A consolidated-undrained triaxial test, usually abbreviated CU triaxial, examines how a saturated soil specimen responds to axial compression after a prescribed consolidation stage while drainage is prevented during shear. If you are trying to read a CU triaxial graph, deviator stress q, pore-pressure response, principal effective stresses or a p′–q stress path, start by identifying which pressure was controlled, which pressure was measured and whether the graph uses total or effective stresses. These quantities can move in different directions.
The important distinction is between consolidation before shearing and the undrained shearing phase. During consolidation, drainage is allowed under a chosen stress state; during subsequent undrained compression, excess pore-water pressure can develop. Measuring that pore pressure makes it possible to calculate effective stresses and understand why the same total loading path can produce different soil-skeleton behaviour. A plotted failure point is not automatically an allowable bearing pressure for a real foundation.
This guide takes the reader through triaxial apparatus, back pressure, saturation checks, consolidation, undrained loading, stress corrections, q, p′, Mohr circles and critical interpretation decisions. Worked calculations use clearly hypothetical values and state their assumptions. The purpose is to translate a specialist laboratory result accurately, not to prescribe construction or replace design by a qualified engineer.
Quick answer: what does a CU triaxial result describe?
A specimen of cohesive soil is enclosed in a triaxial cell, commonly subjected to a confining stress, consolidated under specified conditions and then loaded axially without allowing drainage during shear. The apparatus measures axial force and deformation, while appropriate systems monitor cell and pore pressures. The test can provide stress–strain behaviour, pore-pressure response, and, with suitable calculations, effective-stress information.
The result applies to a particular specimen, saturation quality, consolidation state and strain rate. It can inform engineering models when the field stress path and drainage assumptions are comparable. It does not reproduce every in-situ failure mechanism, soil fabric or spatial variability. Readers should preserve the specimen and test labels alongside any reported strength parameter.
Why the word consolidated comes first
Consolidation is the preparation stage in which the specimen is permitted to drain under an applied stress while volume changes and excess pore-water pressures dissipate according to the method. The chosen consolidation stress influences void ratio, structure and subsequent shear response. A CU specimen is therefore not simply ‘a wet soil squeezed rapidly’; its earlier stress state is part of the result.
Two otherwise similar specimens consolidated to different pressures can develop different strengths and pore-pressure responses in subsequent undrained shearing. If the report lists one strength value without its consolidation history, it is missing crucial context. Compare specimens only after identifying effective consolidation stresses and how each was achieved.
Undrained means a boundary condition, not no water
In the undrained shearing stage, water is not permitted to leave or enter the specimen through the controlled drainage path. Water remains inside the pores, and its pressure may rise or fall as the soil skeleton tends to contract or dilate. The specimen may still experience deformation; undrained is not another word for rigid or incompressible.
The actual quality of undrained conditions depends on equipment, saturation, membrane behaviour, and strain rate. Leakage or partial drainage can alter results. A report should state the intended boundary and support it through the testing method rather than treating a closed valve as proof that all material behaviour followed a perfect theoretical idealisation.
Understanding the cell and drainage lines
A typical triaxial apparatus has a cylindrical soil specimen surrounded by a flexible membrane, a pressure chamber, end platens and drainage connections. Cell pressure applies confinement. Axial loading produces additional deformation and changes the difference between principal stresses. Pore-pressure measurement and back-pressure systems help establish and monitor the hydraulic state.
The presence of several pressure values on a data sheet can be confusing. A cell-pressure set point, back pressure and measured pore pressure are not necessarily the same variable. Label each line before performing arithmetic. Readers should not subtract all printed pressures indiscriminately; the calculation depends on how the apparatus references pressure and what is being reported.
Saturation quality changes effective-stress interpretation
CU tests used to determine effective-stress parameters commonly aim for highly saturated specimens. Air in pores changes compressibility of the pore fluid system and can affect measured pore-pressure response. Saturation is usually assessed through a defined procedure rather than assumed because the sample felt wet. The B parameter, often assessed from a change in pore pressure following an isotropic cell-pressure increment under undrained conditions, is one conventional indicator.
A high B response may support adequate saturation for the test, but the method’s criteria and soil behaviour still matter. A low response does not always reveal one simple problem; specimen stiffness and equipment effects can influence interpretation. Report actual saturation checks rather than adding a generic ‘fully saturated’ claim to a graph.
Total stress and effective stress must not be mixed
In the familiar saturated-soil effective-stress framework, effective principal stress equals total principal stress minus pore-water pressure, when the model and pressure reference apply. Total stress reflects the applied mechanical state; effective stress concerns the stress carried by the soil skeleton. They are closely related but not interchangeable.
Imagine a specimen with total major principal stress σ1 of 300 kPa, total minor principal stress σ3 of 150 kPa, and pore pressure u of 80 kPa at an interpreted point. Effective major stress is 220 kPa and effective minor stress is 70 kPa. The difference σ1′ minus σ3′ is 150 kPa, the same as the total principal-stress difference under this simple isotropic pore-pressure subtraction. Their mean stresses differ.
Deviator stress q is a stress difference
In a conventional triaxial compression representation, deviator stress q equals major principal stress minus minor principal stress. It is often reported in kPa. In an axisymmetric triaxial specimen where the axial stress is major and lateral stresses are equal, q describes the additional difference associated with axial compression.
Using the illustrative stresses 300 and 150 kPa, q equals 150 kPa. That does not mean the specimen experienced a total major stress of only 150 kPa. Nor does q alone tell us the effective confinement or pore pressure. A graph of q versus axial strain is a useful part of the story but not a complete stress-path description.
Mean effective stress p′ gives another coordinate
Under an axisymmetric triaxial state, a common mean effective stress definition is p′ equals (σ1′ + 2σ3′) divided by three. The corresponding total mean stress p equals (σ1 + 2σ3) divided by three; if the pore-pressure contribution is isotropic, p′ equals p minus u. These definitions differ from the alternate s′ variable sometimes used in geotechnical stress-path charts.
With σ1′ equal to 220 kPa and σ3′ equal to 70 kPa, p′ equals (220 + 140) divided by three, or 120 kPa. If a paper instead uses s′ equal to (σ1′ + σ3′) divided by two, the same point has s′ equal to 145 kPa. Confusing those axes makes different papers seem contradictory even though both may be correct within their conventions.
What the p′–q stress path shows
A p′–q graph traces a specimen’s evolving effective mean stress and deviator stress during shearing. Unlike a single failure-strength number, it shows how stress changes over strain under the test conditions. The direction and shape can be influenced by pore-pressure development, confinement, fabric and whether the specimen tends to contract or dilate.
Check whether the curve starts at the consolidated effective state and whether q is plotted vertically or horizontally. Some authors use q–p′ ordering in captions but different axis placement. The labels decide the interpretation. A reader should describe a path from one measured or calculated point to another before assigning mechanistic explanations.
Why pore pressure can rise during undrained shear
A soil skeleton that tends to contract during loading may develop positive excess pore-water pressure when drainage is prevented, in a saturated idealisation. The total applied stress does not necessarily decrease, but increased u reduces the mean effective stress. This can move a stress path leftward in a conventional plot with p′ increasing to the right.
The relationship is not a universal rule for every soil. Different density, structure, overconsolidation and stress paths can lead to different pore-pressure responses. The appropriate wording is conditional: under the reported test, positive excess pore pressure was measured as shear developed. Do not claim that the pressure increase alone proves one specific microscopic mechanism without additional evidence.
Why pore pressure can fall in a dilative response
Some soils tend to expand in volume as particles rearrange under shear. When drainage is prevented in a highly saturated specimen, that tendency can produce reduced or negative excess pore-water pressure relative to the starting state. Mean effective stress may then rise or follow a different path even as deviator stress increases.
A negative excess pressure does not automatically mean the absolute pore pressure is below atmospheric or that the soil has become unsaturated. It describes a change relative to a reference point unless otherwise stated. Always identify whether the figure shows total pore pressure, back-pressure-referenced pressure or an incremental Δu. This distinction prevents wrong conclusions about suction or drainage.
Positive and negative pressure depend on the reference
Suppose the end-of-consolidation measured pore pressure is 200 kPa because a substantial back pressure was applied. During shearing it rises to 240 kPa. The excess pore-pressure increase is positive 40 kPa, not 240 kPa. If the same pressure then falls to 185 kPa, the increment relative to the end-of-consolidation level is negative 15 kPa.
The three numbers can all be physically meaningful, but they answer different questions. A laboratory sheet may record absolute gauge readings while an interpretive plot shows excess pore pressure. Converting between them requires a stated reference. Readers should not treat a positive gauge pressure as evidence of positive excess pressure without checking the baseline.
Volumetric strain and axial strain are not synonyms
During isotropic consolidation, the specimen may change volume as water drains. During the undrained shearing phase, a saturated idealisation may undergo relatively little total volume change while its axial dimensions and shape change substantially. Axial strain describes change in specimen length; volumetric strain describes change in total volume.
A q–axial-strain curve does not directly show volumetric strain. If a paper reports volume change during a nominally undrained stage, inspect membrane compliance, saturation and test setup. Conversely, a CD test may record volume change explicitly during drained shear. The strain label and test phase must travel with the plotted value.
Force measurements become stress after area correction
An axial load cell records force. To obtain the axial stress increment, the force is divided by a representative specimen cross-sectional area under the method’s geometry correction. As a specimen shortens, its cross-section may change. A test report that uses an initial area at large strains without appropriate correction can distort calculated stress.
For a hypothetical force of 120 newtons applied over 1,200 square millimetres, the nominal stress is 0.1 newton per square millimetre, equal to 100 kPa. If corrected area differs from the initial area, the result changes. This simple example demonstrates why a line labelled force in newtons cannot be read as a stress in kilopascals without a geometric conversion.
The axial strain rate influences undrained conditions
A test requires a rate that serves its intended stress path while maintaining the desired drainage boundary and allowing suitable pore-pressure measurement. Too fast a deformation rate can introduce equipment-related errors; too slow a test may allow unintended drainage if the system is imperfect. The standard and the material properties guide the choice.
Different laboratories may use different rates based on the consolidation properties of the soil. A report should identify axial strain rate, specimen dimensions and method. Comparing strength from one rate with another as though time plays no role can obscure important behaviour, especially in fine-grained materials sensitive to strain rate and structure.
The stress–strain curve is not automatically a material constant
A q-versus-axial-strain curve can show initial stiffness, nonlinearity, a peak and sometimes a post-peak decline. The interpretation of ‘failure’ may depend on the observed peak, a defined strain criterion or another documented rule when no sharp peak exists. A curve without an obvious maximum does not license inventing a peak.
Young’s modulus or secant stiffness calculated from a segment also depends on the stress and strain range and the particular loading path. It is not safe to use one curve-derived modulus as a universal field stiffness. Sample disturbance, confinement and drainage alter the apparent response. Give the chosen strain level or tangent definition whenever a stiffness estimate appears.
Failure envelopes require multiple tests
A CU experiment on one specimen under one consolidation condition provides one stress–strain path and potentially a failure point under that state. Estimating a Mohr–Coulomb envelope for a soil commonly requires several specimens at different consolidation stresses and carefully interpreted failure points. A single point cannot uniquely determine both effective cohesion intercept and friction angle.
This distinction is easy to miss in a short report table that lists c′ and φ′ with excessive precision. Check the number of stress states, failure definition and fitting method. If a parameter pair comes from correlations or assumptions rather than direct multi-specimen fitting, it should be labelled accordingly. A neat line is not independent evidence if only one point was measured.
Mohr circles connect principal stresses
A Mohr circle describes stress states graphically using normal and shear stress axes. For a triaxial failure state, a circle can be drawn using major and minor principal stresses. Its centre is half their sum, and its radius is half their difference. One may draw total-stress or effective-stress circles, but they must be labelled distinctly.
Using total principal stresses 300 and 150 kPa, the centre is 225 kPa and the radius is 75 kPa. Using effective values 220 and 70 kPa, the centre is 145 kPa with the same 75 kPa radius under the common pore-pressure subtraction. The circle translates horizontally, illustrating why pore pressure changes effective confinement without changing the principal-stress difference in this idealised example.
Total and effective failure interpretations serve different contexts
The same CU test can be reported in total-stress and effective-stress terms, provided the measurements and model support both. Total-stress interpretations can be useful for conditions approximated as undrained, whereas effective-stress strength parameters may be used in analyses where pore pressures are explicitly considered. The correct framework depends on the field loading and drainage problem.
Do not say that one set is universally stronger or more accurate. An effective-stress Mohr envelope requires good pore-pressure measurement and appropriate saturation, while a total-stress approach still needs a suitable undrained strength and stress-path assumption. Using c′ and φ′ in a total-stress calculation without adjusting the model can be an error even if the numbers were accurately measured.
The friction angle is a fitted parameter
For a simplified Mohr–Coulomb representation, shear resistance along a plane can be described using effective normal stress, an effective cohesion intercept and the tangent of a friction angle. A curve fitted to multiple triaxial failure states may yield interpreted c′ and φ′. Their values can depend on which failure condition, range of confinement and specimens were selected.
Neither intercept nor angle is directly read from one axial load measurement. The model is a practical approximation to material behaviour that can change with stress, strain, fabric and drainage. If the test shows strong curvature across a large stress range, a single straight envelope may poorly represent every point. Readers should distinguish fit quality from the numerical precision of reported parameter values.
Undrained shear strength is also conditional
An undrained strength su is often discussed in short-term stability problems involving fine-grained soils. Yet the value obtained depends on stress history, loading mode, rate and other conditions. A UU triaxial test, CU test and field vane test do not necessarily return identical strengths even for nominally similar samples.
The soil field vane shear guide describes one field test and its limitations. Use it as a comparison of methods, not as an interchangeable source of the CU specimen’s failure stress. A sound report says which apparatus and stress path produced the stated su rather than treating ‘undrained strength’ as one invariant property.
A short stress-path calculation
Imagine a specimen with fixed total confining stress σ3 = 150 kPa. During shearing, the total major principal stress rises from 150 to 300 kPa and measured pore pressure rises from 50 to 80 kPa. At the later point, q equals 300 minus 150 = 150 kPa. Total p equals (300 + 2 × 150) divided by three, or 200 kPa.
Effective p′ equals total p minus pore pressure, so it equals 200 minus 80 = 120 kPa. At the starting point, if total principal stresses were both 150 kPa and pore pressure 50 kPa, p′ was 100 kPa and q was zero. This hypothetical path moved from (100,0) to (120,150) on a p′–q chart. A different pore-pressure history could give a different horizontal trajectory despite the same total applied stress.
Why the same q can have a different p′
Suppose a second otherwise illustrative test reaches the same total principal stresses of 300 and 150 kPa but pore pressure is 130 kPa instead of 80 kPa. The deviator stress is still 150 kPa, because the equal pore pressure subtracts from both principal stresses. Mean effective stress, however, is 200 minus 130, or 70 kPa.
This is a crucial diagnostic result. Two points can share the same q while having effective mean stresses 120 and 70 kPa. Their different p′ values imply different effective confinement and may change how they relate to a strength envelope. Reporting only the peak q would hide the role played by pore pressure in the specimens’ shear response.
A note on Skempton pore-pressure parameters
Some geotechnical analyses describe undrained pore-pressure response using coefficients such as A and B under specified loading assumptions. B is associated with response to an isotropic confining pressure increment, often considered during saturation assessment. A relates an increment of pore pressure to changes in principal-stress difference within a particular formulation.
The values can depend on material state and loading path, and their exact definitions should be shown before comparison. An ‘A at failure’ reported by one laboratory does not mean a universal physical constant for every overconsolidation state. Readers should check whether parameters refer to total or excess pressures, what increments were used and whether the test sequence makes the adopted equations appropriate.
Initial pore pressure is not excess pore pressure
The total pore pressure at a point can include an imposed back pressure and changes produced during shear. The excess pressure usually refers to change from a selected reference state, such as the completed consolidation stage. Conflating these values shifts effective-stress calculations and can reverse conclusions about contractive or dilative tendencies.
A simple audit is to write the end-of-consolidation total pore pressure in one column, the shearing-time total pressure in another, and the difference in a third. Only then compute current effective stress using the appropriate actual total pore pressure. Do not subtract an excess increment from total stress as though it represented the entire u value without adding the baseline where required.
Stress sign conventions are part of the calculation
Geotechnical engineering often treats compression as positive, but other scientific disciplines may adopt different conventions. A strain–stress graph can therefore appear reversed if transferred into a different sign system without explanation. Principal stresses in a triaxial compression test also have an ordering convention; make sure σ1 is the major and σ3 is the minor principal stress.
For the introductory compression framework, q = σ1 − σ3 is nonnegative when σ1 is major. Some alternative definitions of mean and shear invariants use different normalisations. Copy the source’s definitions into explanatory notes. A symbol such as q does not automatically refer to the same numerical convention in every geomechanics paper.
Critical-state terminology should be defined carefully
Critical-state soil mechanics describes idealised conditions where a material can continue deforming in shear at approximately constant stress and, under the chosen framework, without continued volume change. In a p′–q representation, an idealised critical-state line may be expressed as q = M p′ for a suitable material and stress range.
A laboratory curve approaching a steady-looking region at finite strain is not conclusive proof that a full critical state was reached. Strain localisation, sample geometry, boundary friction and test duration matter. If a paper estimates M from a point, ask whether the state satisfied the required assumptions. A fitted straight line may be useful, but its interpretive boundary belongs beside the equation.
Peak and critical strength answer different questions
Some soils show a peak shear resistance followed by softening; others harden over the measured strain range. Critical or large-strain descriptions concern another stage and may be influenced by localisation and specimen constraints. A peak q at five percent axial strain is not numerically identical to a strength chosen at fifteen percent by a reporting rule.
When comparing studies, align failure definitions. A test that ended at a specified strain without a visible peak may report a stress at that strain rather than a true maximum. Avoid treating the highest measured value as an observed global maximum if the test stopped before the curve levelled off. Reporting definitions matter as much as measurement accuracy.
Overconsolidation changes the response
An overconsolidated clay has experienced a stress history that differs from its current reference effective stress under an appropriate interpretation. This can affect stiffness, volume-change tendency and pore-pressure response during undrained loading. Highly overconsolidated materials may show different stress paths from normally consolidated materials.
The oedometer consolidation guide explains one way stress history is inferred from an e–log curve. Do not substitute an oedometer-derived OCR directly for a triaxial consolidation state without considering sample disturbance and relevant definitions. The tests complement each other but use distinct stress paths and boundary conditions.
Why axial strain does not reveal pore pressure alone
A q–strain curve can rise and then flatten, while the pore pressure rises, falls or changes direction. Strain is deformation of the specimen, not a direct indicator of fluid pressure. Two test curves with similar mechanical shape may have very different pore-pressure histories and effective-stress paths.
If a paper includes both q versus strain and Δu versus strain, read them together at matched strain values. This is often more informative than quoting a failure stress alone. Note the pressure reference and axial strain sign. When multiple lines are displayed, verify that colour and symbol assignments refer to the same specimen and consolidation stress in both panels.
Rate effects and imperfect undrained boundaries
Undrained loading is an idealised hydraulic boundary condition. In practice, tubing compliance, tiny leaks, membranes, trapped air and rate selection can change the measured response. A high-quality report describes how pore pressure was monitored and how the testing system was checked. The mere label CU should not replace methodological evidence.
The governing test method requires attention to load, deformation, pore pressure and saturation quality. The official ASTM D4767-11(2020) description states the standard’s purpose and relevant measured quantities. That published scope supports using the method in this article, but a short web summary is not a substitute for the full official laboratory procedure.
Distinguish the CU and CD tests
A consolidated drained triaxial test, often abbreviated CD, allows drainage during shear under sufficiently slow loading to limit excess pore-pressure effects. It can directly follow volume change as shearing proceeds. A CU test, in contrast, prevents drainage during shear and measures pore-pressure response when equipped appropriately.
The active ASTM D7181-20 consolidated drained triaxial test description explains the drained objective. A comparison of CU and CD strengths needs awareness of time scales, specimen histories and stress paths. Their letters are not arbitrary grades of quality: they name deliberate differences in boundary conditions to match distinct engineering questions.
Direct shear is another distinct test geometry
A direct shear box imposes displacement along or near a predetermined plane, while a triaxial specimen can develop shear deformation in a different stress field and failure mechanism. Drained direct shear tests are often used to estimate shear resistance across a prescribed interface or plane under defined normal stresses. They do not simply reproduce triaxial compression with a cheaper apparatus.
When comparing results, ask which soil direction, normal stress, strain or displacement criterion and drainage condition were studied. A parameter from one geometry should not automatically replace a parameter from another in a different design problem. Understanding the test boundary is part of understanding the strength number.
What a field condition must share with the test
A laboratory test becomes relevant to an engineering problem when the sample represents the field material and the loading, consolidation and drainage assumptions reasonably match the field mechanism under consideration. A short-term embankment loading scenario may justify attention to undrained response; a slowly drained long-term problem may require different parameters or analyses.
However, neither scenario can be solved from a single CU curve without subsurface investigation and qualified judgement. Spatial variability, anisotropy, stress rotation, fissures, structure and scale can affect field strength. A textbook test idealises a tractable piece of behaviour, not a complete construction site.
Interpreting a total-stress Mohr circle with one load state
For the illustrative total principal stresses σ1 = 300 and σ3 = 150 kPa, the Mohr-circle centre is (300 + 150)/2 = 225 kPa, while the radius is (300 − 150)/2 = 75 kPa. The largest shear stress represented by this circle in the idealised two-dimensional stress transformation is its radius, or 75 kPa.
The number 75 is not automatically the undrained shear strength of the soil in every application. It describes this circle under one measured state. A triaxial failure interpretation must also identify whether this was the selected failure point and whether other stresses or specimens support the broader parameter. Geometry explains the plot; it does not replace the engineering inference.
An effective-stress circle translates along the normal axis
With pore pressure u = 80 kPa, the effective principal stresses are 220 and 70 kPa. Their circle centre is (220 + 70)/2 = 145 kPa, and the radius remains 75 kPa. Compared with the total circle, the effective circle has moved 80 kPa left on the normal-stress axis while retaining radius.
This simple relationship helps learners understand why equal isotropic pore pressure modifies effective confinement but not the principal-stress difference. The statement depends on the simplified effective-stress subtraction and correct pressure reference. If the soil is partially saturated or the stress model differs, additional definitions are needed rather than mechanically copying the saturated formula.
A two-specimen interpretation exercise
Imagine two specimens reach the same q of 150 kPa but at p′ values of 120 and 70 kPa respectively. Which has the higher deviator stress? Neither; q is equal. Which has the higher effective mean stress? The first. Can we conclude the first soil is categorically stronger? No. Those numbers alone do not provide complete failure envelopes, stress histories or comparable specimen conditions.
This exercise teaches a basic reading skill: say exactly what each coordinate supports and resist ranking a material using one variable. A valid engineering comparison would need the context, full stress paths, failure rules and potentially additional tests. Accurate answers often depend more on limiting claims than on advanced arithmetic.
Diagnose axes before reading a stress-path chart
Start with the horizontal axis: p′, p or s′? Check units, definitions and sign. Then read the vertical axis: q, t, shear stress or another normalised invariant? Identify where consolidation ended and where undrained shear began. Locate individual specimens and their consolidation pressures. Finally, match each path to the same specimen’s pore-pressure and axial-strain curves.
This sequence prevents the common mistake of interpreting a path’s leftward movement as a fall in total confining pressure when it may instead reflect positive excess pore pressure. It also reveals whether axes were normalised by a reference stress. A stress path drawn without definitions is not self-explanatory merely because it looks smooth.
The reader’s most useful questions for an unfamiliar paper
Was the specimen intact, reconstituted or remoulded? What was its initial water content and sampling depth? How was saturation assessed? What effective stress was reached during consolidation? What rate of undrained shearing was used? How were pore pressures referenced and recorded? Which corrections converted force to stress?
A paper answering these questions gives readers the means to judge whether its results support the claimed parameters. If essential values are missing, note the uncertainty. Do not fill gaps with assumptions from another laboratory’s procedure. A technically honest summary can state that the reported q–strain curve is available while effective-stress interpretation is not fully verifiable from the disclosed data.
Common interpretation failures and repairs
The first error is treating ‘consolidated-undrained’ as drainage being prohibited throughout the entire test. Repair it by separating stages. The second is treating total and effective stress as interchangeable. Repair it with explicit subtraction of measured pore pressure. The third is assuming q determines p′. Repair it by also using confining stress and u.
The fourth is fitting two strength parameters from one independent failure point. Repair it by checking the test matrix and model assumptions. The fifth is describing every positive measured pore pressure as positive excess pressure. Repair it by identifying the reference state. The sixth is equating a strain-defined maximum with a universal failure stress. Repair it by naming the adopted failure criterion.
A changed-condition transfer problem
At a hypothetical moment, σ1 = 360 kPa, σ3 = 180 kPa and u = 100 kPa. Compute q and p′ under the standard triaxial formulas. The deviator stress is 360 − 180 = 180 kPa. Total mean stress is (360 + 2 × 180)/3 = 240 kPa. Effective mean stress is 240 − 100 = 140 kPa.
If u rises to 140 kPa while both total principal stresses remain unchanged, q remains 180 kPa but p′ falls to 100 kPa. The stress path moves left by 40 kPa at constant q. This is a precisely stated idealised change; real specimens usually evolve both total stress and pore pressure during continued axial compression.
A second transfer question on stress symbols
A textbook defines s′ = (σ1′ + σ3′)/2 and t = (σ1′ − σ3′)/2, while another defines p′ = (σ1′ + 2σ3′)/3 and q = σ1′ − σ3′. The reader may think the two texts disagree about stresses. They do not necessarily disagree; they use different coordinate choices.
For the earlier effective principal stresses 220 and 70 kPa, the s′–t point is (145,75) kPa, whereas the p′–q point is (120,150) kPa. Both can represent the same physical stress state when definitions are followed. Never copy a slope or failure-line parameter between those plots without transforming the coordinate convention.
Independent practice on pore-pressure increments
A saturated specimen ends consolidation at total pore pressure 160 kPa. During undrained shearing it is measured at 215 kPa. What is Δu? It is +55 kPa relative to the end of consolidation. If a reported effective mean stress needs the current total pore pressure, what u should be used under the stated simple subtraction? Use 215 kPa, not 55 kPa.
Now suppose someone subtracts 55 from total p and obtains a large apparent p′. What did they forget? The pre-existing pore pressure contributed by the testing system and initial state. This mistake can profoundly change the plotted path while all arithmetic looks internally consistent. The repair is to keep gauge values and reference increments in separate columns.
Helpful questions and answers
Does CU mean the specimen never drains? No, it normally consolidates with drainage before undrained shear. Does q always equal axial force divided by initial area? No, geometry and cell-stress terms require proper treatment. Is the B parameter a universal pass/fail number for every soil? Read the applicable method and material context. Can excess pore pressure be negative? Yes under suitable undrained responses and sign conventions.
Are effective stress and total stress equivalent when pore pressure is nonzero? No. Does one test uniquely determine c′ and φ′? No, not without further constraints or observations. Can a CU test provide effective-stress interpretation? Yes, when appropriate pore-pressure measurements are obtained and reliable. Is triaxial failure identical to field failure? Not automatically; boundary conditions and scale differ.
Sources and connected reading
The official ASTM D4767-11(2020) CU triaxial standard description identifies the consolidated-undrained scope, force and deformation observations, and pore-pressure measurements used to evaluate total and effective stress. The official ASTM D7181-20 CD triaxial description provides a comparison with consolidated drained tests. These are scope and purpose references; full laboratory compliance requires consultation of the actual standard and trained personnel.
Continue with oedometer consolidation and stress history, field vane shear and sensitivity and soil Atterberg limits. To strengthen technical language, revisit cross-linguistic transfer and translation, context and meaning. These verified routes concern different reader questions, not interchangeable methods.
Final principle: follow the effective stress, not just the peak
A CU triaxial result is a sequence: saturate and consolidate a specimen, shear without drainage, measure load, deformation and pore pressure, compute stresses under a defined convention, and interpret strength within the limits of the material and apparatus. Leaving out the pore-pressure record can make two physically different responses look identical at the same q.
A reader who tracks q, p′, total stress, pressure reference and failure criterion can explain a stress path accurately and identify which assumptions matter for engineering use. That is the essence of useful translation of technical geomechanics: every plotted number remains connected to a physical measurement and an honest scope of inference.