Geotechnical and Foundation Engineering - short answer questions from AMIE exams (Winter 2020)

Briefly answer the following questions (2 x 5)

How stability number is utilized for slope stability analysis?

Taylor proposed a dimensionless parameter called Taylor stability number.

The stability number method is the method used to evaluate slope stability for homogeneous soils having cohesion.

It is based on the principle resistance of soil mass against sliding, because of cohesion and internal friction acting over the failure plane.

This failure surface is assumed to be a circular arc. The factors affecting the stability of soil slope is expressed with the parameter stability number. 

The stability number is given by, Sn = c/γFH

What do you understand by ‘Liquefaction of soil’?

A Phenomenon whereby a saturated or partially saturated soil substantially loses strength and stiffness in response to applied stress, usually earthquake shaking or other sudden change in stress condition, causing it to behave like a  liquid” is called Soil Liquefaction.

List four important factors that influence the compression behaviour of soils.

  • Moisture content  To achieve the desired density of the soil, the moisture content of that soil has to be controlled properly. If the water content is low, it leads to the soil being stiff which will resist compaction. When the water content is increased, lubrication takes place between the soil particles and the soil becomes more workable.
  • Types of soil  The soil type influences the compaction of that soil to a great extent. The coarse grained soils can be compacted to higher dry density than the fine grained soils. The maximum dry density decreases if the quantity of fines is increased to an amount more than that required to fill voids in the coarse grained soils. Hence it’s safe to say a well graded soil obtains a much higher dry density than a poorly graded soil.
  • Amount of compaction  The optimum water content required for compaction decreases with an increase in the compaction effort. This effect of increase in compaction is significant only until the water content reaches its optimum level. After that level, the volume of air voids becomes almost constant and the effect of increased compaction is not significant. 
  • Contact Pressure  Contact pressure is the pressure between the soil and wheels of the equipment used for compaction. This pressure depends on the weight of the roller wheel and the contact area. A higher contact pressure increases the dry density and lowers the optimum moisture content.

List the various uses of flow net.

  • To determine the amount of seepage and upward lift pressure below the hydraulic structure.
  • Once the flow net is constructed, the same one applies to flows at different discharges.
  • If the velocity of any reference point is known, the flow net gives velocity at any point by using the continuity equation
  • A flow net analysis assists in the design of efficient boundary shapes.
  • It helps in avoiding the separation and the points of stagnation in the design of boundary shapes.

Write two important limitations of the plate load test.

The limitations of the plate load test are:
  • It has a limited depth of influence. It could only give the bearing capacity of soils with depth up to two times the diameter of the plate.
  • It may not provide information on the potential for long term consolidation of foundation soils.
  • There is a scale effect as the size of the test plate is smaller than the actual foundation.
  • To gain access to the test position, excavation is carried out which causes significant ground disturbance. The change in ground stress leads to the change of soil properties which the test is planned to investigate.

Differentiate between the following (2 x 5)

Discharge velocity and seepage velocity

Discharge velocity is the volume of water flowing in unit time across the unit cross-section perpendicular to the direction of flow. Discharge velocity is often known as darcy's discharge velocity and is significantly lesser than the seepage velocity.

Seepage flow is the volume of water flowing in unit time across the unit surface area of available voids. Seepage velocity is the real velocity of water through the soil.


Shallow and deep foundation

Shallow Foundation
  • The depth of the shallow foundation is equal to or less than its width. 
  • In a shallow foundation, the load is transferred to the soil at a relatively small depth.
  • These foundations are constructed in open excavation. 
  • As the depth of foundation is less, visual inspection of construction work at any depth is possible. 
  • The disturbance of soil is negligible. 
  • Shallow foundations are adopted when soil near the surface is capable adequately to support loads of the structure.
Deep Foundation
  • The depth of the deep foundation is greater than its width.
  • In a deep foundation, the load is transferred to the soil at a relatively greater depth.
  • These foundations are constructed by boring or drilling with the help of mechanical arrangements.
  • Visual inspection of construction at greater depth is not possible.
  • Disturbance of soil extends to a large zone all along the length of the deep foundation. 
  • Deep foundations are adopted when soil near the surface is weak and hence loads need to be transferred at greater depths to suitable soil or rock.

Consolidation and compaction

  • Compaction is a process where a 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 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 structure compacted by the transferred load to the soil through the provided foundation system.

Active and passive 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.

Confined and unconfined soil sample.

Confinement in the soil means not allowing water to escape or application of pressure from all sides.

In shear strength tests for soil that is triaxial and unconfined tests soil is confined and unconfined respectively.

In the confinement of soil, there exists an additional confinement pressure from all sides. This pressure is provided as the soil is entrapped or covered with a rubber membrane it does not let soil move in a lateral direction which gives confined pressure.

There is no such membrane in the unconfined compression test therefore no confined stress.

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