Accurate determination of undrained shear strength for cohesive soils using Unconfined Compression (UCS), Triaxial UU, or Field Vane Shear Test. Essential for foundation bearing capacity, slope stability, and soft clay analysis.
Undrained shear strength (Su) represents the maximum shear stress a saturated cohesive soil can sustain under undrained conditions (no pore water drainage). It is a critical parameter for short-term stability analyses in soft clays, foundation design, embankments, and excavation support. For undrained conditions, the total stress friction angle φ=0, and cohesion c = Su. This concept, introduced by Skempton and Terzaghi, is fundamental in modern geotechnics.
These formulations are recognized by ASTM D2166 (UCS), ASTM D2850 (Triaxial UU), and ASTM D2573 (Vane Shear). The calculator implements these exact standards.
| Consistency | Undrained Strength Su (kPa) | Typical Engineering Behavior |
|---|---|---|
| Very soft | < 12 | Fist penetration, extrudes between fingers |
| Soft | 12 – 25 | Molded by light finger pressure |
| Firm | 25 – 50 | Moderate finger pressure required |
| Stiff | 50 – 100 | Indented by thumb with effort |
| Very stiff | 100 – 200 | Indented by thumbnail only |
| Hard | > 200 | Difficult to indent |
Unconfined Compression Test (UCS): A cylindrical clay specimen is loaded axially without confining pressure. Failure occurs when the deviator stress reaches a peak, giving qu. For saturated clays, the Mohr circle at failure has a radius Su = qu/2. This is the simplest index test for routine site characterization. However, it may overestimate Su for fissured or sensitive clays.
Triaxial UU Test: Specimen is saturated, consolidated under a cell pressure (σ3), then sheared rapidly without drainage. The measured deviator stress Δσd at failure gives Su = Δσd/2, independent of the cell pressure. This test provides more reliable strength parameters for clays with low permeability.
Vane Shear Test: A four-bladed vane is pushed into the soil and rotated at a constant rate. The maximum torque T is used to compute Su. The method is ideal for soft to firm clays in-situ and yields the undisturbed undrained strength. Our calculator uses the standard equation for a rectangular vane with both cylindrical and end shear contributions.
Engineers use Su to estimate bearing capacity (Nc·Su), lateral earth pressures, and stability numbers for embankments (Taylor’s stability charts). The value also appears in empirical correlations with SPT N-value and CPT tip resistance for fine-grained soils.
A highway embankment 4m high is to be constructed over a 6m layer of soft clay (Su = 18 kPa). Using the Taylor stability chart, the undrained stability number Ns = γH / Su = (17 kN/m³ × 4m) / 18 ≈ 3.78. For φ=0, this indicates potential failure. The design required prefabricated vertical drains (PVDs) and staged construction to increase Su via consolidation. Our calculator helps rapidly determine Su from field vane or lab tests to support such analysis.
| Soil type / consistency | Su range (kPa) | CPT qc / SPT N60 correlation | Reference |
|---|---|---|---|
| Very soft clay | <12 | qc < 0.3 MPa; N60 0–2 | Lunne et al. (1997) |
| Soft clay | 12–25 | qc 0.3–0.6 MPa; N60 2–4 | CPT in clays (Nkt ≈ 15–20) |
| Firm clay | 25–50 | qc 0.6–1.2 MPa; N60 4–8 | Bowles (1996) |
| Stiff clay | 50–100 | qc 1.2–2.5 MPa; N60 8–15 | Stroud (1974) |
| Very stiff clay | 100–200 | qc 2.5–4.0 MPa; N60 15–30 | Mayne (2007) |
| Hard clay | >200 | qc >4 MPa; N60 >30 | Various |