Undrained Triaxial Compression Test (Lab 3)
Introduction
The triaxial compression test is one of the most common methods of determining the strength of a soil. The test can be done on either a drained or undrained sample to measure the strength of the soil, and in the case of the undrained test, to measure the effect that pore water pressure has on the strength of soil. It is important to know the different strength and types of soil in foundations of buildings so that the likely outcomes can be predicted before construction. The triaxial compression test has some advantages of the shear box test, in that more load combinations can be trialled to see how the soil reacts under different loading conditions.
Apparatus
The triaxial apparatus consists of a soil sample held in place inside a cylinder (in the undrained case, this cylinder is filled with water). The sample is separated from the water by a rubber membrane held in place by the pedestal, and attached o-rings. The pore pressure can be measured, and the back pressure can be applied via drainage through the top and the bottom of the cylinder. The load is applied through the loading arm at the top of the device, simulating in-situ conditions.
Experimental Procedure
The preparation of the sample for the test was reasonable involved. The sample (in its coring tube) was placed onto the extruder, where it was screwed out into a pre-lubricated metal sleeve. The ends were them trimmed, weighed, and placed in the oven to dry (and eventually determine the water content of the sample). The sample was then placed on a porous bronze disc lined with filter paper and then placed on the pedestal. This removed any air trapped within the sample. The top-cap was then placed on the sample, and the rubber membrane was added to the exterior of the sample, providing an air-tight barrier to the water. O-rings were placed over the membrane on the base pedestal and top-cap. The sample was then ready for the triaxial compression test, which takes roughly 6 hours
Results
The results obtained for the “triaxial test data sheet” are as show below: Triaxial Test Data Sheet | Soil description: | Kaolin Clay | Liquid limit: | 62 | Plastic limit: | 30 | | Specific gravity (Gs) | 2.65 | Pre-consolidation pressure: | 400 | | | | | | | Diameter (mm) | 37.9 | 37.5 | 37.7 | Average | Length (mm) | 86.0 | 86.0 | 86.0 | Average | Area (mm2) | 1116.278 | Volume (mm3) | 95999.956 | | Weight (g) | 164.32 | wet density (g/cm3) | 1.71 | | Water Content | Prepared sample | Tin no. | BC32 | Weight tin (g) | 32.42 | Weight tin + wet soil (g) | 83.76 | Weight tin + dry soil (g) | 66.99 | Weight dry soil (g) | 34.57 | Weight water (g) | 16.77 | Water Content (%) | 48.51 | Dry density (g/cm) | 0.38 | Cell Pressure (kPa) | 300 | Back Pressure (kPa) | 200 | Volume change during consolidation (mm3) | 7745 |
The results the group obtained from the B-value test was 0.98.
Calculations, Theory and Discussion
Q1. Describe soil preparation during pre-consolidation, based on the photos in the PPT file.
Refer to the experimental procedure in the introduction for soil preparation.
Q2. Report any important and/or difficult points during laboratory sample preparation exercise.
The main difficulty that the group encountered was ensuring that we didn’t touch the sample too much, as this would reduce the sample’s saturation. This continual manual handling of the sample caused discrepancies in the geometry of the sample. Measuring the correct diameter and length of the sample was also slightly difficult as the calliper occasionally indented the sample. Placing the membrane on the sample and the rubber o-rings required a lot of precision.
Q3. Report observations of the test sample at the end of testing, such as sample deformation, shape and failure plane.
As the group did not actually get to test the sample, they are unable to comment on any observations.
