The primary purpose of this test is to determine the incompressible compressive strength, which is then used to calculate the rough shear strength of the soil under rough conditions. As per ASTM standard, unconfirmed compressive strength (WhyYou) is defined as the compressive stress at which an infinitely cylindrical sample of soil will fail a simple compression test. In addition, in this test method, the unconfirmed compressive strength is taken as the maximum load achieved per unit area, or the load per unit area at 15% axial tension, whichever occurs earlier during the performance of a test.

standard reference

ASTM D 2166 Standard test method for unlimited compressive strength of cohesive soils


For soil, untrained shear strength (sYou) is necessary for the determination of bearing capacity of foundations, dams etc. untrained shear power (sYou) Clay is usually determined by an unconfirmed compression test. untrained shear power (sYou) half of a cohesive clay is equal to unlimited compressive strength (WhyYou) when the soil is at f = 0 position (F = internal friction angle). The most severe conditions for soils usually occur soon after construction, representing untrained conditions, when the untrained shear strength is basically equal to cohesion (C) is expressed as:

sYou = c = qYou/2

Then, as time passes, the pores in the soil gradually lose water, and the intergranular stress increases, leading to the increase in drain shear strength (s), given by s = c + s’tan ‘ , must be used. Where s’ = interplanetary pressure acting perpendicular to the shear plane; And S’ = (S – U), s = total pressure, and You = pore water pressure; C’ And , The shear strength parameters are drained.


  • compression device
  • load and deformation dial gauge
  • Sample Trimming Equipment
  • balance
  • moisture can

Testing Process

(1) Take out the soil sample from the Shelby Tube Sampler. Cut the soil sample so that the ratio (L/D) is approximately 2 and 2.5. in between, Where Took And D are the length and diameter of the soil sample, respectively.

(2) Measure the exact diameter of the top of the sample at 120° apart in three places, and then make the same measurement at the bottom of the sample. Average the measurements and record the average as the diameter on the data sheet.

(3) Measure the exact length of the sample at three locations 120° apart, and then average the measurement and record the average as the length on the data sheet.

(4) Weigh the sample and record the mass on the data sheet.

(5) Carefully place the sample in the compression device and center it on the bottom plate. Adjust the device so that only the upper plate makes contact with the sample and set the load and deformation dials to zero.

Figure 1
Figure 1

(6) Apply load so that the device produces an axial stress at a rate of 0.5% to 2.0% per minute, and then record the load and deformation dial readings on the data sheet every 20 to 50 divisions on the deformation of the dial.

(7) continue to apply load until (1) the load on the sample (load dial) is significantly reduced, (2) the load remains constant for at least four distortion dial readings, or (3) The deformation is significantly higher than the 15% strain that was determined in step 5.

line drawing number 2
line drawing number 2

(8) Draw a diagram to show the sampling failure.

(9) Remove the sample from the compression device and obtain a sample for water content determination. Set the amount of water as per the experiment


(1) Convert the dial readings to the appropriate load and length units, and enter these values ​​in the Distortion and Total Load columns on the data sheet.

(Verify that the conversion has been done correctly, especially by proving the dial gauge reading conversion to load)

(2) Calculate the sample cross-sectional area a0 = *(d2)/4

(3) Calculate the deformation (ΔL) corresponding to the 15% distortion (E).

Tension (E) = L / L0

Where Took0 = original sample length (as measured in step 3).

(4) Calculate the correct area, a’ = a0 / (1-e)

(5) Using A’, calculate the sample strain, sC = p/a’

(Be careful with unit conversions and use constant units).

(6) Calculate the amount of water, w%,

(7) Plot stress vs strain. Display WhyYou As the peak stress of the test (or at 15% stress). Make sure the tension is plotted on the abscissa. (see fig-3)


(8) Calculate the shear power sYou as follows,

sYou = c (or cohesion) = qYou/2

article written by

Pro. Krishna Reddy, UIC

Er. Mukesh Kumar

Photo of author
Er. Mukesh Kumar is Editor in Chief and Co-Funder at Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.