Atteberg Limits Test for Austin's clayey soil

University of Texas at Arlington

Civil Engineering Department

Laboratory Test Report

Atterberg Limits of Soil from Austin, Texas

Written by:

Richard Benda

Joseph Muhirwa

Robert Sargent

June 28th, 2012

Table of Contents                                                                                         Page

Table of Contents ….........................................................................................1
List of Tables and Figures.................................................................................2
Abstract….........................................................................................................3
Introduction........................................................................................................3

Equipment and Materials……………………………………………...............4
Methods and Procedure.....................................................................................4
Data, Results, and Discussion............................................................................6
Conclusion.........................................................................................................8
References..........................................................................................................8

List of Tables and Figures                                                                             Page

Figure 1: Mixed soil and Casagrande Machine....................................................3
Figure 2: Casagrande Cup....................................................................................4
Figure 3: Soil Thread...........................................................................................5
Table 1: Liquid Limit and Plastic Limit Datasheet………………………….......6

Figure 4: Liquid Limit vs. Number of Blows (with outlier)……………….........7

Figure 5: Liquid Limit vs. Number of Blows (without outlier)…………………7

Abstract
            In this laboratory test the liquid limit, plastic limit, and plasticity index of a soil sample from Austin, Texas was determined using ASTM test D4318, Standard Test Methods for Liquid limit, Plastic Limit, and Plasticity Index of Soils. This particular test is applicable for fine-grained soils that can pass through a number 40 U.S. sieve (less than 425 microns in size). These Atterberg limits are important because they are a basic measure of the nature of a fine-grained soil. The soil sample used in this experiment was tan-colored and seemed to absorb plenty of water. The liquid limit of the soil was determined using a Casagrande cup, which consists of placing the moist soil sample into the metal dish, where it is then grooved and dropped repeatedly until the groove closes. The number of blows is recorded, and this is done at least 3 times in order to be plotted and determine the liquid limit. The process of finding the plastic limit includes rolling a small thread of soil multiple times. When the thread cracks, this means the soil has become brittle and has reached the plastic limit. The plasticity index of the soil is merely the plastic limit subtracted from the liquid limit.


Introduction
            In order to find the expansive potential of our soil, we decided to use the Atterberg limits to see if our soil had a high amount of clay. As mentioned the Atterberg limits are very important and can give a good indication of what type of soil you are dealing with. As water is being added to soil, the soil is going through different phases of change. The soil starts out as a solid but as more water is added it goes to semisolid, then plastic, and finally liquid. The plastic limit and liquid limit are the moisture contents when the soil enters the plastic and liquid phases. These Atterberg limits are important because of the fact that they can tell information such as if our soil has a good amount of clay. From these limits the plasticity index can be found which is a direct indication of how much clay is in your soil. The plasticity index shows the range of moisture content that the soil will be in the plastic range. Because clay can absorb so much water before going into the liquid phase, soils with high clay will have a big range for it to be in the plastic phase. Since the soil that we are testing comes from an area with high amounts of clay we expect to get a high plasticity index for this soil.

Equipment and Materials

To carry out this experiment, we used the following materials:

Ø  Fine grained soil and # 40seive (0.425mm opening)

Ø  Distilled water

Ø  Mass scale

Ø  Ceramic soil mixing bowl

Ø  Soil drying oven

Ø  Frosting knife

Ø  Liquid limit device

Ø   Grooving tool (for liquid limit test)

Ø  3 soil moisture containers for liquid limit test.

Ø  1 soil moisture containers for plastic limit test

Ø  Marker for labeling the soil moisture containers

Ø  Plastic limit Device

Methods and Procedure

Below are steps to be followed to determine the liquid limit and plastic limit tests:
1)       Liquid limit:
a)       Pass the soil through a # 40 sieve and use the fraction that passes the sieve
b)       Add distilled water to a 200 g of soil until it has a consistency of peanut butter or frosting
c)       Spread a flat layer of soil in the dropping cup with a frosting knife and use the grooving tool to cut a groove in the soil
d)       Turn the crank on the liquid limit device at 2 cranks per second and closely observe the groove.  Afterwards, count and record the number of cranks that are required to close the groove over a length of 0.5 inch. The tail of the grooving tool can be used to check the 0.5 inch length.
e)       Clean out the cup and repeat steps c)-d) until successive trials yield consistent results that are within a few cranks of each other and record the average number of cranks for soil. This usually requires the increase of moisture or water in the soil sample.
2)       Plastic limit:
a)       Pass the soil through # 40 sieve
b)        Add some distilled water to 50 g of soil to make mud balls that would stick to the wall once thrown.
c)        Take a pea-sized mud ball and roll it out into the frosted plate to form a rod with a diameter of 0.125inches. Use a 0.125 diameter metal rod as a reference. If the soil crumbles at the first time, add some more water and repeat the same process.
d)       Keep rolling the mud ball until a 0.125 inch diameter rod crumbles.
e)       The plastic limit is the moisture content at which the soil rod starts to crumble.
3)       Plastic Index (PI)
The plastic Index is difference between the Liquid limit and the plastic limit. In other words, the Plastic Index can be calculated by subtracting the Plastic Limit from the Liquid Limit.

Data, Results and Discussion

Table 1: Liquid Limit and Plastic Limit Datasheet

Trial Number	1	2	3	4
Container ID	LL 1	LL 2	LL 3	PL 1
Mass container(Mc)	32.4 g	22 g	32.90 g	28.8g
Mass of moist soil + container(M1)	61.9g	46.3 g	58.8 g	38.5g
Mass of dry soil + container( M2)	51.9 g	38.3 g	49.8 g	36.9g
Mass of moisture (Mw)	10 g	8 g	9g	1.6g
Mass of dry soil(Ms)	19.5 g	16.3g	16.9g	8.1 g
Moisture (w)	51.28%	50.96%	53.25%	19.76%
Number of Cranks	24	20	16
Liquid Limit (LL)	51.04%
Corresponding plastic Limit	19.76%
Plasticity index	30.84%

Figure 4: Liquid Limit vs. Number of Blows (with outlier)

Figure 5: Liquid Limit vs. Number of Blows (without outlier)

When doing the graph for the liquid limit test a problem was encountered. Instead of a nice straight line to tell us the moisture content at 25 blows, an incorrect graph was created due to an outlier in our data. We are not sure why this outlier happened. During the test problems with the dropping cup were encountered so the outlier could have been created because of the machine not working properly. To fix this problem we did the graph again without that point which created what we desired. From the new graph we were able to determine the moisture content at 25 blows and find our liquid limit.

Conclusion
            By performing ASTM test D4318 the liquid limit, plastic limit, and then ultimately the plasticity index of this soil sample was able to be determined. This sample had a liquid limit of 50.6 and a plastic limit of 19.76, which resulted in a plasticity index of 30.84. High plasticity clays typically have a plasticity index range of about 20-40, which makes this sample from Austin, Texas a high plasticity clay. This was just as expected, as the sample was very sticky and was able to absorb a very high amount of water before becoming malleable when being mixed. Now that is has been determined that this soil is highly expansive further testing can be done to determine how this soil can be stabilized. For future testing extra precautions will be taken to make sure that all equipment used in the experiment is in perfect working order. Failing to make sure the Casagrande machine was properly calibrated could have easily resulted in poor/inaccurate results which would affect the analysis of the soil. As the Atterberg limits are some of the basic classifications of a soil, it is important that the results are accurate as these results will carry to the rest of the tests that are going to be performed on this sample.

References

ASTM. (n.d.). Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils, ASTM D4318, 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, 6th ed., Oxford University Press, New York, N.Y..