Why Does Soft Clay Call for the Vane Shear Test?

Very soft clay has particular problems that keep conventional tests from giving good answers. The first is sample disturbance — when soft clay is sampled and brought to the laboratory, the specimen is often disturbed during pushing, extraction and transport. Part of the delicate soil structure is destroyed, so laboratory strengths tend to be lower than the true in-situ strength. The softer the clay, the more pronounced this problem becomes.

The second problem is the resolution of the Standard Penetration Test (SPT), which suits sand and medium-stiff or stiffer soils but is too coarse for very soft soil — in Bangkok soft clay the N-value is often only 0–2, so the layers can hardly be told apart (for details on interpreting N-values, see How to Interpret SPT N-Values).

Vane Shear solves both problems by shearing the soil where it lies. The soil is not lifted out of its original stress state, is never transported, and the result is a continuous strength value rather than a blow count. The values therefore reflect the in-situ strength of soft to medium-stiff clay particularly well.

Field Vane Shear Test in a soft clay layer on an actual site
Vane shear testing in a soft clay layer on site to determine undrained shear strength (Photo: SPN Soil Engineering)
📚 Key principle

The softer the soil, the greater the advantage of testing "in place", because sample disturbance is eliminated entirely — this is why soil investigations on the Central Plains and in the Bangkok basin often specify Vane Shear in the scope of work alongside SPT and sampling for laboratory testing.

How Is the Field Vane Test Performed?

The main equipment is a four-bladed cruciform vane fixed to the end of a steel rod, which extends up to a torque-measuring head at the top of the hole. The procedure in brief:

  1. Advance the hole to test depth — drill the borehole, or push the test rods, down to the depth to be tested
  2. Push the vane into undisturbed soil — push the vane below the bottom of the hole by about 5 times the borehole diameter (per ASTM D2573 guidance) to get beyond the soil disturbed by drilling
  3. Rotate at a constant rate — rotate the vane slowly, at no more than about 0.1°/second (6°/minute), while recording torque continuously. Typical soft clay reaches failure in about 2–5 minutes; very soft clay may take up to 10–15 minutes
  4. Read the maximum torque — when the soil around the vane fails in shear, the maximum torque is converted to undrained shear strength (Su) using the known vane dimensions
  5. Measure the remolded strength — after failure, rotate the vane rapidly for about 10 more turns to destroy the soil structure, then test again to obtain the remolded strength, which is used to determine the clay's sensitivity
  6. Repeat at the next level — test at intervals with depth to build a continuous strength profile through the soft clay layer

Testing follows ASTM D2573/D2573M (current title: Standard Test Method for Field Vane Shear Test in Saturated Fine-Grained Soils), which standardizes the procedure, rotation rate and calculation, so results from different boreholes and projects can be checked and compared. All results are reported with the boring log for that borehole.

The figure below summarizes the whole procedure in a single section — from the vane position below the bottom of the hole and the rotation rate to the torque–rotation curve used to read Su and remolded Su.

2D section of a Field Vane Shear Test in a borehole in very soft clay, with a torque–rotation curve and the shear strength formula The left panel is a section through a borehole in very soft clay below a thin stiff surface crust. The hole has a casing almost to the bottom; a shaded zone at the bottom of the hole is soil disturbed by drilling. The torque head sits on a clamp at the top of the casing, and the steel rod runs down to a four-bladed vane pushed into undisturbed soil about five borehole diameters below the bottom of the hole per ASTM D2573. An arrow at the head shows slow rotation of no more than 0.1 degree per second, normally reaching failure in about 2 to 5 minutes. A dotted box below the vane marks the next test level. An enlarged inset shows the vane with height H equal to 2D and a dashed cylindrical shear surface. The right panel is a conceptual torque versus rotation curve: the solid line rises to the peak torque used for undisturbed Su and then drops; after about ten rapid turns, a dashed line shows the lower constant torque used for remolded Su. Below the chart is the formula Su equals 6T divided by 7 pi D cubed for a vane with H equal to 2D, assuming uniform shear stress distribution, and sensitivity St equals Su divided by remolded Su. The bottom strip summarizes undisturbed Su, remolded Su, sensitivity, and Bjerrum's μ correction before use in design. 2D section: Field Vane test in a borehole Not to scale Vane set below borehole bottom Push into undisturbed soil, rotate, read torque at top Stiff crust Clay Very soft Very Soft Clay Repeat test at each depth Rotate ≤ 0.1°/s Failure typ. 2–5 min Torque head Casing Vane rod Drilling-disturbed soil below hole bottom ≈ 5 × hole Ø (ASTM D2573) 4-blade vane H = 2D D Shear cylinder Torque–rotation (conceptual) Rotate to failure, remold, retest Rotation → Torque (T) → Tmax → Su (undisturbed) Tr → Su,r (remolded) Fast ≈ 10 turns Undisturbed (peak) Remolded Su = 6T / (7πD³) T = max. torque · D = vane Ø (vane H = 2D, uniform shear assumed) St = Su / Su,r Sensitivity 1 Su undisturbed From peak torque 2 Su,r remolded Fast ≈ 10 turns, retest 3 Sensitivity St St = Su / Su,r 4 μ correction (Bjerrum) Before design · per PI
Section of a Field Vane Shear Test in a borehole: the vane (H = 2D) is pushed into undisturbed soft clay below the bottom of the hole and rotated slowly to read the peak torque for Su, then the soil is remolded and retested to obtain remolded Su and sensitivity (Diagram: SPN Soil Engineering)

How Is Su Calculated from Torque?

Su from the field vane is the maximum torque divided by a geometric constant of the vane, because the soil around the vane fails along a cylindrical surface — both the side surface and the top and bottom end plates. Assuming shear stress is uniformly distributed on those surfaces, a standard vane with a height of twice its diameter (H = 2D) gives the following formula

Su = 6T / (7πD3)  ·  St = Su / Su,r T = maximum torque · D = vane diameter · Su,r = remolded strength · St = sensitivity

The formula comes from the torque on the side surface, (πD²H/2)·Su, plus the torque on both end plates, (πD³/6)·Su. Substituting H = 2D gives T = (7/6)πD³·Su. The result is a raw field value and must be corrected as described under limitations below before it is used in design.

What Do Vane Shear Results Tell You?

A single test location yields up to 3 usable values:

← Scroll to see the full table →

ValueMeaning and use
Undrained Shear Strength (Su)Undrained shear strength of the clay, used to calculate foundation bearing capacity, pile skin friction, and the stability of excavations/embankments
Remolded StrengthStrength of the soil after its structure is destroyed, used to assess soil behavior around driven piles in the early period after driving
Sensitivity (St)Ratio of Su to remolded strength. Highly sensitive soils lose much of their strength when disturbed and need special care in pile driving and excavation

Su values at several depths are plotted as a strength profile, which in natural soft clay usually increases with depth. This profile is the starting input for soil bearing capacity calculations (see the principles in What Is Bearing Capacity?) and for stability analysis of fills and excavations on soft ground.

Su can be back-calculated from torque because the soil around the vane fails along a cylindrical shear surface. The 3D view below shows how the total torque comes from shear on the side surface and on both end surfaces.

3D model of the cylindrical shear surface around the vane in a Field Vane Shear Test and the derivation of the shear strength formula A conceptual 3D view of a block of very soft clay with the front quarter cut away to reveal the vane at test depth. A steel rod runs from the ground surface down to the vane; an orange arrow at the top of the rod shows torque T applied by slow rotation of no more than 0.1 degree per second. The vane is a thin four-bladed cruciform steel plate inside a translucent cylinder representing the sheared soil column, with height H equal to twice the diameter D. Red arrows around the side surface show shear stress τ equal to Su resisting rotation, and circular arrows on the top and bottom end plates show shear on both ends. A panel on the right explains the derivation: one, four-bladed vane with H equal to 2D; two, torque from the side surface T1 equals pi D squared H over two times Su; three, torque from the top and bottom end surfaces T2 equals pi D cubed over six times Su; four, total T equals T1 plus T2, and with H equal to 2D, Su equals 6T divided by 7 pi D cubed. A note states that the measured value must be multiplied by Bjerrum's correction factor μ according to the plasticity index before use in design, as selected by the engineer. 3D model: cylindrical shear surface around the vane Not to scale H = 2D D 4 Torque T at rod top Rotate ≤ 0.1°/s 1 Vane 4 thin blades, H = 2D 2 Side shear τ = Su resists rotation 3 End shear Top + bottom: τ = Su Steel rod Section: very soft clay (cutaway to show vane) Su formula derivation 1 4-blade vane Height H = 2D · diameter D 2 Side shear T1 = (πD²H/2)·Su 3 Top + bottom end surfaces T2 = (πD³/6)·Su 4 Total T = T1 + T2 With H = 2D: Su = 6T / (7πD³) Measured × μ (Bjerrum) per PI, before design High-PI clay: μ typ. 0.6–0.8 — selected by the engineer Conceptual Uniform τ on shear surface assumed
3D model showing the cylindrical soil column around the vane sheared on both the side surface and the top and bottom end plates, so the measured torque can be back-calculated as Su = 6T/(7πD³) for a vane with H = 2D (Diagram: SPN Soil Engineering)
⚠️ Technical note

Vane shear values usually need to be corrected with a correction factor related to soil plasticity before use in design — an interpretation step carried out by a geotechnical engineer. Raw values should not be entered directly into design equations.

Which Projects Is Vane Shear Suited To?

  • Projects on thick soft clay — such as Bangkok, its surrounding provinces and the Central Plains, where soft clay layers continue for many meters (read about this type of profile in Bangkok Soft Clay and Foundations)
  • Fills, embankments and roads on soft ground — which need stability analysis during construction, because fill loads can cause shear failure of the soft soil beneath
  • Ground improvement works — which need soil strength measured before and after treatment to verify results, such as preloading or PVD, because Vane Shear can be repeated at the same location and gives directly comparable numbers
  • Excavations in soft soil — such as construction pits and drainage systems, which need a detailed strength profile with depth to design the bracing system

The number of boreholes and the locations for Vane Shear depend on project size and the variability of the soil layers — the same principle used to set the number of boreholes in general, explained in How Many Boreholes and How Deep?

What Are the Limitations of Vane Shear?

  • Only for soft to medium-stiff clay — not usable in sand, gravel or very stiff clay, because the vane cannot be pushed in or the values have no meaning
  • Sand lenses or shells distort results locally — hard inclusions in the clay layer can make measured values too high, so interpretation must always be checked against the borehole stratigraphy
  • Gives undrained strength only — long-term (drained) behavior still requires laboratory soil testing as well, such as the consolidation test for settlement
  • Field Su must be corrected before design — the field vane often gives Su higher than the strength actually mobilized in stability problems, so it must be multiplied by a correction factor (μ) based on the clay's plasticity index before analysis (per Bjerrum's work, μ for highly plastic soft clay is typically about 0.6–0.8). The responsible engineer selects the appropriate μ for each soil layer.

For this reason, Vane Shear is not a substitute for soil boring but a powerful complement — the borehole gives an overall picture of the full soil profile plus samples for laboratory testing, while Vane Shear adds accurate strength data in soft clay layers that other methods measure poorly. Using both in the same borehole is the most cost-effective approach for projects on soft ground.

Summary

The Field Vane Shear Test measures the shear strength of soft clay "in place" without taking samples. It provides soil strength (Su), remolded strength and sensitivity, and is especially suited to Bangkok-type soft clay, fills, excavations and ground improvement on soft soil. It should be performed together with soil boring and laboratory testing so that results can be interpreted correctly from every angle.

SPN Soil Engineering provides soil investigation with Field Vane Shear Testing to ASTM standards nationwide across all 77 provinces, reporting results with strength profiles versus depth, certified by civil engineers licensed by the Council of Engineers Thailand (COE). See the full scope of services at Soil Investigation and Testing Services.

📌 Note

This article is provided as engineering knowledge. Test locations, number of test points and interpretation should follow project specifications and the judgment of a licensed engineer.