STRENGTH TESTS

Strength tests are used to determine how soils resist applied loads and shearing forces. These tests provide critical input for foundation design, slope stability analysis, and retaining structure design. Different strength tests are suited to different conditions, for example short-term undrained behaviour or long-term drained behaviour. Strength testing usually requires high-quality samples, with intact samples being preferred, though disturbed samples can be used in some cases.

NOTE: An “[Exp]” prefix signifies the guidance, which is based on professional experience of practicing geotechnical engineers and GEOCompass creators.

'Baby' description:
baby-GC

“Twists a little paddle in the soil to see how much turning it takes to make the soil break apart.”

Applicability:

Cohesive soils.

Parameter Obtained:

Total stress shear strength [kPa] of very soft to soft cohesive soils.

Sample Quality:

Undisturbed.

(or disturbed for remoulded test).

Sample Size:
Scheduling Guidance:
  1. Best suited to soft, fine soils as a strength index only.
  2. Results tend to be higher than other methods due to a small volume tested.
Scheduling Guidance:

BS 1377-7 1990

BS5930:2015

'Baby' description:
baby-GC

“Squishes a small box of soil from the side until it breaks to see how strong it is.”

Applicability:

Cohesive and cohesionless soils (limited to particles <1/6 of the sample height)

Parameter Obtained:

Effective shear strength (c’ and ф’)

Residual strength parameters can be obtained.

Sample Quality:
Disturbed or Undisturbed.
Sample Size:

Sufficient for three test samples required for the test.

Fine- 1kg

Medium- 2kg

Coarse- 10kg

Scheduling Guidance:
  1. 60/100mm x 60/100mm and 20mm or 25mm high.
  2. Cannot be used for undrained tests.
  3. Normal pressure to be specified.

  4. [Exp] Always schedule ‘Shear strength of a set of three 60 mm x 60 mm square specimens by direct shear’

  5. [Exp] Advisable to schedule Atterberg Limits (for cohesive soil, on closest disturbed sample) and Particle Size Distribution (on closest bulk sample) alongside the tests; enables more complete interpretation of results

  6. [Exp] Need to specify:

    (1) normal stresses.  Should be related to the estimated stresses likely to occur in the field condition.  Suggest 0.5x, 1.0x & 2.0x or 1.0x, 2.0x & 4.0x total vertical in-situ stress but this should be dependent on the magnitude of proposed cuttings and embankments.

    (2) for cohesive soil, requirement for peak and residual shear strength

    parameters (usually peak and residual),

    (3) submerged or dry (dependent on field conditions)

Reference Standard:

BS 1377-7 1990

BS EN ISO 17892-10

'Baby' description:
baby-GC

“Squishes a big box of soil from the side until it breaks to see how strong a bigger bit of soil is.”

Applicability:

Cohesive and cohesionless

(limited to particles <1/6 of the sample height)

Parameter Obtained:

Effective shear strength (c’ and ф’)

Residual strength parameters can be obtained.

Sample Quality:
Disturbed or Undisturbed.
Sample Size:

Sufficient for three test samples required for the test

Circa 35kg.

Scheduling Guidance:
  1. 305mm x 305mm and 150mm high.
  2. Cannot be used for undrained tests.
  3. Normal pressure to be specified.

  4. [Exp] Opposed to the Small shear box just one sample/test is required.

  5. [Exp] See Small Shear Box Guidance.

  6. [Exp] The same soil may be re-used for the series of tests if it can be shown that no particle degradation occurs as a result of compaction and shearing,

Reference Standard:

BS 1377-7 1990

BS EN ISO 17892-10

'Baby' description:
baby-GC

“Spins rings of soil past each other a lot to find out how weak the soil gets after slipping many times.”

Applicability:

Cohesive and cohesionless soils (<1.18mm).

Parameter Obtained:

Effective remoulded shear strength ( ф’R). The c’R is assumed as zero.

Sample Quality:

Disturbed.

Sample Size:

>400g

Scheduling Guidance:

1) Normal pressure to be specified.

Reference Standard:

BS 1377-7 1990

'Baby' description:
baby-GC

“Squeezes a cylinder of soil from the ends until it squishes to find out how much push it can handle.”

Applicability:

Cohesive

Largest particle <1/6 of the sample diameter.

Parameter Obtained:

Undrained (total) shear strength of a cohesive soil.

Sample Quality:

Undisturbed (or disturbed for the remoulded strength).

Sample Size:

Cylindrical >34mm in diameter, or rectangular 1000mm2 in area.

Sample height 1.8-2.5 the diameter.

Scheduling Guidance:
  1. Typically, 38mm diameter samples, but up to 100mm can be tested.
  2. It is suitable only for saturated, non-fissured, fine grained soil.
Reference Standard:
BS 1377-7 1990
BS EN ISO 17892-7
BS5930:2015
'Baby' description:
baby-GC

“Squeezes a soil sample all over, without letting water out, to check how much pressure breaks it.”

Applicability:

Cohesive

Largest particle <1/6 of the sample diameter.

Parameter Obtained:

Undrained (total) shear strength of a cohesive soil – Undrained shear strength (Cu)

Sample Quality:

Undisturbed (recommended)

Sample Size:

Cylindrical >34mm diameter.

Sample height 1.8-2.5 the diameter.

Scheduling Guidance:

  1. [Exp] Always schedule ‘Undrained strength of a single 100 mm diameter specimen in triaxial compression without the measurement of pore pressure’

  2. [Exp] Need to specify:

    (1) requirement for single stage,

    (2) specimen diameter (nominal 100mm),

    (3) cell confining pressure (= vertical in-situ stress = sample depth x soil

    unit weight)

  3. [Exp] Advisable to schedule Atterberg Limits alongside the tests (on same U100 sample); enables more complete interpretation of results.

  4. [Exp] Can schedule remoulded tests.
  5. Multistage tests should only be scheduled when limitations of this test are fully understood.
Reference Standard:

BS 1377-7 1990

BS EN ISO 17892-8

BS5930:2015

'Baby' description:
baby-GC

“Lets the soil settle, then squeezes it without letting water escape, to see when it gives way.”

Applicability:

All soils, largest particle <1/6 of the sample diameter.

Parameter Obtained:

Effective strength parameters: c’ and ф’

Sample Quality:

Undisturbed (recommended).

Sample Size:

Cylindrical >34mm diameter.

Sample height 1.8-2.5 the diameter.

Scheduling Guidance:

  1. Typically undertaken on cohesive soils.

  2. Always specify CU test with measurement of pore pressure (100mm diameter single-stage test)

  3. [Exp]

    Need to specify:

    (1) requirement for single stage,

    (2) specimen diameter (nominal 100mm),

    (3) cell confining pressure

  4. [Exp] Advisable to schedule Atterberg Limits alongside the tests (on same U100 sample); enables more complete interpretation of results.

  5. [Exp] Can schedule remoulded tests.

Reference Standard:

BS 1377-8 1990

BS EN ISO 17892-8

'Baby' description:
baby-GC

“Lets the soil settle, then squeezes it gently, allowing water out, to see how much push it can really take.”

Applicability:

All soils, largest particle <1/6 of the sample diameter.

Parameter Obtained:

Effective strength parameters: c’ and ф’.

Sample Quality:

Undisturbed (recommended).

Sample Size:

Cylindrical >34mm diameter.

Sample height 1.8-2.5 the diameter.

Scheduling Guidance:
  1. Typically undertaken on cohesionless soils.
  2. [Exp] Need to specify:

    (1) requirement for single stage,

    (2) specimen diameter (nominal 100mm),

    (3) cell confining pressure.
  3. [Exp] Advisable to schedule Atterberg Limits alongside the tests (on same U100 sample); enables more complete interpretation of results.
  4. Can schedule remoulded tests.
Reference Standard:

BS 1377-8 1990

BS EN ISO 17892-8

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