CE 410 SOIL MECHANICS 1. SOIL AND SOIL MECHANICS (INTRODUCTION)

a. Definition of Soil Mechanics b. Development of Soil Mechanics HISTORICAL PERSPECTIVE

c. Fields of Application d. Soil Definition (agricultural and engineering) d. Soil Formation

e. Residual and Transported Soils f. Some commonly used Soil Designations g. Structure of Soils

h. Texture of Soils 2 COMPOSITION OF SOIL TERMINOLOGY AND DEFINITIONS

2.1 COMPOSITION OF SOIL (PHASE RELATION AND DEFINITION) Soil is a complex physical system. A mass of soil includes accumulated solid particles or soil grains and the void spaces that exist between the

particles. The void spaces may be partially or completely filled with water or some other liquid. Void spaces not occupied by water or any other liquid are filled with air or some other gas.

Fig (a) Actual soil mass, (b) Representation of soil mass by phase diagram

2.2 BASIC TERMINOLOGY

Figure: Soil-phase diagram (volumes and weights of phases)

Volumetric Ratios 1. Porosity of a soil mass is the ratio of the volume of voids to the total volume of the soil mass.

2. Void ratio of a soil mass is defined as the ratio of the volume of voids to the volume of solids in the soil mass.

3. Degree of saturation of a soil mass is defined as the ratio of the volume of water in the voids to the volume of voids.

4. Air void ratio of a soil mass is defined as the ratio of the volume of air voids to the total volume of the soil mass.

5. Air content of a soil mass is defined as the ratio of the volume of air voids to the total volume of voids.

Mass-Volume (or Weight-Volume) Relationships

6. Water content or Moisture content of a soil mass is defined as the ratio of the weight of water to the weight of solids (dry weight) of the soil mass.

7. Bulk unit weight or Mass unit weight of a soil mass is defined as the weight per unit volume of the soil mass.

8. Unit Weight of Solids is the weight of soil solids per unit volume of solids alone. It is also sometimes called the absolute unit weight of a soil.

9. The Saturated unit weight is defined as the bulk unit weight of the soil mass in the saturated condition.

10. The Submerged unit weight or Buoyant unit weight of a soil is its unit weight in the submerged condition.

( )

11. The Dry unit weight is defined as the weight of soil solids per unit of total volume; the former is obtained by drying the soil, while the latter would be got prior to drying.

12. The Mass specific gravity of a soil may be defined as the ratio of mass or bulk unit weight of soil to the unit weight of water at the standard temperature (4C).

This is also referred to as bulk specific gravity or apparent specific gravity. 13. The specific gravity of soil solids is defined as the ratio of the unit weight of solids (absolute unit weight of soil) to the unit weight of water at the standard temperature (4C).

2.3 CERTAIN IMPORTANT RELATIONSHIPS 14. Porosity

15. Void Ratio

16. S, e, w, Gs

17. Total Unit Weight

( )

18. Saturated Unit Weight

19. Dry Unit Weight

19. Submerged Unit Weight

20. Swell factor (SF) or free swell factor is the ratio of the volume of excavated material to the volume of in situ material (sometimes called borrow pit material or bank material):

2.3 EXAMPLES

DERIVING SOIL CONSTITUENT RELATIONSHIPS

EXAMPLE 1: Prove the following relationships: a.

b.

c.

( )

CALCULATION OF VOID RATIO AND POROSITY EXAMPLE 2: A container of volume 2.84x10

-3 m

3 weighs 9.81 N. Dry

sand was poured to fill the container. The container and the sand weigh 52.4 N. Calculate the following:

a. Void ratio b. Porosity c. Describe the condition of the soil (loose or dense). Assume

Gs = 2.70.

d. Dry density

CALCULATING SOIL CONSTITUENTS EXAMPLE 3: A sample of saturated clay was placed in a container and weighed. The weight was 6 N. The clay in its container was placed in

an oven for 24 hours at 105C. The weight reduced to a constant value of 5 N. The weight of the container is 1 N. Use Gs = 2.70. Determine the following:

a. Water content b. Void ratio c. Bulk unit weight

d. Dry unit weight e. Effective unit weight

CALCULATION OF WATER CONTENT OF AN UNSATURATED SOIL EXAMPLE 4: The void space in a soil sample consists of 80% air and 20% water. The dry unit weight is d = 15.8 kN/m

3 and Gs = 2.7.

Determine the water content.

DETERMINATION OF AGGREGATE REQUIREMENT FOR ROADWAY EXAMPLE 5: Aggregates from a material storage site are required for

the embankment of a roadway. The porosity of the aggregates at the storage site is 80%, and the desired porosity of the compacted aggregates in the embankment is 20%. For a section of the

embankment 7.6 m wide X 0.65 m compacted thickness X 315 m long, calculate the volume of aggregates required.

APPLICATION OF SOIL CONSTITUENT RELATIONSHIPS TO A PRACTICAL PROBLEM EXAMPLE 6: An embankment for a highway is to be constructed from

a soil compacted to a dry unit weight of 18 kN/m3. The clay has to be

trucked to the site from a borrow pit. The bulk unit weight of the soil in the borrow pit is 17 kN/m

3 and its natural water content is 5%. Calculate

the volume of clay from the borrow pit required for 1 cubic meter of embankment. The swell factor is 1.2 (20% free swell). Assume Gs = 2.7.

APPLICATION OF SOIL CONSTITUENT RELATIONSHIPS TO A PRACTICAL PROBLEM EXAMPLE 7: If the borrow soil in Example 6 were to be compacted to

attain a dry unit weight of 18 kN/m3 at a water content of 7%, determine

the amount of water required per cubic meter of embankment, assuming no loss of water during transportation.

ASSIGNMENT NO. 2 SOIL COMPOSITION Discuss/Illustrate/Explain/Enumerate:

a. Overview of the Mineralogy b. Silicates c. Silicate Frameworks and Composition of Granular Soils

d. Building Blocks of Clay Minerals e. Types of Bonds f. Common Clay minerals with two sheets per layer

g. Common Clay minerals with three sheets per layer h. Relationship between soil composition and Engineering

Properties of Soil

ASSIGNMENT NO. 3 PHASE-RELATIONSHIPS Solve the following problems:

1. For a given soil, show that

2. For a given soil, show that

(

)

3. For a given soil, show that

( )

4. A 0.4-m3 moist soil sample has the following:

Moist mass = 711.2 kg Dry mass = 623.9 kg

Specific gravity of soil solids = 2.68 Estimate: a. Moisture content

b. Moist density c. Dry density d. Void Ratio

e. Porosity 5. In its natural state, a moist soil has a volume of 0.33 ft

3 and

weighs 39.93 lb. The oven-dry weight of the soil is 34.54 lb. If Gs = 2.67, calculate the moisture content, moist unit weight,

dry unit weight, void ratio, porosity, and degree of saturation. 6. The moist weight of 0.2 ft3 of a soil is 23 lb. The moisture

content and the specific gravity of the soil solids are determined in the laboratory to be 11% and 2.7, respectively. Calculate the following:

a. Moist unit weight (lb/ft3)

b. Dry unit weight (lb/ft3)

c. Void ratio

d. Porosity e. Degree of saturation (%) f. Volume occupied by water (ft

3)

7. The saturated unit weight of a soil is 19.8 kN/m3. The

moisture content of the soil is 17.1%. Determine the following: a. Dry unit weight

b. Specific gravity of soil solids c. Void ratio

8. The unit weight of a soil is 95 lb/ft3. The moisture content of

this soil is 19.2%. when the degree of saturation is 60%. Determine: a. Void ratio

b. Specific gravity of soil solids c. Saturated unit weight

9. For a given soil, the following are given: Gs = 2.67; moist unit

weight, = 112 lb/ft3; and moisture content, w = 10.8%.

Determine: a. Dry unit weight b. Void ratio

c. Porosity d. Degree of saturation

10. Refer to Problem 9. Determine the weight of water, in pounds,

to be added per cubic foot of soil for: a. 80% degree of saturation b. 100% degree of saturation