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TABLE OF CONTENTS

Table of contents 1I. The objectives of instrumentations 2

II. Scopes of the instrumentation work 2

III. Inclinometer 21. What is inclinometer? 2

2. Inclinometer components 2

3. How can inclinometer be installed? 6

4. Commissioning and Base reading 75. Installation Records 7

6. Data Reduction 8

7. Reducing Data Manually 8

8. The accuracy of measurement 10IV. Piezometer 18

1. Applications 182. Types of Piezometers 19

V. Magnet extensometers 25

1. Overview 25

2. Instrument Description 263. Installation 27

4. How can the data of equipment be read? 28

References 29



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INSTRUMENTS FOR FIELD MONITORING OF

DEEP EXCAVATION

I. THE OBJECTIVES OF INSTRUMENTATIONS

To verify the design assumptions including the geotechnical parameters and modeling

techniques by providing a means of comparing measured and predicted ground movements sothat the ground model may be modified accordingly.

To early identify and prevent the detrimental environmental impacts when they occur.

To identify early the potential construction hazards. Provide the data for objectively access the feasibility for adjustment of construction methods.

II. SCOPES OF THE INSTRUMENTATION WORK

i) Settlement markers measure the vertical movement of ground surface close to the

works.ii) Inclinometers in the sub-soil and diaphragm wall to monitor their respective lateral

displacements as works progress.

iii) Piezometers to monitor the pore water pressure regime in the sub-soil.

III. INCLINOMETER

1. What is inclinometer?

Inclinometers are defined as devices for monitoring deformation normal to the axis of a pipe by means of probe passing along the pipe. The probe contains a gravity-sensing transducer

designed to measure inclination with respect to the vertical. The pipe may be installed either in

the borehole or in fill, and in most applications is installed in a near vertical alignment, so that theinclinometer provides data for defining subsurface horizontal deformation. Inclinometers are also

referred to as slope inclinometers, probe inclinometers, and slope indicators.

Typical applications include the following:

i. Determining the zone of landslide movement.ii. Monitoring the extent and rate of horizontal movement of embankment dams,

embankment on soft ground, and alongside open cut excavations or tunnels.

iii. Monitoring the deflection of bulkheads, piles, or retaining walls.

2. Inclinometer componentsMost inclinometer systems have four major components:

i. A permanently installed guide casing, made of plastic, aluminum alloy, fiberglass,or steel. When horizontal deformation measurements are required, the casing is

installed in a near vertical alignment. The guide casing usually has tracking

grooves for controlling orientation of the probe.ii. A portable probe containing a gravity-sensing transducer.

iii. A portable readout unit for power supply and indication of probe inclination.

iv. A graduated electrical cable linking the probe to the readout unit.



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2.1. I nclinometer Casing

The inclinometer casing provides access for subsurface measurements,

controls the orientation of the sensors, and moves with the surrounding

ground. In vertical installations, the inclinometer casing is installed in a borehole that passes through a suspected zone of movement. One set of

grooves is aligned in the expected direction of movement (downhill, for

example).

Inclinometer casing is a special purpose, grooved pipe used ininclinometer installations. It is typically installed in boreholes, but can also be

embedded in fills, cast into concrete, or attached to structures. Inclinometer

casing provides access for the inclinometer probe, allowing it to obtainsubsurface measurements. Grooves inside the casing control the orientation ofthe probe and provide a surface from which repeatable tilt measurements can

be obtained.

Casing DiameterCasing is designed to deform with movement of the adjacent ground

or structure. The useful life of the casing ends when continued movement of



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the ground pinches or shears the casing, preventing passage of the inclinometer probe. Larger

diameter casing generally provides longer life.

Large Diameter Casing (85 mm, 3.34 inch) is suitable for landslides and long term

monitoring. It is also appropriate for monitoring multiple shear zones or very narrow shear zones,and it is required for the horizontal Digitilt inclinometer probe.

Medium Diameter Casing (70 mm, 2.75 inch) is suitable for construction projects. It canalso be used for slope stability monitoring when only a moderate degree of deformation isanticipated.

Small Diameter Casing (48 mm, 1.9") is suitable for applications where small

deformations are distributed over broad zones. It is generally not installed in soils.

2.2. Portable Probe

The inclinometer probe consists of a stainless steel body, a

connector for control cable, and two pivoting wheel assemblies. When properly connected to the control cable, the probe is waterproof and has

been used deeper than 1000 feet. The wheel assemblies consists of a yoke

and two wheels. One of the wheels in each assembly is higher than theother. This wheel is called the “upper wheel” and has special significance,as explained below.

Measurement Planes

The inclinometer probe employs two forcebalanced servo-accelerometers to measure tilt. One-

accelerometer measures tilt in the plane of the

inclinometer wheels. This is the “A”axis. Theother accelerometer measures tilt in the plane that is perpendicular to

the wheels. This is the “B” axis. The drawing at left shows the probe

from the top. When the probe is tilted toward the A0 or B0 direction,

readings are positive. When the probe is tilted in the A180 or B180directions, readings are negative.

Orientation of the ProbeInclinometer casing is installed so that one set of grooves

is aligned with the expected direction of movement. One groove,

typically the “downhill” groove should be marked A0. In astandard inclinometer survey, the probe is drawn from the bottom to the top of the casing two times. In the first pass, the

upper wheels of the probe should be inserted into the A0 groove.

This ensures that movements are positive values.

2.3. Portable readout unitPortable readout unit or The

Digitilt DataMate is a recording readout that is used with Digitiltinclinometer probes (vertical or horizontal), Digitilt tiltmeters, and

the spiral sensor. It works with both metric and English unit versions

of these sensors.

Readings stored in the DataMate are transferred to a PC using

the DMM software supplied with the DataMate.



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2.4. Graduated electrical cableGraduated electrical cable or Control cable is used to

control the depth of the inclinometer probe. It also conducts

power to the probe and returns signals to the readout.• Metric control cables are graduated with yellow marks

at 0.5-meter intervals and red marks at 1-meter intervals. There are numeric marks at 5-meterintervals.• English control cables are graduated with yellow markers at 2-foot intervals and red

marks at 5-foot intervals. There are numeric marks at 50-foot intervals. In addition, there are

yellow bands of tape at 10-foot intervals. Each band represents 10 feet from the last numericmark. For example, four bands represent 40 feet from the last numeric depth mark.

Depth Control

Accurate inclinometer measurements depend on consistent placement of the inclinometer

probe. Always align the depth marks on the control cable with the same reference. Aim for placement repeatability of 6 mm (1/4 inch) or better. We recommend using a pulley assembly to

assist with depth control. The jam cleat on the pulley assembly holds the cable

and the top edge of the chassis provides a convenient reference for cable depthmarks. The small pulley assembly is used with 48 mm and 70 mm casing (1.9

and 2.75 inch). The large pulley assembly is used with 70 mm and 85 mm

casing (2.75 and 3.34 inch).

Using the Pulley Assembly

1. Remove the pulley from the

chassis.

2. Clamp the chassis to the topof the casing.

3. Insert the inclinometer probe and control cable.

4. Replace the pulley.

The distance between the top edge of the pulleychassis and the top of the casing is one foot. Your

data reduction software can automatically adjust for

this, so keep your survey procedure simple: use themarks on the cable and the top edge of the pulley

chassis for reference. Let the software do any extra

work required. Check that operators consistentlyuse the pulley assembly. If the pulley is used forone survey and not for the next, the resulting data

sets will not be directly comparable. Sometimes a

monument case or a protective pipe makes it

impossible to attach the pulley assembly to thecasing. In this case, you can make a removable

adapter for the pulley assembly. If you use an

adapter, be sure to use it consistently.

3. How can inclinometer be installed?

The access tube, which is made from ABS plastic, is a self-aligning casing. Inside the access,

tube contains four grooves forming two guiding



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paths, perpendicular each other, for a reading probe traveling during measurement. Displacement

of inclinometer access tube shall be monitored by the RST inclinometer system that could report

the displacement, in both graphical and numerical, on site.

Outline of installation procedure of inclinometer is given below as well as illustrated in Fig.1

o Locate the installation position by the survey team.

o

Drill or clean the borehole using soil-boring machine.o After the borehole, preparation is finished, grouting of cement: bentonite mixture

containing sufficient water to achieve a pump able mix shall be carried out.

Grouting mix of cement: bentonite shall be 20:1 and 3:1 for inclinometer installed

in bored pile and in ground, respectively.

o Lower down first length of the access tube with an end cap into the borehole by keeping one

transit path is in direction of movement to be anticipated.

o Connect next length of the access tube with one in borehole by method recommended by

manufacturer.

o Lower down the casing and then connect the next access tube as per the above details.

o Connect and lowering down the access tubes until reaching the designed depth.

o Put the top cap to prevent anything falling down into the access tube.

o Install the protective casing to protect the inclinometer access tube.

o Fill the sand around the access tube exposed above ground (if necessary) in order to reduce

sway of the casing during monitoring.

4. Commissioning and Base readingAfter installation, the function of each inclinometer will be checked. Initial reading of

each instrument shall be carried out to form base reading of each one.For inclinometer, the base reading shall be taken at minimum two days after installation. As a

part of the commissioning, three sets of reading shall be taken and compared. If significant

differences or anomalies are found, then further readings shall be averaged to form the based

readings representing conditions prior tostart of the filling work.

The instrumentation records shall contain the

following information:

o Instrument reference number and

type

o Location co-ordinate

o Dates of installation

o Initial reading

5. Installation Records

Records shall be presented as required bythe inspection, comprising drilling logs, and

installation data sheet provided in the

attachments. The following items shall berecorded:

o Existing ground level at time of installation

o Weather conditions

o Sketch of instrument location reference to the site layout

o Instrument details such as length, diameter, orientation and depth



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o Soil boring details such as equipment used, borehole size, type of drilling mud, and

any casing used

o Simplified log of ground conditions (in each drill hole)

o Type of back fill used

o Problem encountered, delays, unusual features of installation, and any events that may

be have a bearing on the instrument behavior.o Commissioning information and readings.

o Photographs showing installation activities of each instrument.

6. Data Reduction Inclinometer Measurements

The inclinometer probe measures tilt, rather than lateral movement. How does

tilt provide information about lateral movement? The basic principle involves the

sine function, an angle, and the hypotenuse of a right triangle. We are interested inthe length of the side opposite the angle θ.

hypotenuse

oppositesidesin

side opposite = hypotenuse × sinθ

DeviationIn the drawing at right, the hypotenuse of the right triangle is the

measurement interval. The measurement interval is typically 0.5 m with

metric-unit inclinometers or 2 feet with English-unit inclinometers.The side opposite the angle of tilt is deviation. It is calculated by

multiplying the sine of the angle of tilt by the measurement interval. This

calculation translates the angular measurement into a lateral distance and is

the first step to calculating lateral movement.

Cumulative Deviation

By summing and plotting the deviation values obtained at eachmeasurement interval, we can see the profile of the casing. The black squares

at each measurement interval represent cumulative deviation values that

would be plotted to show the profile of the casing.

Displacements

Changes in deviation are called displacements, since the change

indicates that the casing has moved away from its original position. Whendisplacements are summed and plotted, the result is a high-resolution

representation of movement.

7. Reducing Data Manually Normally, computer software is used to reduce inclinometer data. Here,

we show only a simple overview.



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Displayed Readings

Slope Indicator’s readouts display “reading units” rather than angles or deviation.

Reading units are defined below:

Displayed Reading = sinθ × Instrument ConstantReading English = sinθ × 20,000

Reading Metric = sinθ × 25,000Combining Readings The standard two-pass survey provides two readings per axis for each interval. The probe

is oriented in the “0” direction for the first reading and in the “180” direction for the second

reading. This two-pass system has several advantages. First, it eliminates the sensor offset, whichcan change from survey to survey. Second, it provides a means of detecting error through

checksums and other routines. Third, it tends to smooth the effect of random errors. At some

point during data reduction, the two readings are combined and averaged. For example:

A0 Reading = 359 A180 Reading = – 339

3492

(-339)-359 ReadingAveraged

Calculating DeviationTo calculate lateral deviation, we average the A0 and A180 readings, divide by the

instrument constant, and multiply by the measurement interval. In the example below, we show

an English-unit calculation:Lateral Deviation = Measurement Interval x sin θ

000,202

)339(359inches24

= 0.4188 inches

Calculating Displacement

Displacement, the change in lateral deviation, indicates movement of the casing. To

calculate displacement, we find the change in (combined and averaged) reading units, divide bythe instrument constant, and multiply by the length of the measurement interval.

Combined Reading current = 700 Combined Reading initial = 698

Displacement = Measurement Interval ×Δsinθ

000,202

698700inches24

= 0.0012 inches

Calculating ChecksumsA checksum is the sum of a “0” reading and a “180” reading at the same depth.

A0 reading = 359 A180 reading = -339

Checksum = 359 + (-339)

= 20 Bias (zero offset)

If you hold your inclinometer probe absolutely vertical and check the reading, you will

typically see a non-zero value for each axis. The non-zero value is the result of a slight bias in theoutput of the accelerometers. The bias (or zero offset) may be negative or positive and will

change over the life of the probe. This is not normally a matter for concern, because the zero

offset is effectively eliminated by the standard two-pass survey and the data reduction procedure.

Divide reading unit by instrumentconstant to obtain sine of angle.

Combine the A0 & 180 readings and

divide by 2 to average them



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Below, we show an readings that have a zero offset of 10. During the first pass, the probe

measures a tilt of 1 degree. During the second pass, the probe measures a tilt of -1 degree,

because it has been rotated 180 degrees. See how the offset increases the positive reading and

decreases the negative reading, even though the measured angle has not changed. However, whenthe two readings are combined, as discussed in

“Combining Readings” above, the offset iseliminated and the correct value emerges.Tilt angle = 1 degree.

Theoretical reading unit = 349 (20,000 x sin (1))

Offset = 10Displayed A0 reading = 359 (349 + 10)

Displayed A180 reading = -339 (-349 + 10)

Combined reading = 698 (359 - (-339))

Averaged reading = 349

8. The accuracy of measurement

Inclinometer gives the horizontal movement determined from differential displacementrelative to its toes. Wood (1984) reported that the accuracy of the inclinometer based on the

manufacturer’s specification, check on repeatability and calibration is of order of ±0.1 mm over a

500 mm gauge length. Although this does not include the indeterminate effects of wear and

corrosion of the duct, it is comparablewith published data (Dunnicliff, 1971).

Hence, for a duct 20 m an error bound

of ±4.0 mm for the location of the toprelative to the toe may occur.

Although such error bounds are large

in comparison with the recorded wall

movements, the frequency of thereadings together with precautions

taken to minimize the build-up of error

should have ensured lower actualerrors than those of obtained from

these considerations. Moreover, the

joints in the ducts and damage of tubeduring excavation also can ruin theresult.

Strut should install thermistor

for temperature measurement. The

perturbations in the axial loads may bealmost wholly attributed to the

changes in ambient temperature and

clearly demonstrate the inability of thestrut to expand and relieve the induced

stress.

The other method for measuring wallmovement apart from inclinometer is

sliding micrometer. Fi ure 3. Measurable/ Immeasurable deflection



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Figure 4. Justification of inclinometer reading



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Figure 5. Back-calculated bending moment from field measurement



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Figure 6. First step of install the inclinometer



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Figure 7. Second step of install the inclinometer



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Figure 8. Third step of install the inclinometer

Figure 8. Inclinometer in in-situ.



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Figure 9. Installation of inclinometer on steel sheet pile wall.



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Figure 9. Installation of inclinometer on diaphragm wall.



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IV. PIEZOMETER

4.1. Applications

There are some typical applications of piezometers:

Monitoring pore water pressures to determine safe rates of fill

or excavation.

Monitoring pore water pressures to evaluate slope stability.

Monitoring dewatering systems used for excavations.

Monitoring ground improvement systems, such as vertical

drains and sand drains.

Monitoring pore pressures to check the performance of

earthfill dams and embankments.

Monitoring pore pressures to check containment systems at

landfills and tailings dams.



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4.2. Types of Piezometers

4.2.1 Standpipe Piezometers

a) Applications

Standpipe piezometers are used to monitor piezometric water

levels. Observation wells are used to monitor ground water levels.

Readings are obtained with a water level indicator.

Typical applications include:

Monitoring pore-water pressure to determine the stability of

slopes, embankments, and landfill dikes.

Monitoring ground improvement techniques such as vertical

drains, sand drains, and dynamic compaction.

Monitoring dewatering schemes for excavations and

underground openings.

Monitoring seepage and ground water movement in

embankments, landfill dikes, and dams. Monitoring water drawdown during pumping tests.

b) Installation

The standpipe piezometer, which is installed in a borehole, consists of a filter tip joined to

a riser pipe. The filter tip is placed in a sand zone and a bentonite seal is placed above the sand to

isolate the pore water pressure at the tip. The annular space between the riser pipe and the

borehole is backfilled to the surface with a bentonite grout to prevent unwanted vertical migration

of water. The riser pipe is terminated above ground level with a vented cap.

The observation well uses the same components as the standpipe piezometer, but is installed

differently. No bentonite seals are placed and the borehole is backfilled with gravel or sand ratherthan a bentonite grout. The top of the borehole is sealed to prevent the entry of surface runoff,

and the riser pipe is terminated above ground level.c) Operation

Water levels in either the standpipe piezometer or the observation well are measured with

a water level indicator. The water level indicator consists of a probe, a graduated cable or tape,

and a cable reel with built-in electronics. The probe is lowered down the standpipe until it makes

contact with water. This is signaled by a light and a buzzer built into the cable reel. The depth-to-

water reading is taken from the cable or tape. The Water Level Indicator features a sensitivity

adjustment which helps the user obtain consistent measurements and eliminates false triggering

in different well and water conditions.d) Advantages

Economical components.

Simple to read.

Very good long-term reliability.

No electrical, no calibrated components.

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e) Limitations

Accuracy depends on skill of operator; reading requires a man on site; remote reading not

possible; slower to show changes in pore-water pressure.

4.2.2. Pneumatic Piezometers

The pneumatic piezometer consists of a pneumatic pressure

transducer and pneumatic tubing. It can be installed in a borehole,

embedded in fill, or suspended in a standpipe. Readings are

obtained with a pneumatic indicator.a) Applications

Pneumatic piezometers are used to measure pore water

pressure in saturated soils. Applications include:

Monitoring pore pressures to determine safe rates of fill or excavation.

Monitoring pore water pressures to determine slope stability.

Monitoring the effects of dewatering systems used for excavations.

Monitoring the effects of ground improvement systems such as vertical drains and

sand drains.

Monitoring pore water pressures to check the performance of earth fill dams and

embankments.

Monitoring pore water pressures to check containment

systems at land fills and tailings dams.b) Advantages

Pneumatic piezometers employ a simple and reliable

transducer that is free from zero drift. Long term performance isenhanced by corrosion-resistant plastic construction, polyethylene

tubing, and in-line filters in all connectors. Compatible with both

flow and no-flow reading techniques.c) Operating Principle

In a typical installation, the piezometer is sealed in a borehole,

embedded in fill, or suspended in a standpipe. Twin pneumatic tubes

run from the piezometer to a terminal at the surface. Readings are

obtained with a pneumatic indicator.

The piezometer contains a flexible diaphragm. Water pressure acts on one side of the diaphragm

and gas pressure acts on the other. When a reading is required, a pneumatic indicator is connected

to the terminal or directly to the tubing. Compressed nitrogen gas from the indicator flows down

the input tube to increase gas pressure on the diaphragm.

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When gas pressure exceeds water pressure, the diaphragm is forced away from the vent tube,

allowing excess gas to escape via the vent tube. When the return flow of gas is detected at the

surface, the gas supply is shut off.

d) Advantages

Reliable, remote reading possible, not electrical, indicator can be calibrated at any time.

e) Limitations

Accuracy depends on skill of operator; difficult and expensive to automate, so reading

requires man on site; reading time increases with length of tubing; pneumatic tubing can be

blocked by condensation if not frequently charged with dry nitrogen gas.

Gas pressure in the piezometer decreases until water pressure forces the diaphragm to its original

position, preventing further escape of gas through the vent tube. At this point, gas pressure equals

water pressure, and the pneumatic indicator shows the reading on its pressure gauge.

4.2.3. Vibrating Wire Piezometers

The vibrating wire piezometer consists of a vibrating wire

pressure transducer and signal cable. It can be installed in a

borehole, embedded in fill, or suspended in a standpipe. Readings

are obtained with a portable readout or a data logger.a) Applications

Typical applications for the VW piezometer are:

Monitoring pore water pressures to determine safe rates of

fill or excavation.

Monitoring pore water pressures to determine slope stability.

Monitoring the effects of dewatering systems used for excavations.

Monitoring the effects of ground improvement systems such as vertical drains and sand

drains.

Monitoring pore pressures to check the performance of earth fill dams and embankments.

Monitoring pore pressures to check containment systems at land fills and tailings dams.b) Operation

The VW piezometer converts water pressure to a frequency signal via a diaphragm, a

tensioned steel wire, and an electromagnetic coil. The piezometer is designed so that a change in

pressure on the diaphragm causes a change in tension of the wire. When excited by theelectromagnetic coil, the wire vibrates at its natural frequency. The vibration of the wire in the

proximity of the coil generates a frequency signal that is transmitted to the readout device. The

readout device processes the signal, applies calibration factors, and displays a reading in the

required engineering unit.c) Installation Overview

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Grout-In Method: The piezometer is lowered, filter-end up, to the specified depth in the

borehole. Then the borehole is filled with a bentonite-cement grout.

Sand Filter Method: The borehole is flushed with water or biodegradable drilling mud. A

sand filter is placed around the piezometer which is positioned at the specified depth. A bentonite

plug is formed at the top of the sand filter. Then the remainder of the borehole is filled with a

bentonite-cement grout.

Push-In: The special-body, push-in piezometer is pushed into soft, cohesive soil at the

bottom of a borehole. The piezometer must be monitored to ensure that it is not overpressured as

it is pushed in. The borehole is then filled with a bentonite-cement grout.

Embankments: The piezometer is embedded in sand and then covered with hand-

compacted select fill. Signal cables are routed though trenches and covered with compacted fill.

Bentonite water stops are placed at appropriate locations. Readings become available when the

surrounding soil becomes saturated.d) Advantages

High Resolution: VW piezometers provide a resolution of 0.025% of full scale.

High Accuracy: Slope Indicator's automated, precision calibration system ensures that all

VW piezometers meet or exceed their accuracy specifications.

Groutable: The VW piezometer can be installed without a sand filter or a bentonite seal.

This greatly simplifies same-hole installation of multiple piezometers or piezometers with

inclinometer casing.

Rapid Response: VW piezometers offer rapid response to changes in pore water pressure,

whether they are grouted in, pushed into cohesive soils, or embedded in a sand filter zone. Reliable Signal Transmission: With properly shielded cable, signals from the VW

piezometer can be transmitted long distances.

Temperature Measurement: All VW piezometers are equipped with a temperature sensor.

e) Advantages and Limitations

It is easy to read, very accurate; good response time in all soils; easy to automate; reliable

remote readings. But, it must be protected from electrical transients.

4.2.4. Multi-Level Vibrating Wire Piezometer

The multi-level VW piezometer system is used to monitor pore-water pressure at multiple

zones in a borehole. It consists of a number of VW piezometers in special housings, signal cable,

a grout fitting, and some user supplied components (mainly PVC pipe). The system is grouted

into a borehole. Readings are obtained with a portable readout or a data logger.a) Application

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Multi-Level VW Piezometers are used to monitor pore-

water pressure at different zones in the borehole.b) Operation

The multi-level system consists of VW piezometers in

multi-level housings, as shown in the photograph at right, andPVC placement pipe. The piezometers are assembled in-line

with the PVC pipe and installed downhole. Signal cables are

brought to the surface through the PVC placement pipe. The

borehole is then backfilled with a bentonite-cement grout,

using the placement pipe to deliver the grout. When the grout

cures, each piezometer is isolated from the zones above and

below it, but is highly responsive to changes in pore-water

pressures at its own elevation.

c) Advantages

Slope Indicator's multilevel system solves or avoids the

problems associated with traditional multi-level piezometer

installations:

In the traditional method, placing sensors at their

intended depth is difficult, and the difficulty increases with

the number of sensors. With the multi-level system,

piezometers are installed in-line with PVC pipe. The pipe

controls the elevation and relative spacing of the

piezometers. In the traditional method, placement of sand intake

zones and bentonite seals is time-consuming and uncertain.

The multi-level system entirely eliminates sand intake zones

and bentonite seals. Instead, the entire borehole is filled with

a bentonite-cement grout.

In the traditional method, signal cables from the

piezometers pass through the seals that isolate the various

intake zones. The cables can form channels for migration of

water between zones. With the multi-level system, signal

cables from each piezometer run to the surface through thePVC placement pipe. The pipe itself is watertight and is later

filled with grout. This completely eliminates communication

between zones.

In the traditional method, cables can be twisted and permanently damaged in the process

of withdrawing drill casing or auger sections. With the multi-level system, cables are encased

in the PVC pipe and are much less likely to be twisted or damaged.



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But its limitations are same as VW piezometers.

4.2.5. Vented Vibrating Wire Pressure Transducers

The vibrating wire piezometer consists of a vibrating

wire pressure transducer, a vented signal cable, and adesiccant chamber. It is designed for monitoring water levels

in wells, stilling basins, and wiers. Readings are obtained

with a portable readout or a data logger.a) Applications

The vented pressure transducer is designed specifically for monitoring changes in water

levels in wells and stilling basins.b) Operation

The VW pressure transducer converts water pressure to a frequency signal via a patented*

arrangement of diaphragm, a tensioned steel wire, and an electromagnetic coil. The pressuretransducer is designed so that a change in pressure on the diaphragm causes a change in tension

of the wire. An electromagnetic coil is used to excite the wire, which then vibrates at its natural

frequency. The vibration of the wire in the proximity of the coil generates a frequency signal that

is transmitted to the readout device. The readout or data logger stores the reading in Hz.

Calibration factors are then applied to the reading to arrive at a pressure in engineering units.c) Advantages

Two Ranges: The vented pressure transducer is available in 22 and 50 psi ranges.

Large Diameter Vent Tube: The large diameter vent tube provides quick response to

changes in atmospheric pressure and cannot be blocked by droplets of condensation.

Oversize Desiccant Chamber: The large capacity, low maintenance desiccant chamber

keeps the vent tube dry for 3 to 6 months.

It is easy to read, accurate, and can be connected to data loggers and requires no barometric

pressure compensation.

Its Limitation is electrical noise from a pump in the same well can interfere with operation.

4.2.6. Titanium Pressure Transducer

The titanium pressure transducer is a 4-20mA device

that is compatible with industrial data loggers. It is use for

monitoring water levels in pumping wells and for monitoring

pore-water pressure in environments that would corrode

stainless steel.

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a) Applications

Designed for compatibility with industrial data loggers, the titanium pressure transducer is used

to monitor pore-water pressure and water levels. Typical applications include:

Monitoring pore-water pressures in corrosive environments such as salt water and

landfills.

Monitoring rapid changes in pore-water pressure such as those produced by earthquakes.

Monitoring water levels in pumping tests.b) Operation

The pressure transducer may be sealed in a borehole or suspended in a well. Signal cable from

the transducer is terminated at a readout station, where it can be connected to a data logger or

readout device. Water pressure acts on diaphragm of the transducer. Semiconductor strain gauges

bonded to the inside of the diaphragm sense the pressure and output a signal that is proportional

to the pressure on the diaphragm. The signal is transmitted to the data logger or readout device

via a 4-20mA loop circuit.c) Advantages

High Resistance to Corrosion: All metal parts, including the diaphragm, are made from titanium.

High Resistance to Noise: The electronics of the transducer are highly resistant to electrical noise,

such as that generated by pumps.

Compatible with PLCs: The titanium pressure transducer incorporates a 4-20mA transmitter to

provide compatibility with standard industrial data acquisition systems.

Suitable for Dynamic Monitoring : The transducer can be read continously.Excess of pore water pressure versus time

This equipment is suitable for dynamic measurements, easy to read, highly resistant to electrical

noise.

Its Limitation are a 4-20mA circuits require more power so are less suitable for battery-operation

and long term stability may not be good enough for some applications.

V. MAGNET EXTENSOMETERS

5.1. Overview

The magnet extensometer is a multipoint extensometer

that can be installed with inclinometer casing or with 1-inch

access pipe. The system consists of a probe, a steel measuring-tape, a tape reel with built-in light and buzzer, and a number of

magnets positioned along the length of an access pipe.

The magnet extensometer is used to monitor settlementand heave in excavations, foundations, dams, and embankments.

It can also be installed behind retaining structures, such as sheet

piles and slurry walls, and above underground openings, such astunnels and shafts. Data from the extensometer indicate the



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depths at which settlement has occurred as well as the total amount of settlement.

Magnet extensometers have been used to monitor settlement and heave in various

geotechnical applications. A test embankment was constructed to failure in Rio de Janeiro with

observation by two magnet extensometer columns (Ramalho-Ortigao, 1983).Magnet extensometers were fitted to vertical inclinometer casing along a portion of the

Central Artery/Tunnel for monitoring of a tied- back deep excavation (O’Rourke and O’Donnell,1997). The performance verification of lime-cement columns at this same I-15 Project was basedon magnet extensometer observations (Saye, et al., 2001). Magnet plates were installed in the

bedding sand below the 100 South geofoam embankments at alternate block layer intervals. The

plates were sequenced along a central riser PVC access pipe and move with the surrounding fill.

5.2. Instrument Description

The magnet extensometer system consists of settlement plates and permanent magnets,

PVC riser pipe segments and a sensing probe and measuring tape. Each settlement plate is asquare of 305 mm sides and 12.5 mm thickness with an annular permanent magnet collar, of 60

mm outside diameter, fitted at the center. The magnet collar opening is about 34 mm in diameter

to accept a schedule 40 PVC pipe of nominal 25 mm inside diameter. The plate and magnet collarassembly slide freely along the stem of the PVC riser pipe to position at the desired level.

Access Pipe: Inclinometer casing or one-inch access pipe can be used. After pipe and

magnets are installed, the borehole is backfilled with grout.

Datum Magnet: The datum magnet is fixed directly to the bottom section of access pipeto serve as a reference. A datum magnet is used when the bottom of the pipe is anchored in stable

ground.

.

Spider Magnet: The spider magnet, named for its spring-steel legs, is used in boreholes.

The legs are compressed for installation and are released when the magnet is positioned at the

Figure 9. Magnet extensometer probe inserted in the PVC riser



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specified depth. The spider magnet is typically attached to the access pipe prior to installation,

but can also be pushed into place after the pipe is installed.

Plate Magnet: The plate magnet is used in fill. It is positioned at the specified elevation

and then covered with fill material that is compacted to the same specifications as the

surrounding fill.

Telescoping Sections: Telescoping sections are installed when settlement or heave is

expected to exceed 3%.The dead zone between the north and south poles of the permanent magnet is a narrow

section and is fixed relative to the position of the plate. A magnet probe suspended by agraduated tape with conductors is lowered from surface through the PVC pipe to detect the

location of the dead zone and thus the position of the attached plate. The depth location from

surface to each magnet plate can be read to the 1 mm graduation on the measuring tape, and

readings are repeatable to ± 3 mm. Figure 7 shows the depth measuring tape and the magnet probe inserted within a PVC riser pipe. A gas-powered post-hole augur was used to bore through

the geofoam blocks at selected locations to accommodate the PVC riser pipe (Figure 8). The

geofoam block was then raised and lowered passing the riser pipe through the augured hole(Figure 9). Several settlement plates were nested vertically over the height of one PVC riser pipeand at different elevations within the fill. Plastic sheeting was placed over the final or top magnet

plate to provide a bond free interface between the load distribution slab and the underlying

geofoam. As the fill settles, the plate positions adjust accordingly. Successive changes in depth ofmagnet plate positions in reference to an initial baseline survey represent the rate and amount of

movement over a depth profile.

5.3. InstallationAccess pipe, magnets, and grout backfill must all move with the

surrounding ground in order for the magnet extensometer to work. Access

pipe must be installed with telescoping sections in zones where settlementis expected to occur. Spider magnets are coupled to the soil by the grout

that surrounds the pipe and the magnets. Thus, grout must be fairly weak,

so that it can deform or crumble as settlement occurs.

1. Number magnets and sections of access pipe with their intendeddepth. Prepare release cords for spider magnets. Release cord must be

of sufficient length to extend between intended magnet depth and

surface. Allow a minimum of 3m (10ft) of extra cord for surfacehandling

2. Fix the datum magnet to the bottom section of access pipe. The

datum magnet is usually installed at least 0.5 meter or 2 feet above the bottom of the pipe.

3. Compress and attach spider magnets to pipe. Spider magnets must

be securely attached to the pipe so that they reach their requiredlocations as the pipe is installed.



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4. Check that pipe sections are marked for order of installation, magnets are fixed to each

section of pipe, and release cords are labeled, coiled and taped to pipe sections.

5. Install pipe with magnets attached. As each section of pipe is installed with its magnet,

uncoil release cord and lay out in straight line. Check that cord will not be snagged, sincethis could release legs prematurely. Plan to lay out release cords from other magnets as well

and take care to avoid tangling cords. If possible, assign someone to feed cords down hole as pipe is lowered.

6. Check depth of each magnet using magnet extensometer probe. Pull drill casing, if used,to an elevation that is above the upper legs of the deepest magnet. If legs are released into

drill casing, the entire installation will have to be replaced.

7. Release legs of the magnet, pulling upwards on release cord. If necessary, pull drill casing

above next magnet. Then pull release cord to release the legs. Repeat this step until all spider

magnets are anchored.

8. Backfill borehole with a weak bentonite-cement grout as specified by project engineer.

5.4. How can the data of equipment be read? The system consists of a probe, a steel measuring-tape, a tape reel with built-in light and

buzzer, and a number of magnets positioned along the length of an access pipe. The magnets are

coupled to the surrounding soil and move up or down as heave or settlement occurs. Readings are

obtained by drawing the probe through the access pipe to find the depth of the magnets. When the probe enters a magnetic field, a reed switch closes, activating the light and buzzer. The operator

then refers to the 1 millimeter or 0.01 foot graduations on the tape and notes the depth of the

magnet.

When the access pipe is anchored in stable ground, the depth of each magnet is referenced to adatum magnet that is fixed to the bottom of the access pipe. If the bottom of the access pipe is not

in stable ground, the depths of the magnets must be referenced to the top of the pipe, which is

optically surveyed before readings are taken.It needs a reedswitch probe on a tape that is long enough to achieve the full depth of the

installation.

After installation, base readings can be taken using the reedswitch probe. The probe is passed

inside the access tube to the base of the installation. Make a note of the base depth.

Raise the probe up the access tube until an audible tone is heard. Pull the probe up slowly

until the tone stops. Take a reading on the tape, against the lip/top of the access tube. Slowly pull

the probe up again until another tone is heard. Take this reading. The difference between the tworeadings should be 10-15 mm. There will be two tones heard for each magnet, you should notedown the reading at the "end" of the first tone and the reading at the "start" of the second tone.

Continue to pull the probe up the access tubes, repeating the reading procedure for each

magnetic target. To determine the exact location of the spider magnet, take the mean reading for each magnet.

The mean is taken from the two readings taken for each magnet as above.

As the datum magnet is fixed to the tube, this will not move. To determine any movement of

any spider magnet, deduct each magnet reading from the datum magnet reading.



References

Dunnicliff, John. (1988). “Geotechnical instrumentations for monitoring field performance”. A

Wiley-Interscience Publication, John Wiley & Sons, Inc., USA.

Tamrakar, S.B. (2001). “Design parameters for elasto- plastic FE analysis of soft clay ground”.Ph.D. thesis.

Thirapong P. “Application of FEM to Excavation of Soft Clay”, Master thesis.

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