8th International Congress on Civil Engineering, May 11-13, 2009, Shiraz University, Shiraz, Iran
Pavement Subsurface Drainage Design Procedure for Iran N. Tabatabaee, M.R. Pouranian* Department of Civil Engineering, Sharif University of Technology, Tehran, Iran
[email protected] Abstract Providing adequate drainage to a pavement system is an important design consideration to prevent premature failure due to water related problems such as pumping action, loss of support and rutting. This study focuses on subsurface drainage issues in Iran. Evaluation parameters for subdrainage are traffic load, permeability coefficient of subgrade, freezing index and annual precipitation. The last two are climatic parameters used in the procedure proposed in this paper. The time-to-drain method was used to evaluate subdrainage efficiency. SH-DRAIN' Software was developed to facilitate pavement subsurface drainage design Keywords: Freezing index, subsurface drainage, Permeability coefficient
1.
INTRODUCTION
Excess water content in a pavement base, subbase, and subgrade soils can cause early distress and lead to structural or functional failure if counter measures are not taken. Water-related damage can cause reduced subgrade and base/subbase strength, differential swelling in expansive subgrade soils, stripping of asphalt in flexible pavements, frost heave and reduced strength during frost melt, and movement of fine particles into the base or subbase course materials resulting in a reduction of the hydraulic conductivity [1]. Laboratory and field tests indicate that the moduli of base and subgrade materials are strongly affected by moisture content. Furthermore, a relatively rapid decrease in the level of serviceability can occur because the pavement’s ability to transmit dynamic loads imposed by traffic is weakened. [2, 4] The movement of a wheel on pavement with a saturated subgrade can produce a moving pressure wave, which in turn can create large hydrostatic forces within the structural section. These pulsating pore pressures significantly influence the load-carrying capacity of all parts of the pavement structure. [4] Freeze-thaw cycles can also cause moisture-induced pavement damage when the moisture migrates through the capillary fringe toward the freezing front to increase ice lenses. The presence of water in the pavement is mainly caused by infiltration through the pavement surfaces and shoulders, melting of ice during freezing/thawing cycles, capillary action, and seasonal changes in the water table. Sources of moisture in pavement systems are shown in Figure 1 [2].
Figure 1. Sources of moisture in pavement systems [2].
2.
STEP 1: ASSESSING THE NEED FOR DRAINAGE
Identifying the need for subdrainage for a given project situation is an important step in the pavement design process. Ideally, the need for subdrainage should be based on a cost/benefit analysis where the benefit (extended life, reduced maintenance) is greater than the added cost of installing and maintaining the system. In the absence of a universally acceptable procedure to perform such analysis, the practical approach outlined in Table 1. Subgrade permeability (Ksubgrade) was determined using AASHTO T 215 for coarse-
8th International Congress on Civil Engineering, May 11-13, 2009, Shiraz University, Shiraz, Iran
grained soils (clean sands and gravels) and U.S. Army Corps of Engineers Engineering Manual (EM- 11102-1906) procedure for permeability determination of fine grained soils (falling head; clays and silts). Table 1. Assessment of need for subsurface drainage in new or reconstructed pavements.
< 2.5 million heavy trucks Climatic Condition
30 m/day NR
2.5-12 million heavy trucks Ksubgrade 30 m/day m/day m/day R R F
> 12 million heavy trucks 30 m/day F
R
F
F
NR
NR
NR
The following factors should to be considered when deciding upon the feasibility of providing subdrainage: Past pavement performance and experience in similar conditions. Cost differential and anticipated increase in service life through the use of drainage alternatives Anticipated durability and/or erodibility of paving materials This procedure is based primarily on site conditions that affect the decision-making process. Heavy trucks are defined as vehicles of FHWA Class 4 or higher as described in Table 2. [8] Table 2. Vehicle class definition.
Vehicle Class 4 5 6 7 8 9 10 11 12 13
Description Buses: All vehicles manufactured as passenger buses with two axles and six tires or three or more axles (including school buses). Two-axle, six-tire, single-unit trucks: All vehicles on a single frame including trucks, camping and recreational vehicles, and motor homes with two axles and dual rear wheels. Three-axle, single-unit trucks: All vehicles on a single frame including trucks, camping and recreational vehicles, and motor homes, with three axles. Four or more axle, single-unit trucks: All trucks on a single frame with four or more axles. Four or fewer axle, single-trailer trucks: All vehicles with four or fewer axles consisting of two units, one of which is a tractor or straight truck power unit. Five-axle, single-trailer trucks: All five-axle vehicles consisting of two units, one of which is a tractor or straight truck power unit. Six or more axle, single-trailer trucks: All vehicles with six or more axles consisting of two units, one of which is a tractor or straight truck power unit. Five or fewer axle, multi-trailer trucks: All vehicles with five or fewer axles consisting of three or more units, one of which is a tractor or straight truck power unit Six-axle, multi-trailer trucks: All six-axle vehicles consisting of three or more units, one of which is a tractor or straight truck power unit. Seven or more axle, multi-trailer trucks: All vehicles with seven or more axles consisting of three or more units, one of which is a tractor or straight truck power unit
The following criteria were used to define the four climatic regions shown, similarly to the long term pavement performance (LTPP) system [2]. Wet climate: Annual precipitation > 508 mm
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8th International Congress on Civil Engineering, May 11-13, 2009, Shiraz University, Shiraz, Iran
Dry climate: Annual precipitation < 508 mm Freeze: Annual freezing index > 83 degree days C No freeze: Annual freezing index < 83 degree days C The freezing index (FI) is the summation of the degree days for a freezing season: n
FI=
(1)
(0-Ti ) i=1
where Ti = average daily air temperature on dayi, °C; i = counter for number of days below freezing; n = last day in specified period when average daily air temperature is below freezing. Where drainage is recommended in Table 1, moisture-related problems are anticipated. The incorporation of subsurface drainage features to alleviate these problems should be considered in such situations [2]. Where drainage is feasible, it is possible that moisture could affect pavement performance. However, the use of subdrainage should be governed by cost considerations, past performance history, and material quality. Where drainage is not recommended in Table 1, the addition of drainage probably will not be costeffective. However, special considerations for a given project such as cut sections, flat grades, sag curves, or poor construction materials should be evaluated on a case-by-case basis. Overall, adjusting the recommendations in Table 1 to local experience is important. Table 3 shows FI, annual precipitation and climatic conditions for major Iranian cities. Approximately 10% of the country falls into the wet climatic category.SH-DRAIN'software was can estimate freezing index and annual precipitation for anywhere in Iran Table 3. Climatic parameters for major Iranian cities.
City Ardebil Ahvaz Arak Bandar Abbas Birjand Boushehr Bujnurd Esfahan Qazvin Qom Gorgan Hamedan Ilam Kerman Kermanshah Khoramabad Mashhad Orumieh Rasht Sanandaj Sari Semnan Shahrekord Shiraz Tabriz Tehran Yasooj Yazd Zahedan Zanjan
FI (degree days C) 590 0 355 0 30 0 130 90 260 38 15 540 30 20 182 40 160 410 40 350 5 42 360 4 385 108 45 40 0 475
Annual precipitation (mm) 302 233 337 172 164 255 270 122 320 137 595 320 602 142 463 514 256 333 1374 460 761 143 336 328 290 237 870 58 80 310
3
Climate freeze, dry no freeze, dry freeze, dry no freeze, dry no freeze, dry no freeze, dry freeze, dry freeze, dry freeze, dry no freeze, dry no freeze, wet freeze dry no freeze, wet no freeze, dry freeze dry no freeze, wet freeze, dry freeze, dry no freeze, wet freeze, dry no freeze, wet no freeze, dry freeze, dry no freeze, dry freeze, dry freeze, dry no freeze, wet no freeze, dry no freeze, dry freeze, dry
8th International Congress on Civil Engineering, May 11-13, 2009, Shiraz University, Shiraz, Iran
3.
STEP 2: SELECTION OF DRAINAGE ALTERNATIVES
When it has been determined that subsurface drainage is needed, the designer must select the drainage type that will be most effective for a given pavement design. The type of subsurface drainage required should be based on the weighted need for a given design situation. When site conditions are such that drainage is recommended in Table 1, the following options are available for this pavement type [2]: A permeable base system with pipe edge drains (Fig. 2). The permeable base could be an asphalt treated permeable base (ATPB) or a cement treated permeable base (CTPB) layer. A separator layer must also be provided. A daylighted permeable base (Fig. 3) could be considered for flat grades (< 0.5% longitudinal grade) or at the bottom of sag curves when maintenance of the daylighted edge can be assured. A partial drainage system with a non-eroding base and pipe edge drains (Fig. 4).
Figure 2. Permeable base system with pipe edge drains [2].
Figure 3. Daylighted permeable base system [2].
Figure 4. Non-eroding base with pipe edge drains [2].
When site conditions are such that drainage is feasible, the provision of a thick, daylighted dense aggregate base can be considered in addition to the two options listed above.
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STEP 3: HYDRAULIC DESIGN 4
8th International Congress on Civil Engineering, May 11-13, 2009, Shiraz University, Shiraz, Iran
The issues involved in designing the main components of a permeable base system are permeable base design, separator layer design, and edge drain design. 4.1.
PERMEABLE BASE
The recommended approach for performing hydraulic design of permeable bases is the time-to-drain procedure. This procedure is based on the following assumptions [2]: Water infiltrates the pavement until the permeable base is saturated. Excess runoff will not enter the pavement section after it is saturated. After a rainfall event ceases, water is drained to the side ditches or storm drains through edge drains or by daylighting. The main parameter of interest in the time-to-drain procedure is the time required to drain the permeable base to a pre-established moisture level. Table 4 presents guidelines for selecting the permeable base quality of drainage based in this method [3]. Table 4. Permeable base quality of drainage rating based on time taken to drain 50% of the drainable water [3].
Quality of Drainage excellent good fair poor very poor
Time to Drain 2 hr 1 day 7 days 1 month does not drain
The parameters for the time-to-drain design procedure include basic pavement design and material properties such as roadway geometry (cross-slope, longitudinal slope, lane width), thickness of the permeable base, porosity and effective porosity of permeable base aggregate, and permeability of the base material. Using these inputs, the time-to-drain parameter is calculated for a given degree of drainage (U). The final design is then chosen on the basis of this information. Among all input parameters, permeability has the greatest influence and permeable base thickness the least influence on the time-to-drain parameter. The time required to drain a permeable base decreases exponentially with an increase in permeability. It is recommended that the permeability be increased by a reduction in fines (a minimum of 304 m/day is required for permeable bases) [2]. However, care must be taken to maintain adequate stability in the permeable base while effecting a reduction in fines. To guarantee reasonable stability, a minimum coefficient of uniformity, Cu, of 3.5 is required for an untreated permeable base. 4.1.1. PERMEABILITY Various empirical methods have been used to estimate the coefficient of permeability. The most common method is the following statistical relationship, developed by Moulton [4]: k (m/day) = [(1.894 x 105) D101.478 n6.654] / P2000.597
(2)
where n = porosity; D10 = effective grain size (mm) corresponding to 10% passing; P200 = percent passing no. 200 sieve. The base and subbase gradations used in Iran are shown in Table 5. These gradations have been defined by the Management and Planning Organization of Iran. [5]. To satisfy the minimum permeability criteria (304 m/day or 1000 ft/day) for permeable drainage, minimum porosity, plasticity index(PI) and minimum thickness of the drainage layer for each of five layer categories of base and subbase gradation for Iran were calculated. Table 6 shows the results. SH-DRAIN estimates permeability coefficient base on equation 2. 4.1.2. INFLOW It is recommended that inflow rate be estimated by the water-carrying capacity of a pavement crack or joint. An equation to compute the infiltration rate for normal conditions of pavement with no cracking is [6]: qi = Ic [Nc/W + Wc/(WCs)] + kp
5
(3)
8th International Congress on Civil Engineering, May 11-13, 2009, Shiraz University, Shiraz, Iran
Table 5. Common granular base and subbase gradations in Iran [5]. Type I % passing SubBase base 100 100
Sieve 50 (mm) 37.5 (mm) 25 (mm) 19 (mm) 9.5 (mm) #4 # 10 # 30 # 40 # 200
Type II % passing SubBase base 100 100
Type III % passing SubBase base 100 100
Type IV % passing SubBase base -------
Type V % passing SubBase base --------
95-100
----
----
100
----
-----
100
100
----
-----
----70-92 50-70 35-55 ----12-25 ----0-8
------30-65 25-55 15-40 ---8-20 2-8
------30-65 25-55 15-40 ---8-20 2-8
75-90 ----40-70 30-60 20-50 ----10-30 0-12
75-95 ---40-75 30-60 20-45 ---15-30 2-8
75-90 ----40-75 30-60 20-45 ----15-30 5-12
70-100 60-90 45-75 30-60 20-50 ---10-30 2-8
90-100 ----55-80 40-60 28-48 ----14-28 5-12
100 ---50-85 35-65 25-50 ---15-30 2-8
100 ----50-85 35-65 25-50 ----15-30 5-12
where: qi = rate of pavement infiltration, m3/day/m2 (ft3/day/ft2) Ic = crack infiltration rate, m3/day/m (ft3/day/ft) Nc = number of longitudinal cracks Wc = length of contributing transverse joints or cracks, m (ft) W = width of permeable base, m (ft) Cs = spacing of contributing transverse joints or cracks, m (ft) kp = pavement permeability, m/day (ft/day) A value of Ic = 0.223 m3/day/m (2.4 ft3/day/ft) is suggested for computations based on studies of saturated joints/cracks. This value approximates the average infiltration rate measured through cracks in AC1 surfaces underlain by open-graded materials. Consideration of all probable combinations of inflow and outflow leads to the following relationship for computing net design inflow (qn) [6]: qn = qi + qg
(4)
qn = qi + qm
(5)
where qn = net design inflow (m3/day/m2) qm = melt water inflow (m3/day/m2) qg = groundwater inflow (m3/day/m2) If the inflow of Eq.5 is greater than Eq.6, the inflow of Eq.5 should be selected for design and vice versa. Table 6. Minimum values of porosity, PI, Cu for five categories of base and subbase gradation for Iran.
Minimum porosity Type I II III IV V
1
Base .45 .66 .54 .48 .54
Subbase .74 .80 .85 .87 .90
Minimum PI Base .25 .5 .45 .25 .45
Subbase .30 .60 .54 .25 .50
Minimum Cu Base 56.38 61.69 50.47 50.47 38.64
Asphalt Concrete
6
Subbase 61.69 50.47 94.97 94.97 95.12
Cu > 3.5 stability criteria
Minimum thickness (cm)
Base yes yes yes yes yes
Base 10 10 10 10 10
Subbase yes yes yes yes yes
Subbase 12 12 12 12 12
8th International Congress on Civil Engineering, May 11-13, 2009, Shiraz University, Shiraz, Iran
4.2.
SEPARATOR LAYER
The following requirements are necessary to ensure that the dense-graded aggregate separator layer does not contain too many fines and is well-graded [2]: Maximum percentage of material passing the no. 200 sieve should not exceed 12%. Coefficient of uniformity should be greater than 20, preferably greater than 40. All five base and subbase gradations pass these criteria and can be used as a separator layer. SH-DRAIN can be determined aggregated and fabricated separator layers specifications based on filtering criteria. 4.3.
EDGE DRAIN DESIGN
The hydraulic design of edge drains is basically a four-step process as outlined below. Determine pavement discharge rate (based on time-to-drain approach) Determine edge drain flow capacity Determine outlet spacing (for pipe edge drains, maximum outlet spacing should not exceed 75 m for maintenance purposes) Determine trench width Figure 5 shows the difference between three conditions (without edge drain, with edge drain and fouled edge drain). The evaluation criterion is 85% saturation. This shows that usage of edge drains reduces drainage time. However, some agencies do not allow the use of drainage layers unless necessary maintenance is guaranteed. This requires video inspection just after construction and at regular intervals [7]. Detailed hydraulic features of edgedrains can be calculated and determined by SH-DRAIN'software.
Figure 5. Influence of edge drain on drainage time.
3.
CONCLUSION
Pavement drainage systems remove infiltrating water as quickly as possible to prevent such detrimental effects as pumping of fines, frost- heave and strength reduction in the base, subbase and subgrade. Typical drainage provisions include the placement of a free draining aggregate layer or blanket, as well as the installation of longitudinal and transverse drains. The subdrainage is essential where climatic condition is wet. The climatic condition of Iran is variable and the use of subdrainage is not necessary in all areas. The current study presents a three-step method for the design of subsurface drainage for pavements. A cross section of components of the subdrainage scheme is defined. The designer selects the most viable
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8th International Congress on Civil Engineering, May 11-13, 2009, Shiraz University, Shiraz, Iran
option or options based on availability of materials, economic feasibility, construction experience, projected maintenance and localized factors for structural evaluation. Structural analysis is aided by the preparation of a cross-section of the pavement structure including factors in the hydraulic design process such as layer type, thickness, relative arrangement, pavement geometry, pipe slope and elevation above the ditch line. Base and subbase layers have been defined for Iran and drainage capacity criteria for base and subbase gradation are supplied in Table 6. Care must be taken during the structural design process to ensure that the final design solution matches the assumptions made in the hydraulic design process. Edge drains are suitable only if regular maintenance will be performed. The pavement maintenance is a critical factor in the design of the subdrainage. Software was developed, called SH-DRAIN which is written in visual studio c#.net and includes all of the above steps, proving a systematic approach to designing subdrainage for pavements.
4.
REFERENCES
1. Cedergren, H. R., Arman, J. A., and O'Brien, K. H., (1973), “Development of Guidelines for the Design of Subsurface Drainage Systems for Highway Pavement Structural Sections”, Report No. RD-73-14, Federal Highway Administration, U. S. Department of Transportation, Washington, D.C. 2. AASHTO, (2002), “Guide for Design of Pavement Structures” (draft), Washington, D.C. 3. AASHTO, (2002), “Guide for Design of Pavement Structures”, Washington, D.C. 4, Huang, Y. H., (1003), “Pavement Analysis and Design”, Prentice-Hall, New Jersey. 5. Management and Planning Organization, (2001), “General Specification of Roads”, Publication 101 (in Persian). 6. Moulton, L. K., (1980), “Highway Subdrainage Design”, Report FHWA-TS-80-224, Federal Highway Administration, Washington, D.C. 7. Stivers, M. L., Smith, T. E., Hoerner, T. E., and Romine, A. R., (1999), “Maintenance QA Program Implementation Manual”, NCHRP Report 422, Transportation Research Board, National Research Council, Washington, D.C. 8. AASHTO, (1992), “Guidelines for Traffic Data Programs”, Joint Task Force on Traffic Monitoring Standards, Washington D.C.
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