Aspects of geotechnical engineering

ASPECTS OF GEOTECHNICAL ENGINEERING

 

1. SOIL DEPOSITS

During the planning, design, and construction of foundation, embankments, and earth-retaining structures, engineers find it helpful to know the origin of the soil deposit over which the foundation is to be constructed. It is because each soil deposit has its own unique physical attributes.

The primary mechanism of soil creation is the weathering of rock. There are two general types of weathering, viz., Mechanical weathering and Chemical weathering.

Mechanical weathering is the process by which rocks are broken into smaller and smaller pieces by physical forces including running water, wind, ocean waves, glacier ice, frost, and expansion and contraction caused by the gain and loss of heat.

Chemical weathering is the process of chemical decomposition of the original rock. In the case of mechanical weathering, the rock breaks into smaller pieces without a change in the chemical composition. However, in chemical weathering, the original material may be changed to something entirely different. For example, the chemical weathering of feldspar can produce clay minerals. Rock weathering is mostly a combination of mechanical and chemical weathering.

Soils as they are found in different regions can be classified into two broad categories, viz., (i) Residual soils and (ii) Transported soils. Residual soils exist at the same location as the rock from which they were formed. Decomposed granite is a common example for residual soil.

Generally, the depth of residual soils varies from 5 to 20 m. Residual soils show considerable variation of engineering properties form top layer to bottom layer. The transition observed is gradual. Relatively finer materials are found near ground surface and they become coarser with depth to reach larger fragment of stone.;

Transported soils form from weathered material deposits, which are transported by natural forces to a new site, away from the site of origin. The type of transport soil is determined by the agent, such as wind, water, ice or snow that assists in its transportation.

 

2. SOIL COMPACTION

In the construction of highway embankments, earth dams, and many other engineering structures, loose soils must be compacted to increase their unit weights. Compaction increases the strength characteristics of soils. Hence, it increases the bearing capacity of foundation constructed over them. Compaction also decreases the amount of undesirable settlements of structures and increases the stability of slopes of embankments.

Smooth-wheel rollers, sheep foot rollers, rubber-tired rollers, and vibratory rollers are generally used in the field for soil compaction. Vibratory rollers are used mostly for the densification of granular soils.

 

3. HYDRAULIC CONDUCTIVITY AND SEEPAGE

Soils have interconnected voids through which water can flow from points of high energy to points of low energy. A study on the flow of water through porous soil media is important in soil mechanics. It is necessary for estimating the quantity of underground seepage under various hydraulic conditions. It is required for investigation of problems involving the pumping of water for underground construction. It is also required for making stability analysis of earth dams and earth-retaining structures that are subjected to seepage forces.

 

4. STRESSES IN SOIL

When a building is constructed, its weight is transmitted to the ground through its foundation, thus inducing stresses in the underlying strata. These induced stresses might cause problems such as excessive settlement or shear failure and thus are important to geotechnical engineers. Stresses at a point in a soil layer are induced by:

(i) Self-weight of the soil layers (Geostatic stresses) and

(ii) Added load (such as buildings, bridges, dams, etc.)

The vertical geostatic stresses increase with depth. As the stress increases in the soil, the soil can be deformed by the stress.

Soils are multiphase systems. In a given volume of soil, the solid particles are distributed randomly with void spaces in between. The void spaces are continuous and are occupied by water, air or both. When a load is applied to soil, it is carried by the water in the pores as well as the solid grains.

 

5. COMPRESSIBLITY OF SOIL

A stress increase caused by the construction of foundation or other loads compress the soil layers. The compression of soil is caused by

1. Deformation of particles

2. Relocation of soil particles

3. Expulsion of water or air from the void spaces

In general, the soil settlement caused by load may be divided into three broad categories:

1) Immediate settlement: It is caused by the elastic deformation of soils without any change in the moisture content.

2) Primary consolidation settlement: It is the result of a volume change in soils because of the expulsion of water that occupies the void spaces.

3) Secondary consolidation settlement: It follows the primary consolidation settlement under a constant stress.

The total settlement of a foundation is the sum of above three settlements. When foundations are constructed on very compressible clay, the consolidation settlement can be several times greater than immediate settlement.

The leaning tower of Pisa is the classic example of differential settlement (Fig. 1). The failure is primarily due to the location of the tower and the instability of the ground underneath the foundation.

The weight of the tower compressed the ground beneath beginning with a seven meter fayer of silt and thirty meters of clay under the foundation. An area of clay under the south side of the structure is softer than its counterparts causing the infamous lean towards the south. Further depression of the clay below the foundation is a result of consolidation.

 

6. SHEAR STRENGTH OF SOIL

The shear strength of a soil mass is the internal resistance per unit area that the soil mass can offer to resist failure and sliding along any plane inside it. It is primarily derived from friction between the particles and interlocking. Engineers must understand the nature of shearing resistance in order to analyse soil stability problems such as bearing capacity, slope stability and lateral pressure on earth-retaining structures.

 

7. SLOPE STABILITY

An exposed ground surface that stands at an angle with the horizontal is called an unrestrained slope. The slope can be natural or constructed. If the ground surface is not horizontal, a component of gravity will cause the soil to move downward, as shown in Fig. 1. If the component of gravity is large enough, slope failure can occur; that is the soil mass in zone abcdea can slide downward. The driving force overcomes the resistance from shear strength of the soil along the rupture surface.

Civil engineers often are expected to make calculations to check the safety of natural slopes, slopes of excavations and compacted embankments. This check involves determining the shear stress developed along the most likely rupture surface and comparing it with the shear strength of the soil. This process is called slope stability analysis. The most likely rupture surface is the critical surface that has the minimum factor of safety.

Factor of Safety

The task of the civil engineer who analyses slope stability is to determine the factor of safety,

fs = qf / qd

where, qf, = average shear strength of the soil

qd = average shear stress developed along the potential failure surface

When fs is equal to 1, the slope is about to fail. The acceptable factor of safety is 1.5 for the design of a stable slope.

 

8. LATERAL EARTH PRESSURE

Retaining structures such as retaining walls and basement walls are commonly encountered in foundation engineering as they support slopes of earth masses. Proper design and construction of these structures require a thorough knowledge of the lateral forces that act between the retaining structures and the soil masses being retained. These lateral forces are caused by lateral earth pressure.

Lateral earth pressure is a function of several factors such as, (a) type and amount of wall movement, (b) shear strength parameters of the soil, (c) unit weight of the soil.

 

9. SUBSURFACE EXPLORATION

The purpose of subsurface exploration is to obtain information that will aid the geotechnical engineer in the tasks given below:

1. Selecting the type and depth of foundation suitable for a given structure.

2. Evaluating the safe load bearing capacity of the soil.

3. Estimating the likely settlement of the selected foundation and making allowance for the same in the design

4. Determining potential foundation problems (Expansive soil, collapsible soil, sanitary landfill and so on)

5. Determining the underground water level.

6. Evaluating the earth pressure against the walls, basements, abutments, etc. and to make provision against difficulties during construction.

7. Establishing construction methods for changing subsoil conditions

Subsurface exploration is also necessary for underground construction and excavation. It may be required when additions or alterations to existing structures are contemplated. It usually ... involves soil sampling and laboratory tests of the soil samples retrieved. The depth of exploration or a trial pit or bore hole will depend upon the characteristics of the soil as well as the type of structure, its shape, size and loading condition.

 

10. FOUNDATIONS - BEARING CAPACITY AND SETTLEMENT

The lowest part of a structure is generally referred to as foundation. Its function is to transfer the load of the structure to the soil on which it is resting. A properly designed foundation is one that transfers the load throughout the soil on which it is resting. It should ensure that load transfer does not overstress the soil. Overstressing the soil can result in either excessive settlement or shear failure of the soil, both of which cause damage to the structure.

The geotechnical and structural engineers who design foundations must evaluate the bearing capacity of soils. Depending on the structure and soil encountered, various types of foundations are used. Further details on requirements of foundations, foundation soils and types of foundation are provided in Chapter 5.