Geotech Interview questions

 

               Geotechnical Engineering Basic Interview Questions

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1.What are the basic principles of foundation design?

1. The foundation must be stable against shear failure of the supporting soil. 

2. The foundation must not settle beyond a tolerable limit to avoid damage to the structure. 

2.How do you determine the bearing capacity of soil?

The ultimate bearing capacity , or the allowable soil pressure, can be calculated either from bearing capacity theories or from some of the in situ tests. 

The most popular bearing capacity calculation methods are 

1. Terzaghi's bearing capacity theory 

2. The general bearing capacity equations

3. Field tests

According to Terzaghi's bearing capacity theory the soil's ultimate bearing capacity is dependent on density  , cohesion, angle of friction of soil, shape, breadth and depth of footing. It also depends on the depth of water table..

Field tests:

There are few popular field tests to derive the bearing capapcity of soil like

i) Standard Penetration Test(SPT)

ii) Plate load test  (PLT)

iii) Cone penetration Test (CPT)

iv)Vane shear test( for clay soil)

Field Test	Purpose	Procedure / Measurement	Bearing Capacity Estimation	Remarks
Standard Penetration Test (SPT)	Indirect estimate of soil strength	Split-barrel driven into soil; count blows (N-value) for 30 cm penetration	Sand: ( qu ≈ 50 N ) kPa (dense sand) Clay: (qu ≈ 10 N ) kPa (stiff clay)  ( qa = q u / FOS (3 usually)	Simple, widely used; suitable for granular & cohesive soils; correlations are empirical
Plate Load Test (PLT)	Direct measurement of bearing capacity & settlement	Rigid plate at foundation level; apply incremental load; measure settlement	Ultimate bearing capacity = (Load at failure ÷ Plate area)	Most reliable field test; gives actual settlement behaviour ; requires space & equipment
Cone Penetration Test (CPT)	Continuous profile of soil resistance	Cone pushed into soil at constant rate; measure cone resistance (q_c) & sleeve friction (f_s)	Allowable bearing capacity: ( qa = qc / Ns ), where ( Ns ) depends on soil type	Fast, continuous data; better for sand & silt; less disturbance
Vane Shear Test	Measures undrained shear strength of soft clay	Four-blade vane rotated in soil; torque measured	( qu = cu x Nc )     (cu = undrained shear strength;       Nc ≈ 6–9)	Best for soft clays; simple & portable; not suitable for sand
Pressure meter Test	Measures in-situ stress-strain response	Cylindrical probe expanded in borehole; pressure vs. expansion recorded	Ultimate bearing capacity calculated from pressure  meter modulus & limit pressure	Accurate; works for all soil types; requires specialized equipment

3.How much eccentricity is allowed in load on footing?

A maximum of B/6 eccentricity is allowed in load application on footing where B is breadth of footing. Which means a 100 cm of eccentricity allowed in a footing of 600 X 1000 cm size..

4.What is the difference between compaction and consolidation? 

Compaction is a quick process of expelling air from the soil by mechanical means(like vibrators, rollers, rammers) to increase the dry density of soil which in turn improves the shear strength of the soil and imparts better resistance to settlement . It is suitable for cohesionless soils like sand and gravel. Compaction ultimately improves the load taking capacity of soil or in other terms it improves the bearing capacity.

Compaction increases soil density → increases shear strength → increases bearing capacity.    

Consolidation is a natural process which expels pore water from the soil over a long period of time there by increasing effective stress of soil. 

Consolidation increases effective stress → increases shear strength → improves bearing capacity, especially in cohesive soils, but it takes time to fully develop.

5.What is liquefaction and when does it occur?

Liquefaction is a phenomenon in which saturated soil temporarily loses its strength and behaves like a liquid when subjected to sudden stress or vibration.

• The soil particles are loose and water-saturated.

• Under shaking or sudden load, pore water pressure increases, and effective stress drops to nearly zero.

• As a result, the soil cannot support structures, causing sinking, tilting, or lateral spreading,

Liquefaction occurs when loose, saturated soils are suddenly shaken or loaded, causing loss of strength and fluid-like behavior. Earthquakes are the most common trigger.

6.What is the importance of Atterberg limits?

Atterberg limits are super important in soil engineering because they help us understand how fine-grained soils (like clay and silt) behave under different moisture conditions. Basically, they tell us how soil changes from solid → semi-solid → plastic → liquid as water content increases.

Purpose	Why It’s Important

 

1. Soil Classification

Helps classify cohesive soils (using systems like ISCS or USCS) based on Plasticity Index and Liquid Limit. This tells us if soil is low plasticity clay, high plasticity clay, silt, etc.

 

2. Predicts Soil Behavior

Indicates how soil will behave in the field when moisture changes — shrinkage, swelling, stickiness, etc.

 

3. Determines Plasticity & Workability

Engineers need to know if soil can be molded (like for embankments) or if it becomes too soft or brittle.

 

4. Identifies Expansive Soils

A high Liquid Limit and large Plasticity Index indicate expansive clay, which can cause cracking in buildings, roads, and foundations.

 

5. Helps in Construction Decisions

Determines whether a soil is suitable for compaction, foundation support, road subgrade, or landfill liner.

 

6. Used for Quality Control

Atterberg limits help compare natural soil with modified or stabilized soil (e.g., lime or cement treated).

 

If PI is high → soil is highly plastic and unstable when wet (bad for foundation without treatment)

If PI is low → soil behaves more like silt, low strength, poor compaction

If LL is very high → soil may swell or shrink dramatically with moisture changes

7.What are expansive soils? How do you treat them?

They are soils with high clay content, especially montmorillonite clay, that:

• Absorb water → expand (heave)

• Lose water → shrink (settle, crack)

This movement can damage:

• Foundations

• Pavements

• Floors

• Retaining walls

They cause differential settlement, which is a structural engineer’s nightmare.

Why They Expand?

Because of:

• High plasticity

• High shrink–swell index

• High liquid limit and plasticity index

• Define permeability. What factors affect it?

• How do you design a retaining wall?

• What is soil stabilization? List various techniques.

• Explain seepage and flow nets.

The expansive soils can be treated with

1. Chemical Stabilization

Mix with:

• Lime (most common, reacts with clay → reduces plasticity)

• Cement

• Fly ash

• GGBS

• Polymers (modern)

Reduces swelling and increases strength.

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2. Replacement

 The expansive soil/clay can be removed  (top 1–2 m usually) and replaced with:

• Sand

• Gravel

• Non-expansive soil/Muram

This is for small sites usually because it’s expensive for big ones.

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3. Pre-wetting / Moisture Conditioning

Flood the area before construction so the soil expands early. Then build when it's at “full swelling” so it won’t expand later.

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4. Soil Reinforcement

• Geotextiles

• Geogrids

• Fibres
  These reduce cracks and improve load-carrying capacity.

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6. Deep Foundations

If the soil is too stubborn to fix:

• Pile foundations

• Drilled piers

• Under-reamed piles (specifically for expansive soils)

Under-reamed piles are like anchors—they resist uplift from swelling soil.

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7. Provide Flexible Structural Elements

Design tweaks like:

• Flexible pavements

• Raft foundations

• Isolated footings with gaps (These help accommodate movement.)

8. Explain the difference between bulk density, dry density, saturated density, and submerged density?

Bulk Density: It is the weight of soil (solids + water + air) per unit volume in its natural/moist state.

                                                                 γ=(Ws​+Ww)/V       

  where γ= Bulk density, Ws = Weight of soil, Ww= Weight of water in the pores,                                         V= Volume​​

​​Dry Density: It is the weight of only soil solids per unit volume. It represents soil with no water. Dry density is generally used to evaluate compaction quality. 

                                                                  γd​=Ws/V

Saturated Density:​ It is the unit weight when all voids are filled with water. No air present in the pores of the soil. It represents worst-case heavy condition of the soil, when soil loading needs to be considered.

                                                              γsat​=(Ws​+Ww(voids full))/V

Submerged Density:​  It is the effective unit weight of soil when fully submerged in water. It accounts for buoyancy, so the soil “feels lighter”. It is used in bearing capacity, slope stability, and earth pressure calculations when soil is below groundwater level.

                                                           γsub​=γsat​−γw

9. What is meant by well-graded and poorly graded soils?

A well-graded soil contains a good range of particle sizes (from large to small) with no abrupt gaps.

A poorly-graded soil has either:

• particles of nearly the same size (uniformly graded), or

• missing certain size ranges (gap-graded).

Gradation strongly affects soil engineering properties:

Well-graded soils

• Dense packing
•  Higher shear strength
•  Lower permeability
• Less compressibility
• Good for foundations, embankments, pavements

Poorly-graded soils

•  Loose packing
• Lower shear strength
•  Higher permeability
• More compressible
•  Problematic for construction if not compacted properly

Uses of Soil Gradation:

a. Road construction

Well-graded soil → better compaction and stability.

b. Embankments & dams

Well-graded soil gives higher strength and lower voids.

c. Filters & drainage layers

Poorly-graded sand (uniform sand) is good for filtering because of:

• high permeability

• stable void structure

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