Geotechnical and Foundation Engineering - short answer questions from AMIE exams (Winter 2019)

Differentiate between (any four) (4 x 5)

Active and Passive Earth Pressure

Active pressure is the condition in which the earth exerts a force on a retaining system and the members tend to move toward the excavation. Passive pressure is a condition in which the retaining system exerts a force on the soil.

General and Local Shear failure

General Shear Failure

• The general shear failure is characterized by rupture of the underlying soil followed by bulging of the soil surface around the footing.  
• In the field conditions, the soil is often heaved up on only one side of the footing followed by tilting of the structure.  
• The failure surfaces are well-formed as shown in the figure.
• A general shear failure occurs in soils that are in a dense or hard states.

Local shear failure:

• In local shear failure, there is significant compression of the soil under the footing and failure planes are not fully formed.
• This type of failure generally occurs in medium dense sand or clayey soil of medium consistency. In load settlement plot, there is no distinct point that defines the ultimate load.
• This type of failure mode may be considered as an intermediate or transitional phase between general shear and punching shear failure modes (discussed in the subsequent section).

Consolidation and Compaction

Compaction

• Compaction is a process where mechanical pressure is used to compress the soil mass for the purpose of soil improvement.
• Dynamic loads by rapid mechanical methods like tamping, rolling and vibration are applied for a small interval in soil compaction.
• In the compaction process, soil volume is reduced by removing air void from the saturated and dry soil.
• Compaction of soil is mainly used for sandy soil.
• Compaction is intentionally done to produce a high unit weight of soil and consequently improve other soil properties.

Consolidation

• Consolidation is a process where steady and static pressure causes compression of saturated soil.
• Static and sustained loading is applied for a long interval in soil consolidation.
• In the consolidation process, soil volume is reduced by squeezing out pore water from the saturated soil.
• Consolidation of soil is mainly used for clayey soil.
• Consolidation is a natural process where soil below the building and other structures are compacted by the transferred load to the soil through the provided foundation system.

Cantilever Retaining Wall and Counterfort Retaining Wall

Cantilever Retaining Wall

These are the reinforced concrete walls in which lateral earth pressure is resisted by the structural action of its members. The base of the wall is extended into the backfill on the heel side and is known as heel slab, as shown in the figure. The backfill over the heel slab provides significant additional lateral stability to the wall. The back of the wall on the heel side is also given a slope. This increases the width of the wall with depth, similar to the increase in lateral earth pressure with depth.

The vertical wall (known as the stem), the heel slab, and the toe slab act as cantilevers fixed at their junction and spanning to the other end. The stem is subjected to lateral earth pressure, causing bending away from the backfill. The heel slab and the toe slab are subjected to resultant upward soil pressure from the bottom and bend upward. Reinforcement is therefore provided on the tension side, that is, vertically on the backside of the stem and horizontally at the bottom of the heel slab and the toe slab.

Counterfort Retaining Wall

When the height of a cantilever retaining wall is more than about 7 m, it is economical to provide a vertical bracing system, known as counterforts, on the backfill side above the heel slab. The counterforts are triangular beams of variable depth and uniform width, connecting the heel slab and the stem, provided at regular spacing along the length of the wall, as shown in the following figure.

The stem and the heel slab act as continuous slabs spanning horizontally along the length of the wall between the counterforts. The use of counterforts reduces the bending moment due to earth pressure and hence the size and reinforcement of the stem and the heel slab. Counterforts are subjected to tension due to the action of lateral earth pressure of the backfill on the stem.

Net, Safe and allowable bearing capacity

Allowable bearing capacity

It may be defined as the net load intensity at which no failure occurs is called allowable bearing capacity.

Calculating the gross allowable load-bearing capacity of shallow foundations requires the application of the factor of safety (FS) to the gross ultimate bearing capacity.

q_allowable = q_ultimate/FS

Net ultimate bearing capacity

It is defined as the ultimate pressure per unit area of the foundation that can be supported by the soil in excess of the pressure caused by the surrounding soil at the foundation level.

If the difference between the unit weight of the soil and the concrete is negligible, then

q_net(u) = q(u) – q

Safe bearing capacity

It may be defined as the maximum Pressure that a soil bears without shear failure is known as safe bearing capacity.

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