Geotechnical Instrumentation

Geotechnical Instrumentation

19.1

Inclinometer

A slight m o v e m e n t of soil has been observed prior to most landslides, earthquakes, and slope failures. Some believe that insects, horses, and pigs feel these slight movements. An inclinometer is designed to measure slight ground movements that occur prior to a slope failure. See Figs. 19.1 and 19.2.

Figure 19.1 Inclinometer

Figure 19.2 Working of the inclinometer

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Geotechnical Engineering Calculations and Rules of Thumb

An inclinometer is shown in Fig. 19.2. The inclinometer is lowered into the well. The wheels are fixed at the side to guide t h r o u g h the well casing. The p e n d u l u m has an electrical potential. W h e n the inclinometer is vertical, b o t h detectors (detector A and B) feel the same potential. W h e n the inclinometer is inclined as shown on the right in Fig. 19.2, by an angle of u, the p e n d u l u m is closer to detector A t h a n to detector B. Hence detector A would record a higher reading t h a n detector B. This electrical potential difference can be utilized to obtain the angle of inclination (or angle u). 19.1.1

Procedure

9 The well is installed. 9 Most inclinometers are approximately 2 ft long. The inclinometer is inserted into the well and a reading (angle ~) is taken from 0 ft to 2 ft. If the well casing is vertical, the reading should be 90 ~. 9 Next, the inclinometer is inserted 2 ft more and another reading is taken. The process is continued to the b o t t o m of the well. The obtained readings would look like those shown in Table 19.1. Table 19.1

Inclinometer readings

Depth (ft) Angle ~

0-2 90.0

2-4 90.0

4-6 90.2

6-8 90.3

8-10 90.3

10-12 90.4

12-14 90.1

Depth (ft) Angle c~

14-16 90.5

16-18 90.3

18-20 90.5

20-22 90.1

22-24 90.2

24-26 90.7

26-28 90.3

9 The next set of readings were taken a day or two later. If the soil had moved, the inclinometer would record different readings. A typical set of readings is shown in Table 19.2. Table 19.2 Second set of inclinometer readings

Depth (ft) Angle u

0-2 90.0

2-4 90.0

4-6 90.1

6-8 90.2

8-10 88.3

10-12 88.1

12-14 87.3

Depth (ft) Angle c~

14-16 87.2

16-18 89.1

18-20 88.5

20-22 88.3

22-24 89.1

24-26 89.0

26-28 89.2

Chapter 19

Geotechnical Instrumentation

271

Therefore, at a depth of 8 to 10 ft, the inclinometer has bent, since the angle shows a deviation from 90 ~ See Fig. 19.3. Slope

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-~8 f,ss SsSSI

9 At a depth of 8 ft, the inclinometer has bent as shown. This would indicate that the slope is not stable. 9 Above slope indicator, results could be used to identify the failure plane.

Figure 19.3 Measuring soil movement It is possible to identify the slip circle by installing m a n y inclinometers.

19.2

Tiltmeter

The operation of the tiltmeter is similar to a level. In a regular level, an air bubble is used to identify the tilt. If the air bubble is at the center, t h e n the tilt is zero. Tiltrneters are filled with an electrically sensitive liquid instead of water. W h e n the tiltmeter tilts, the bubble moves. The electrical signal coming from the tiltmeter is d e p e n d a n t u p o n the location of the bubble. Tiltmeters are used to measure the tilt in buildings, bridges, tunnels, and so on. 19.2.1

Procedure

9 Tilt plates are attached to the building wall, as shown in Fig. 19.4.

Figure 19.4

Tilt plates

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Geotechnical Engineering Calculations and Rules of Thumb

9 A tiltmeter is temporarily fixed to the plate. The tilt is measured. The tiltmeter is removed and t h e n m o v e d to measure the tilt in the next plate. One tiltmeter can be used to measure the tilt of m a n y tilt plates. See Fig. 19.5.

Figure 19.5

Horizontal tiltmeters

Horizontal tiltmeters can be installed to investigate soil m o v e m e n t due to foundations or e m b a n k m e n t s .