE DRAINAGE SYSTEMS
II. DESIGN OF SURFACE DRAINAGE SYSTEMS t c =distance/velocity of flow t c is then used to determine the rainfall intensity (I)
II. DESIGN OF SURFACE DRAINAGE SYSTEMS This chart can also be alternatively used to determine t c.
III. DESIGN OF SIDE DITCHES AND OPEN CHANNELS The Manning s Formula Once the quantity of runoff is known, the design of ditches and similar structures is based on the principles of open channel flow. Mannings s formula assumes steady flow in a uniform channel. V 1 n R 2 / 3 S 1 / Where: V= mean velocity (m/sec) R= hydraulic radius (m)= Area/wetted perimeter S=slope of the channel (m/m) n=manning s roughness coefficient 2 Q V A
III. DESIGN OF SIDE DITCHES AND OPEN CHANNELS The Manning s Formula
III. DESIGN OF SIDE DITCHES AND OPEN CHANNELS Capacity of a Trapezoidal Channel
III. DESIGN OF SIDE DITCHES AND OPEN CHANNELS Examples: 1. The maximum quantity of water expected in one of the open longitudinal drains on clayey y soil is 0.9 m3/sec. Design the cross section and longitudinal slope of trapezoidal drain assuming the bottom width of the trapezoidal section to be 1.0 mand cross slopes to be 1V:1.5H. The allowable velocity of flow in the drain is 1.2 m/sec and Manning s roughness coefficient is 0.02.
III. DESIGN OF SIDE DITCHES AND OPEN CHANNELS Examples: 2. The surface water from road side is drained to the longitudinal side drain from across one half a bituminous pavement surface of total width 7.0 m, shoulder and adjoining land of width 8.0 m one side of the drain. On the other side of the longitudinal drain, water flows across from reserved land with grass and 2% cross slope towards the side drain, the width of this strip of land being 25 m. The run off coefficients of the pavement, shoulder and reserve land with grass surface are 0.8, 0.25, and 035 respectively. The length of the stretch of land parallel to the road from where water is expected to flow to the side drain is about 400 m. Estimate the quantity of run-off flowing in the drain assuming 25 years period of frequency.
IV. SUBSURFACE DRAINAGE 1. Lowering of Water Table Highest level of water table should be below the subgrade. Practically 1.0 to 1.2 m below subgrade Relatively permeable soil- Longitudinal drains are mainly used Impermeable soils- T d b dd Transverse drains may be necessary in addition to longitudinal drains
IV. SUBSURFACE DRAINAGE 1. Lowering of Water Table Fig. Symmetrical longitudinal drains used to lower the groundwater table and to collect water infiltrating the pavement.
IV. SUBSURFACE DRAINAGE Lowering of Water Table Longitudinal Drain Transverse Drains F L f t t bl Fig. Lowering of water table using Transverse Drains (Plan View)
IV. SUBSURFACE DRAINAGE 2. Seepage Control If seepage zone is at a depth less than 0.6 to 0.9 m below subgrade level, Use longitudinal pipe drain in trench with filter material to intercept the seepage flow. This phenomenon can be explained This phenomenon can be explained using figures.
IV. SUBSURFACE DRAINAGE 2. Seepage Control Fig. Longitudinal interceptor drain used to cut off seepage and lower the groundwater table.
IV. SUBSURFACE DRAINAGE 2. Seepage Control Groundwater seeps through the slope where the water table intersects the land slope, and Groundwater flows beneath the pavement while also entering the pavement foundation materials. Fig. (A) Illustration of ground water flow along a sloping impervious layer toward a roadway.
IV. SUBSURFACE DRAINAGE 2. Seepage Control Fig. (B) Illustration of interceptor drain on the drawdown of the groundwater table.
IV. SUBSURFACE DRAINAGE 2. Seepage Control Fig. Longitudinal collector drain used to remove water seeping into the pavement structural section.
IV. SUBSURFACE DRAINAGE 3. Control of Capillary Rise Capillary rise can be controlled by Using a layer of granular material of suitable thickness. Using a layer of impermeable capillary cutoff. Capillary water should not rise above the thickness of the granular layer Granular material Capillary rise Highest water table Fig. Granular capillary cutoff
IV. SUBSURFACE DRAINAGE 3. Control of Capillary Rise Bituminous layer or other geo-textiles can be used as an impermeable layer. Impermeable layer Capillary rise Highest water table Fig. Impermeable layer capillary cutoff
IV. SUBSURFACE DRAINAGE 4. Design of Filter Material Proper filter material should be used for: Subsurface drainage system and backfilling the drainage trenches and Criteria: Permeability and Piping D15 of filter 5 D15 of foundation Permeability criteria D 85 D of of filter foundation 15 5 Piping criteria
IV. SUBSURFACE DRAINAGE 4. Design of Filter Material D P =size of perforation in drain pipe D 85 Filter = 2D P 10 0 80 60 40 Per rcent pas ssing 20 0. 0 1. 0 Filter Material 2D P 5D 8 5 Foundation soil 5D 1 1.0 0. 0.0 0.00 Particle size 1 (mm), log 1 scale 1 Fig. Design of Filter Material 5 The area between the two red curves represents the filler material.
References: 1. Highway Engineering, 7 th Ed. Paul H. Wright and Karen K. Dixon. Wiley (2004) 2. Highway Engineering, 8 th Ed. S.K. Khanna and C.E.G. Justo. (2001)
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