Benkelman Beam Deflection Technique

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REPORT ON PAVEMENT DESIGN FOR THE SECTION OF ROAD FROM CLUB GATE TO

SAFILGUDA

1.0 Introduction:

The section of the road from Club Gate to Safilguda is a two lane road carrying heavy

commercial traffic on both sides. It is proposed to widen the entire stretch of the road.

The pavement design for the widening portion is proposed based on the existing

carriageway.

Based on the soil investigation performed by Centre for Transportation Engineering, JNTU,

the design is proposed. The existing road structure is evaluated through Benkelman Beam

Deflection technique and the overlay for the existing road is proposed using BBD test as

per IRC 81 : 1997 – Guidelines for Strengthening of flexible road pavements using

Benkelman Beam Deflection technique. The proposed widening portion is designed as

per IRC 37 : 2001 – Guidelines for the design of flexible pavements.

2.0 Soil condition:

The soil under the pavement structure is tested and the results are furnished below. This

data is used in determination of moisture / seasonal correction factor in overlay design.

1. Moisture content = 6.02%

2. Liquid limit = 23 %

3. Plastic Limit = 7%

4. Plasticity index = 16

5. CBR = 6 %

3.0 Traffic:

The commercial traffic (greater 3 tonnes ) of 933 cv / day has been reported on the

stretch. The design traffic calculations for overlay design as well as widening portion are

presented below.

The design traffic is considered in terms of the cumulative number of standard

axles to be carried during the design life of the road. Its computation involves estimates

of the initial volume of commercial vehicles per day, lateral distribution of traffic, the

growth rate , the design life in years and the vehicle damage factor (number of standard

axle per commercial vehicle) to convert commercial vehicles to standard axles.

The following equation is used to make the required calculation:

Ns = { 365 x A [(1 + r )x – 1]} x F / r

Where Ns = The cumulative number of standard axles to be catered for in the

A = Initial traffic , in the year of completion of construction, in terms of no. of commercial

vehicles per day duly modified to account for the lane distribution

r= Annual growth rate of commercial vehicles

x = Design life in years

F = Vehicle Damage factor.

3.1 Calculations:

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Initial traffic on the stretch = 933 cv / day

A = Initial traffic x Lane distribution factor

Lane distribution factor = 0.75 (As per IRC 81-1997 and IRC 37 – 2001) for two lane single

carriage way roads.

Hence A = 933 x 0.75 = 699.75.

r= 7.5 % (assumed growth rate)

Design life for the overlaying portion as suggested by IRC 81 is 10 years x=10 years

F= 3.5 for plain roads carrying traffic between 500- 1500 cv /day.

Hence Ns = 12.64 msa. (for Overlaying portion)

For the widening portion which is a new formation, the design life of 20 years is

considered (Design life for urban roads recommended as per IRC 37 : 2001 is 20 years) .

Hence the design traffic considering x = 20 years is :

Ns = 38.71 msa. (For widening portion)

4.0 Benkelman Beam Deflection Test:

This method is used to know the deflection of the pavement which is inturn used in

determining the overlay thickness for an existing flexible pavement

Equipment used in the survey:

1. Thermometer

2. Auger

3. Mandrel

4. Glycerol

5. Truck

6. Benkelman Beam

7. Dial gauge

8. Tape

Three types of data are required for knowing the deflection.:

1. Temperature data:

The standard temperature for doing the experiment is 35oC . Since it is not possible to

conduct the test at the standard temperature, a correction factor has to be applied for

the deflection. The correction factor is determined by knowing the temperature at the

time of the survey. If the depth of the BT surface is more than 40mm , then correction

factor has to be applied. If the depth is less i.e if it is a thin bituminous surfacing like

premix carpet and surface dressing, then no correction is required.

The procedure for determining the temperature is given below.

a. A hole has to be drilled into the pavement with the help of a mandrel. The depth

of the hole is 45 mm and the diameter of the hole at the top is 1.25 cm and at the

bottom is 1 cm.

b. The hole has to be filled with glycerol and the temperature has to be recorded

after 5 minutes with the thermometer (range of temperature between 0 -100o)

with 1o division.

c. The temperature readings have to be measured for every hour during the survey.

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BT layer

Base

Subgrade

50 cm

Test point where deflection is measured which is

located @ 0.6 m from edge of pavement if it is a

single lane road; 0.9 m if it is a two lane and 1.5 m if

it is a four lane road.

5 -10 cm

Shoulder Hole to be drilled

in shoulder

0.6 – 1.5 m

15cm

Auger

2. Soil data:

Deflection measurements should be made during the monsoons when the pavement is

in its weakest condition. Hence a correction for seasonal variation has to be applied for

the deflection which is a function of the soil subgrade. The data required is :a) Average

annual rainfall in that area , (b) Soil classification – sandy / gravelly, clayey with low

plasticity and clayey with high plasticity. (c) Field moisture content.

Hence the soil tests that have to be conducted are Moisture content test (Standard

Proctor test), Sieve analysis (for soil classification) and Atterberg limit tests (for

Determination of PI value).

The procedure for soil collection is given below:

(i) Make a test pit in the shoulder to a depth upto 15 cm below the subgrade

level in every km. Using an auger collect the soil sample from the subgrade

beneath the deflection points (These are the points which are at a distance

of 0.6m from the edge of the pavement if the carriageway width < 3.5 m

{single lane road} ; 0.9 m if the carriageway width is greater than 3.5 m {Two

lane road} and 1.5 m if it is a four lane road) The view of the sample collection

is given below in the drawing.

(ii) Minimum sample weight must be 100 gm.

3. Truck specifications for conducting the test:

Rear axle weight of the truck = 8170 kg

Tyre pressure = 5.6 kg / cm2

Spacing between the tyre walls = 30 -40 mm.

Sub base

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Procedure :

1. Pavement condition survey has to be done for dividing the road sections into

uniform sections

2. Minimum of 10 points have to be selected. In each road section of uniform

performance minimum of ten points should be marked at equal distance in each

lane of traffic for making the deflection measurements in the outer wheel path.

The interval between the points should not be more than 50 m . On roads having

more than one lane, the points marked on adjacent lanes should be staggered.

In the present study the distance of 25 m have been considered for

measurement.

3. The point on the pavement to be tested is selected and marked. For highways,

the point should be located 60 cm from the pavement edge if the lane width is

less than 3.5 m and 90 cm from the pavement edge for wider lanes. For divided

four lane highway, the measurement points should be 1.5 m from the pavement

edge.

4. The dual wheels of the truck are centered above the selected point.

5. The probe of the Benkelman beam is inserted between the duals and placed on

the selected point

6. The locking pin is removed from the beam and the legs are adjusted so that the

plunger of the beam is in contact with the stem of the dial gauge. The beam

pivot arms are checked for free movement

7. The dial gauge is set at approximately 1 cm . the initial reading is recorded when

the rate of deformation of the pavement is equal or less than 0.025 mm / min

8. The truck is slowly driven a distance of 270 cm and stopped.

9. An intermediate reading is recorded when the rate of recovery of the pavement

is equal or less than 0.025 mm / min

10. The truck is driven forward a further 9m

11. The final reading is recorded when the rate of recovery of pavement is equal to

or less than 0.025mm / min

12. Pavement temperature is recorded atleast once every hour inserting

thermometer in the standard hole and filling up the hole with glycerol.

13. The tyre pressure is checked at two or three hour intervals during the day and

adjusted to the standard , if necessary.

5. 0 Overlay design Calculations:

1. If Dintermediate – Dfinal ≤ 0.025 mm and Dinitial - Dfinal ≤ 0.025 mm

Actual deflection (XT) = 2 (Dinitial – Dfinal)

2. If Dinitial - Dfinal > 0.025 mm,

Actual deflection XT = XA + 2.91 Y

= 2(Dinitial – Dfinal) + 2.91 [ 2 ( Dfinal ~ Dintermediate)].

3. Rebound deflection = 2 x XT

4. Correction for temperature:

The stiffness of bituminous layers changes with temperature of the binder and

consequently the surface deflections of a given pavement will vary depending on the

temperature of the constituent bituminous layers. Correction for temperature variation

on deflection values measured at temperature other than 35 oC should be 0.01 mm for

each degree centigrade change from the standard temperature of 35 oC. The

correction will be positive for pavement temperature lower than 35 oC and negative for

pavement temperature higher than 35 oC.

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5. Correction for seasonal variation:

The correction for seasonal variation is determined from the graph below.

The deflection values corrected for temperature shall be multiplied by the appropriate

values of seasonal correction factors to obtain corrected values of deflection.

4. Mean deflection = x = ∑ x / n

5. Standard deviation = Q = sqrt (∑(x - x)2 / n-1)

6. Characteristic Deflection = Dc = x + 2 Q

The overlay thickness in terms of Bituminous macadam is obtained from the following

graph.

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The test data and the analysis are presented below

The Characteristic deflection from the analysis of the test data = 2.51 mm

Design traffic = 12.64 msa.

Pavement overlay details :

Alternative 1: Provide Bituminous concrete of 40 mm thickness over a DBM course of 100

mm thickness

Alternative 2: Provide a DBM / AC/ SDBC thickness of 140mm

Alternative 3: Provide a Bituminous Macadam of 190 mm thickness.

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6.0 Design of widening portion:

The widening portion is a new formation . Hence it is designed as per IRC 37 – 2001. The

design traffic is calculated for a design life of 20 years . Based on the CBR values and the

design traffic, the pavement structure is proposed. The plates used for the design are

enclosed below. The structure of the proposed pavement is given below.

Structure of the pavement in widening portion (designed for CBR = 6%):

The design is based on the design traffic of 38.71 msa.

Total proposed thickness = 665 mm

1. Bituminous Surfacing – a. BC = 40 mm

b. DBM = 115 mm

2. WMM = 250 mm

3. GSB = 260 mm