|THE COOPER UNION
Albert Nerken School of Engineering
Soil Mechanics Laboratory
Experiment No. 9 - Consolidation of Clays
|To determine the stress-strain characteristics of a laterally confined sample of
|1) 'Soil Testing for Engineers' by T. W. Lambe; Chapter IX.
2) "Engineering Properties of Soils and Their Measurements", 4th ed. by Joseph E. Bowles; Experiment No. 13.
3) A.S.T.M. Standards, 1997; (4.08) Designation D2435-96.
|1) 2.5 in.
diameter fixed ring consolidometer
2) a. Loading frame
7) Sample extruder
8) Sample trimmer
9) Miscellaneous apparatus:
|1) Determine the height and diameter of the consolidation ring,
and record it on Data Sheet (A).
2) Weigh the ring and two watch glasses on the triple beam balance and record this weight on Data Sheet (A).
3) Using the sample extruder, extrude approximately a 2.0 in. length of sample from the sampling tube.
4) Using a wire saw, cut this 2.0 in. piece from the remainder of the sample still in the sampling tube. Recap the sampling tube and seal with a liberal coating of hot parrafin.
5) Set the consolidation ring into the sample trimmer and place the 2.0 in. disc of soil on top of the ring.
6) Insert the trimmer tool in its holder, and adjust it horizontally so that it just makes contact with the vertical edges of the soil sample.
7) Rotate the sample and the ring about their vertical axis, and proceed to take a light cut with the trimming tool. After this first cut, advance the trimming tool horizontally for another light cut and rotate again to take the cut. Each successive cut will reduce the diameter of the sample until it becomes 2.5 in., and will just fit into the ring. Insert it into the ring by gently pressing the sample using a saturated porous stone.
8) Repeat this trimming process until the sample is protruding from the lower end of the ring by approximately 0.50 in..
9) Remove the ring and soil sample from the sample trimmer, and using the wire saw, trim the portions of the sample which are protruding from the ends of the ring. Perform this trimming in such a manner as to leave the two soil surfaces absolutely flush with the top and bottom of the ring.
10) Place the ring containing the trimmed soil sample between the same two watch glasses from Step 2, and weigh it on the triple beam balance. Record this the weight on Data Sheet (A).
11) Take a saturated porous stone and set it into the base of the consolidometer.
12) Place the ring containing the soil into the consolidometer, and attach the clamp ring and gasket with the six screws.
13) Take a second saturated porous stone, and center it very carefully in the ring. If this centering is not done carefully, the stone will be in contact with the ring and the full load will not be applied to the sample during the test.
14) Pack cotton batting around this upper porous stone, and moisten it.
15) Place the dial indicator on the supporting rods of the consolidometer, and set the entire assembly into the loading frame following the procedure in either (A) or (B), depending on the loading system to be used:
(A) Lever System Loading Frame ( Note: record data on Data Sheet (B) ):
1) Adjust the sliding counterweight on the overhead beam of the lever system until it is positioned to completely balance the weight of all the other lever system components which come to bear on the soil sample.
2) Hold the lower lever arm in such a position that the loading plate is almost in contact with the top porous stone of the soil sample. Then adjust, vertically, the position of the dial indicator so that the maximum amount of dial run will be available during the test.
3) Move the lower lever arm until the loading plate makes contact with the top porous stone.
4) Holding a 1/2 kg. weight just above contact with the weight pan of the lever system, record an initial dial reading, making certain to record both the dial and counter readings, and set a stopwatch for 5 seconds before the full minute.
5) Start the stopwatch and count from the -5 second reading to zero, and at the exact zero, apply the 1/2 kg. weight to the loading pan.
6) Record the dial readings for 0 (recorded in Step 4), 1/4, 1, 21/4, 4, 61/4, 9, 121/4, 16, 201/4, 25, 301/4, 36, 421/4, 49, 56 1/4, 64, and 1440 minutes. [The odd times for the early recordings are based upon the fact that they are the perfect squares of 0, 1/4, 1, 1 1/2, etc. minutes]
7) At the end of the 24 hour (1440 minutes) period, apply an additional 1/2 kg. load, and again record dial readings for 0, 1/4, 1, 2 1/4, etc. minutes, as in Step 6. Note that the zero reading of this step corresponds exactly with the 1440 minute reading of Step 6.
8) At the end of the 48 hour period, apply an additional 1 kg. load, and record the dial readings for the designated times.
9) It can be stated, as a general rule, that at the end of 24 hour period, a load is applied to the weight pan equal to the sum of all weights previously added.
10) During the first 48 hours of loading, make certain that the cotton batting is kept moistened. This is to ensure that the sample does not dry.
11) When the third increment of load is applied, the batting may be removed, and the well surrounding the top porous stone filled with water. This well must be kept filled with water for the duration of the test.
12) Continue to apply a new increment every 24 hours until the total applied loading on the pan is 16 kg.
(B) Pneumatic Loading System ( Note: record data on Data Sheet (C) ):
1) Set the regulator (according to the calibration provided) for a pressure of 1/8 tsf., leaving the valve closed. Record an initial dial reading, being certain to record both the dial and counter readings, then set a stopwatch for 5 seconds before the full minute.
2) Start the stopwatch and count from the -5 second reading to zero, and at the exact zero, open the valve.
3) Record the dial readings for 0 (recorded in Step 1), 1/4, 1, 21/4, 4, 61/4, 9, 121/4, 16, 201/4, 25, 301/4, 36, 421/4, 49, 56 1/4, 64, and 1440 minutes. [The odd times for the early recordings are based upon the fact that they are the perfect squares of 0, 1/4, 1, 1 1/2, etc. minutes]
4) At the end of the 24 hour (1440 minutes) period, set the regulator for a pressure of 1/4 tsf., and again record dial readings for 0, 1/4, 1, 2 1/4, etc. minutes, as in Step 3. Note that the zero reading of this step corresponds exactly with the 1440 minute reading of Step 3.
5) At the end of the 48 hour period, set the regulator for 0.5 tsf., and record the dial readings for the designated times.
6) It can be stated, as a general rule, that at the end of 24 hour period, a pressure is applied to the sample equal to the sum of all pressures previously added.
7) During the first 48 hours of loading, make certain that the cotton batting is kept moistened. This is to ensure that the sample does not dry.
8) When the third increment of load is applied, the batting may be removed, and the well surrounding the top porous stone filled with water. This well must be kept filled with water for the duration of the test.
9) Continue to apply a new increment every 24 hours until the total applied pressure is 8 tsf.
16) Depending on the nature of the problem for which the consolidation characteristics of this clay are being obtained, one or more unloading and reloading cycles may have to be performed. For example, the sample may be loaded to a particular value as outlined in Steps A6-A8 or B3-B5, possibly the third or fourth increment of loading, when, instead of applying an additional increment, one or more of the previously applied increments will be removed and the sample permitted to expand. These "unloading cycles" generally do not require 24 hours, often lasting only 4 - 6 hours. Following an unloading cycle, a "reloading cycle" may be begun, again using the 24 hour increments from before. The exact nature of these unloading and reloading cycles will be outlined by the instructor, and will be based upon the type of settlement analysis being contemplated.
17) When all loading and unloading cycles have been completed, remove the consolidometer from the loading frame.
18) Remove the ring containing the consolidated soil from the consolidometer, place it between the same two watch glasses from Step 2, and weigh it on a triple beam balance. Record the weight on Data Sheet (A).
19) Carefully remove every bit of soil from the ring, place it between the same two watch glasses from Step 2, and dry the sample in the oven. Record this weight on Data Sheet (A).
20) Using the dry weight of this consolidation sample, together with the initial wet weight from Step 10 and the final wet weight from Step 18, both the initial and final moisture contents may be computed and recorded on Data Sheet (A).
21) Using the sample diameter and its initial thickness (Step 1), the specific gravity of the soil (using the Procedure of Experiment 2), the initial wet weight (Step 10), and the dry weight (Step 19), compute the initial voids ratio and % saturation.
22) Using the sample diameter and the final thickness of the sample (computed by deducting the total compression accumulated during the entire loading program - Steps A5-A12 or B2-B9, inclusive, and Step 16 - from the initial thickness of the sample from Step 1.), the specific gravity of the soil, the final wet weight (Step 18), and the dry weight (Step 19), compute the final voids ratio and % saturation.
|1) For all loading increments where the time vs, compression
readings were taken, prepare the following plots:
a) On cartesian coordinates, plot for eacc loading increment the compression as ordinate vs. the sqaure root of the time elapsed for each reading as abcissae.
For example: at the very beginning of the test, the applied pressure was, of course, zero t.s.f. The first load was applied and began to have its effect by producing a compression pattern varying with time. The compression vs. square root of time plot for this first loading increment will therefore be labeled "0 To X t.s.f.", where X indicates the t.s.f. actually being applied to the sample.
b) As a check, all of the loading increments will be plotted on semi-logarithmic paper with compression as ordinate vs. time on the logarithmic scale. Note that in both a. and b., a further graphical construction will be performed (top be described later) and exactly what will be checked will be obvious then.
2) If a known amount of settlement or compression takes place in a sample of known total thickness initially (assuming the cross-sectional area remains constant) and known initial voids ratio, then the voids ratio at the end of the compression may be computed. Making use of this fact then, the following plot may then be assembled:
a) Compute the voids ratio exisitng at the end of the loading period for the first load increment by using the initial thickness of the sample (step 1), the initial voids ration of the sample (step 21) and the total compression accumulated during this first 24-hour loading period (step A6 or B3).
b) Compute the voids ratio existing at the end of each 24-hour loading period by using the initial thickness of the sample (step 1), the initial voids ratio of the sample (step 21) and the total compression accumulated during all increments of loading previously applied.
c) Plot on semi-logarithmic paper the values of the various voids ratios as arithmetic ordinate vs. the intergranular pressure in t.s.f. effective at the end of each 24-hour loading period, as logarithmic abcissa.
d) Determine the value of the Maximum Past Consolidated Pressure to which the soil has been consolidated under, using the Casagrande construction.
e) Using the voids ratio determined in a) and b), create a cartesian coordinates plot of the voids ratio as ordinate and the intergranular pressures as abcissa. Determine the coefficient of compresibility, av, and the coefficient of volume compressibility, mv, for each load increment.
3) Working with the plots of step 1, the following constructions will be performed:
a) Note that each of the compression vs. square root of time plots are assembled using on ly the compressions accumulated during the particular load increment being studied, and not the compressions accumulated from all previously applied increments.
b) On each of the plots, establish a straight line through as many of the plotted points as possible, and extend this line back to intersect at zero time. If, as possible, this intersection does not agree with the first of the plotted points, use the new "corrected zero time" as the true value.
c) Draw a smooth curve through all remaining points, and make a smooth transition with the straight line of b.
d) Through this "corrected zero", draw a straight line having an inverse slope 15% greater than the line through the data. This can be easily done by multiplying any value of the abcissae on the straight line through the data by 1.15, plotting the value and drawing the new line from the "corrected zero" through this point.
e) Where this new straight line intersects the test curve, both the compression and time (actually the square root of time) for the theoretical 90% consolidation can be picked off.
f) Repeat b. through e. for all load increments.
g) For those plots which were made on the semi-logarithmic paper, a somewhat different graphical construction will be applied:
1. Establish a straight line through both the early and final portions of the data, i.e., two separate stright lines.
2. Since this plot is logarithmic, and time is on the log scale, it will not be possible to locate a zero time point. However, assuming the early portion of the curve to be parabolic, select a point t1 at another point corresponding to 0.25t1, lay off the curve an ordinate equal to the difference between the ordinates at t1 and 0.25t1.
3. Select another point t2. At a point corresponding to 0.25t2, lay off above the curve an ordinate equal to the difference between ordinates at t2 and 0.25t2.
4. Repeat step 3. once again.
5. Connnect the points established by laying of the "differences in ordinates." This horizontal line represents the line of zero compression.
6. The intersection of the two straight lines from 1. represents both the compression and time for 100% consolidation or compression.
7. Repeat 1. - 6. for all load increments.
4) For each of the plots of compression vs. square root of time, compute the coefficient of consolidation using:
Cv = (T90H2) / (t90)
Cv = coefficient of consolidation in cm2/sec.
T90 = time factor for 90% consolidation (obtained from charts prepared from theoretical consolidation equatons)
H = average length of the longest drainage path during the particular loading increment, in cm.
t90 = time for 90% consolidation in sec. (obtained from step 3e.)
5) For each of the plots of compression vs. log of time, compute the coefficient of consolidation using:
Cv = (T50H2) / (t50)
Cv = coefficient of consolidation in cm2/sec.
T50 = time factor for 50% consolidation (obtained from charts prepared from theoretical consolidation equatons)
H = average length of the longest drainage path during the particular loading increment, in cm.
t50 = time for 50% consolidation in sec. (obtained from steps 3g-6 and 3g-7.)
6) Prepare a plot of Cv vs. log s using the values of Cv computed in steps 4 and 5.
7) Compute the coefficients of permeability for each pressure increment, and plot the values vs. log of pressure in the same manner as step 6.
8) For the plots in steps 1(a) and 1(b), determine the primary consolidation ratio, rp, and the secondary consolidation ratio, rs, for each loading increment.