SOIL REPORT: Consolidation test

 SOIL REPORT

Consolidation test

Purpose:

• This test is performed to determine the magnitude and rate of volume decrease that a laterally confined soil specimen undergoes when subjected to different vertical pressures. 
• From the measured data, the consolidation curve (pressure-void ratio relationship) can be plotted. This data is useful in determining the compression index, the recompression index, and the preconsolidation pressure (or maximum past pressure) of the soil.
• In addition, the data obtained can also be used to determine the coefficient of consolidation and the coefficient of secondary compression of the soil.

 

Significance:

The consolidation properties determined from the consolidation test are used to estimate the magnitude and the rate of both primary and secondary consolidation settlement of a structure or an earth fill, used in the design of engineered structures and the evaluation of their performance.

 

Equipment:

Consolidation device (including ring, porous stones, water reservoir, and load plate), Dial gauge (0.0001 inch= 1.0 on the dial), sample trimming device, glass plate, metal straight edge, clock, moisture can, filter paper.

 

Test Procedure:

(1) Weigh the empty consolidation ring together with the glass plate.

(2) Measure the height (h) of the ring and its inside diameter (d).

(3) Extrude the soil sample from the sampler.

Determine the initial moisture content and the specific gravity of the soil.

(4) Cut approximately a three-inch-long sample.

Place the sample on the consolidation ring and cut the sides of the sample to be approximately the same as the outside diameter of the ring.

(5) Press the sample into the ring. Be careful throughout the trimming process to ensure that there is no void space between the sample and the ring.

(6) Turn the ring over carefully and remove the portion of the soil protruding above the ring.

Using the metal straight edge, cut the soil surface flush with the surface of the ring. 

Remove the final portion carefully.

(7) Place the previously weighed Saran-covered glass plate on the freshly cut surface

Turn the ring over again, and carefully cut the other end in a similar manner.

(8) Weigh the specimen plus ring plus glass plate.

(9) Carefully remove the ring with a specimen.

Center the porous stones on the top and bottom surfaces of the test specimen.

Place the filter papers between porous stones and soil specimens.

Lower the assembly carefully into the base of the water reservoir.

Fill the water reservoir with water until the specimen is completely covered and saturated.

(10) Place the load plate centrally on the upper porous stone and adjust the loading device.

(11) Adjust the dial gauge to a zero reading.

(12) With the toggle switch in the down (closed) position, set the pressure gauge dial (based on calibration curve) to result in an applied pressure of 0.5 tsf (tons per square foot).

(13) Simultaneously, open the valve (by quickly lifting the toggle switch to the up (open) position) and start the timing clock.

(14) Record the consolidation dial readings at the elapsed times given on the datasheet.

(15) Repeat Steps 11 to 13 for different preselected pressures (generally includes loading pressures of 1.0, 2.0, 4.0, 8.0, and 16.0 tsf and unloading pressures of 8.0, 4.0, 2.0, 1.0 and 0.5 tsf)

(16) At the last elapsed time reading, record the final consolidation dial reading and time, release the load, and quickly disassemble the consolidation device and remove the specimen. Quickly but carefully blot the surfaces dry with paper toweling. (The specimen will tend to absorb water after the load is released.)

(17) Place the specimen and ring on the Saran-covered glass plate and, once again, weigh them together.

(18) Weigh an empty large moisture can and lid.

(19) Carefully remove the specimen from the consolidation ring, being sure not to lose too much soil, and place the specimen in the previously weighed moisture can. Place the moisture can containing the specimen in the oven and let it dry for 12 to 18 hours.

(20) Weigh the dry specimen in the moisture can.

 

Analysis:

(1) Calculate the initial water content and specific gravity of the soil.

(2) For each pressure increment, construct a semi-log plot of the consolidation dial readings versus the log time (in minutes).

Determine D0, D50, D100, and the coefficient of consolidation (cv) using Casagrande’s logarithm of time fitting method.

Also, calculate the coefficient of secondary compression based on these plots.

(3) Calculate the void ratio at the end of primary consolidation for each pressure increment.

Plot log pressure versus void ratio. Based on this plot, calculate compression index, recompression index, and preconsolidation pressure (maximum past pressure).

(4) Summarize and discuss the results.