A flat dilatometer test, usually shortened to DMT, produces a sequence of in-situ pressure readings while a thin blade is pushed through soil. If you want to understand DMT p0 and p1 readings, the material index ID, horizontal stress index KD or dilatometer modulus ED, begin with the membrane measurements and their calibration corrections. The indices are calculated from measured pressures; they are not independent sensors that directly identify clay, sand, stress history and soil stiffness without interpretation.
DMT uses a small membrane expanded against surrounding soil at chosen depths. Raw A and B readings are corrected for membrane effects and gauge zero to obtain interpreted contact and expansion pressures. Those pressures, a pore-water reference and effective vertical stress are then combined into dimensionless indices and a pressure-valued modulus. Each derived quantity has a particular mathematical definition, and some widely used engineering correlations are empirical.
This guide explains how to read flat plate dilatometer records, interpret p0 and p1, compute ID, KD and ED, examine assumptions about groundwater and stress, and avoid confusion with CPTu, SPT or laboratory consolidation. The examples use hypothetical numbers with clearly stated units. The aim is to keep the chain from blade and membrane to pressure readings and cautiously inferred soil properties intact.
Quick answer: what does the DMT actually record?
A flat dilatometer blade is inserted into the ground, usually by static pushing, at planned test depths. A circular membrane on one side is inflated pneumatically. The first characteristic pressure reading is associated with the membrane beginning to move from its flush position, while another is associated with a specified outward deflection. These readings are converted through calibration relationships into p0 and p1.
The instrument does not directly report a definitive soil name or Young’s modulus. A series of readings creates a depth profile, from which indices are calculated and used with established correlations. The distinction between measured pressure and interpreted soil parameter is the central rule for reading a DMT figure.
Why a thin blade can reveal soil response
The blade changes the local stress state as it penetrates. At a test depth, inflating its membrane applies lateral pressure to the surrounding soil over a small displacement. The pressure required depends on material response, in-situ stress, stiffness, drainage, fabric and how insertion disturbed the ground.
Different deposits may therefore produce characteristic pressure profiles. Yet the experiment is not simply an unaltered observation of natural horizontal earth pressure. Inserting a blade necessarily changes local stresses. Interpretation methods attempt to relate the instrument response to soil conditions through physical reasoning and empirical calibration; they do not make insertion disturbance disappear.
Blade movement and membrane inflation are separate stages
First the blade is driven or pushed to the selected depth. Then the membrane is inflated to obtain readings; it is deflated before advancing to the next location. The test spacing may be regular, but it is not a continuous measurement of pressure at every millimetre of depth.
This matters when drawing a DMT profile. Two observations spaced 200 mm apart are two tests, not an uninterrupted measurement in between. If a thin seam lies between them, its behaviour may be missed. The plotted line connecting points is a visual aid and should not be mistaken for a sensor trace through every intervening centimetre.
What the A and B readings describe
Conventional DMT procedures refer to an A pressure related to membrane lift-off and a B pressure related to further outward deflection. The readings are made with a particular membrane and gauge setup. They must be corrected for the resistance of the membrane and other instrumental effects before being treated as p0 and p1.
An export that contains only letters A and B should not be interpreted as already corrected material pressures unless the method says so. The corrections are not optional cosmetic changes. A relatively small pressure error can have a substantial effect on calculated ratios when their denominators are small, especially near soft layers or uncertain pore-pressure conditions.
Membrane calibration is part of the physics
The metal membrane has its own stiffness and requires pressure to move even outside soil. Calibration estimates the pressure offsets associated with its movement. The measuring system can also have a zero offset. Corrected p0 and p1 attempt to remove those instrument effects in a defined way.
A reader should look for the calibration record, zero-reading convention and formula version used by the processing software. Membrane replacement or damage can change the behaviour. Treating A and B as if they were perfect soil pressures silently mixes instrument stiffness with the ground response that the method seeks to estimate.
The corrected p0 has a specific meaning
The quantity p0 is commonly termed corrected lift-off or contact pressure. It corresponds to the interpretation of pressure when the membrane begins to move from its flush condition after calibration corrections. Because the blade has already displaced soil, p0 should not simply be called ‘undisturbed horizontal earth pressure’ without a suitable model.
Its value is used in ID, KD and other empirical correlations. A report that labels it only as ‘horizontal pressure’ may create the false impression that direct in-situ K0 was measured. Describe the procedural nature of the reading and keep the engineering interpretation separate.
The corrected p1 is associated with expansion
The quantity p1 is commonly the corrected pressure at the specified outward membrane deflection. Its difference from p0, Δp=p1−p0, represents the additional pressure associated with the membrane’s movement within the instrument’s definition. This positive difference is used to calculate the dilatometer modulus and material index.
The measured displacement is small and controlled; it does not describe how the entire soil mass will deform under a full-size foundation. A modulus computed from Δp is an intermediate index that may be transformed to other stiffness quantities using context-specific correlations. Do not replace one modulus name with another merely because both have pressure units.
Pore-water pressure u0 must be distinguished
The DMT material and horizontal-stress indices use an in-situ pore-water pressure reference u0. This is generally estimated from groundwater evidence and relevant hydrostatic or other assumptions, rather than being one of the basic A/B membrane readings. Groundwater conditions can therefore affect derived indices even if the membrane measurements remain unchanged.
If a dataset assumes groundwater is two metres below the surface, ask whether that was observed during the investigation, measured later or inferred from regional information. An incorrect u0 shifts p0−u0, potentially changing both ID and KD. Pressure units and sign conventions also have to be consistent.
Effective vertical stress is another independent input
Effective overburden stress, commonly written σ′v0, can be estimated as total vertical stress minus pore-water pressure under the stated stress convention. Soil unit weights, layer depths, groundwater and surcharge can affect it. The DMT blade does not measure σ′v0 directly in the way it records A/B response.
A value of KD cannot be reproduced without knowing the σ′v0 used in its denominator. This makes a ground model part of the calculation. Two analyses with identical p0 may produce different KD if their effective-stress assumptions differ. The method should state those assumptions rather than hiding them under a classification colour.
Define the DMT material index ID
One common formula is ID=(p1−p0)/(p0−u0). The index is dimensionless because its numerator and denominator are both pressures. It is related empirically to soil material behaviour and is often used in a stratigraphic interpretation alongside other DMT data.
If p1−p0 or p0−u0 is small, the ratio can become sensitive to measurement uncertainty. A large calculated ID should not be translated automatically into a precise grain-size percentage. Behaviour-based interpretation is different from laboratory soil classification using sieves or plasticity tests. State the measured values and the formula before assigning a material category.
Worked example: calculating ID
For a fictional DMT observation, take corrected p0=300 kPa, corrected p1=550 kPa and reference u0=50 kPa. The expansion difference is 250 kPa and the contact pressure above pore pressure is also 250 kPa. Thus ID=250/250=1.00.
This example gives a reproducible ratio without requiring a soil name. The corresponding material-index category depends on the adopted empirical chart and whether the test conditions meet its assumptions. A person who writes ‘ID=1.00 means exactly 30% silt’ has exceeded what the calculation measures. The ratio is an observed-and-derived response index, not an automated grain-size analysis.
Define the horizontal stress index KD
A common expression is KD=(p0−u0)/σ′v0. It compares contact pressure above the pore reference with effective vertical overburden. The result is dimensionless. The index is often related to stress history and at-rest lateral pressure through empirical relationships.
Despite its name, KD is not simply the in-situ K0 earth-pressure coefficient. Blade insertion changes stress, and correlations are needed to infer geological state. The similar letters are a frequent source of technical confusion. Report KD as a dilatometer index and label any subsequently estimated K0 or overconsolidation ratio as a separate interpretation.
Worked example: calculating KD
Continue the fictional observation with p0=300 kPa and u0=50 kPa. Suppose an independent stress analysis estimates σ′v0=125 kPa. Then KD=(300−50)/125=2.00. The ratio has no pressure units because 250 kPa is divided by 125 kPa.
If the same membrane pressures were interpreted with an effective stress of 100 kPa, KD would become 2.50. No change in recorded DMT pressures would be required for that difference. This is why effective-stress assumptions matter. A stress-history conclusion should not be stronger than the evidence supporting both numerator and denominator.
The dilatometer modulus ED uses the pressure difference
The conventional DMT dilatometer modulus is often expressed ED=34.7(p1−p0), when input pressures and output follow consistent units. The numerical coefficient comes from the assumed membrane geometry and an idealised mechanical interpretation. ED has pressure units, unlike ID and KD.
With p1−p0=250 kPa in the fictional example, ED=34.7×250=8,675 kPa, or 8.675 MPa. This is a valid application of the stated formula. It does not mean a field foundation has Young’s modulus exactly 8.675 MPa. The quantity is a specific dilatometer modulus associated with the test.
ED and Young’s modulus are not synonyms
Young’s modulus is a constitutive stiffness concept linked to a specified stress–strain behaviour. Soil stiffness depends on strain level, stress state, drainage, time and soil structure. ED is constructed from a membrane pressure difference at a limited movement using the DMT procedure and its assumptions.
For settlement analysis, practitioners may use a constrained modulus or other calibrated stiffness estimate derived from DMT indices, not ED substituted directly without interpretation. A spreadsheet that renames the ED column ‘Young’s modulus’ may produce a plausible number while changing its meaning. Precision in the name prevents incorrect use in later calculations.
Constrained modulus M_DMT requires a correlation
Some DMT interpretation frameworks estimate a constrained modulus M_DMT by combining ED with a multiplier dependent on other indices and empirical calibration. Different studies and regional methods may choose different coefficients or restrictions. The output is a parameter for a particular deformation model, not a second direct membrane reading.
If a report gives both ED and M_DMT, do not assume they must be numerically identical. Ask for the adopted equation, soil behaviour and relevant stress conditions. The constrained-modulus idea connects to one-dimensional deformation, but its usefulness for foundation settlement depends on whether the correlation is appropriate for the site and the loading.
A typical profile is a sequence of pressure and index columns
A DMT log may show p0, p1, ID, KD and ED as parallel curves against depth, sometimes alongside estimated OCR or other derived properties. The first pair are corrected test pressures. The next three are computed indices or modulus. Subsequent plotted strength or consolidation estimates add further model assumptions.
Begin reading from left to right in that measurement order. Where a sharp index change occurs, inspect whether p0, p1 or the groundwater reference drove it. The coloured classification column is the end of an inference chain. A reader who starts there can miss a calibration or stress-input issue that changes the geological story.
Material index is not an exact soil taxonomy
Published DMT charts often use the ID index to distinguish broad clay-like, silt-like and sand-like behaviours. Such boundaries were established empirically, and they do not provide the grain-size distribution or plasticity classification that a laboratory can determine. Cemented soils, organic materials, structured clays and mixed deposits may behave unusually.
When a study claims ‘sand’ from DMT, a careful summary may say ‘sand-like DMT behaviour’ unless verified samples support the geological identification. The Atterberg limits guide addresses plasticity measurements that DMT does not perform. Different assays can agree without becoming interchangeable.
KD can suggest stress history but does not date the soil
Stress history concerns how a soil’s present state relates to past effective stresses and deformation. KD can correlate with overconsolidation ratio under selected frameworks, yet such estimates are empirical and may be influenced by ageing, cementation, fabric, drainage and insertion disturbance. A higher KD is not a direct reading of the maximum historical vertical load.
A report that prints OCR from KD should identify its calibration and whether the soil falls within the relationship’s domain. An OCR of four is not four centuries of age or proof of one particular past glacier or excavation. Geological causes require separate contextual evidence.
The meaning of overconsolidation ratio OCR
OCR is conventionally a ratio of a past maximum effective vertical stress reference to the present effective vertical stress, under a defined interpretation of preconsolidation. It is dimensionless. Laboratory oedometer interpretation may estimate a yield or preconsolidation stress, whereas DMT offers a different in-situ route through correlations.
The oedometer consolidation guide helps keep these distinct. Neither laboratory OCR nor DMT-correlated OCR is a universal geologic clock. Compare outputs together with sampling disturbance and soil fabric, rather than forcing them to agree to the last decimal place.
DMT tests small lateral movement, not full foundation behaviour
The membrane moves a small controlled distance relative to its blade. A foundation loads a much larger volume and can cause more complex stress paths over longer intervals. Comparing stiffness estimates therefore requires a mechanical model that translates the local response to the larger loading geometry.
A high ED may support an interpretation of relatively stiff response under test conditions, but it does not guarantee negligible settlement under any footing. Soil layers, drainage, loading rates and possible stress redistribution still matter. A meaningful engineering conclusion identifies which test conditions the parameter represents and what additional calculations were performed.
Calibration drift can resemble a soil transition
If a membrane calibration or pressure gauge changes during a sounding, the corrected p0 and p1 may shift independently of the ground. Membrane condition, gauge zero, gas supply and operator procedure can influence readings. A sudden change in multiple derived indices should prompt a check of raw A/B and calibration before assigning a geological cause.
Quality review may include membrane calibration records before and after field use, zero checks and instrument response. Processing software should retain the raw readings and correction settings. A reader should be able to reproduce the displayed pressures rather than accepting a final index curve as an untestable black box.
DMT is different from a cone penetration test
CPTu records tip resistance, sleeve friction and pore pressure during near-continuous pushing. DMT inserts a blade and inflates a membrane at discrete depths. The instruments probe different mechanical responses and use different empirical interpretation schemes. A DMT material-index curve is not simply another way to plot CPTu soil-behaviour type.
The CPTu interpretation guide explains q_t and pore-pressure corrections. A useful comparison may confirm similar layer changes between soundings, but numeric index values should not be directly equated. Agreement can strengthen a site interpretation while the separate physical measurements remain visible.
A changing material index does not always mean a new layer
An ID curve can rise or fall because p1−p0 changes, because p0−u0 changes, or because both move in different directions. A sudden spike therefore needs more investigation than simply reading the colour assigned by a chart. A membrane pressure change could reflect material response, calibration variation or a different pore-pressure assumption.
Plot p0 and p1 beside ID. If both pressures remain smooth but ID jumps sharply, inspect the denominator and input reference. If the jump coincides with a consistent change in other field measurements and recovered specimens, a geological transition becomes more plausible. Trace the arithmetic before telling a story about stratigraphy.
Ratios become unstable near a small denominator
Consider ID=(p1−p0)/(p0−u0). If p0 is only slightly above u0, the denominator is small and the ratio can become very large. A small measurement error in either pressure can then cause a substantial shift in ID. The resulting number may be mathematically valid yet poorly constrained for material interpretation.
For example, a numerator of 80 kPa divided by a denominator of 10 kPa gives ID=8. If uncertainty changes the denominator to 5 kPa, ID doubles to 16 even though the numerator is unchanged. Such sensitivity should be reported as a limitation rather than translated into a dramatically more precise soil class.
What a negative calculated index might indicate
Under typical assumptions some DMT indices are expected within meaningful ranges, but an unexpected negative value may occur when corrected pressures or reference inputs become inconsistent with the model. This is a prompt to review gauge offsets, membrane calibration, data import, pore pressure and whether the calculation is applicable at that depth.
Do not silently replace all negative values with zero simply to satisfy a chart. A correction convention or an unusual material response may need examination. The original p0, p1, A, B and calibration information should remain available, because the arithmetic alone cannot establish whether the value is physical, procedural or erroneous.
Hydrostatic conditions cannot always be assumed
The usual u0 model may use water depth below a groundwater level and a hydrostatic gradient. But perched water, hydraulic gradients, artesian pressure and time-dependent conditions can make the actual pore-pressure field more complex. DMT has no direct guarantee that its input u0 represents every depth correctly.
A useful case report explains whether the groundwater level was observed, inferred from nearby piezometers or taken from a design assumption. If two soundings were performed in different seasons, their stress normalisation may use different pore pressures even when soil properties did not change. A precise ratio is no more reliable than its pressure reference.
Worked groundwater sensitivity example
Take p0=300 kPa, p1=550 kPa and σ′v0=125 kPa. With u0=50 kPa, ID=1.00 and KD=2.00. If u0 were 75 kPa and all other inputs were held constant solely for sensitivity illustration, ID=250/225≈1.11 and KD=225/125=1.80.
In a real ground model, changing u0 may also change effective stress, so holding σ′v0 fixed would not always be physically consistent. The controlled exercise isolates the numerator and denominator effects; a complete reinterpretation must update the stress calculation too. Explain the difference between mathematical sensitivity and physically consistent recalculation.
The test has a defined material range
A flat dilatometer blade is generally suited to soils that it can penetrate without severe damage, including many sands, silts and clays. Gravel, cobbles, cemented layers and debris can obstruct or damage the blade and membrane. A test termination or missing reading is not proof that the ground at that depth has a particular strength.
Before extrapolating a profile through a refusal interval, read the field log. Did the blade fail to advance, was the membrane damaged, or was the test deliberately stopped? The reason limits what can be inferred. Complementary drilling or sampling may be required for material outside the equipment’s effective range.
Installation disturbs the lateral stress field
Blade insertion creates a cavity-like disturbance and changes local stresses. The corrected lift-off pressure therefore includes an instrument–soil interaction, not a direct untouched horizontal stress measurement. Empirical relationships linking KD to K0 or stress history account for this only under their own assumptions and datasets.
The distinction matters in structured or sensitive soil, where disturbance can affect the measured response. A high KD could be related to stress history, cementation, ageing or other fabric effects. The figure does not carry labels identifying those causes. Interpret the pattern against geology, sampling evidence and relevant laboratory measurements rather than declaring one mechanism inevitable.
K0 and KD sound similar but are different quantities
K0 commonly denotes an at-rest earth-pressure coefficient expressing horizontal-to-vertical effective stress under a specified geotechnical convention. KD is a DMT horizontal stress index derived from corrected membrane contact pressure, a pore reference and effective overburden. It is influenced by blade installation.
They may be connected through empirical relationships, but KD=K0 is not a general identity. A technical report should carry separate columns and explicitly identify any equation used to infer K0 from KD. The difference is especially important when designing retaining structures where horizontal stresses strongly influence the outcome. A name resembling an established variable does not make the measurement equivalent.
Stress-history inference needs independent context
An overconsolidation ratio estimated from KD may be compared with a laboratory preconsolidation estimate, geological evidence or previous loading history. Agreement can support an interpretation, yet disagreement is not automatically evidence that one instrument failed. Laboratory sampling disturbance, natural structure and the calibration’s geological domain all affect estimates.
For clays, it is often useful to compare stress-history indicators with consolidation behaviour and plasticity. Do not interpret a single high KD as proof of one specific historical event. A former load, chemical bonding or ageing can produce some similar qualitative tendencies while involving different underlying mechanisms.
ED does not uniquely determine a settlement value
A foundation settlement calculation needs loading intensity, influence geometry, layer thickness and a suitable deformation model. ED is based on the pressure required for a small membrane displacement. Translating it into a settlement parameter typically involves further correlations and often additional site investigation.
A report claiming ‘ED=10 MPa, therefore settlement=10 mm’ skips the loading system entirely. The units alone show the omission: a modulus is pressure, while settlement is length. An equation must connect strain and geometry before a length can be obtained. That dimensional check can expose an otherwise plausible-looking but unsupported design statement.
How M_DMT relates to deformation interpretation
The constrained modulus estimated through some DMT methods is commonly used for one-dimensional compression or settlement-oriented interpretation. It may be calculated using ED and empirical multipliers that depend on indices such as ID or KD. Thus two soils with the same ED can receive different estimated constrained moduli under a chosen method.
When comparing reports, check whether the displayed modulus is ED, M_DMT, a small-strain wave modulus or a model-derived Young’s modulus. They address different deformation regimes. One should not substitute them freely because all are expressed in kilopascals or megapascals. The physical meaning of the variable matters as much as its dimension.
Loading rate and drainage influence test behaviour
DMT measurements are performed during a particular inflation and reading sequence, while soils may respond in drained, undrained or intermediate ways depending on permeability and testing rate. Time-dependent behaviour can also influence the pressure response. A comparable soil tested with substantially different timing may not yield identical readings.
The methods section should therefore describe the procedure and whether it followed an applicable standard. A field profile recorded under uncertain timing cannot always be made equivalent to another through a single scaling factor. The reader needs to understand the operational conditions, especially where soft clays or partially drained silts are involved.
Distinguish the regular DMT from seismic DMT
Some systems pair DMT pressure measurements with downhole seismic travel-time measurements, often referred to as seismic dilatometer or SDMT testing. The seismic part can provide shear-wave velocity information, while the membrane still provides pressure-derived indices. The outputs are complementary but distinct.
Shear-wave velocity can support small-strain stiffness analysis through density and appropriate elastic relationships. It does not transform ED into a direct measurement of the same small-strain modulus. If a log includes both ED and Vs, state what each instrument channel measured. The combination can be valuable precisely because it probes different aspects of soil behaviour.
A small-strain shear modulus is not ED
A commonly used elastic relationship for a material under suitable assumptions is G0=ρVs², where ρ is mass density and Vs is shear-wave velocity. This equation concerns a small-strain shear modulus, unlike ED from membrane expansion. For an illustrative ρ=1,800 kg/m³ and Vs=200 m/s, G0=1,800×40,000=72,000,000 Pa or 72 MPa.
It would be incorrect to insist that ED in the same soil must equal 72 MPa. The measurements involve different strains, directions and mechanical assumptions. To compare them, engineers need a context-specific stiffness framework. Their difference may be informative rather than erroneous.
Compare DMT with SPT while retaining each method’s limitations
SPT records hammer blow resistance and recovers a disturbed sample at selected borehole depths. DMT records membrane pressure response at selected pushed-blade depths. The two may corroborate stratigraphic changes, but there is no universal conversion from SPT N to DMT KD or ED.
The SPT blow-count guide explains how equipment energy and stress normalisation affect N. A comparison should align depth, soil type and groundwater rather than forcing an index-by-index agreement. If one test indicates an unusual horizon, the other may help investigate it without rendering it automatically true.
DMT can support the choice of further investigations
When DMT suggests a change in response or stress history, an engineer may target laboratory sampling, pressuremeter testing or more detailed cone investigation to resolve uncertainties. A well-planned field programme uses different tests to answer complementary questions. A strong investigation does not require every test to generate identical numbers.
For example, DMT may suggest stiffness differences with depth, while laboratory consolidation tests describe a stress–strain response under controlled one-dimensional loading. Their interpretations can be compared to refine a ground model. The key is to document which question each test answered rather than treating all numbers as alternative readings of one hidden soil property.
Reconcile depth and elevation datums
A DMT profile may be plotted against depth below a test location, while a borehole log uses elevation relative to a survey datum. If the ground surfaces differ, comparing numerical depths directly can place supposed layers at the wrong elevation. This is a spatial reference error, not a soil mechanics issue.
Document location, ground-level elevation, datum and any inclination corrections before aligning profiles. If a DMT test ends early at an obstruction, note that coverage too. A thoughtful interpretation connects measured points in a shared coordinate system before deciding whether observations support the same geological unit.
Data smoothing can conceal important variation
A DMT dataset may contain fluctuations in p0 and p1 that software smooths to improve visual readability. Some fluctuations reflect noise; others may correspond to genuine thin layers or local fabric differences. An aggressive smoothing window can erase potentially useful evidence or shift the apparent position of a boundary.
Retain raw corrected pressures and the processing method. When a surprising ID or KD spike appears, inspect the underlying unsmoothed points and calibration record. A smooth chart should not be considered more trustworthy merely because it looks professional. Graph preparation is an interpretation step that can alter the reader’s impression of the ground.
A complete worked set of DMT indices
Take an illustrative observation with corrected p0=400 kPa, p1=600 kPa, hydrostatic u0=100 kPa and effective overburden σ′v0=150 kPa. The pressure difference is 200 kPa. ID=200/(400−100)=0.667. KD=(400−100)/150=2.00. ED=34.7×200=6,940 kPa, or 6.94 MPa.
Each calculation uses pressures consistently. ID and KD are dimensionless, while ED has units of stress. The numerical indices may support classification under a specified chart, but none uniquely identifies an in-situ shear strength, preconsolidation stress or settlement. The exercise keeps direct, corrected and inferred variables separate.
A data-quality scenario involving a changed gauge zero
Suppose identical raw A and B readings are processed twice, but the second computation uses a different gauge-zero correction. The resulting p0 and p1 may differ, and the derived indices change. This could be a legitimate correction if the second zero is supported by calibration; it could also be a processing error if it was entered arbitrarily.
Do not choose whichever interpretation gives the expected geological sequence. Retrieve the calibration sheet and processing method and recalculate consistently. A transparent technical record explains which inputs were revised and why. The objective is to correct the measurement chain, not to make the final index curves agree with a preferred hypothesis.
Independent transfer: change only p1
Hold corrected p0=300 kPa, u0=50 kPa and σ′v0=125 kPa. Increase p1 from 550 to 650 kPa in a hypothetical second measurement. The difference p1−p0 increases from 250 to 350 kPa. ID changes from 1.00 to 1.40, and ED changes from 8.675 to 12.145 MPa.
KD remains 2.00 because its formula does not contain p1. This is an effective reasoning check: different indices respond to different variables. If software reported KD increasing solely because p1 changed while every KD input stayed fixed, examine the processing formula. The exercise tests structural understanding, not merely memory.
Independent transfer: change effective stress
Keep p0=300 kPa and u0=50 kPa but change effective overburden from 125 to 100 kPa in an isolated arithmetic example. KD increases from 2.00 to 2.50, while ID and ED remain unchanged if p1 is also held fixed. This demonstrates the dependence of KD on its denominator.
In a physically consistent field reinterpretation, changes in groundwater or unit weights might affect other inputs too. The exercise deliberately isolates one variable to expose the equation’s structure. A real analysis should update all linked quantities and then evaluate whether the empirical correlation is still appropriate.
Frequent mistakes and direct repairs
One mistake is applying ID and KD formulas to uncorrected A and B readings. Repair it by first documenting membrane calibration and calculating p0 and p1. A second is treating KD as direct K0. Repair it by naming the empirical correlation and its domain. A third is treating ED as Young’s modulus. Repair it by preserving the dilatometer-specific modulus name.
A fourth is assuming ID supplies exact grain-size classification. Repair it with independent samples or laboratory tests. A fifth is using an uncertain groundwater reference without disclosure. Repair it by reviewing groundwater observations and sensitivity. Finally, do not interpret a refusal depth as a continuous hard layer without geological evidence.
Questions about DMT readers often ask
Does DMT recover an undisturbed sample? No; its principal output is in-situ membrane pressure response. Is p0 the same as raw A? Not necessarily; correction is required. Does p1 mean total vertical pressure? No, it is a membrane expansion pressure. Is ID a soil particle-size percentage? No, it is a dimensionless material-behaviour index. Does KD equal K0? No.
Is ED directly the Young’s modulus for every design? No. Can groundwater affect ID and KD? Yes. Can the blade penetrate cobbles reliably? No, obstructions can invalidate or terminate testing. Are two neighbouring soundings expected to agree perfectly? No; geology and instrumentation vary. Does high ED guarantee negligible settlement? No; loading and deformation modelling are still required.
Method sources and related reading
The FHWA Flat Dilatometer Test technical report provides an official introduction to the blade, membrane measurements and interpretation. The open foundation-engineering chapter on the flat dilatometer explains pressure-derived indices and distinctions among stress-history concepts. The FHWA Geotechnical Aspects of Pavements manual discusses DMT alongside other in-situ methods and their limitations.
For connected eduKateSG learning, use CPTu cone profiles, SPT blow-count corrections, soil oedometer consolidation and cross-linguistic transfer. Each helps distinguish measurements from terms that only appear equivalent.
Closing lesson: a DMT index is a calculation with a history
The flat dilatometer test offers valuable in-situ evidence about pressure response and soil behaviour. Yet every ID, KD or ED plotted on a report depends on corrected membrane readings and appropriately defined reference conditions. The result becomes misleading when calibration is hidden or when empirical estimates are presented as directly measured soil properties.
The reliable reading route is straightforward: confirm where and how the blade was tested, inspect A and B, verify membrane and gauge corrections, calculate p0 and p1, identify u0 and effective overburden, then compute and interpret each index. When those steps remain visible, the final geological and engineering conclusions can be examined and improved rather than merely accepted.