Experiment 13-Unconfined Compression.pdf

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Krishna Reddy UIC Laboratory TEST Prof Prof Krishna Reddy TEST Prof Krishna TEST soil Engineering Properties
 
Engineering Properties of Soils Based on Laboratory Testing 
Prof. Krishna Reddy, UIC 
 
145 
EXPERIMENT 12 
UNCONFINED COMPRESSION (UC) TEST 
 
 
Purpose: 
 
The primary purpose of this test is to determine the unconfined compressive 
strength, which is then used to calculate the unconsolidated undrained shear 
strength of the clay under unconfined condi tions.  According to the ASTM standard, 
the unconfined compressive strength (qu) is defined as the compressive stress at 
which an unconfined cylindrical specimen of soil will fail in a simple compression 
test.  In addition, in this test method, the unconfined compressive strength is taken 
as the maximum load attained per unit area, or the load per unit area at 15% axial 
strain, whichever occurs first during the performance of a test. 
 
 
Standard Reference: 
 
ASTM D 2166 - Standard Test Method for Unconfined Compressive Strength 
of Cohesive Soil 
 
 
Significance: 
 
For soils, the undrained shear strength (su) is necessary for the 
determination of the bearing capacity of foundations, dams, etc.  The undrained 
shear strength (su) of clays is commonly determined from an unconfined 
compression test.  The undrained shear strength (su) of a cohesive soil is equal to 
one-half the unconfined compressive strength (qu) when the soil is under the f  = 0 
condition (f  = the angle of internal friction).  The most critical condition for the soil 
usually occurs immediately after construction, which represents undrained 
conditions, when the undrained shear strength is basically equal to the cohesion 
(c). This is expressed as: 
 
Engineering Properties of Soils Based on Laboratory Testing 
Prof. Krishna Reddy, UIC 
 
146 
 
 
 
 
 
su = c = 2
u
q
 
Then, as time passes, the pore water in the soil slowly dissipates, and the 
intergranular stress increases, so that the drained shear strength (s), given by s = c 
+ s‘tan f , must be used. Where s‘ = intergranular pressure acting perpendicular to 
the shear plane; and s‘ = (s - u), s = total pressure, and u = pore water pressure; c’ 
and ϕ’ are drained shear strength parameters. The determination of drained shear 
strength parameters is given in Experiment 14 
 
 
Equipment: 
 
Compression device, Load and deformation dial gauges, Sample trimming 
equipment, Balance, Moisture can. 
 
 
 
 
 
Engineering Properties of Soils Based on Laboratory Testing 
Prof. Krishna Reddy, UIC 
 
147 
 
 
 
Test Procedure: 
(1) 
Extrude the soil sample from Shelby tube sampler. Cut a soil specimen so 
that the ratio (L/d) is approximately between 2 and 2.5.  
 
Where L and d are the length and diameter of soil specimen, respectively. 
 
(2) 
Measure the exact diameter of the top of the specimen at three locations 
120° apart, and then make the same measurements on the bottom of the 
specimen.  Average the measurements and record the average as the 
diameter on the data sheet. 
 
(3) 
Measure the exact length of the specimen at three locations 120° apart, and 
then average the measurements and record the average as the length on 
the data sheet.   
 
 
Engineering Properties of Soils Based on Laboratory Testing 
Prof. Krishna Reddy, UIC 
 
148 
(4) Weigh the sample and record the mass on the data sheet. 
 
(5) 
Calculate the deformation (∆L) corresponding to 15% strain (ε). 
Strain (e) = 
oL
L
∆
 
 
Where L0 = Original specimen length (as measured in step 3). 
 
(6) 
Carefully place the specimen in the compression device and center it on the 
bottom plate.  Adjust the device so that the upper plate just makes contact 
with the specimen and set the load and deformation dials to zero. 
 
(7) 
Apply the load so that the device produces an axial strain at a rate of 0.5% to 
2.0% per minute, and then record the load and deformation dial readings on 
the data sheet at every 20 to 50 divisions on deformation the dial. 
 
(8) 
Keep applying the load until (1) the load (load dial) decreases on the 
specimen significantly, (2) the load holds constant for at least four 
deformation dial readings, or (3) the deformation is significantly past the 15% 
strain that was determined in step 5. 
 
(9) 
Draw a sketch to depict the sample failure. 
 
(10) Remove the sample from the compression device and obtain a sample for 
water content determination.  Determine the water content as in Experiment 
1. 
 
 
Engineering Properties of Soils Based on Laboratory Testing 
Prof. Krishna Reddy, UIC 
 
149 
Analysis: 
(1) 
Convert the dial readings to the appropriate load and length units, and enter 
these values on the data sheet in the deformation and total load columns. 
(Confirm that the conversion is done correctly, particularly proving dial gage 
readings conversion into load) 
 
(2) 
Compute the sample cross-sectional area 
( )2
d
4
0
A
×
π
=
 
(3) 
Compute the strain, 
0
L
?L
e =
 
(4) 
Computed the corrected area, 
e
1
0
A
'A
−
=
 
 
(5) 
Using A’, compute the specimen stress, s c = 
'
A
P
 
 
(Be careful with unit conversions and use constant units). 
 
(6) 
Compute the water content, w%. 
 
(7) 
Plot the stress versus strain.  Show qu as the peak stress (or at 15% strain) 
of the test.  Be sure that the strain is plotted on the abscissa. See example 
data. 
 
(8) 
Draw Mohr’s circle using qu from the last step and show the undrained shear 
strength, su = c (or cohesion) = qu/2.  See the example data. 
 
 
 
 
 
 
Engineering Properties of Soils Based on Laboratory Testing 
Prof. Krishna Reddy, UIC 
 
150 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
EXAMPLE DATA 
 
 
 
Engineering Properties of Soils Based on Laboratory Testing 
Prof. Krishna Reddy, UIC 
 
151 
UNCONFINED COMPRESSION TEST 
DATA SHEET 
 
 
Date Tested: August 30, 2002 
Tested By: CEMM315 Class, Group A 
Project Name: CEMM315 Lab  
Sample Number: ST-1, 8’-10’ 
Visual Classification: Brown silty clay, medium plasticity, moist CL. 
 
Sample data: 
 
Diameter (d) = 7.29 cm 
 
Length (L0) = 14.78 cm 
Mass = 1221.4 g 
Table 1: Moisture Content determination 
Sample no. 
ST-1, 8-10 
Moisture can number - Lid number 
A 
MC = Mass of empty, clean can + lid (grams) 
15.6 
MCMS  = Mass of can, lid, and moist soil (grams) 
45.7 
MCDS = Mass of can, lid, and dry soil (grams) 
39.5 
MS = Mass of soil solids (grams) 
23.9 
MW = Mass of pore water (grams) 
6.2 
W = Water content, w% 
25.94 
 
Area (A0)  = 
(
)
7.29
4
×
p
 = 41..74 cm2      
Volume = 
(
)
14.78
2
7.29
4
×
×
p
  = 616.9 cm3 
Wet density = 
616.9
1221.4
= 1.98 g/cm3 
Water content (w%) = 25.9 % 
Dry density (?d) = 





 +
100
25.9
1
1.98 = 1.57 g/cm3 
 
 
 
Engineering Properties of Soils Based on Laboratory Testing 
Prof. Krishna Reddy, UIC 
 
152 
Table 2: Unconfined Compression Test Data (Deformation Dial: 1 unit = 0.10mm; 
Proving Ring No: 24691; Load Dial: 1 unit = 0.3154 lb) 
Deformation 
Dial Reading 
Load Dial 
Reading 
Sample 
Deformation 
∆L (mm) 
Strain 
(ε) 
% 
Strain 
Corrected 
Area A' 
Load 
(lb) 
Load 
(KN) 
Stress 
(kPa) 
0 
0 
0 
0.000 
0.000 
41.739  
0.000 
0.000 
0.000 
20 
4 
0.2 
0.001 
0.135 
41.796 
1.262 
56.131 
1.343 
40 
9 
0.4 
0.003 
0.271 
41.853 
2.839  126.295 
3.018  
60 
12 
0.6 
0.004 0.406 
41.909 
3.785 168.393 
4.018  
80 
19 
0.8 
0.005 
0.541 
41.966 
5.994 266.622 6.353 
100 
21 
1 
0.007 0.677 42.024 
6.625 294.687 7.012 
120  
24 
1.2 
0.008 
0.812 
42.081 
7.571 336.786 
8.003 
140 
26 
1.4 
0.009 
0.947 
42.138 
8.202 364.851 
8.658  
160 
29 
1.6 
0.011 
1.083 
42.196 
9.148 406.949 
9.644 
180 
33 
1.8 
0.012 
1.218  
42.254 
10.410  463.080 10.959  
200 
36 
2 
0.014 
1.353 
42.312 
11.356  505.178  11.939  
250  
45 
2.5 
0.017 
1.691 
42.457 
14.196 631.473 14.873 
300 
54 
3 
0.020  
2.030  
42.604 17.035 757.768 17.786 
350  
64 
3.5 
0.024 2.368 
42.752 20.189 898.095 21.007 
400 
74 
4 
0.027 2.706 
42.900 23.344 1038.422 24.205 
450  
84 
4.5 
0.030  
3.045 
43.050  26.498 1178.750  27.381 
500 
93 
5 
0.034 
3.383 
43.201 
29.338 1305.044 30.209 
550  
102 
5.5 
0.037 3.721 
43.353 32.177 1431.339  33.016 
600 
112 
6 
0.041 4.060 
43.505 
35.331 1571.666 36.126  
650  
120  
6.5 
0.044 4.398 
43.659  37.855 1683.928  38.570  
700 
129  
7 
0.047 4.736 
43.814 40.694 1810.223 41.316 
750  
138 
7.5 
0.051 
5.074 
43.971 43.533 1936.517 44.041 
800 
144 
8 
0.054 
5.413 
44.128  45.426  2020.714 45.792 
850  
152 
8.5 
0.058  
5.751 
44.286 47.950  2132.976 48.163 
900 
160 
9 
0.061 
6.089 
44.446 50.473 2245.237 50.516 
950  
166 
9.5 
0.064 6.428  
44.606 52.366 2329.434 52.222 
1000 
171 
10 
0.068 6.766 44.768 53.943 2399.598 53.600 
1100 
182 
11 
0.074 7.442 
45.096 57.413 2553.958  56.634 
1200 
192 
12 
0.081 
8.119 
45.428  60.568 2694.285 59.309 
1300 
202 
13 
0.088 
8.796 45.765 63.722 2834.612 61.939  
1400 
209 
14 
0.095 
9.472 46.107 65.931 2932.841 63.610  
1500 
217 
15 
0.101 
10.149 46.454 68.454 3045.103 65.551 
1600 
223 
16 
0.108 10.825 46.806 70.347 3129.300 66.856  
1700 
229  
17 
0.115 11.502 47.164 72.240 3213.496 68.134 
1800 
234 
18 
0.122 12.179  47.527 73.817 3283.660 69.090 
1900 
240 
19 
0.129  12.855 47.896 75.710  3367.856  70.315 
2000 
243 
20 
0.135 13.532 48.271 76.656  3409.954 70.642 
2200 
250  
22 
0.149 14.885 49.039  78.864 3508.184 71.539  
2400 
253 
24 
0.162 16.238 49.831 
79.811 3550.282 71.247 
2600 
255 
26 
0.176 17.591 50.649 80.442 3578.347 70.650  
2800 
256  
28 
0.189 18.945 51.495 80.757 3592.380 69.762 
3000 
254 
30 
0.203 20.298 52.369 
80.126  3564.314 68.062 
 
Engineering Properties of Soils Based on Laboratory Testing 
Prof. Krishna Reddy, UIC 
 
153 
SAMPLE: ST-1, 8'-10'
0
10
20
30
40
50
60
70
80
0
5
10
15
20
25
Axial Strain (%)
Axial Stress (kPa)
 
 
From the stress-strain curve and Mohr’s circle: 
Unconfined compressive strength (qu) = 72.0 KPa 
Cohesion (c) = 36.0 KPa 
 
Engineering Properties of Soils Based on Laboratory Testing 
Prof. Krishna Reddy, UIC 
 
154 
 
 
 
 
 
 
 
 
 
 
BLANK DATA SHEETS 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Engineering Properties of Soils Based on Laboratory Testing 
Prof. Krishna Reddy, UIC 
 
155 
UNCONFINED COMPRESSION TEST 
DATA SHEET 
 
 
Date Tested:  
Tested By:  
Project Name:  
Sample Number:  
Visual Classification:  
 
Sample data: 
 
Diameter (d) = 
 
Length (L0) =  
Mass =  
 
Table 1: Moisture Content determination 
Sample no. 
 
Moisture can number - Lid number 
 
MC = Mass of empty, clean can + lid (grams) 
 
MCMS  = Mass of can, lid, and moist soil (grams) 
 
MCDS = Mass of can, lid, and dry soil (grams) 
 
MS = Mass of soil solids (grams) 
 
MW = Mass of pore water (grams) 
 
W = Water content, w% 
 
 
Area (A0)  =  
Volume =    
Wet density =  
Water content (w%) =  
Dry density (?d) =  
 
 
Engineering Properties of Soils Based on Laboratory Testing 
Prof. Krishna Reddy, UIC 
 
156 
Table 2: Unconfined Compression Test Data (Deformation Dial: 1 unit = 0.10mm; 
Proving Ring No: 24691; Load Dial: 1 unit = 0.3154 lb) 
Deformation 
Dial Reading 
Load Dial 
Reading 
Sample 
Deformation 
∆L (mm) 
Strain 
(ε) 
% 
Strain 
Corrected 
Area A' 
Load 
(lb) 
Load 
(KN) 
Stress 
(kPa) 
0 
 
 
 
 
 
 
 
 
20 
 
 
 
 
 
 
 
 
40 
 
 
 
 
 
 
 
 
60 
 
 
 
 
 
 
 
 
80 
 
 
 
 
 
 
 
 
100 
 
 
 
 
 
 
 
 
120  
 
 
 
 
 
 
 
 
140 
 
 
 
 
 
 
 
 
160 
 
 
 
 
 
 
 
 
180 
 
 
 
 
 
 
 
 
200 
 
 
 
 
 
 
 
 
250  
 
 
 
 
 
 
 
 
300 
 
 
 
 
 
 
 
 
350  
 
 
 
 
 
 
 
 
400 
 
 
 
 
 
 
 
 
450  
 
 
 
 
 
 
 
 
500 
 
 
 
 
 
 
 
 
550  
 
 
 
 
 
 
 
 
600 
 
 
 
 
 
 
 
 
650  
 
 
 
 
 
 
 
 
700 
 
 
 
 
 
 
 
 
750  
 
 
 
 
 
 
 
 
800 
 
 
 
 
 
 
 
 
850  
 
 
 
 
 
 
 
 
900 
 
 
 
 
 
 
 
 
950  
 
 
 
 
 
 
 
 
1000 
 
 
 
 
 
 
 
 
1100 
 
 
 
 
 
 
 
 
1200 
 
 
 
 
 
 
 
 
1300 
 
 
 
 
 
 
 
 
1400 
 
 
 
 
 
 
 
 
1500 
 
 
 
 
 
 
 
 
1600 
 
 
 
 
 
 
 
 
1700 
 
 
 
 
 
 
 
 
1800 
 
 
 
 
 
 
 
 
1900 
 
 
 
 
 
 
 
 
2000 
 
 
 
 
 
 
 
 
2200 
 
 
 
 
 
 
 
 
2400 
 
 
 
 
 
 
 
 
2600 
 
 
 
 
 
 
 
 
2800 
 
 
 
 
 
 
 
 
3000 
 
 
 
 
 
 
 
 
 
Engineering Properties of Soils Based on Laboratory Testing 
Prof. Krishna Reddy, UIC 
 
157 
0
20
40
60
80
100
120
140
160
180
0
5
10
15
20
25
Axial Strain (%)
Axial Stress (kPa)
0
10
20
30
40
50
60
70
80
90
100
0
10
20
30
40
50
60
70
80
90
100
Normal Stress (kPa)
Shear Stress (kPa)
 
From the stress-strain curve and Mohr’s circle: 
Unconfined compressive strength (qu) =  
Cohesion (c) =