Geotechnical and Foundation Engineering - short answer questions from AMIE exams (Summer 2018)

Define the following (1 x 8)

Stress path

The stress path is used to represent the successive states of stress in a test specimen of soil during loading or unloading. A series of Mohr circles can be drawn to represent the successive states of stress, but it is difficult to represent the number of circles in one diagram. The following figure shows the number of Mohr circles by keeping σ3 and increasing σ1 on the σ  - t  plane. The successive states of stress can be represented by a series of stress points, and a locus of these points (in the form of straight or curve) is obtained. The locus is called the stress path. 

Quicksand condition

Quicksand condition is the floatation of particles of cohesionless soil, like fine gravel and sand, due to vertical upward seepage flow. As sand boiling occurs, the bearing capacity and shear strength of the cohesionless soil decrease and the agitations of soil particles become apparent.

Quicksand condition is not a type of soil, but a flow condition that occurs in cohesionless soils. Practically, boiling conditions may occur when excavations are made below the water table and water is pumped out from the excavation pit to keep the area free from water.

Partially saturated soils

The fully saturated soil contains only sand and water, Vw = Vv, hence the Degree of saturation S = 100%.

Site investigation

Site Investigation is the process of collecting information, assessment of the data and reporting potential hazards beneath a site that is unknown.

Site Investigation is grouped into the following stages:
  • Soil Investigation
  • Geologic survey maps.
  • Preliminary investigation
  • Detailed Site Investigation
  • Supplementary investigation and construction control.


Vibro-compaction is a ground improvement method that uses a specialized vibrating probe for in-situ subsurface compaction of loose sandy or gravelly soils at depths beyond which surface compaction efforts are effective.

The vibrating probe densifies loose granular, cohesionless soils by using mechanical vibrations and, in some applications, water saturation to minimize the effective stresses between the soil grains which then allows the oil grains to rearrange under the action of gravity into a denser state.

Generally, Vibro-compaction can be used to achieve the following enhanced soil performance or
  • Increased soil bearing capacity
  • Reduced foundation settlements
  • Increased resistance to liquefaction
  • Compaction to stabilize pile foundations driven through loose granular materials
  • Densification for abutments, piers and approach embankment foundations
  • Increased shear strength
  • Reduced permeability
  • Filling of voids in treated areas

Reinforced earth

It is a composite material formed by cohesionless soils and flexible metal reinforcing strips. The reinforcement and the earth are joined together through friction. One alternate arrangement to avoid the conventional arrangement will be the Reinforced earth wall. This method will be used to support soil embankments. This arrangement is useful to support the higher depths of soil embankment with minimized cost.

A typical section of a Reinforced earth structure is shown below.

Differentiate between the following (2 x 6)

Shallow and deep foundations

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.

Coulomb and Rankine's theories

Rankine Earth Pressure theory
  • In this, Rankine's considered the real, frictionless face of the retaining wall which is in contact with backfill.
  • Here retaining wall is only vertical
  • In this theory, elemental failure is considered.
  • We can find both active and passive earth pressure.
  • This is related to plastic equilibrium

Coulomb's Earth Pressure Theory
  • In this, Coulomb is considered a retaining wall with friction.
  • It may be vertical, inclined or both.
  • Here wedge failure is considered.
  • It is not suitable for the passive stage.
  • This is related to elastic equilibrium.

Ultimate and allowable pressures

Ultimate bearing capacity is the theoretical maximum pressure that can be supported without failure; allowable bearing capacity is the ultimate bearing capacity divided by a factor of safety. 

Confined and unconfined flows

  • A confined aquifer is an aquifer below the land surface that is saturated with water. Layers of impermeable material are both above and below the aquifer, causing it to be under pressure so that when the aquifer is penetrated by a well, the water will rise above the top of the aquifer.
  • A water table or unconfined aquifer is an aquifer whose upper water surface (water table) is at atmospheric pressure and thus is able to rise and fall. Water table aquifers are usually closer to the Earth's surface than confined aquifers are, and as such are impacted by drought conditions sooner than confined aquifers.

Dilation and contraction

  • A sample of a material is called dilative if its volume increases with increasing shear and contractive if the volume decreases with increasing shear.
  • Dilatancy is a common feature of soils and sands. Its effect can be seen when the wet sand around the foot of a person walking on the beach appears to dry up. The deformation caused by the foot expands the sand under it and the water in the sand moves to fill the new space between the grains.

Flocculent and dispersed structure of the soil.

  • In a flocculent structure, clay particles connect edge to edge or edges to face. These soils form due to attractive forces.
  • In dispersed structure, particles pint face to face and repulsive forces play an important role in the deformation of such soils. Dispersed structures develop in clays that have been remoulded.
  • Flocculent structure enclosed a large volume of voids whereas dispersed structure has relatively fewer voids.
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