GEO-35

CPT undrained strength from qc

su = (qc − σv) / Nkt. Nkt ≈ 10–18 in clay.

Reading speed

In situRobertsonNkt

Governing equation

s_u=(q_c-\sigma_v)/N_{kt}

where

q_c: Cone resistance (MPa)
\sigma_v: Total overburden (kPa)
N_{kt}: Cone factor Nkt (—)
s_u: Undrained strength (kPa)

Lecture brief

Historical brief

Soil mechanics became a quantitative laboratory after Karl von Terzaghi’s 1925–1943 work on effective stress, consolidation and bearing. These sheets still size shallow foundations, retaining walls, piles and drainage in SI. This sheet (GEO-35 — CPT undrained strength from qc) is the form associated with Robertson · Nkt. Working symbols: $q_c$ , $\sigma_v$ , $N_{kt}$ $\rightarrow$ $s_u$ . A cone is a deep undrained bearing test. Nkt plays the role of Nc, calibrated on nearby su.

Purpose

Purpose: compute

s_u

from

q_c

\sigma_v

N_{kt}

in Geotechnical engineering via

s_u=(q_c-\sigma_v)/N_{kt}

su = (qc − σv) / Nkt. Nkt ≈ 10–18 in clay. Use it when a real geotechnical engineering question must be answered in SI before a code check.

Live realistic example

In symbols

Live case. Given

q_c = 1.200\,\mathrm{MPa}

\sigma_v = 80.000\,\mathrm{kPa}

N_{kt} = 14.000\,\mathrm{—}

, the governing relation

s_u=(q_c-\sigma_v)/N_{kt}

yields

s_u = 80.0\,\mathrm{kPa}

. A cone, a tip resistance, an su estimate. Move a slider: the numbers are this situation, not a canned story.

Calculator

Inputs

Cone resistance q_c1.2 MPaTotal overburden \sigma_v80 kPaCone factor Nkt N_{kt}14 —

Outputs

Undrained strength s_u80.0 kPa

Reading speed

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Narration of this film

A cone, a tip resistance, an su estimate.

A cone is a deep undrained bearing test. Nkt plays the role of Nc, calibrated on nearby su.

Reading speed

Watch on YouTube