Basic Soil Test On Sub-Grade Soil Material & Flexible Pavement Design

Project and Seminar Materials for Civil Engineering CE

Basic Soil Test On Sub-Grade Soil Material & Flexible Pavement Design


This case study is an existing pathway or fort path linking Ugwuji and Campus 111 (IMT) for a proposed road construction by the government of Enugu State.

This project encloses the following

  • The condition of the existing sub-grade and its suitability for use as a good sub-grade soil material by processes of different lab test.
  • Using the group index method and CBR method to design a flexible pavement on the pathway under consideration.
  • The optimum moisture content (OMC) and the maximum Dry Density (MDD) which are obtained by various laboratory test with a view of applying corresponding values in the field.

Table of Content

  • Title Page
  • Approval Page
  • Certification
  • Dedication
  • Preface
  • Acknowledgement
  • Table Of Contents

Chapter One

  • 1.0 Collection Of Sample And Laboratory Testing Program

Chapter Two

  • 2.0 Mechanical Analysis

Chapter Three

  • 3.0 Compacting Test

Chapter Four

  • 4.0 Atterberg Limit Test
  • 4.1 Liquid Limit
  • 4.2 Plastic Limit

Chapter Five

  • 5.0 Califorms Bearing Ration (Cbr) Test

Chapter Six

  • 6.0 Analysis Conclusion And Recommendation
  • 6.1 Flexible Pavement Design
  • Bibliography

Chapter One

Collection of Samples and Laboratory Testing Program

The test carried out in this project was on distributed soil sample.

This implies collecting soil samples that are disfigured their natural shape.

Soil samples were collected on three (3) trial pit along Ugwuaji and Campus 111 IMT Enugu. A total of three (3) samples were collected from the trial pits (one from each pit) using pick are and spade. The samples were taken at 1.0m depth in each pit. The samples were put in polythene bags and labelled A, B, C. The samples were spread on the floor of the soil laboratory and stones removed for seven (7) days to allow for air drying.

The laboratory testing program comprises mechanical analysis, compaction test and atterberg limit tests, and the California bearing ratio (CBR) test.

The objectives of the laboratory testing program were

  1. To obtain a general information regarding the nature of the soil and their variation with depth throughout the length of the road under consideration.
  2. To obtain information regarding the strength characteristics of the soil.
  3. To obtain information which will enable engineers to group soils according to their appearances of the purposes of complying different soils.

Chapter Six


From the analysis of test results on the sample, it was found that sample A and B are generally made up of fine grains with a greater percentage of it passing sieve No 200, while sample C is excellent and very good for pavement design.

Sample A was collected some metres into the pathway then sample was collected about half way into the pathway (CH A – B = 124.7m), but than sample C was collected at the end of the pathway very close to the express linking Ugwuaji (CH B – C = 331m).

Since the sample C part of the pathway is okay, burrow pit material could be collected from there to cover or blanket the entire sum of the pathway, but since material will not be enough, we now collected burrow pit material from the area where it would be enough to sum for the burrow pit material was based on particle size distribution, plastic limit liquid limit, compaction and CBR test to show that the sample is okay for pavement design. The essence of sub base is to help spread and reduce the losd distribution over the sub grade.

Conclusion and Recommendation

From the results of the tests, it was found that the samples A and B are generally composed of sility day, which does not allow easy drainage of water. It rather retained a lot of water, which result in the swelling of the soil with consequent collapse under load. Sample C was found to be excellent.

Further study shows that when the soils are stabilized by the mechanical stabilization method, their plasticity indices reduced this rendering the soil more stable with variations in its moisture contents.

The CBR values are consequently increased. Any type of soil can be stabilized with cement but much concern is attached on the economic implication of cement stabilization.

The stabilization with cement raises the CBR values of the soil, but from the economic point of view, unless where soil cement has an exceptional advantage, in the construction project in question. The mechanical stabilization method can always be used where it is found to be a good alternative since this will be cheaper in term of cost.
In the soil studied in this project based on experiments and past studies, it is concluded that sample C is good as sub – base material and good for pavement design.

The following recommendation are thus made regarding construction with stabilized materials.

  1. There should be adequate and full field control especially in the soil – cement construction.
  2. Careful compaction control.
  3. There should be adequate drainage of the road.
  4. Use a proper material for the base course and good placing of sealing cost to minimize water absorption from the surface.
  5. In embankments, the sloping should not be too steep. A slope of I vertical to 2 horizontal is recommendation.

Considering the current high cost of cement and the economy of construction, the use of cement for stabilization should be encouraged mostly when the percentage of it needed for stabilizing, the soil is not high and the use of soil cement material is very necessary – when the mechanical method is found to be a good substitute, it should be preferably employed.

Flexible Pavement Design

Several methods of designing a road pavement exists. We shall concenbate on the two international methods.

  1. The group index method (Daniel sketches)
  2. The CBR method
– The group index method (G.I)

The group index of a soil reflects the characteristics of the soil and is based on the sieve analysis and atterberg limit test.

G.I   =  0.29 + 0.005ac + 0.01bd

Where  a  = % passing sieve No. 200

35 La ³ 75 (1 – 40)

b  =   % passing sieve No. 200

15 < b ³ 55 (1 – 40)

C =   Liquid limit

40 < C ³ 60 (1 – 20)

The advantage of the G.I method is that the overall comparative meant of a given soil sample is readily ascertained.

The G.I method number is recorded to greater whole number.

Typical soil rating table

CBR (%)General rating Material
0 – 3Very poorMaterial
3 – 7Poor to fairSub – grade
7 – 20FairSub – grade
20 – 50GoodSub – grade
50 and aboveExcellentSub – grade

To design a suitable pavement using he CBR design chart

  1. CBR sub grade soil = 8.2%
  2. CBR sub – base material = 27%
  3. CBR base = 80%
  4. NO of heavy vehicle 2000 = 500
  5. Design life = 20 years
  6. Annual rate of increase = 7.5%

In heavy vehicles

The road is proposed to be completed in December 2004.


A = P (1 + r)n + 10
= 500 (1 + 75/100)4 + 10
= 500 (1 + 75/110)14
= 500 (1 + 075)14
= 500 (1.075)14

Corresponding to traffic intensity of 2380 of 1380, curve E of chart is used

  1. Over 8.2% CBR sub grade = 33cm
  2. Over 27% CBR sub base = 17cm
  3. Over 80% CBR base = 7cm

:. Depth of sub base – 33 – 17 = 16cm
Depth of base = 17 – 7 = 10cm
Depth of bituminous
Base course = 7/2 = 3.5cm

Depth of wearing

Surface = 7/2 = 3.5cm

Basic Soil Test On Sub-Grade Soil Material & Flexible Pavement Design

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