Geotech 2 conceptual

Immediate Settlement (S_e)

Elastic deformation of dry/moist soils without any change in moisture content

Primary Consolidation (S_c)

Drainage of water from voids leading to a volume change in the cohesive soil. Rate largely depends on the permeability of soil and drainage distance

Secondary Consolidation (S_s)

Plastic adjustment of cohesive soil fabric under a constant effective stress - creep movement

1-D Consolidation Test

Geotechnical test designed to capture the soil consolidation behaviors

Terzaghi’s Spring-Mass analogy

a perforated piston in a water-filled cylinder with internal springs provides a direct analogy for soil consolidation

Before Consolidation of Terzaghi (at t=0)

New Δσ load applied to saturated clay carried by pore water pressure and creating excess pore water pressure (Δu)

After Consolidation of Terzaghi (at t=∞)

effective stress (Δσ) increases, Δu approaches 0, particles reach settlement as water is expelled

Terzaghi theory assumptions for saturated clay soils

system is homogeneous, fully saturated, compressibility of water/soil grains is negligible, water flows in one direction, Darcy’s law is valid

Coefficient of consolidation methods

Logarithm-of-time method, Square-root-of-time method, and Hyperbola method

Coefficient of Consolidation (C_v)

Expresses how quickly excess pore-water pressure dissipates and settlement develops with time after loading

How does failure occur?

Shear stress along the failure surface reaches the shear strength

Soil element does not fail

if the Mohr circle is contained within the envelope

MC failure envelope for granular (non-cohesive) soils

MC failure envelope for Saturated, plastic clays

Types of Triaxial tests

CD, CU, and UU

CD Test

no excess pore pressure throughout the test, very slow shearing (multiple days long) to avoid build-up of pore pressure, and gives c’ and Φ’ for analyzing fully drained situations

CU Test

pore pressure develops during shear to measure σ’, gives c’ and Φ’ faster than CD test and is therefore more preferable for determining the c’ and Φ’ value

UU Test

pore pressure develops during shear but σ’ is not measured but rather unknown, Φ’ is 0 and the failure envelope is horizontal leaving us to analyze the test in terms of σ to give us c_u and Φ_u very quickly