Soil Mechanics and Foundation Engineering Key Concepts

Shear Strength

Soil's ability to resist sliding along a failure plane

Shear Strength Equation

τ = c + σ′ tan(φ)

Cohesion (c)

Stickiness of soil particles (mainly in clay)

Internal Friction Angle (φ)

Resistance due to particle interaction (mainly in sand/gravel)

Normal Stress (σ′)

Stress acting perpendicular to the failure plane

Clean Sand

Cohesion Value is 0 (no cohesion)

Effect of Normal Stress on Shear Strength

Increasing normal stress increases shear strength

Failure Plane

Diagonal plane along which soil fails

Failure Plane Angle Formula

θ = 45° + φ/2

Failure Plane in Sand

Angled (diagonal)

Failure Plane in Saturated Clay

Nearly vertical

Cohesive Soil

Soil with cohesion (e.g., clay)

Cohesionless Soil

Soil with no cohesion (e.g., sand, gravel)

Which Soil Has Higher Friction Angle

Sand and gravel

UU Test (Unconsolidated Undrained)

Rapid loading, no drainage, valves closed

CU Test (Consolidated Undrained)

Partial drainage before loading

CD Test (Consolidated Drained)

Slow loading, full drainage allowed

Triaxial Test for Rapid Loading

UU Test

Direct Shear Test Purpose

Determine shear strength and friction angle

Unconfined Compression Test Formula

c = qu / 2

Bearing Capacity

Maximum load soil can support

Dense Sand vs Loose Sand

Dense sand has higher bearing capacity

Effect of Water on Bearing Capacity

Reduces bearing capacity

Contact Pressure

Pressure between foundation and soil

Contact Pressure Formula

q = P / A

Effect of Area on Pressure

Smaller area = higher pressure

Shallow Foundation

Foundation less than ~10 ft deep

Deep Foundation

Foundation that transfers load to deeper soil layers

End-Bearing Pile

Load transferred to soil at the tip

Friction Pile

Load transferred through side friction

Pile Capacity Equation

Qult = Qfriction + Qtip

Retaining Wall

Main structure used to hold back soil

Retaining Wall Failure #1 Cause

Water

Retaining Wall Must Resist

Sliding, overturning, bearing failure

Best Way to Improve Retaining Wall Stability

Provide proper drainage

Gravity Wall

Uses its own weight to resist soil pressure

Cantilever Wall

Reinforced concrete wall, thinner design

MSE Wall

Modular panels (tile-like) with reinforcement strips

Backfill Material

Material placed behind retaining wall

Good Backfill

Granular, free-draining soil

Bad Backfill

Clay (holds water)

Factor of Safety (FoS)

Safety margin to prevent failure

Purpose of Factor of Safety

Ensure stability under uncertain conditions

Water in Soil Effect

Reduces strength and stability

1 psi to psf Conversion

1 psi = 144 psf

10 psi to psf

1440 psf

1 ft to inches

12 inches

1 square foot to square inches

144 in²

1 cubic foot to cubic inches

1728 in³

Caisson

Watertight chamber used for underwater construction

Drilled Shaft / Pier

Deep foundation drilled into ground

Vibratory Hammer

Equipment used to drive piles into soil

Uniform Settlement

Even settlement across a structure