11 Unconfined Compressive Strength Test

Introduction

The unconfined compression test is the most popular method of soil shear testing because it is one of the fastest and least expensive methods of measuring shear strength. It is used primarily for saturated, cohesive soils recovered from thin-walled sampling tubes. The test is not applicable to cohesionless or coarse-grained soils.
The unconfined compression test is strain-controlled, and when the soil sample is loaded rapidly, the pore pressures (water within the soil) undergo changes that do not have enough time to dissipate. Hence it is representative of soils in construction sites where the rate of construction is very fast and the pore waters do not have time to dissipate.

Practical Application

The test is used in all geotechnical engineering designs (e.g., design and stability analysis of foundations, retaining walls, slopes, and embankments) to obtain a rough estimate of the soil strength and determine the viable construction techniques.

Objective

The objective of this experiment is

  • To determine the unconfined compressive strength (qu) of the soil

Equipment

  • Unconfined compression testing machine (triaxial machine)
  • Specimen preparation equipment
  • Sample extruder
  • Balance

Standard Reference

  • ASTM D2166: Standard Test Method for Unconfined Compressive Strength of Cohesive Soil

Method

  • Remolded specimens are prepared in the laboratory and are dependent upon the Proctor data pertaining to the required molding water content.
  • If testing undisturbed specimens retrieved from the ground by various sampling techniques, trim the samples into regular triaxial specimen dimensions (2.8 inch x 5.6 inch)
    Measuring the size of the speciment
    Figure 11.1: Measuring the specimen width
    Measuring the height of the specimen
    Figure 11.2: Measuring the specimen height
  • There will be significant variations in the strength of undisturbed and remolded samples.
    Measure the diameter and length of the specimen to be tested
  • If curing the soil samples (treated soils), wrap them in a geotextile and put them in a ziplock bag. Place the sample in a humidity room maintained at a relative humidity of 90%.
    Unconfined compression apparatus
    Figure 11.3: Unconfined compression
  • Prior to testing, avoid any moisture loss in the sample, and place it on an acrylic triaxial base. The ends of the sample are assumed to be frictionless.
  • Without applying confinement, place the triaxial cell above the sample.
  • Maintain the rate of strain at 1.2700 mm/min, as per ASTM specifications.
    Specimen after test
    Figure 11.4: Broken specimen
  • Stop the test when you observe a drop in the strain versus load plot. The data acquisition system collects real-time data.

Video Materials

Lecture VIdeo

A PowerPoint presentation is created to understand the background and method of this experiment.

Demostration Video

A short video is executed to demonstrate the experiment procedure and sample calculation.

Results and Discussions

Sample Data Sheet

Diameter (d) = 7.29 cm
Length (L0) = 14.78 cm
Mass = 1221.4 g

Table 11.1: Moisture content determination

 \begin{tabular}{|c|c|} \hline Sample No. & 1\hspace{3cm} \\ \hline Moisture can number- Lid number & A \\ \hline $M_C$= Mass of empty, clean can + lid (grams) & 15.6 \\ \hline $M_CMS$= Mass of can, lid, and moist soil (grams) & 45.7 \\ \hline $M_CDS$= Mass of Can, lid, and dry soil (grams) & 39.5 \\ \hline $M_S$= Mass of soil solids (grams) & 23.9 \\ \hline $M_W$ = Mass of pore water (grams) & 6.2 \\ \hline W = Water content & 25.94 \\ \hline \end{tabular}

Area (Ao)= p/ × (7.29)= 41.74 cm2
Volume= p/4 × (7.29)2 × 14.78$= 616.9 cm2
Wet density= 1221.4/616.9 = 1.98 g/cm3
Water content (w%) = 25.9%
Dry density (γd) = 1.98/(1+25.9/100) =1.57 g/cm3

Table 11.2: Unconfined Compression Test Data (Deformation Dial: 1 unit = 0.10mm; LoadDial: 1 unit = 0.3154 lb)

Deformation

Dial Reading

Load

Dial

Reading

Sample

Deformation

(mm)

Strain %

Strain

Corrected

Area, A

Load

(lb)

Load

(kN)

Stress

(kPa)

0 0 0 0 0 41.7 0.0 0.0 0.0
20 4 0.2 0.001 0.1 41.8 1.3 56.1 1.3
40 9 0.4 0.003 0.3 41.9 2.8 126.3 3.0
60 12 0.6 0.004 0.4 41.9 3.8 168.4 4.0
80 19 0.8 0.005 0.5 42.0 6.0 266.6 6.4
100 21 1 0.007 0.7 42.0 6.6 294.7 7.0
120 24 1.2 0.008 0.8 42.1 7.6 336.8 8.0
140 26 1.4 0.009 0.9 42.1 8.2 364.9 8.7
160 29 1.6 0.011 1.1 42.2 9.1 406.9 9.6
180 33 1.8 0.012 1.2 42.3 10.4 463.1 11.0
200 36 2 0.014 1.4 42.3 11.4 505.2 11.9
250 45 2.5 0.017 1.7 42.5 14.2 631.5 14.9
300 54 3 0.020 2.0 42.6 17.0 757.8 17.8
350 64 3.5 0.024 2.4 42.8 20.2 898.1 21.0
400 74 4 0.027 2.7 42.9 23.3 1038.4 24.2
450 84 4.5 0.030 3.0 43.1 26.5 1178.8 27.4
500 93 5 0.034 3.4 43.2 29.3 1305.0 30.2
550 102 5.5 0.037 3.7 43.4 32.2 1431.3 33.0
600 112 6 0.041 4.1 43.5 35.3 1571.7 36.1
650 120 6.5 0.044 4.4 43.7 37.9 1683.9 38.6
700 129 7 0.047 4.7 43.8 40.7 1810.2 41.3
750 138 7.5 0.051 5.1 44.0 43.5 1936.5 44.0
800 144 8 0.054 5.4 44.1 45.4 2020.7 45.8
850 152 8.5 0.058 5.8 44.3 48.0 2133.0 48.2
900 160 9 0.061 6.1 44.4 50.5 2245.2 50.5
950 166 9.5 0.064 6.4 44.6 52.4 2329.4 52.2
1000 171 10 0.068 6.8 44.8 53.9 2399.6 53.6
1100 182 11 0.074 7.4 45.1 57.4 2554.0 56.6
1200 192 12 0.081 8.2 45.4 60.6 2694.3 59.3
1300 202 13 0.088 8.8 45.8 63.7 2834.6 61.9
1400 209 14 0.095 9.5 46.1 65.9 2932.8 63.6
1500 217 15 0.101 10.1 46.5 68.5 3045.1 65.6
1600 223 16 0.108 10.8 46.8 70.3 3129.3 66.9
1700 229 17 0.115 11.5 47.2 72.2 3213.5 68.1
1800 234 18 0.122 12.2 47.5 73.8 3283.7 69.1
1900 240 19 0.129 12.9 47.9 75.7 3367.9 70.3
2000 243 20 0.135 13.5 48.3 76.7 3410.0 70.6
2200 250 22 0.149 14.9 49.0 78.9 3508.2 71.5
2400 253 24 0.162 16.2 49.8 79.8 3550.3 71.2
2600 255 26 0.176 17.6 50.6 80.4 3578.3 70.7
2800 256 28 0.189 18.9 51.5 80.8 3592.4 69.8
3000 254 30 0.203 20.3 52.4 80.1 3564.3 68.1

Unconfined compressive strength, qu = 71.5 kPa
Undrained cohesion, cu = qu/2= 35.75 kPa

A graph showing axial stress in vertical axis and axial strain in horizontal axis. Peak axial stress was determined from this graph.
Figure 11.5: Axial stress vs axial strain graph

Report

Use the template provided to prepare your lab report for this experiment. Your report should include the following:

  • Objective of the test
  • Applications of the test
  • Apparatus used
  • Test procedures (optional)
  • Analysis of test results – Complete the table provided and show one sample calculation.
  • Summary and conclusions – Comment on the cohesion value of the tested sample.

 

 

 

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Properties and Behavior of Soil - Online Lab Manual by MD Sahadat Hossain, Ph.D., P.E.; Md Azijul Islam; Faria Fahim Badhon; and Tanvir Imtiaz is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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