The pore pressure measurement in triaxial test

The most common pore pressure measurement is making use of the electrical pressure transducer to measure the pressure of sample. However, the ordinary arrangement of pore pressure transducer is to connect the transducer to the bottom of the sample through nylon tube. In other words, the pore pressure measured by transducer was the pressure at the bottom of the sample, and there were lots of connectors and tubes in the path of measurement. Therefore, there are two major possible errors in measuring the pore pressure using the above method:

I. The pore pressure measured from the bottom of sample cannot represent the true pore pressure of the sample, due to the un-uniformity of the distribution of the pore pressure in soil samples, especially for clay.

II. The measurement path consisted of many connectors and nylon tubes. In the process of measuring, there might be a few air bubbles hide in connectors.

The bubbles will cause un-estimative errors in pore pressure measuring.

In order to reduce the errors and to measure the pore pressure at different positions of sample, we adopt one kind of miniature pore pressure transducer to achieve the above objects. The feature of the miniature pore pressure transducer is the very mini size, so as to place it in any position we want.

2.2.1 Standard Pore Pressure Transducer-PMP 1400

The base pore pressure measurement is making use of the pore pressure transducer PMP1400 from Druck. The range of pressure measurement is -1 bar to 4 bar. The word “standard” is meant that the transducer is commonly used in conventional triaxial test apparatus to measure the base pore water pressure. In addition to use the base pore pressure transducer, we also adopt the miniature pore water transducer to measure the pressure at the middle of sample. Therefore, the pore pressure measured at different position through two kinds of pore pressure transducer can be monitored and get comparison.

2.2.2 Miniature Pore Pressure Transducer-PDCR81

The miniature pore pressure transducer used in this research is PDCR81 from Druck. The PDCR81, shown in Fig. 2.17, consists of a 0.09-mm-thick, single-crystal, and silicon diaphragm with a fully active strain gauge bridge diffused into the surface.

A high air entry porous stone is placed in the tip of the transducer, just overlying the diaphragm. One side of the diaphragm is exposed to the atmosphere via the transducer cabling, while the other side is exposed to the pore water via the porous stone. The deformation of the diaphragm causes a change in voltage measured across the strain gauge, which is equated to pressure.

The small size of the PDCR81 allows it to be inserted easily into soil samples while causing minimal interference. The small size also leads to a quick response time since only a small amount of fluid is required to flow into or out of the device for a given change in pressure. This quick response time allows the PDCR81 to be better used for real time monitoring of pore pressure changes during testing, including dynamic events. Therefore, miniature pore pressure transducers, such as the PDCR81, are currently used in a variety of geotechnical testing applications for measuring pore water pressure.

The kit supplied comprises the following components: Druck PDCR 81 pressure transducer, 8.5mm drill and 3/8" UNF Tap, Cutter and Rubber Block, Flanged Grommet, Pair of o-rings and o-ring stretcher, Right Angle Bracket. Their purpose and installation is described below.

1. Druck PDCR 81 pressure transducer. The transducer is provided with a small ceramic tip. This is the sensing end of the transducer and is placed against the surface of the triaxial test specimen. The transducer is shrouded in heat-shrink sleeving which is connected to a bulkhead gland. The bulkhead gland is provided with an o-ring seal to seal against the outside of the triaxial cell.

2. Cutter and Rubber Block. The cutter is provided to cut a small hole in the middle of the rubber sleeve subsequently used to jacket the triaxial test specimen. Lay the rubber sleeve on a flat surface. The Rubber Block is positioned inside the rubber sleeve at the midpoint. Hold the cutting tool vertically above the block with the brass serrated end touching the rubber sleeve. Press down on the brass top of the cutter against the reaction of the rubber block while rotating the outer body of the cutter. Rotate a few times until a hole is cut in the sleeve through to the rubber block. This hole is exactly the right size to take the flanged grommet. In order to make the sleeve airtight, we used a small self-adhesive label to cover the hole while we use a suction sleeve stretcher to apply the sleeve to the test specimen.

3. Flanged Grommet. The grommet is designed to fit exactly through the hole bored in the rubber membrane jacketing the triaxial test specimen. The flange is designed to lie under the membrane against the surface of the test specimen. The grommet pokes through the membrane to receive the transducer tip.

4. Pair of o-rings and o-ring stretcher. Slide the pair of o-rings over the sensor and onto the transducer lead. Slip the o-ring stretcher over the thinner part of the transducer lead between the sensor and the bulkhead gland with the tapered end away from the sensor. Slide the two o-rings onto the stretcher from the tapered end. Push the sensor into the grommet so that the ceramic tip is against the surface of the test specimen. Slide the o-ring stretcher along the cable, over the sensor, and up against the test specimen. Roll the o-rings off so that the grommet grips the sensor and holds it in place against the surface of the test specimen.

5. Right Angle Bracket. Close to the sensor, slip one end of the Right Angle Bracket onto the cable. Then bend the cable gently around into a right angle and slip the cable into the other end of the Right Angle Bracket. This leads the cable nicely away from the wall of the triaxial cell and down to the bulkhead gland.

The advantage of using midheight pore pressure transducer was point out by Sheahan, Ladd and Germaine (1996). The response times of base and midheight pore pressure devices were measured during B-value checks. The midheight device response fully (i.e. a B-value of at least 95% is measured) within 5-7 seconds and the base device within 45-60 seconds.

2.3 Load and Temperature Measurement in Triaxial Test System