18  Consistence

Consistence is a general term for the field-assessed behaviour of soil materials in response to applied stress over a range of soil-water states from wet to dry. The consistence properties described are strength, failure, fluidity, penetration resistance, packing, sensitivity, induration, plasticity, stickiness, dilatancy, and dry coherence.

Consistency tests should be performed down-profile, with at least one sample tested per horizon. Samples should be taken from the mid-point of each horizon. Additional samples in transition zones may also be of interest.

Note 18.1: Consistence tests and the New Zealand Soil Classification (NZSC)

For classifying using the NZSC, particular tests down to a certain depth may be required. At the order level:

• Gley soils: must not have any moderately or very fluid (Section 18.10) layers within 30 cm of the soil surface. Unconfined soil strength (Section 18.2.1) and cementation (Section 18.11) tests may be required to confirm absence of an underlying fragipan.
• Ultic soils: need an assessment of soil strength (Section 18.2.1) in the B horizon under moist conditions. May need assessment of failure mode (Section 18.9) over a range of moisture conditions to rule out oxidic or cutanoxidic horizons. May need assessment of sensitivity (Section 18.3) and stickiness (Section 18.5) to rule out allophanic soil materials.
• Podzols: may need unconfined strength (Section 18.2.1) and failure mode (Section 18.9) tests to help identify a Podzolic B horizon, as well as the presence of humus- and ortstein-pans.
• Allophanic soils: may need assessment of sensitivity (Section 18.3) and stickiness (Section 18.5) to help confirm allophanic soil materials.
• Melanic soils: may require stickiness assessment (Section 18.5) in the B horizon.
• Semiarid soils: may require cementation tests (Section 18.11) to confirm fragipan absence.
• Oxidic soils: May need assessment of ped strength (Section 18.2.2), soil strength (Section 18.2.1), and failure mode (Section 18.9) over a range of moisture conditions to confirm presence of an oxidic horizon. Stickiness (Section 18.5) and plasticity (Section 18.4) may also be helpful.
• Granular soils: May need assessment of ped strength (Section 18.2.2), soil strength (Section 18.2.1), and failure mode (Section 18.9) over a range of moisture conditions to confirm presence of a cutanoxidic horizon. Stickiness (Section 18.5) and plasticity (Section 18.4) may also be helpful.
• Pallic soils: may require cementation tests (Section 18.11) to confirm fragipan presence. May require assessment of failure mode (Section 18.9) to help confirm Brittle-B horizon presence. May require ped strength assessment in the B horizon.
• Recent soils: must not have any moderately or very fluid (Section 18.10) layers within 30 cm of the soil surface.

18.1 Preparing for consistence assessment

The nature of the soil materials in each horizon determines what tests can and should be done. The first step is to determine if the sample is cohesive.

• Cohesive soil materials may have plasticity and stickiness characteristics and have a strength which can be measured in unconfined compression. All listed properties, except particle packing, can be described for non-gravelly, cohesive soils.
• Non-cohesive soil materials have no shear resistance other than that developed by internal friction between grains. Non-cohesive soil materials, by definition, have no unconfined dry strength or plasticity, and the only property that can be meaningfully described is degree of packing or particle packing.

The range of properties which can be described in gravelly, cohesive soils varies with gravel content. In practice, this depends on the feasibility of obtaining samples for the required tests.

The distinction between cohesive and non-cohesive is recognised by performing moulding, plasticity, and dilatancy tests in the following sequence (Figure 18.1). Where doubt still remains after these tests have been completed, the dry coherence test (Section 18.1.1) may be necessary.

flowchart TD
  q1["Can a sample cube be formed from remoulded material?"] -->|yes| q2["Is the material plastic?"]
  q2 ---->|yes| a1(["**Cohesive**"])
  q1 -->|no| q3["Is the material plastic?"]
  q3 -->|yes| q4
  q2 -->|no| q4["Does the material have a fast positive reaction to the dilatency test?"]
  q4 -->|no| q5["Does the material have a distinctly negative reaction to the dilatency test?"]
  q4 --->|yes| a2
  q3 ---->|no| a2(["**Non-cohesive**"])
  q5 -->|yes| a1
  q5 -->|no| a2

Figure 18.1: Determining sample cohesion

18.1.1 Dry coherence

Procedure

1. Remould a 30 mm test sample into a cube, and air-dry.
2. If the cube collapses under its own weight or after being lightly touched, treat the soil as non-cohesive.

18.2 Strength

Strength is also known as ‘resistance to crushing’. This is assessed using hand or body-weight strength, so the user must calibrate themselves against the classes in Table 18.1. Drohan et al. (2020) discuss various options for calibration, including the use of pinch gauges or push-pull gauges to learn what different levels of force feel like.

Each of the tests in this section are evaluated against Table 18.1.

Table 18.1: Strength (resistance to crushing) classes

Code	Name	Failure condition	Force required (N)
1	Very Weak	Very gentle force in hand	< 8
2	Weak	Gentle force in hand	8-20
3	Slightly firm	Moderate force in hand	20-40
4	Firm	Strong/maximum force in hand	40-75
5	Very firm	Gentle force underfoot	75-150
6	Hard	Full body weight (~80 kg) applied slowly	150-800
7	Very hard	Withstands full body weight	> 800

18.2.1 Unconfined soil strength

• Test at observed field moisture condition.
• If a test cube cannot be formed, the result is 1 ‘Very Weak’ (see Table 18.1). This can happen when soil pedality is weak or absent, but also when pedality is strong and peds are small.
• This test cannot be reliably performed on auger-extracted samples.

Procedure

1. Perform the test on a minimally disturbed 30 mm cube.
2. Hold the cube between the thumb and forefinger. Both digits must be straight and fully extended.
3. Pinch down. If the cube can be crushed, rate the difficulty using categories 1-4 in Table 18.1.
4. If the cube cannot be crushed by hand, attempt to crush it slowly underfoot, against a hard surface. Rate the difficulty using categories 5-7 in Table 18.1.

18.2.2 Unconfined aggregate strength

• Test at observed field moisture condition.
• Test on whole peds where they are under 50 mm in size. Larger peds should be shaved down to 30 mm.
• This test cannot be reliably performed on auger-extracted samples.

Procedure

1. Perform the test on an extracted ped.
2. Hold the ped between the thumb and forefinger. Both digits must be straight and fully extended.
3. Pinch down. If the ped can be crushed, rate the difficulty using categories 1-4 in Table 18.1.
4. If the ped cannot be crushed by hand, attempt to crush it slowly underfoot, against a hard surface. Rate the difficulty using categories 5-7 in Table 18.1.

18.2.3 Remoulded strength

Some soils lose strength to a notable degree when remoulded, and their apparent clay content as estimated with hand texturing increases. This has implications for engineering.

• Test at observed field moisture condition.
• Perform the test on an extracted 30 mm cube or sample of equivalent volume.
• This test can be reliably performed on auger-extracted samples.

Procedure

1. Perform the test on an extracted 30 mm cube or sample of equivalent volume.
2. Place the sample in a small plastic bag and seal with the air excluded (this maintains field moisture content while remoulding). Work the sample until its structure is completely broken down. This may take several minutes.
3. Still in the bag, remould the soil into a cube and assess its strength against Table 18.1.
4. Use Table 18.2 to also record the ratio of unconfined to remoulded strength (‘sensitivity’, Section 18.3).

18.2.4 Strength at plastic limit

Soil strength at the plastic limit has engineering implications.

• Test at the soil’s plastic limit.
• Perform the test on an extracted 30 mm cube or sample of equivalent volume.
• This test can be reliably performed on auger-extracted samples.

Procedure

1. Work the sample until its structure is completely broken down. This may take several minutes. Keep adding small amounts of water so that the sample is almost, but not quite, saturated.
2. Remould the soil into a cube and assess its strength against Table 18.1.

18.3 Sensitivity

In soils with an unconfined strength class of 4 firm or greater, sensitivity is reported as the ratio of unconfined soil strength (Section 18.2.1) to remoulded strength (Section 18.2.3). For weaker soils, the ‘shear test’ is performed. The two methods are assigned equivalent classes in Table 18.2.

• Test soil at a moist or wet state.
• Perform the test on a minimally disturbed 30 mm sample cube.
• This test cannot be reliably performed on auger-extracted samples.

Procedure

1. Hold the sample cube between extended thumb and forefinger as for Section 18.2.
2. Pinch and press the sample to create a lateral, ‘shear’ force, moving the finger forward and down over the thumb.
3. Assess the sample behaviour against Table 18.2.

Note that this test is not analogous to the shear vane test of New Zealand Geotechnical Society (2005) and cannot be used as a substitute.

Table 18.2: Sensitivity (strength loss on remoulding) classes

Code	Name	Relative strength loss on remoulding (classes)	Shear test behaviour
1	Non-sensitive	0-1	the sample does not smear under shear force, and may soften only slightly
2	Weakly sensitive	2	the sample smears and skids under moderate shear force (20-40 kPa)
3	Moderately sensitive	3	the sample suddenly turns fluid under moderate shear force (20-40 kPa) and the fingers skid. Some free water is evident on fingers after shearing
4	Very sensitive	>3	the soil suddenly turns fluid and slippery under only gentle shear force (<20 kPa). Free water is easily seen on the fingers after shearing

18.4 Plasticity

Plasticity is the capacity of soil material to change shape continuously when stress is applied and to retain that shape after the removal of stress.

• Test soil at a wet state.
• Perform the test on 3-4 cm² of soil.
• This test can be reliably performed on auger-extracted samples.

Procedure

1. Thoroughly remould the sample, keeping it in a wet state.
2. Adjust the water content to slightly above the plastic limit. In practice this water content is slightly above that at which a 4-mm diameter roll of sample crumbles but less than that when sample becomes sticky (Section 18.5), if stickiness is exhibited.
3. Attempt to roll the sample out into a rod of 40 mm length. Assess the minimum achievable diameter of the rod and its behaviour using Table 18.3.

Table 18.3: Plasticity classes

Code	Name	Description
1	Non-plastic	A roll 40 mm long and 6 mm thick that supports its own weight cannot be formed.
2	Slightly plastic	A roll 40 mm long and 6 mm thick can be formed and will support its own weight, but a roll 4 mm thick will not support its own weight.
3	Moderately plastic	A roll 40 mm long and 4 mm diameter can be formed, and will support its own weight, but a roll 2 mm thick will not support its own weight.
4	Very plastic	A roll 40 mm long and 2 mm thick can be formed and will support its own weight.

18.5 Stickiness

Stickiness is the degree to which remoulded soil adheres to foreign objects.

• Test soil at a wet state.
• Perform the test on 3-4 cm² of soil.
• This test can be reliably performed on auger-extracted samples.

Procedure

1. Thoroughly remould the sample, keeping it in a wet state.
2. Press the remoulded material between thumb and forefinger, and note its adherence to the fingers.
3. Adjust the water content by working the sample in the hand to remove water, or by adding water as necessary to achieve the maximum stickiness that the material will exhibit
4. Assess maximum stickiness and record using Table 18.4.

Table 18.4: Stickiness classes

Code	Name	Description
1	Non-sticky	After release of pressure, practically no soil material adheres to the thumb or forefinger
2	Slightly sticky	After release of pressure, soil material adheres perceptibly to both thumb and forefinger but, as the digits are separated, it tends to come off one or the other cleanly. The material is not stretched appreciably when the thumb and finger are separated
3	Moderately sticky	After release of pressure, soil material adheres to both thumb and forefinger and tends to stretch somewhat and pull apart rather than pulling free from either digit
4	Very sticky	After release of pressure, soil material adheres so strongly to both thumb and forefinger that it is decidedly stretched when they are separated, and finally pulls apart leaving part of the soil on thumb and part on the forefinger.

18.6 Dilatency

Dilatancy occurs due to volume change during shearing and is used as a criterion for the distinguishing non-cohesive from cohesive soil materials.

• Test soil at a moist to wet state.
• Perform the test on 3-4 cm² of soil.
• This test can be reliably performed on auger-extracted samples.

Procedure

1. Adjust the soil to a moist to wet state.
2. Roll into a ball ~30 mm diameter.
3. Hold sample in upturned palm and slap the back of the hand containing the sample several times with the other hand.

A dilatant reaction is shown by a shiny surface caused by water extrusion, and by a dull surface when the palm of the hand is folded to partly remould the sample, causing water to be drawn into the sample. Record dilatent behaviour as a binary, true/false.

18.7 Penetration resistance

Penetration resistance is the capacity of the soil, in its natural confined state, to resist penetration by rigid objects. This property is commonly related to strength by empirical means (Table 18.5).

• This test requires a large pit or exposure

Procedure

1. Expose a soil profile.
2. Assess ease of penetrating a vertical face using Table 18.5.

Table 18.5: Penetration resistance classes

Code	Name	Description	kpa
1	Extremely low	Easily penetrated by fist	0-500
2	Very low	Easily penetrated by thumb	500 - 1000
3	Low	Readily indented by thumb but penetrated with considerable effort. Imprinted by finger with some effort.	1000 - 1500
4	Moderate	Readily indented by thumbail	1500-2200
5	High	Barely imprinted by thumb or forefinger	2200 - 3000
6	Very high	Indented by thumbnail. No imprint by thumb or finger	3100 - 4000
7	Extremely high	Barely indented by thumbnail	> 4000

18.8 Particle Packing

Particle packing assesses the stability of a gravelly soil.

• Test only on gravelly soils where strength cannot be assessed
• This test requires a large pit or exposure
• Do not test at wet or saturated field moisture conditions, as this is unsafe

Procedure

1. Expose a gravelly soil profile.
2. Assess ease of excavating and maintaining a vertical face using Table 18.6.

Table 18.6: Particle-packing classes for gravelly soils

Code	Name	Description
1	Loose	Easily dislodged by spade or pick; does not maintain a vertical face
2	Compact	Intermediate between loose and dense
3	Dense	Difficult to dislodge except with spade and by removing individual fragments; maintains a stable vertical face
4	Very dense	Very difficult to dislodge except by pick and removing individual fragments; maintains a stable vertical face

18.9 Mode of failure

Failure is the manner in which unconfined soil yields to increasing stress.

• Test at observed field moisture condition.
• Perform the test on a minimally disturbed 30 mm cube or aggregate.
• If diagnosing Oxidic or Cutanoxidic soil material for the NZSC, repeat the test under dry to wet moisture conditions.

Procedure

1. Perform the unconfined strength test according to Section 18.2.1.
2. At strengths of 4 or less, mode of failure can be observed. Assess and record per Table 18.7.

Table 18.7: Failure mode classes

Code	Name	Description
N	Not determinable	Soil is either too dry or too wet to reliably test
V	Very friable	Test specimen cannot be formed, or crumbles under very slight stress on crushing within the hand, into aggregates predominantly <2 mm in size. In most instances the test specimen is difficult to obtain.
F	Friable	Test sample cannot be formed, or crumbles under very slight stress on crushing within the hand, into aggregates predominantly >2 mm in size, or crumbles under slight stress on crushing within the hand into aggregates predominantly <2 mm in size.
B	Brittle	Under slowly increasing pressure between the extended thumb and forefinger, a test specimen retains its size and shape with few or no cracks until it fractures abruptly into aggregates predominantly > 2 mm in size.
S	Semi-deformable	Under increasing pressure between the extended thumb and forefinger the test specimen as a whole is perceptibly compressed in the direction along which pressure is exerted, but cracks develop and the specimen ruptures before it has been compressed to half its original thickness. Before rupture, parts crumble and fall away if the specimen is shaken while held between the thumb and forefinger.
D	Deformable	Under slowly increasing pressure between the extended thumb and forefinger, the test specimen can be compressed to at least half its original thickness without the appearance of cracks and without rupture.

18.10 Fluidity

Fluidity tests whether saturated soils have an unconfined strength that is largely eliminated by saturation. Such materials were generally emplaced under water, have never been drained, have a fine texture that minimises gravity-based settling, and are not overlain by enough other sediment to have been compressed.

• Test soil at a saturated state.
• Only test soils that are saturated under field conditions.
• Perform the test on ~50 cm² of soil (a large handful).
• This test can be reliably performed on auger-extracted samples.

Procedure

1. Extract the sample and squeeze firmly in the hand.
2. Evaluate the sample behaviour using Table 18.8.

Table 18.8: Fluidity classes

Code	Name	Description
1	Slightly fluid	Some material tends to flow into the spaces between the fingers, but after full pressure is exerted most of the residue is left in the hand
2	Moderately fluid	Soil material flows easily between the fingers, but a small residue is left in the hand after full pressure is exerted
3	Very fluid	Soil material flows like a slightly viscous liquid between the fingers, and very little or no residue is left in the hand after full pressure is exerted

18.11 Induration

Induration is the degree to which soil particles are held together by some combination of cementation and close packing.

• Perform the test on a minimally disturbed 30 mm cube or aggregate.
• This test may also be performed on individual nodules and other solids < 30 mm.
• This test cannot be reliably performed on auger-extracted samples.
• This test requires a minimum one hour to complete so may be done immediately following fieldwork.

Equipment

• Sponge (~50 mm diameter, flat)
• Deionised (DI) water

Procedure

1. Test the soil strength at field-moist condition according to Section 18.2.1.
2. Place the sample cube on a wet sponge until thoroughly moistened, then immerse the sample in DI water for 1 hour.
3. Retest the soil strength of the wet sample according to Section 18.2.1.
4. Assess the behaviour of the sample using Table 18.9.

Table 18.9: Induration classes

Code	Name	Description
1	Non-indurated	A wet test specimen slakes within one hour when placed in water
2	Very weakly indurated	A wet test specimen that has been immersed for one hour and has not slaked has strength class of 2-4
3	Weakly indurated	A wet test specimen that has been immersed for one hour has a strength class of 6
4	Strongly indurated	A wet test specimen that has been immersed for an hour has a strength class of 7
5	Very strongly indurated	A wet test specimen after immersion for an hour has a strength class of >7 and does not break when struck by a sharp blow with a hammer.

Drohan PJ, Thompson JA, Lindbo DL, Beaudette DE, Dadio SD 2020. Redefining the fragipan to improve field recognition and land use relevance. Soil Science Society of America Journal [Internet]. 84(4):1055–1066. https://doi.org/10.1002/saj2.20098

New Zealand Geotechnical Society 2005. Field description of soil and rock: Guideline for the field classification and description of soil and rock for engineering purposes. Burns D, Farquar G, Mills M, Williams A, editors. Wellington, New Zealand: New Zealand Geotechnical Society, Inc.