subgrade soil investigation for designing of whitetopping...

Subgrade Soil Investigation for Designing of Whitetopping Overlays on Existing Flexible Pavement—A Case Study of Pune City, India

31

SUBGRADE SOIL INVESTIGATION FOR DESIGNING OF WHITETOPPING OVERLAYS ON EXISTING FLEXIBLE PAVEMENT—A CASE STUDY

OF PUNE CITY, INDIA

D.R. Jundhare Research Scholar, Civil Engineering Department, Sinhgad College of Engineering, Vadgaon (Bk.), Pune–411 041, M.S., India. E-mail: [email protected] K.C. Khare Prof. (Dr.), Civil Engineering Department, Sinhgad College of Engineering, Vadgaon (Bk.), Pune–411 041, M.S., India. E-mail: [email protected] R.K. Jain Prof. (Dr.), Pad. Dr. D.Y. Patil Institute of Engineering and Technology, Pimpri, Pune–411 018, M.S., India. E-mail: [email protected]

ABSTRACT: The subgrade provides a foundation for supporting the pavement. As a result, much of the required pavement thickness and the performance obtained from the pavement during its design life depend on the strength and uniformity of the subgrade. It is desirable a thorough investigation of the subgrade be made so that the design and construction will insure uniformity of support for the pavement and realization of the maximum strength potential for the particular subgrade soil type. The purpose of field testing and sampling is to identify the extent and properties of the various subgrade materials. The subgrade soil and its properties are important in the design of pavement structure. In Pimpri Chinchwad (twin city of Pune) city a stretch of 3.000 km of Mumbai Pune Arterial Road of existing flexible pavement has been rehabilitated by providing overlay of cement concrete (whitetopping) of 320 mm thickness. To design this whitetopping overlay different field testing of subgrade soil have been carried out. This paper discusses about the field tests of subgrade soil samplings of case study road to find out index properties of soil, modulus of subgrade reaction (k) by plate bearing test and field California Bearing Ratio (C.B.R.) Test for designing required parameters of rigid overlay thickness. Subgrade soil investigation helps in saving of natural resources, human resources, time and cost of the project. Keywords: Subgrade, Moisture Content, Plasticity Index, C.B.R. Test., Plate Bearing Test, Modulus of Subgrade Reaction. 1. INTRODUCTION

All transportation systems are built either on, in, or with soil and products from the ground. Soil is arguably the most critical component of the transportation system, since most construction is dependent upon project soil properties and characteristics. The characterization and evaluation of soil is critical to the performance of pavement structures. The case study is of Mumbai Pune arterial road which has four lanes. The main carriageways are rigid overlays of two lanes of 9 m wide and service roads are flexible pavement of two lanes of 11.50 m wide. The length of this road, to be strengthened is 12 km, out of which the case study is carried out for 3 km from Nasik-Phata to Pimpri. For design purpose of design of overlay, it is necessary, site soil investigation of subgrade. In this case study, sub-grade soil samplings to confirm type of soil, modulus of subgrade reaction (k) by plate bearing test and field California Bearing Ratio (C.B.R.) for designing required parameters of rigid overlay thickness were conducted.

2. SOIL SAMPLING AND TESTING

For many engineering projects it is very useful to take a sample of soil, and to investigate its properties in the laboratory. The investigation may be a visual inspection (which indicates the type of material: sand clay or peat), mechanical tests such as C.B.R. Test, sieve analysis, moisture contents, plasticity index etc. Hence soil sampling in field is carried out and is shown in Figure 1. A simple method to take a sample is to drive a tube into the ground and then recovering tube with the soil in it. The tube may be 1 meter long and have valve at its bottom, to prevent loosing the sample. The tube may be brought into the soil by driving it into the ground using a falling weight or hammer. An advantage of this method is that it does not require heavy equipment. The sub grade soil samples were collected from four different locations of study area. The tests were conducted on collected samples and the test results are presented in Table 1. Trial pits were taken and soil sample collected for assessed 4 days soaked CBR. This will be the basis of design for new pavement.

IGC 2009, Guntur, INDIA


32

Fig. 1: Soil Sampling in Trial Pits

Table 1: Soil Sample Collection Details and Moisture Contents Location

No. Moisture content

Plasticity index

Sub-grade soil type

1. 15.19% 9 Silty Sand 2.

22.00% 12 Inorganic Clays of Medium plasticity

3. 12.30% 12.43 Clayey sand 4 15.47% 9 Silty-Clayey sand

3. PLATE BEARING TEST

The plate bearing test is used to evaluate the supporting power of subgrade for use in pavement design by using relatively large diameter plates. The plate bearing test was originally devised to find the modulus of subgrade reaction in the Westergaard’s analysis for wheel load stress in cement concrete pavement. The test set up consists of a set of plates of diameter 30, 45, 60, and 75 cm; a loading device consists of jack and proving ring arrangement and a reaction frame against which a jack gives a thrust to the plate. A datum frame resting far from the loaded area and dial gauges from this frame are used to measure the settlement of the loaded plate. The load- settlement curve is plotted on a graph paper and the load value P1 corresponding settlement value of ? = 0.125 mm is read and Modulus of Subgrade Reaction (k-value) is calculated by reaction, k = P1/p × (150)2 × 0.125 KN/mm3. The load values corresponding mean settlement values of trial pit no. 1 and 2 are shown in Tables 2 and 3 respectively and graphs (Figs. 2 & 3) are also plotted.

Table 2: The Load Values Corresponding Mean Settlement of Trial Pit No. 1

(Size of plate = 300 mm diameter)

Load (KN) Plate Area (m2 ) Settlement (mm)

0 0 10 0.07 0.64 20 0.07 1.41 30 0.07 1.99 45 0.07 3.09

Table 3: The Load Values Corresponding Mean Settlement of Trial Pit No. 2

(Size of plate = 450 mm × 450 mm and 300 mm × 300 mm) Load (KN)

Plate area (m2 )

Load intensity (KN / m2)

Settlement (mm)

0 0 0

25 0.2025 123.456 0.57

45 0.2025 222.222 1.35

Test resumed with 0.3 m × 0.3 m plate 30 0.09 333.333 2.05 45 0.09 500.000 2.73

0

0.5

1

1.5

2

2.5

3

3.5

1 2 3 4 5

Load in KN

Set

tlem

ent

in m

m

Settlement (mm)

Fig. 2: Graph of Load versus Settlement of Pit No. 1

0

0.5

1

1.5

2

2.5

3

1 2 3 4 5

Load in KN

Set

tlem

ent

in m

m

Series1

Fig. 3: Graph of Load versus Settlement of Pit No. 2

Modulus of subgrade reaction k is defined as the pressure sustained per unit deformation of subgrade at specified deformation or pressure level, using specified plate size. The test site is leveled and the plate is properly seated on the prepared surface. The stiffening plates of decreasing diameter are placed and the jack and proving ring assembly are fitted to provide reaction against the frame. Four dial gauges are fixed on the periphery of the plate, from the independent datum frame for measuring settlements.


33

4. CALIFORNIA BEARING RATIO TEST

California Bearing Ratio is a test widely used to evaluate soil and soil aggregate support values for structural pavement design and abbreviated as the C.B.R. Test.

The test is basically a penetration test, in which the load required to cause a plunger of standard size to penetrate a specimen of soil at standard rate is measured.

In this case study, in situ test was conducted. For this field test a loaded truck is used to provide the reaction and loading is achieved by a screw jack (Fig. 4).

The existing road requiring strengthening, moisture content should be the field moisture content, preferably after rains. The density of the specimen should in such cases be field density. The field moisture content and density are determined at a distance of 0.6 to 1 meter from the pavement edge below the pavement. Field C.B.R. tests results are given in Tables 4 and 5.

Table 4: Field CBR Test Report (I) Type of Layer: Sub Base Moisture content: 5%

Penetration (mm)

Providing ring dial gauge reading (Div.) Load (kg)

0.5 20

1.0 28 1.5 32 2.0 37 2.5 40 220.64 3.0 42 4.0 46 5.0 58 319.928

Bearing ratio at 2.5 mm penetration = 16.05%. Bearing ratio at 5.0 mm penetration = 15.57%.

Table 5: Field CBR Test Report (II) Type of Layer: Sub Base Moisture Content: 5%

Penetration (mm)

Providing ring dial gauge reading (Div.) Load (kg)

0.5 29.60

1.0 36.00

1.5 37.00

2.0 37.00

2.5 37.00 204.092

3.0 37.00

4.0 37.00

5.0 37.00 204.092

Bearing ratio at 2.5 mm penetration = 16.05%. Bearing ratio at 5.0 mm penetration = 15.57%.

If value at 2.5 mm is greater than that at 5 mm penetration, the former is adopted. If not, then it is repeated and the new value of load is 5 mm penetration is still greater; this value is used for calculation of CBR.

Fig. 4: Field California Bearing Ratio (CBR) Test

5. SUBGRADE PERFORMANCE

Performance of subgrade generally depends on the following three of its basic characteristics (all of which are interrelated).

5.1 Load Bearing Capacity

The subgrade must be able to support loads transmitted from the pavement structure. This load bearing capacity is often affected by degree of compaction, moisture content, and soil type. A subgrade that can support a high amount of loading without excessive deformation is considered good.

5.2 Moisture Content

Moisture tends to affect a number of subgrade properties including load bearing capacity, shrinkage and swelling. Moisture content can be influenced by a number of things such as drainage, groundwater table elevation, infiltration, or pavement porosity (which can be assisted by cracks in the pavement). Generally, excessively wet subgrades will deform excessively under load.

5.3 Shrinkage and/or Swelling

Some soils shrink or swell depending upon their moisture content. Additionally, soils with excessive fines content may be susceptible to frost heave in northern climates. Shrinkage, swelling and frost heave will tend to deform and crack any pavement type constructed over them.


34

The overall strength and performance of a pavement is dependent not only upon its design (including both mix design and structural design) but also on the load-bearing capacity of the subgrade soil. Thus, anything that can be done to increase the load-bearing capacity (or structural support) of the subgrade soil will most likely improve the pavement load-bearing capacity and thus, pavement strength and performance. Additionally, greater subgrade structural capacity can result in thinner (but not excessively thin) and more economical pavement structures. Finally, the finished subgrade should meet elevations, grades and slopes specified in the contract plans.

6. CONCLUSION

1. For design the whitetopping overlay on existing flexible pavement, soil investigation is must. The subgrade soil and its properties are important in the design of pavement structure.

2. Sub-grade soil types—silty sand, clayey sand and inorganic clays of medium plasticity were found in study area stretch.

3. CBR values of sub-grade were observed more than 8% and range between 15.50% to 16%. Modulus of Subgrade Reaction (k) was found 20 kg/cm3. The modulus of subgrade reaction (k) is used as a primary input for rigid pavement design. It can be estimated from CBR or calculated from field tests like the plate bearing test.

4. Plate bearing test is quick and easy to perform and gain understanding of foundation behavior which will enable the evaluation of foundation bearing capacity and settlement under loading condition. Results from the test shall consist of raw data, load-settlement curve, yield pressure, recommended allowable pressure for foundation design, and modulus of subgrade reaction (k) for road design.

5. The purpose of field testing and sampling is to identify the extent and properties of the various subgrade materials. Subgrade soil is an integral part of the road pavement structure as it provides the support to the pavement from beneath.

6. Plasticity index is defined as the difference between liquid limit and plastic limit which is a useful measure for the possibility to process the clay.

7. Greater subgrade structural capacity can result in thinner (but not excessively thin) and more economical pavement structures.

8. Subgrade properties are essential pavement design parameters. Materials typically encountered in subgrades are characterized by their strength and their resistance to deformation under load (stiffness).

9. Subgrade characteristics and performance are influential in pavement structural design. Characteristics such as load bearing capacity, moisture content and expansiveness will influence not only structural design but also long-term performance and cost.

REFERENCES

Arnold, Verruijt (2001). Soil Mechanics, Delft of Technology.

Kadiyali, L.R. and Lal, N.B. (2004). Principles and Practices of Highway Engineering, Khanna Publishers, New Delhi.

Khanna, S.K. and Justo, C.E.G. (2001). Highway Engineering, Nem Chand and Bros., Roorkee.

Santosh, Kumar Garg (2001). Soil Mechanics and Foundation Engineering, Khanna Publishers, New Delhi.

subgrade soil investigation for designing of whitetopping...

Documents