Unconfined Compressive Strength Test

University of Texas at Arlington

Civil Engineering Department

Laboratory Test Report

Unconfined Compressive Strength on Soil from Austin, TX.

Written by:

Richard Benda

Joseph Muhirwa

Robert Sargent

July 11, 2012

Table of Contents                                                                                                        Page
Abstract...........................................................................................................................3
Introduction.....................................................................................................................4
Equipment and Materials................................................................................................4
Methods and Procedure...................................................................................................5
Data, Results, and Discussion.........................................................................................6
Conclusion.......................................................................................................................7
References.......................................................................................................................8

List of Tables and Figures                                                                                          Page
Figure 1 – Samples ready for curing………………………………………………….…4

Figure 2 – Sample being tested……………………………………………………….…5

Figure 3 – Samples after testing………………………………………………………...6

Figure 4 – Stress vs. Axial Strain…………………………………………………….....7

Abstract

One very important property of a soil is its compressive strength, which is the soil’s ability to withstand axial forces. The compressive strength of a soil is very important in nearly all geotechnical engineering designs because it is used in obtaining an estimate of the soil strength. For a soil sample from Austin, Texas, ASTM D2166-06 Standard Test Method for Unconfined Compressive Strength of Cohesive Soil was used to determine the unconfined compressive strength (q_u) of the soil. This particular test is applicable for cohesive and fine-grained soils only. The soil used in this test was a high-plasticity clay from Austin, Texas. A control sample with no lime was compared to three samples with 2, 4, and 6 percent lime mixed into them in order to determine what mixture of lime increases unconfined compressive strength the most. The unconfined compressive strength test first consists of preparing a sample at its optimum moisture content in a cylindrical mold. The sample is then extruded from the mold using a sample extruder. Next, the sample can be tested. However, in this test the lime samples were allowed to cure in a moisture room for 7 days. When the samples were finished curing an unconfined compressive strength machine (triaxial machine) was used to compress the samples. A triaxial cell is placed over the sample without any compression applied. The attached data acquisition device is used to collect information, and the test is stopped when there is a drop observed in the strain versus load plot. The plot can then be used to determine the unconfined compressive strength of the soil, and from that the most effective lime mixture can be obtained.

Introduction

The UCS test was done to help determine the best percentage of lime to stabilize a soil by finding the unconfined compressive strength. This value is used in all engineering design because it is a quick way to gain a good idea of the soil’s strength. It allows important values to be obtained such as the shear strength and cohesion of the soil being tested. The test involves putting a triaxial load on the specimen and then waiting for it to fail. After all the data was collected the stress and strain was calculated by using the load and deformation data and the known dimensions of the specimen. The expectations for this test were that the UCS value would go up as the percentage of lime increased in the sample, meaning the 6 percent lime sample should give the highest strength. At the end of this test data was obtained that showed how much each percentage of lime affected the strength, giving the ability to make an economical decision about what percentage of lime to use.   

Equipment and Materials

• Unconfined compression testing machine
• Sample extruder
• Balance
• Sample preparation equipment

Methods and Procedure

1.      Determine the amount of soil to be used from the optimum moisture content, dry density of the soil (obtained prior to the experiment from the standard Proctor test), and the size of the mold to be used in the experiment. The mold should have a height-to-diameter ratio (L/D) of between two and three.

2.      Mix the water into the sample. For the 2, 4, and 6% lime samples, add the proper amount of lime and additional water to compensate for the hydration of the lime.

3.      Using a proctor hammer, begin to compact the soil into the mold. This should be done in 5 layers. After one layer is done, the top of that layer is grooved with a spatula. This is repeated for the remaining 4 layers.

4.      Use the specimen extruder to remove the sample from the mold.

5.      Carefully wrap the specimens in saran wrap and place them in the moisture room to cure for a week. This step is not necessary if no lime is used.

6.      After the curing process, take the first sample and place it on the triaxial machine and make sure it is centered. Place the triaxial cell over the sample.

7.      Begin the test and the data logger.

8.      Repeat steps 6 and 7 for the remaining samples.

9.      The data from the acquisition system can then be used to determine the unconfined compression strength (q_u) of each sample.

Data, Results and Discussion

The unconfined compressive strength test was used as a simple means to measure the unconfined compressive strength of a non-treated specimen and three lime-treated specimens of Austin’s soil. From this information the bearing capacity of lime treated soil and control soil could be compared. The highest unconfined compressive strength was obtained from the specimen with 6% lime. The unconfined compressive strengths of all four specimens are shown in figure 4. Based on the failure plane of the specimens and the relationship between the consistency of cohesive soils and unconfined compressive strength (Das 2002), the control soil was able to be classified as a soft soil and the three lime-treated soils as very stiff soils. Based on figure 4 it can be said that as the lime percentage increases, the compressive strength increases as well.  Since the undrained shear strength is directly proportional to the unconfined compressive strength, an increase of lime percentage would also lead to high undrained shear strength, therefore resulting in a high bearing capacity of the soil. In overall, it can be said that the lime-treated soil with 6% lime has the highest bearing capacity.

Figure 4 - Stress vs. Axial Strain

       

Conclusion

By performing the unconfined compressive strength test, the compressive strength of each sample was determined. From this data, the most effective amount of lime to add to a sample to increase its strength was obtained. The sample with six percent lime had the overall highest bearing capacity, though the sample with four percent came in a very close second. As the four percent sample still was quite strong, it could still be used in situations where a very high bearing capacity is not required, or a lower cost is required. However, even paying for additional lime in the soil is more economical than paying for additional maintenance costs if the soil fails or the foundation atop the soil is damaged. The main source of error in this laboratory test most likely resulted from improper trimming during sample preparation. Improper trimming during preparation can often result in the load from the triaxial machine getting distributed unevenly, which could cause the sample to fail early. In turn, this causes us to obtain faulty readings and a lower unconfined compressive strength is obtained. Also, this same situation could occur if the sample was not completely centered on the triaxial machine. Though adding lime to a sample does make the soil more brittle, it does significantly increase the bearing capacity of the soil for use in foundations.

References

ASTM. (n.d.). Standard Test Method for Unconfined Compressive Strength of Cohesive Soil, ASTM D2166-06, West Conshohocken, PA.

Das, Braja M. (2009). Principles of Geotechnical Engineering, 25th ed., Cengage Learning, Stanford, CT.

Das, Braja M. (2002). Soil Mechanics Laboratory Manual, 7th ed., Oxford University Press, New York, N.Y..