Cive 334 - Dr Song - Experiment\': Consolidation Test Report - 2018 April PDF

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Download Cive 334 - Dr Song - Experiment': Consolidation Test Report - 2018 April PDF

Description

Consolidation Test CIVE 334 Dr. Song Due April 10th, 2018

Purpose This test is designed to determine the rate and magnitude of volume decrease that a soil sample undergoes when subjected to different vertical loads. The data helps to determine the compression and recompression index, coefficient of consolidation and coefficient of secondary consolidation, as well as the preconsolidation pressure.

Equipment Used

Figure 1. Consolidation test machine with load cell and LVDT

Figure 2. Fixed ring consolidation cell

Other Equipment :

-

Soil Sample

Test Procedure 1. Prepare soil specimen for the test by trimming extruded sample from Shelby tube, collect excess soil for determination of moisture content and specific gravity 2. Determine mass of the consolidation ring 3. Place soil specimen on consolidation ring, trim off excess so that the specimen is flush with the top of the ring, record height and diameter of specimen 4. Weigh the consolidation ring and the specimen, record the mass 5. Determine moisture content of excess soil collected in step 1 6. Saturate the bottom porous stone on the base of the consolidation cell, and place soil specimen on the ring over the porous stone 7. Place the upper porous stone on the specimen and attach the top ring to the base of the consolidation cell. 8. Add water to the consolidation cell to submerge the soil and keep it saturated, ensure it is saturated for the entire test 9. Place the consolidation cell in the test machine 10. Apply load to the specimen such that the magnitude of pressure p is 1 psi. The loads applied will be step loading, increasing the load after a specified amount of time. The step loads will be, 1, 4, 8, 16, 32, 64, 128, 32, 8, 32, 128, 256, 64, 16, 4, 1, and 0 psi, in that order. 11. After testing, remove soil and determine moisture content.

Results Table 1. Pressure, Void Ratio, and Calculation of Coefficient of Consolidation

Load Step

Pressure (psi)

1

1

Final Dial Reading (in) -0.0052

2

4

0.0023

3

8

Change in Height ΔH (in)

Final Specimen Height Ht(f) (in)

Height Of Voids Hv (in)

Final Void Ratio e

1.0000

0.4158

0.715077

0.9925

0.4083

0.702202

0.9854

0.4012

0.689935

0.0075 0.0071 0.0094 0.0127 4

16

0.0221

0.9727

0.3885

0.668094

0.9547

0.3705

0.637139

0.9298

0.3456

0.594318

0.8965

0.3123

0.537051

0.0180 5

32

0.0401

6

64

0.065

7

128

0.0983

0.0249 0.0333 Unloading 8 32

-0.0146 0.0837

0.9111

0.3269

0.562159

0.9299

0.3457

0.59449

0.9201

0.3359

0.577637

0.8912

0.3070

0.527937

-0.0188 9 Loading

8

0.0649

10

32

0.0747

11

128

0.1036

0.0098

256

0.139 0.1217

14

64

0.1031

15

16

0.0862

0.8558

0.2716

0.467059

0.8731

0.2889

0.49681

0.8917

0.3075

0.528797

0.9086

0.3244

0.55786

-0.0173

1

-0.3424

0.012

0.01175

0.9889

32.33

5.88

0.006

0.00819

0.9790

40.21

11.77

0.005

0.00401

0.9637

51.01

14.73

0.004

0.0031

0.9422

62.35

15.49

0.003

0.00282

0.9131

72.11

16.73

0.003

0.00245

0.9038

75.36

16.21

0.002

0.00248

0.9056

49.02

35.18

0.004

0.00115

0.8735

72.18

51.80

0.002

0.00073

0.9115 0.9145

0.3303

0.568006

1.3372

0.7530

1.294932

-0.4227 17

t50

4.16

0.9001

-0.0059 0.0803

t90

18.28

0.8824

-0.0169

4

t50

0.9963

0.8644

-0.0186

16

t90

cv * 103 (in2/s) from

0.9250

0.0354 Unloading 13 64

Fitting time (s)

0.9205

0.0289

12

Avg. Height during Consolidation Ht(av) (in)

1.1258

Table 2. Sample and Ring Data

Initial Weight of Ring

212.50

Wet Wt. of Sample + Ring Wet Wt. Of Sample Tare Wt.

366.39 153.89 109.73

Wet Wt. Of Soil + Tare Dry Wt. Of Soil + Tare

238.60 212.61

% Moisture Final Wet Sample + Ring

25.30%

Dry Sample Only

121.44

Dry Sample + Ring % Moisture

335.94 27.52

Description – Lean clay, mottled dark brown and gray, firm and moist (Fill) Specimen Diameter, D = 2.5 in

369.36

Height of Solids, Hs = 0.5815 in Cc, Cs = 0.19024, 0.0391(respectively) Preconsolidation Pressure, pc = 6.89psi

Initial Specimen Height, Ht(i)= 1 in Specific Gravity, Gs = 2.68 Figure 1. Total Disp. Vs Log(time) 0.0080 0.0100

Total Disp. (in)

0.0120 0.0140 0.0160 0.0180 0.0200 0.0220 0.10

1.00

10.00

log(time) (min)

100.00

1000.00

Figure 2. Total Disp. vs Square Root Time

0.0085 0.0105

Total Disp. (in)

0.0125 0.0145 0.0165 0.0185 0.0205 0.0225 0.0000

1.0000

2.0000

3.0000

4.0000

5.0000

Square Root time (min0.5)

Figure 3. Void Ratio vs. log(Pressure)

6.0000

7.0000

8.0000

Figure 4. Cv vs log(Pressure)

0.01

cv x103 (in2/s)

0.01 0.01 t9 0 t5 0

0.01 0.01 0 0 0.15

1.5

15

Pressure (psi)

Sample Calculations

Height of Solids, Hs Mass of dry soil specimen =H s π 2 D G s ρw 4

( ) 2.54

cm ∗2.5∈¿ ¿ ¿

π g ∗( ¿ 2¿ )∗2.68∗1 3 =1.477 cm =0.5815 inches 4 cm ¿ 122.41 g ¿

Difference in Height, ΔH ΔH =Final Height of Step Load−Initial Height of Step Load ¿ 0.0747∈−0.0649∈¿ 0.0097∈¿

Height at end of step load, Ht(f)

150

Height at beginning of step load− ΔH=H t (f ) 0.9925 ∈−0.0071 ∈¿ 0.9854 inches

Average Height during consolidation H t (av)=

Height at beginning of stepload−Height at end of step load 2 H t (av)=

0.9925−0.9854 =0.9889 inches 2

Height of Voids H v =H t ( f −H s=¿ ) 0.9925 ∈−0.58149 ∈¿ 0.4083 ∈¿ Void Ratio, e e=

H v 0.4083 = =0.689935∈¿ H s 0.58149

Coefficient of Consolidation, cv 2

2 0.848∗0.9889∈¿ =0.006 ¿ s 4∗32.33 s 2 0.848 H t (av ) =¿ For t 90 −c v = ¿ 4 t 90

2 0.197∗0.9889∈¿ 2 =0.00819 ¿ 4∗5.88 s s 2 0.197 H t ( av) =¿ For t50 −c v = 4 t 50¿

* t90 and t50 determined graphically, according to Soil Mechanics Lab Manual, Braja M. Das

Discussion

At first glance, figures 3 and 4 match up very closely with what was expected and match up closely to the example graphs within the lab manual. The loading of the soil curves downward with a linear region eventually beginning around a void ratio of e = 0.595, allowing the Cc to be calculated, then upon unloading, the graph curves up slightly. The estimated linear region fits in nicely with the graph and allows quick determination for the Cs value. The values for the Compression Index, Cc, and Swell Index, Cs, determined through testing were near those estimated from the sample equations 17.9 – 17.12 in the lab manual. Using those equations, values of 0.15906 and 0.0367 were obtained, with determined values of 0.19024 and 0.0391, showing that these equations are good enough to get a preliminary estimation of values to be obtained before testing the soils. The range of cv values appears to be rather low when compared to the example values in the lab manual. The low value for cv means that the soil likely generally very compressible, meaning a greater decrease in void space for a soil must occur to expel the water from the soil. Due to more water having to be expelled from the voids, it will take a longer amount of time, consequently leading to a lower rate of consolidation. The void ratio of the soil began at 0.715 prior to loading and finished at 0.568 at the end of unloading at the end of consolidation, with a low void ratio of 0.467 at a loading of 256psi. The finishing void ratio was only about 80% of the initial void ratio. The drainage of the soil appears to be low, leading to a higher settlement time.

Error

This test appears to have quite a bit of error, even just glancing over the graphs. When comparing Figures 1 and 2 to the figures within the lab manual, they look off. The curve in figure two is not an even curve and has bumps in it. This would lead to an inaccurate value for the t90 because the initial tangent that has to be found returns a very steep sloped line, much steeper than that in the lab manual. This in turn leads to a steeper second line than what is expected. The problem with the two lines being steep is that when the second line is steep, it crosses the curve sooner than it should, giving a t90 value that is much lower what it is. The curve in figure 1 is not as pretty as the curve in the lab manual but returned a value for t50 that appeared to be in a semi-acceptable range. The error in the value for t90 leads directly to a low value calculated for cv. The low value for cv would lead one to believe the soil is less compressible than it might actually be and may deter the soil from being using in a certain scenario even though the true value is within the acceptable range for the scenario. One anomaly in the data was the final void ratio at the very end of the test. The value was over 1, which is not possible, because it would mean the amount of voids within the soil exceed the height of soil there, which does not make sense. This value is calculated using a series of

variables, beginning with the ΔH. The ΔH at the end of the last step is a rise of 0.4226 inches. This rise in the soil does not make sense, because it exceeds the total amount the soil was compressed. The soil cannot grow about the initial height, which it does in this case. If the soil was consolidated on a faster schedule, so less time during each step, the soil would not have had the same loss in void ratio because the amount of water being expelled in each step would be less. This would lead to the overall consolidation of the soil to be much less, leading the coefficient of consolidation to be even lower than it truly is....