Pavement Design Revised

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    Pavement design forms an important part of detailed engineering study. The economic returns are

    directly dependent on the performance of the pavement. The scope of pavement design in this project

    can be divided into the following sections.

    Design of Flexible Pavement in Concentric/ Eccentric Widening portions,

    Design of Rigid Pavement

    Design of overlay on the existing pavement

    Design of service roads

    Flexible Pavement Design

    Design methods outlined in IRC: 37-2001 has been used for design of the new pavement.

    Flexible pavement design in our country generally is based on the guidelines given in the IRC: 37-

    2001. The revised code is a modification to the IRC-37-84 incorporating a mechanistic approach. In

    this new code, pavement designs have been extended to cover upto 150 Msa and CBR strengths

    upto 10%.

    Flexible pavement design has been modified as a layer structure. Stresses and strains at critical

    locations have been computed using the linear elastic model developed under the aegis of MORT&H.

    To give proper consideration to the aspect of performance, the following three types of pavement

    distress resulting from repeated application of traffic loads are considered.

    Vertical compressive strain at the top of subgrade

    Horizontal tensile strain at the bottom of bituminous layer

    Pavement deformation within the bituminous surface

    By meeting the mix design requirements, the permanent deformation within the bituminous

    layer can be controlled, whereas the thicknesses of granular and bituminous layers are

    selected using the analytical design approach so that the strains at the critical points are

    within the allowable limits.

    Appropriate designs can be furnished based on the following data:

    Design traffic in terms of cumulative number of standard axles

    CBR value of Subgrade

    Traffic Growth and calculation of Msa

    The method considers traffic in terms of cumulative number of standard axles (8160 Kg) to be carried

    by the pavement during the design life. Design traffic will be estimated based on the following

    parameters:

    Initial traffic after construction in terms of number of commercial vehicles per day

    Traffic growth rate during the design life in percentage

    Design life in number of years

    Vehicle damage factor (VDF)

    Distribution of commercial traffic over the carriageway

    Soil Properties

    Subgrade: The soil used for subgrade should be well compacted to utilise its full strength toeconomise the overall thickness of the pavement. Most of the specifications prescribe use of selected

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    material and stiffer standards of compaction in the subgrade, and the top 500mm portion of the

    roadway. As per the MORT&H specifications, the subgrade has to be compacted to 97% of dry

    density, using heavy compaction (modified Proctor density).

    Subbase: Granular base materials conforming to Clause 401 of MORT&H specifications for Road and

    Bridge works are recommended for use. These specifications suggest three gradings each for close

    and coarse graded granular sub-base materials and specify that the materials passing 425 microns

    sieve should have liquid limit and plasticity index more than 25 and 6 respectively. These

    requirements and the specified grain size distribution of the sub-base material should be strictly

    enforced to meet stability and drainage requirements of the granular sub-base layer. The sub-base

    should have a minimum CBR of 20% for cumulative traffic upto 2 Msa, and 30% for traffic exceeding 2

    Msa. The thickness of Subbase should not be less than 150mm for design traffic less than 10 Msa,

    and 200mm for design traffic of 10 Msa and above.

    Base: Materials to be used in base course must satisfy the grading and physical requirements

    prescribed in the IRC/MORT&H specifications. The recommended minimum thickness of granular

    base is 225mm for traffic upto 2 Msa and 250mm for traffic exceeding 2 Msa.

    Bituminous Surfacing: Bituminous surfacing shall consist of either a wearing course or a binder

    course with a wearing course depending upon the traffic to be carried. The most commonly used

    wearing courses are surface dressing, open graded premix carpet, mix seal surfacing, semi-dense

    bituminous concrete and bituminous concrete. Choice of the appropriate type of bituminous wearing

    course will depend on several factors like design traffic over the service life, the type of base/binder

    course, rainfall and other related factors. The grade of bitumen will be selected keeping in view the

    traffic, rainfall and other environmental and climatic conditions

    Design

    Flexible pavement design has been carried out using IRC: 37-2001. The revised code is a

    modification to the IRC: 37-1984 incorporating a mechanistic approach. In this new code, pavementdesigns have been extended to cover upto 150Msa and CBR strength upto 10%. The design values

    are compared with the values obtained by the AASHTO method for their adequacy.

    a. Design Life

    A 20 year design life has been considered for the flexible pavement.

    b. Vehicle Damage Factors (VDF)

    The vehicle damage factor is a multiplier to convert the number of commercial vehicles of different

    axle loads and axle configuration. To the number of standard axle load repetitions. It is defined as

    equivalent number of standard axles per commercial vehicle. Since, the axle load data was not

    available, a value of 4.5, as given in IRC: 37-2001 has been adopted.

    c. Growth Rate

    Traffic growth rate should be estimated from either by studying the past trends of the traffic growth or

    by establishing the econometric models. However average growth rate of 7.5% has been assumed for

    carrying out the design.

    d. Traffic Considerations

    Classified volume count was conducted from 18 th Feb to 24th Feb of 2006 at km 413+100. The details

    of the volume count are given below.

    Date Car/Jeep/Van Bus Mini Bus LCV2 Axle

    Truck

    Multi-Axle

    Vehicles18.02.06 11532 1451 546 1174 1604 180

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    19.02.06 13200 1517 329 888 1210 100

    20.02.06 8759 1045 185 639 1702 172

    21.02.06 8234 1026 149 738 1645 245

    22.02.06 8183 1014 228 800 1617 187

    23.02.06 7710 1024 162 786 1683 172

    24.02.06 6206 894 101 632 819 116Total 63824 7971 1700 5657 10280 1172

    ADT 9118 1139 243 808 1469 167

    After projecting the traffic with the above discussed parameters, a design lane Msa of 100 has been

    considered for the pavement design. The projected traffic is given below.

    YearProjected Traffic

    MsaCumm

    Msa

    2006 3826 2.36 2.36

    2007 3984 2.45 4.81

    2008 4148 2.56 7.37

    2009 4321 2.66 10.03

    2010 4519 2.78 12.81

    2011 4726 2.91 15.72

    2012 4943 3.04 18.77

    2013 5196 3.20 21.97

    2014 5462 3.36 25.33

    2015 5742 3.54 28.87

    2016 5972 3.68 32.54

    2017 6211 3.83 36.372018 6459 3.98 40.35

    2019 6717 4.14 44.49

    2020 6986 4.30 48.79

    2021 7236 4.46 53.25

    2022 7496 4.62 57.86

    2023 7765 4.78 62.65

    2024 8043 4.95 67.60

    2025 8332 5.13 72.73

    2026 8632 5.32 78.05

    e. Subgrade Strength

    The widened/new pavement will have to be laid on embankment constructed by the material from the

    borrow areas. Soil with subgrade CBR of 8% is considered in the design. The test results of the

    borrow samples is given below.

    CHAINAGE SIDEDISTANCEFROM NH-1

    FSI(%)

    GRAVEL(%)

    SAND(%)

    SILT /CLAY (%)

    LL(%)

    PI(%)

    MDD(gm/cc)

    OMC(%)

    CBR(%)

    433+100 RHS 1.10 Km 11.00 0 56.10 43.90 18 2.0 2.12 7.50 33.50

    430+000 LHS 0.40 Km 13.64 0 39.80 60.20 19 2.0 2.13 8.50 20.60

    432+000 LHS 0.60 Km 15.00 0 51.70 48.30 19 2.0 2.03 9.00 8.90

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    431+000 RHS 4.20 Km 11.11 0 64.00 36.00 - NP 2.13 8.50 18.20

    431+000 LHS 5.50 Km 0.00 0 61.80 38.20 - NP 2.14 8.00 42.50

    431+000 LHS 5.50 KM 0.00 0 62.16 37.84 - NP 2.12 6.50 60.00

    f. Flexible Pavement Design Approach

    New pavement is designed for subgrade strength of 10%. Pavement will be laid on subgrade, which

    has a 4 days soaked CBR more than 10%. IRC pavement designs are given for a maximum sub-

    grade CBR of 10%. In the present case, a thickness of 500mm select sub grade with 10% is assumed

    as sub grade, and over which the pavement crust will be built. The flexible pavement for new

    carriageway has been designed as per the IRC: 37-2001. The pavement composition has been

    designed for a life of 20 years. The pavement composition has been designed for a traffic level of 100

    Msa and subgrade CBR of 10%.

    The proposed pavement composition for the main carriageway is given below:

    BC 50mm

    DBM 130mm

    WMM 250mm

    GSB 200mm

    The pavement is designed for 20 years and supposed to withstand the traffic till the end of the design

    life. But the pavement needs functional overlays every 5 th year from date of commencement. Hence it

    is proposed to provide the thickness for 50msa in the beginning and the balance thickness soon after

    the traffic reaches 50msa, which shall be verified during 1st and 2nd overlay.

    The proposed pavement composition for the main carriageway for 50 msa is given below:

    BC 40mm

    DBM 120mm

    WMM 250mm

    GSB 200mm

    However, the concessionaire has to follow the O&M operations given in Schedule L. As per the

    requirement he has to carry out the Roughness and Benkelman Beam deflection studies from time to

    time to ensure a good riding surface.

    g. Design of Service Roads

    As per the technical schedules, service roads should be designed for 25msa, which seems to be in

    higher side. While reviewing the DPR it was found that the following thickness is provided for service

    roads, hence the same is followed.

    The following pavement is recommended for Service Roads:

    SDBC 25 mm

    BM 50 mm

    WMM 250 mm

    GSB 200 mm

    DESIGN OF RIGID PAVEMENT

    IRC: 58-2002

    This design method is based on limiting the flexural stress of the cement concrete layer. The critical

    stress is a combination of stress induced by traffic and temperature. The fatigue effects in the

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    concrete, due to repetitive loading of the pavement, are incorporated in the design method by an

    adjustment factor, dependent on the appropriate traffic class.

    Traffic Parameters

    According to IRC: 58-2002, the roads have been classified under various categories depending on the

    number of commercial vehicles (>3 Ton) plying on them in a day. The 98% weight of commercialvehicles, as recorded in the axle load survey, will be considered and the design calculations will be

    carried out.

    Environmental Parameters

    h. Temperature Effects

    Temperature differential between the top and bottom of the concrete pavement is dependent upon the

    slab thickness, wind velocity, and also depends upon the geographical location. Recommendation

    regarding the temperature differentials and the geographical locations is given in the IRC: 58-2002.

    Spacing of the expansion and contraction joints is also dependent on the changes in temperature.

    i. Sub-grade and Sub-base

    The strength of the subgrade is expressed in terms of K, which is defined as pressure per unit

    deflection of the foundation determined by the plate-bearing test. The subgrade soil strength and

    strength of foundation will be effected by the presence of moisture. A correlation between the K value

    and the CBR strength has been given in the IRC: 58. From this table one can estimate the

    approximate K value, depending on the CBR of soil.

    Design Procedure

    After choosing the input values for the various parameters and a selected design thickness, the

    temperature stress in the critical edge region will be computed. The residual strength of concrete

    strength for supporting traffic loads will be calculated. Finally, the edge load stress will be calculated

    and compared with the available strength. In case the thickness is not adequate the calculation has tobe repeated with an adjusted slab thickness.

    Design

    Dry lean concrete (DLC) subbase is suggested for use. For the expected traffic situation a 150mm

    thick sub-base layer is deemed appropriate. The DLC should have a characteristic 7 day compressive

    strength not less than 10 MPa which corresponds to a flexural strength of approximately, 2 MPa.

    Below DLC, GSB layer of 150mm thick will be provided which will not only act as drainage layer but

    also serves as a stable working platform on which the construction equipment can be operated.

    The combined subgrade and subbase support is taken in terms of Westergaards subgrade modulusK, the value of which has to be obtained from plate bearing tests. The Austroads design procedure

    utilizes the corresponding CBR strength derived from the CBR of subgrade and the thickness of the

    subbase to estimate the K-value

    After estimating the K-value as 48.667 N / mm2 / mm x 10-3, the Axle load spectrum as obtained from

    the Axle load survey was taken and the percentage category of commercial vehicles in each load

    category was calculated. The projected traffic for the design period of 30 years has been used to

    estimate number of repetitions of individual category of load.

    The following composition is adopted for rigid pavement in toll plaza.

    GSB 200mm

    Dry lean concrete (DLC) 150mm

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    Pavement quality concrete (PQC) 280mm

    Length of Dowel bar 500mm

    Diameter of Dowel bar 32mm

    Spacing of Dowel bar 300mm

    DESIGN OF OVERLAY

    Benkelman Beam Deflection Survey

    General

    Pavement evaluation involves a thorough study of various factors such as sub-grade support,

    pavement composition and its thickness, traffic loading and environmental conditions. The primary

    objective of pavement condition evolution is to assess as to whether and to what extent the pavement

    fulfils the intended requirements so that the maintenance and strengthening jobs could be planned in

    time. A well compacted pavement section or one which has been well conditional by traffic deforms

    elastically under each wheel load application such that when the load moves away, there is an elastic

    recovery or rebound deflection of the deformed pavement surface. This is the basic principle ofdeflection method pavement evaluation or overlay design.

    The maximum deflection under a design wheel load depends on several factors such as sub-grade

    soil properties, moisture in the sub-grade, pavement thickness and its composition, temperature of the

    pavement, loading particulars, etc. Therefore the amount of pavement deflection under a design

    wheel load or its rebound deflection on removal of this load is a measure of the structural stability of

    the pavement system under the prevailing condition of the test. Larger rebound deflection indicates

    weaker pavement structure, which may require earlier strengthening or higher overlay thickness. It is

    desirable to carry out the deflection studies soon after the monsoon when the pavement system may

    be at the weakest condition due to maximum sub-grade moisture content.

    Structural Evaluation of Pavement

    Field investigations and tests carried out in various countries have shown that the performance of a

    flexible pavement is closely related to be elastic deflection under loads or its rebound deflection.

    Measurement of transient deflection of pavement under design wheel loads serves as an index of the

    pavement to carry traffic loads under the prevailing conditions. Out of the various equipment used for

    the purpose, Benkelman Beam is most commonly used, as the measurements are simple and

    reliable. Assessment of flexible pavement overlay thickness requirement by Benkelman Beam method

    is explained below. It is one of the non-destructive testing techniques for assessing the load carrying

    capacity of the pavement.

    Benkelman Beam

    Benkelman Beam is a device, which can be conveniently used to measure the rebound deflection of a

    pavement due to a dual wheel load assembly or the design wheel load. The equipment consists of a

    slender beam of length 3.66m, which is pivoted to a datum frame at a distance 2.44m from the probe

    end. The datum frame rests on a pair of firm levelling legs and a rear leg with adjustable height. The

    probe end of the beam is inserted between the dual rear wheels of the truck and placed on the

    pavement surface at the centre of the loaded area of the dual wheel load assembly. A dial gauge is

    fixed on the datum frame with its spindle in contact with the other end of the beam in such a way that

    the distance between the probe end and the fulcrum of the beam is twice the distance between the

    fulcrum and the dial gauge spindle. Thus the rebound deflection reading measured at the dial gauge

    is to be multiplied by two to get the actual movement of the probe end due to the rebound deflection of

    the pavement surface when the dual wheel load is moved forward. A loaded truck with rear axle load

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    of 8170 kg is used for the deflection study. The design wheel load is a dual wheel load assembly of

    gross weight 4085kg with an inflation pressure of 5.6 kg/cm2.

    Procedure

    The stretch of road length to be evaluated is first surveyed to assess the general condition of the

    pavement with respect to the ruts, cracks and undulations. The loading points on the pavement fordeflection measurements are located along the wheel paths on a line 0.9metre from the pavement

    edge in the case of pavements of total width more than 3.5 m; the distance from the edge is reduced

    to 0.6metre on narrower pavements. Deflection measurements were taken at every 500m staggered

    in each lane. After marking the deflection observation points, the study is carried out in the following

    steps:

    The truck is driven slowly parallel to the edge and stopped such that the left side rear dual

    wheel is centrally placed over the first point for deflection measurement.

    The probe end or the Benkelman Beam is inserted between the gap of the dual wheel and is

    placed exactly over the deflection observation point.

    When the dial gauge reading is stationary or when the rate of change of pavement deflection

    is less than 0.025 mm per minute, the initial dial gauge reading D0 is noted. Both the readings

    of the large and small needles of the dial gauge may be noted; the large needle may also be

    set to zero if necessary at this stage. (The least count of the large needled is 0.01 mm and

    that of small needle is 2.0 mm).

    The truck is moved forward slowly through a distance of 2.7 m from the point and stopped.

    The intermediate dial gauge reading D1 is noted when the rate of recovery of the pavement is

    less than 0.025 mm per minute.

    The truck is then driven forward through a further distance of 9.0 m and the final dial gauge

    reading D2 is recorded as before.

    The three-deflection dial reading D0 D1 and D2 form a set of readings at one deflection point

    under consideration. Similarly the truck is moved forward to the next deflection point, the

    probe of the Benkelman Beam inserted and the procedure of noting the set of three-deflection

    observations is repeated. The deflection observations are continued at all the desired points.

    The temperature of the pavement surface is recorded at intervals of one hour during the

    study. Tyre pressure is checked and adjusted if necessary, at intervals of about three hours

    during the deflection study. The moisture content in the sub-grade soil is also to be

    determined at prescribed locations.

    The rebound deflection value D at any point is given by one of the following two conditions:

    (a) If D0-D1 < 2.5 divisions of the dial gauge or 0.025 mm, D = 2 (Do D2) divisions of

    0.01 mm units = 0.02 (Do- D2) mm

    (b) If D0- D1 > 2.5 division, this indicates that correction is needed for the vertical

    movement of the front legs. Therefore, D = 2 (D0- D2) +2 K (D1 D2) divisions.

    The value of K is to be determined for every make of the Benkelman Beam and is given by the

    relation:

    f

    edK

    23 =

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    Where

    d = distance between the bearing of the beam and the rear adjusting leg

    e = distance between the dial gauge and rear adjusting leg

    f = distance between the front and rear legs

    The value of K of Benkelman Beam generally available in India is found to be 2.91. Therefore, the

    deflection value D in case of leg correction is given by:

    D = 0.02 (D0- D2) + 0.0582 (D1 D2) mm

    Correction for Pavement Temperature and Sub-Grade Moisture Variations

    When the pavement consists of relatively thick bituminous layers like the bituminous macadam or

    asphaltic concrete in the base/binder/surface course, variations in temperature of pavement surface

    course cause variation in pavement deflection under the standard load. The IRC has suggested a

    standard pavement temperature of 35 C and a correction factor of 0.0065 mm per C to be applied

    for the variation from this standard pavement temperature. The correction will be negative when the

    pavement temperature is above 35 C and positive when it is lower. However, it is suggested that

    deflection studies should be carried out when the pavement temperature is above 30 C, if this

    correction factor is to be applied.

    The seasonal variations cause variation in sub-grade moisture. It is always not possible to conduct

    deflection studies during monsoon season when sub-grade moisture content is the highest. IRC has

    suggested tentative correction factors of 2.0 for clayey soils and 1.2 to 1.3 for sandy sub-grade soils

    may be adopted if the deflection observations are made during dry seasons. The deflection under the

    worst sub-grade moisture may therefore be estimated by multiplying the summer deflection value by

    the appropriate correction factor.

    Analysis of Data

    The rebound deflection values are determined in millimeters after applying the leg corrections. The

    mean value of the deflections and the standard deviations are calculated. The Characteristic

    deflection is arrived by using the following equation.

    tDDe +=

    Here the value of t is to be chosen depending upon the percentage of the deflection values to be

    covered in the design. When t = 1.0, De = D + covers about 84 percent of the cases; when t = 2.0,

    De = D + 2 covers about 97.7 percent of the cases of deflection values on the pavement section,

    assuming normal distribution of rebound deflection values. The necessary corrections for pavement

    temperature and sub-grade moisture may be applied to the characteristics deflection values, D e

    before designing the overlay thickness.

    Design

    Benkelman Beam Deflection Survey & Analysis of Data

    Benkelman Beam deflection measurements were made in the month of July 05. The testing approach

    and measurement was essentially according to IRC: 81-1997, the test points being located 0.9 metre

    inside the edge of the road. Testing was carried out both for main line and control sections. In main

    line, the deflections reading were taken at 500m interval, and in control sections, measurements were

    taken at 10m interval, staggered for a length of 100m per kilometer. Pavement and air temperatures

    were recorded for the purpose of temperature correction. Subgrade moisture data was also obtained

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    from field moisture measurements in the test pits for applying seasonal corrections to deflection

    measurements.

    Benkelman beam deflections were carried out at sections where the condition of the pavement is

    good to fair. Where the condition of the pavement is distressed, testing was not carried out as the

    deflections obtained in the distressed sections will not give consistent deflection measurements. In

    stretches where the pavement is severely cracked, such sections were considered for overlay by

    scarifying the top distressed layer.

    (a) Temperature Correction

    Pavement temperatures at the time of BBD measurements were on an average above the standard

    temperature (IRC) of 350C. Appropriate temperature corrections were made based on the

    recommendations in IRC: 81-1997.

    j. Correction for Seasonal Variation

    The pavement for most of the length is laid on embankment. From the test pits, soil samples were

    collected and the field moisture content was estimated. Based on the field moisture content, type of

    soil, and the average intensity of the rainfall, appropriate moisture correction factors were applied assuggested in IRC: 81-1997.

    k. Homogeneous Sections

    Based on the cumulative differences approach, entire project corridor has been divided into 10

    homogeneous sections. For these sections, characteristic deflections have been calculated taking the

    mean plus 2 standard deviations. The homogeneous sections and the overlay thickness for each

    section are given in the Table.

    Table: Homogeneous Sections and Overlay Thickness

    Sl. NoChainage Length

    (Km)

    Overlay Thickness (mm)

    From To BC DBM Total

    1 407.00 415.00 8.00 50 75 125

    2 415.00 417.00 2.00 50 85 135

    3 417.00 418.00 1.00 50 90 140

    4 418.00 425.00 7.00 50 100 150

    5 425.00 431.00 6.00 50 75 125

    6 431.00 433.00 2.00 50 105 155

    7 433.00 436.00 3.00 50 75 125

    8 436.00 438.00 2.00 50 70 120

    9 438.00 445.00 7.00 50 75 125

    10 445.00 449.00 4.00 50 50 10011 449.00 452.00 3.00 50 75 125

    12 452.00 456.00 4.00 50 55 105

    BBD analysis is given in Appendix-1