576 CEE Lecture Notes Date: 12/2/99 By: Vaishal Sheth Subject: Drainage Design Source: Text by Huang Section 8.1,8.2,8.3 8.1 Need for Drainage Layer Misconception- Good drainage is not required if the thickness design is based on saturated conditions. Required when Infiltration of the pavement and drainage capacity of the base,subbase and subgrade are not equal. Detrimental Effects of Water Reduces strength of unbounded layers. Causes pumping of Rigid pavement. Pumping of fines in Flexible pavements. Frost heave and swelling. Stripping of HMA. Sources of Water Infiltration through cracks, joints etc. Groundwater Ice melting (Frost Heave) Movement of Water Caused by: Gravity (Granular materials) Darcy's law v=ki v = Discharge velocity k = Coefficient of permeability i = Hydraulic gradient Capillary action (Fine grained materials)
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Vapor pressure Methods of Controlling Water in Pavements Prevention Sealing surface and interception of ground water. Transverse slopes are used to provide surface drainage Feature Slop (in/ft) Slope (%) Pavement 3/16 3/8 1.5 3.0 Shoulder 3/8 3.0 6.0 Ditch 3-6 25.0 50.0
Quick Removal Common drainage installations for Quick Removal - Drainage Blanket - Longitudinal drain: Collector pipe, or Daylighted construction - Transverse drain Stronger Pavements - Thicker HMA or PCC reduce hydrodynamic pressure and it's detrimental effects. 8.2 Drainage Materials Aggregates - Sound, clean open-graded materials. - High Permeability - Protected from clogging by filter Filter Criteria Clogging Criterion D15 filter / D85 soil 5 (Dx : grain size corresponding to x % passing) Permeability Criterion D15 filter / D15 soil 5 Additional Criteria - D50 filter / D50 soil 25 (U.S. Army Corps of Engineers) - Cu =D60/D10 for filter materials 25 (To minimize segregation) - P200 5 % ( To prevent Infiltration of fines) - D5 of filter 0.0029 in.
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Geotextiles Filter fabrics to protect the drainage layer from clogging. Requirements - Retain soil - Allow water to flow - Clogging proof - Made from strong, tough, polymeric fibers - Dimensionally stable Filter Criteria - Apparent opening size (AOS) Size of glass beads when 5 % pass through the geotextile. - Maximum Retention and Minimum Clogging Criteria - Fine grained soils with more than 50% passing through a N. 200 sieve: Woven : AOS D85 Nonwoven: AOS 1.8 D85 AOS No. 50 sieve or opening smaller than 0.297 mm. Granular Materials with 50 % P200 AOS B * D85 B=1 when Cu 2 or 8 B= 0.5 Cu when 2 Cu 4 B= 8/Cu when 4 < Cu < 8
Permeability Criteria Permittivity * thickness = Permeability (ASTM) k(fabric) k(soil) Clogging Criteria Woven: Percent open area 4 % Nonwoven: Porosity 30 % Applications of Geotextiles Envelope of trench drains Wrapping of pipe drains
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Filter of drainage layers Prefabricated "fin" drains. Pipes Could be made up of - Concrete - Clay - Bituminized fiber - Metal - Plastics Peforated or slotted with open joints when used as underdrains Do not need perforations, slots or open joints when used as outlet pipes Filter Criteria - For slots : D85 of filter material / Slot width > 1.2 - For circular holes : D85 of filter material / Hole diameter > 1.0 - For Open-jointed pipes : D85 of filter material / Maximum opening of pipe 2 Pipes should be durable under the physical and chemical environments to which they will be exposed.
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8.3 Design Procedures Estimation of Inflow Sources of Inflow Surface infiltration Most important design for Groundwater seepage should be kept lower by longitudinal drains Meltwater from ice lenses For northern climates, consider only if larger than groundwater seepage. Surface Infiltration 3 - Infiltration rate Ic = 2.4 ft /day/ft of crack - Infiltration rate per unit area
qi
= Ic
(
Nc W c W p / + W pC s
)+
Kp
Ic = crack infiltration rate Nc = number of longitudinal cracks Wp = Width of pavement subjected to infiltration Wc = length of transverse cracks or joints Cs = spacing of transverse cracks or joints Kp = rate of infiltration through uncracked pavement surface = permeability of HMA or PCC (very small) N = No. of traffic lanes
Assuming Nc = N + 1 (number of lanes) Wc = Wp Kp = 0.0 Ic = 2.4ft2/day/ft of crack 8 cracking = 0.1 ft3/hour/ft Inflow rate q = qiWp = 0.1( N + 1 + Wp / Cs) q is in ft3/h/linear ft Cs = Joint spacing for concrete pavements = 40 ft for asphalt pavements. OR: Figure 8.13 multiply by .33 to .50 for asphalt , .50 to .67 for PCC.
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Groundwater Seepage Figure 8.14 Steps to calculate groundwater inflow 1) Calculate Li = 3.8(H-Ho) Li = Distance of influence 2 2) Calculate qi = k(H Ho) / 2Li H= Initial height of groundwater table above the impervious layer Ho = Vertical distance between the bottom of drainage layer and the impervious layer 3) Determine q2 from figure 8.14 4) Determine lateral flow qL = q1 + q2 ( Multiply by 2 if one side drain) 5) Ground water inflow per unit area qg = 2q2 / W ( W= Width of roadway) Meltwater from Ice lenses Design Inflow rate qm = average during first day after thawing Depends on - Average rate of heave - Permeability K of subgrade - Consolidation pressure p Calculated by getting Heave rate from Table 8.5 and then looking up Figure 8.15 Design Inflow Design inflow = sum of all inflows - outflow Inflow qd = qi + qg (when there is no frost action) Inflow qd = qi + qm Outflow - Depends on K of soil and permeability. Can be determined using flownets. - Assuming outflow rate = Permeability (when subgrade is not affected by water table) 3 2 - Therefore outflow rate = 0.10 m /day/m (Assuming HG =1.0) 8.3....