Field instrumentation for foundation soils and buildings

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Field instrumentation for foundation soils and buildings

Bozozuk, M.

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FIELD INSTRUMENTATION FOR FOUNDATION

1

SOILS AND BUILDINGS

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Reprinted from

Analysis and Design of Building Foundations Edited by Hsai-Yang Fang

Lehigh Valley, Pa.

,

Envo Publishing Co. 1976 h.

p. 181

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208

BUILDING RESEARCH

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LIBRARY

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RES6ARCn C O U W ~ L

DBR Paper No. 753

Division of Building Research

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I I S _{OMMA I} R E Le comportement d l u n e c o n s t r u c t i o n e t c e l u i du s o l de f o n d a t i o n d e v r a i e n t G t r e c o n s i d d r d s ensemble l o r s q u ' i l s ' a g i t d ' d v a l u e r l e u r rende-

ment. L ' a u t e u r examine l e pourquoi d e s mesures

s u r p l a c e e t d d c r i t l e s i n s t r u m e n t s q u i permet- t e n t de mesurer les m o u v e m e n t s v e r t i c a u x e t l a t b r a u x du s o l , l e s p r e s s i o n s de l ' e a u i n t e r - s t i t i e l l e , l e s p r e s s i o n s des t e r r e s , l e s p r e s - s i o n s du v e n t , l e s mouvements e t l e s dbforma- t i o n s d e s b d i f i c e s e t l e s p r o p r i b t d s dynamiques des b d i f i c e s .

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ANALYSIS AND DESIGN OF BUILDING FOUNDATIONS

FIELD INSTRUMENTATION FOR FOUNDATION SOILS AND BUILDINGS

by

M.

Bozozuk

Research Officer, Geotechnical Section Division of Building Research National Research Council of Canada

Ottawa

ABSTRACT

1 The behavior of a structure and the supporting soil should be considered

together in assessing their performance. This paper discusses the reasons for field measurements and describes the instrumentation for measuring

vertical and lateral ground movements, pore water pressures, earth pressures, wind pressures, building movements and deformations, and the dynamic

properties of buildings.

* * * * * *

The function of a foundation is to provide a suitable base for a

building and to transmit its load to the foundation soil without overstressing it and lead to excessive settlements, deformations or failure. "If a

foundation fails, all above it fails," warned G.F. Sowers (1974). It is imperative, therefore, that the foundations be designed for the soil conditions that exist at the site.

After a building is erected it may be subjected to external forces from winds, earthquakes, and ground vibrations (Fig. 1). These influences are in addition to the live and dead loads resulting from the weight of the structure and its contents, and are also transmitted to the underlying soil through the foundation. A building, therefore, has to be designed and constructed with its foundations and the foundation soil as one complete system. Field instrumentation should be installed to measure the interaction of the structure and the soil in order to understand the behavior of a building. This paper reviews some of the various types of instruments and measurement techniques that can be used to monitor the performance of foundation soils, the foundations and the superstructure.

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PURPOSE OF INSTRUMENTATION

I

Observations on an engineered structure may be required for any or all

I of the following reasons:

To determine the engineering behavior of the foundation soils and components of the structure which cannot be obtained reliably from laboratory tests. Such information may then be used for future

design. I

To check the behavior of either temporary or permanent structures during

,

construction.

_'

To check that design criteria are satisfied. To assure its safety during its lifetime.

As a precautionary measure to give advance warning of possible failures or problems which may be encountered during construction.

Feld (1974) presented ten useful rules for avoiding failures during construction. A brief review of some of these rules suggests problem areas where field instrumentation would be extremely useful to help reduce the risks involved.

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Gravity always works

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if permanent support is not provided, something

will fail.

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Chain reaction will turn a small failure into a large failure.

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A small error or oversight, in design, in material strength, in assembly

or in protective measures, can cause a large failure.

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Vigilance is necessary to avoid small errors. If there are no capable

foremen on the job and in the design office, then supervision must take over the chore of control.

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There is no foolproof design or construction method, without the guidance

of proper and careful control.

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The best way to generate a failure on a job is to disregard the lessons

to be learned from someone else's failures.

FIELD MEASUREMENTS

When the foundation soil is unloaded by excavation, loaded by a

structure, or has piles driven into it, reactions such as soil movements and stress changes are produced that are directly related to the magnitude of loads, to the size of the loaded area and to the engineering properties of the soil. Some of these reactions may be rapid and take place during

construction; others are time dependent and will continue to occur long after the structure is completed. Instrumentation and techniques to measure some of the reactions on various engineering structures have been given by

Bozozuk (1970), Dunnicliff (1971) and Hanna (1973). A description follows of the types of field instrumentation required to measure the performance of tall buildings, their foundations and foundation soils.

Vertical Ground Movements Bench Mark

A stable datum point or bench mark is required to measure the absolute or total settlements over long periods. The best bench mark is a point located on a rock outcrop or on a building supported on end bearing piles. If this is not readily available, a stable bench mark should be installed in the soil. Bozozuk et a1 (1962) described the design and installation of a deep bench mark that can be installed through the overburden and set on bedrock or other suitable material. The bench mark consists of a long central pipe supported by a steel foot at the lower end (Fig. 2(a)). A stainless steel ball is accessible at the upper end above ground surface for referencing the

engineering surveys. The central pipe is shielded from the surrounding ground by an oil- or grease-filled casing which protects the instrument from vertical ground movements, frost heave and corrosion. A protective casing and cover completes the installation.

Single Point Settlement Gauge

The simplest settlement gauge to measure vertical ground movements (Fig. 2(b)) is similar to the bench mark. It consists of a settlement plate or point installed at a predetermined depth and extended to the ground surface by a vertical pipe shielded from the surrounding soil by an oil-filled casing. Most installations of this type differ only in the kind of settlement point used. Some use a plate or a cement plug in the bottom of a borehold. Bjerrwn

et a1 (1965), described the Borros point which is anchored at the desired

depth with three fins driven into undisturbed soil. A screw point described

by Bozozuk (1968) has the added advantage that it can be located precisely by turning it into undisturbed soil at the bottom of the borehole. Its depth can also be increased at any time by turning it further into the ground and adding extensions to the center pipe.

A separate settlement gauge is required for each depth of interest. i

Measurements are made by level surveys to the center pipe which must be

referenced to a stable bench mark. An accuracy of k0.15 mm can easily be

obtained, but it is directly related to the materials used in its construction (steel or invar rods), annual temperature variations, and to the instrument used to carry out the surveys.

Multiple Point Settlement Gauge

This gauge was designed for locations where there was not enough space

to install a number of single point gauges. A simple, relatively inexpensive,

multipoint gauge is shown in Fig. 2(c). Described by Fellenius (1969), the

gauge consists of a corrugated flexible hose, 32 mm I.D., similar to those used on vacuum cleaners, fitted with annular copper rings at predetermined locations. The hose is extremely flexible in the longitudinal direction but stiff and strong radially. When installed in a vertical borehole and grouted

1

with a weak bentonite-cement grout (Tavenas et al, 1973) the copper rings move freely with the soil. Measurements are made by lowering an electric probe mounted on a rod or tape down the tube to make contact with the copper

ring. The accuracy of the system is about +3 mm.

i

The gauge shown in Fig. 2(d) (Burland et al, 1972) consists of circular

magnetic markers installed around a PVC guide pipe in a 10-cm-dim borehole and located with electric reed switches mounted on a measuring tape or rod. The circular magnets are installed in succession starting from the bottom of the borehole with a magnet holder that slides over the plastic guide tube. When the magnet is at the required depth, the insertion tool is withdrawn; this releases springs mounted on the magnets. The springs then bite into the side of the borehole, anchoring the magnets in undisturbed soil. Collapse of the borehole is prevented by grouting with a soft flexible grout around the guide tube. Measurements on the locations of the magD.ets can be made to

within +1 mm by lowering a reed switch mounted on a steel tape down the guide tube. A more elaborate measuring system consists of reed switches mounted on measuring rods at the level of each magnet and left permanently in the guide tube. Each switch is connected to separate electric circuits. Only small axial movements of the measuring rod made with a screw micrometer at the surface is required to sense each magnet. Burland considers that this measuring system produces an accuracy of better than 0.1 nun.

Penman (1969), Dunnicliff (1971) and Hanna (1973) described the electrical-impedance-type multipoint settlement gauge for use in filled ground. It consists of steel plates placed concentrically around telescoping plastic pipe installed vertically in the soil as the fill is placed and compacted. The plates move freely with the soil as they are not attached to the pipe. A measuring tape fitted with an induction coil locates the steel plates to an accuracy of +3 mm as it is lowered down the pipe.

Dunnicliff (1971) and Hanna (1973) described the "cross-arm" multipoint gauge for measuring vertical movements in filled ground developed by the U.S.

Bureau of Reclamation (1960). Cross arms welded to short lengths of pipe are positioned over each other at predetermined locations as the fill is

constructed. The pipes telescope over each other, allowing the cross arms to move freely with the soil. A measuring "torpedo" lowered down the pipe

locates the ends of the pipe sections. The accuracy of this system is about

+5 _mm.

Heave Gauge

Soil rebound or heave due to stress release from excavation can sometimes be measured with the instruments as described. These devices, however,

extend to the ground surface and may interfere with the excavation process. Frequently they are damaged and lost and no further measurements can be made. This problem may be overcome with heave gauges described by Bozozuk (1963) and Dunnicliff (1971).

The gauge described by Bozozuk (1963) was patterned after those used in Norway (Norwegian Geotechnical Institute, 1962). It consists of four 6-mm- thick steel fins welded together to form a vane 9 cm in d i m by 30 cm long supporting a circular plate 2 cm thick (Fig. 3). A keyhole in the center of

the plate aids installation. The gauge is lowered down a borehole 10 cm in diam and pushed into undisturbed soil with rods fitting the keyhole. After the rods are removed, the hole is filled with a bentonite slurry (w/c 800 to 900%) colored with red Erythrosine dye. The bentonite prevents the hole from collapsing; the red color facilitates location of the boreholes while

excavation is in progress. Measurements are made by sounding through the bentonite to the gauge with specially machined rods and using standard leveling techniques. The accuracy is of the order of 2 3 mm.

1

Remote-reading settlement gauges are required at locations where projecting instruments would interfere with construction.

Burn (1959) developed the electric settlement gauge shown in Fig. 4. The instrument measures relative movements between a bearing plate, which is located at the ground surface supporting the measuring head, and a spiral anchor reference point which is turned into the ground to the desired depth. The measuring device consists of a chain and sprocket keyed to the reference pipe which turns a spirally wound variable electric resistance coil. Readings in ohms are converted to vertical displacement. A number of these instruments have been installed to measure vertical settlements beneath earth embankments on clay subsoils. Those that were not damaged by construction are still providing measurements eleven years after installation. Burn (1959) states that the accuracy of the measurements are within 51 mm.

The simplest remote-reading settlement gauge is the water tube level. The settlement gauge and readout are placed at the same elevation and connected with a hose. The hose is then filled with water and when it overflows at the gauge end, its elevation is measured by recording the level of the water at the readout end. The accuracy of the system is improved by using a micrometer to measure the elevation of the water.

When the settlement gauge and readout have to be placed at different elevations, an auxiliary pressure system is required. If the settlement gauge is higher than the readout, the water can be pumped to the gauge until it overflows and spills into a return drain line added to the system (Wilson,

1967).

A

pressure gauge or manometer measures the hydraulic head, and hence the elevation of the gauge.

I A number of remote reading instruments are available where the settlement gauges have to be located below the level of the readout. The hydraulic settlement gauge or tassomstre (Peignaud, 1973) that has been used in France

I

for a number of years is shown in Fig. 5. The cell, which is partially filled

I with water, is connected to the readout with water and gas lines. The gas

pressure in the cell is increased, forcing the water to flow from the cell into the burette; the gas pressure and water level in the burette are then I _{measured and recorded. In subsequent measurements the same amount of gas}

pressure is applied, and the difference in the burette readings indicates the soil movements. The cells shown in Figs. 5(b) and (c) are for shallow and deep installations respectively. The cell shown in Fig. 5(c) can be installed in vertical boreholes. The cells are made of polyvinyl chloride to eliminate corrosion problems. Antifreeze solutions are used instead of water in

freezing conditions. Dunnicliff (1971) and Hanna (1973) described other gauges such as the electrical mercury settlement gauge developed by Irwin

(1964) and the multipoint hydraulic settlement gauge incorporating transducers developed by Ward et a1 (1968).

Horizontal Ground Movements

,

Lateral movements at shallow depths can be measured with marker plates and tension wires fastened to indicators; at greater depths inclinometers and

E vertical tubes must be used. A typical plate gauge, shown in Fig. 6 (Bozozuk, I 1972), consists of a steel plate which is installed by driving it vertically

I

into the soil. The plate is connected to the measuring head with a flexible steel cable pretensioned with a lead weight and protected from the surrounding

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soil with an oil-filled polyethylene tube. A sliding joint is provided in the tube near the plate to ensure that the plate can move freely with the soil without being affected by stresses that may be generated in the tubing.

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Extensometers similar to those described by Dunnicliff (1971) and Hanna (1973) are also used. Tavenas et a1 (1974) installed the multiple-point

I

gauge (Fig. 2(c)) horizontally beneath an embankment to measure lateral

movements. Penman's impedance gauge (1969), which uses steel plates, was also used to measure horizontal movements.

Horizontal movements at greater depths are usually measured with

inclinometers lowered down a cased borehole (Fig. 7). The Wilson (1967)

inclinometer operating as a Wheatstone bridge requires specially extruded aluminum casing with guide grooves. The casing is installed in a prebored hole and grouted. Measurements of lateral movements and their directions are made by lowering the indicator down the casing.

Kallstenius and Bergau (1961) described a strain gauge inclinometer developed by the Swedish Geotechnical Institute. This instrument differs from

the Wilson inclinometer mainly in that a PVC pipe casing is used. Because

there are no guide grooves, the indicator is attached to 1-m-long extension tubes connected with flexible couplings to guide it down the casing. At the top of the casing the tubes are connected to a diopter dial. This dial, which indicates the orientation of the inclinometer, enables measurement of the movements in a predetermined direction or direction of maximum deflection.

Hanna (1973) described other inclinometers such as the Digitilt instrument which measures inclinations by a pair of servo-accelerometers. Bromwell et a1 (1971) described an automatic recording inclinometer which provides a photographic record of the measurements.

Pore Water Pressure Measurements

Water pressures are measured with piezometers to provide information on groundwater conditions, permeability of the soil, seepage gradients and changes in pore water pressures due to construction. Many kinds of

piezometers are available; the choice depends on the subsoil conditions and the measurement required.

The earliest piezometer, which is still in use, was the open standpipe which consists of a tubular porous stone tip embedded in sand and sealed down

a borehole (Casagrande, 1949). A plastic riser tube connects the tip with

the ground surface. The hydraulic head is measured by sounding down the tube 188

to the water. Other piezometers of this type consist of porous plastic or porous bronze filter tips. The Geonor piezometer developed at the Norwegian Geotechnical Institute consists of a porous bronze filter that fits onto "El'

size drill rods that are left in place to protect the standpipe. A number of them can be installed in one borehole (Bjerrum et al, 1965). In soft clays, a borehole is often unnecessary as they can be pushed directly into the ground to the desired level (Bozozuk, 1960). These piezometers are used in saturated soils where the changes in pore water pressures are gradual or where the permeability is great enough so that water can flow into or out of the piezometers quickly.

The open standpipe piezometers are not suitable to measure rapidly changing pore water pressures because of the long time lag. The no-flow or constant volume piezometers such as the hydraulic, pneumatic or electric piezometers should be used under these conditions.

The hydraulic piezometer (U.S. Bureau of Reclamation, 1960) consists of a moulded plastic tip containing a porous disc connected by two tubes to a pressure gauge which measures the head directly. The two tubes permit the flushing of entrapped air from the lines.

The pneumatic piezometer consists of a sealed porous tip containing a pressure sensitive valve and diaphragm (Lauffer and Schober, 1964; Warlam and Thomas, 1965). The porous tip is connected to a readout with two tubes

(Fig. 8(a)). Air, gas, water or oil is pumped down one tube until the line pressure equals water pressure acting on the opposite side of the diaphragm. Additional pumping causes the diaphragm to deflect slightly, permitting the gas or fluid to flow into the return line. These piezometers are simple to operate, have long-term stability and negligible time lag.

The electric peizometer has an electric sensing device mounted on the flexible diaphragm of the porous tip. They are extremely sensitive and respond immediately to pore pressure changes. They have performed well in short-term observations, but their long-term record has not been satisfactory.

The Division of Building Research, National Research Council of Canada, has used the Geonor electric vibrating wire piezometers for short-term

observations in marine clays. Most have performed satisfactorily for about a year, but only a few provided good data for the four years since they were installed. Hanna (1973) and Peignaud (1973) have published excellent reviews of other pneumatic and electric peizometers.

Earth Pressures ₊

The design of foundations for tall structures or retaining walls for excavations depends upon the magnitude and distribution of the contact earth pressures. The instrumentation for these structures is limited to those that can be placed at the contact between the structure and soil. Two main types are commonly used: the electric and hydraulic cells.

The electric cells normally consist of a circular housing supporting a flexible diaphragm. The soil causes the diaphragm to deform and this indicates the soil pressure which is measured either with strain gauges

(Sparrow and Tory, 1966; Carlson and Pirtz, 1952) or with vibrating wire transducers (Oien, 1958; Scott and Kilgour, 1967; Thomas and Ward, 1969). Vibrating wire cells described by Oien (1958) have been used to measure total pressures on a tunnel for a period up to three years (Eden and Bozozuk, 1969); the vibrating wire cells described by Thomas and Ward (1969) were used in a test embankment (Bozozuk and Leonards, 1972).

The Gloetzl earth pressure cell (Lauffer and Schober, 1964) is a

hydraulic cell consisting of two flat, stiff, metal plates joined at the rim to form a cavity which is connected to a flexible diaphragm and bypass valve (Fig. 8(b)). The cavity is filled with oil and prepressured to about 1 atm. Two tubes connect the valve to a measuring instrument. In operation, light oil or gas is pumped through one tube until its pressure equals that pressing against the opposite side of the diaphragm. The pressure exceeding the prestress gives a measure of the earth pressure acting on the cell. Applying more pressure only forces the diaphragm to deform, allowing the gas or fluid to return via the return line. This cell has provided excellent long-term i

service and has been used to measure contact pressures beneath a raft foundation (Eden et a1

,

1973)

.

A photoelastic load cell (Hooper, 1973) was developed to measure raft contact pressures and the loads on piles from a building. The cell was designed to be sturdy to give long-term stability. It has functioned

satisfactorily for several years. The cell consists of a load-carrying steel column containing a transverse hole at mid-height. A solid glass cylinder is located at the center of the hole which is preloaded across the diameter by a wedge mechanism. In operation the load applied to the steel column deforms the transverse hole and compresses the glass cylinder. Circularly polarized light passed through the glass cylinder is viewed through an analyser. The isochromatic fringe pattern is then measured; this gives a measure of the applied load.

Normal and shear stresses have been measured successfully along the base of a test embankment (Bozozuk and Leonards, 1972) using the Cambridge earth pressure cells (Arthur and Roscoe, 1961). The cell developed by Agarwal and Venkatesen (1965) was used to measure normal pressures and shear stresses along the face of a pile subject to skin friction loading. The cells on both projects performed satisfactorily.

Settlement Measurements Inside Buildings

Building settlements are usually measured by conducting engineering level surveys on fixed or stable points at preselected locations on the structure. Points located inside the building often become obstructed after construction is completed and the building is occupied, so that these surveys are difficult to carry out. To overcome this problem the Division of Building Research, National Research Council of Canada, constructed a water tube level for performing level surveys inside buildings (Peckover, 1952).

The principle of the water tube level is illustrated in Fig. 9. It consists of a suitable length of clear plastic hose filled with deaired water, and used as a manometer. Each end of the level contains a valve and a burette fitted with a vernier scale, levelling bubble and levelling screw (Fig. 10ra)).

The observation points shown in Fig. 10(b) are installed at about the same elevation on the walls at desired locations. In operation the water tube level is suspended on two observation points and the level of the water in the burettes recorded. Interchanging the ends and repeating the observation corrects the measurement for temperature effects. Differences in elevation can be measured to within 20.1 mm.

Mounting the burettes on specially machined rods of equal length converts the water tube level into a "walking stick" level. Elevations of any flat surface can then be measured without the use of the observation points shown in Fig. 10(b). The accuracy of this apparatus is about 21 mm.

Dynamic Measurements

Tall buildings are subject to wind loads and may be subject to earthquake forces. As these external forces are transmitted to the foundations they should be considered in the design of both the foundations and the

superstructure. Instrumentation to measure external forces and the dynamic properties of the structure should be included in plans for instrumenting a tall building.

A strong motion seismograph is used to measure ground or structural acceleration levels during an earthquake. It is a long-term installation, usually for the life of a structure, recording the acceleration-time history of an earthquake when the accelerations exceed a preassigned trigger level of, for example, 0.01 gravity. From these records the lateral and vertical

forces induced in a structure and its performance can be ascertained. If only one instrument is used it is usually placed in the lowest basement level. Occasionally additional seismographs located at mid-height and on the top

fleor are used to monitor the performance of the structure during an earthquake.

Wind forces are measured with differential pressure transducers mounted flush on the exterior walls of the building (Dalgliesh et al, 1967; Dalgliesh, 1974; Eaton and Mayne, 1974). They are usually located at approximately 85, 65, 45 per cent of the building height and occasionally at 25 per cent. The observations are related to wind speed which serves as a bench mark for the

calculations. Wind speeds are measured with anemometers which should be located outside the influence of the structure. Dalgliesh (1974) mounted anemometers on fixed masts from 150 to 200 ft (48 to 61 m) above the roof of a building.

The dynamic properties of buildings are measured with accelerometers placed at preselected points within a structure (Fig. 11) (Crawford and Ward, 1964; Ward and Crawford, 1966; Ward and Rainer, 1972). This type of distribution enables the modal frequencies of vibration and their associated mode shapes and damping ratios to be determined from the recorded data. Any motion of the structure, whether produced by wind, street traffic, movement of elevators, or people, is sufficient to activate the accelerometers.

The recording system and the systems for analysing the test data are illustrated in Fig. 12. The accelerometer output is amplified and recorded on tape. The record is returned to the laboratory where it is analysed. Once the dynamic characteristics are known, it is possible to calculate the elastic response of the structure to a known system of external forces. After the tests are completed, the accelerometers and recording equipment are removed and are available for use in other structures.

The lateral displacement of a building subject to an external force, e.g., wind, can be computed from these measurements. It is also possible to measure it directly by aiming a stable laser beam whose source is stationary onto a photoelastic target mounted directly on the structure. The photoelastic cells generate a voltage proportional to the distance the center of the target moves relative to the beam. The Division of Building Research has measured bridge deflections caused by a passing train to within 0.3 mm from a distance of about 800 ft (244 m).

DISCUSSION

Field measurements are often the only reliable means of obtaining information for design, to control construction, check on safety, verify design theories and to assess the over-all performance of a structure.

Measurements must be accurate, reliable and meaningful; field instrumentation should be accurate, reliable and carefully located.

Installing instrumentation and taking measurements is more difficult in the field than in a laboratory. In the field, the climate and environment are generally far from ideal. Furthermore, once the instruments are installed it is usually impossible to check them or to make adjustments or repairs. Consequently, all field instruments should be assembled and checked in the laboratory before they are taken to the field and installed.

Field instruments should never be installed without having been calibrated; the calibration procedure should duplicate, as closely as

possible, their intended use in the field. For example, earth pressure cells that are to be installed in sand should be calibrated in a calibration box using sand obtained from the site (Bozozuk, 1972).

The choice of instruments is very important. Mechanical gauges are usually the most reliable if they can be protected from corrosion or

external damage. The main disadvantage is that, as readings have to be taken manually, access to them has to be provided. On the other hand, with

electrical instrumentation it is possible to take readings electronically and store them on tape for subsequent analysis. Electric gauges, unfortunately, when subject to an adverse environment with severe wetting and temperature conditions, have the poorest service life. The proven performance record of any device is the best guide to the selection of field equipment.

The mortality rate of most field instrumentation is high because, in addition to losses due to adverse environmental conditions, they are vulnerable to damage during construction of the building and also to vandalism while they are exposed. Part of the problem can be solved by selecting the most reliable instrumentation and providing adequate

protection. In addition it is recommended that the proposed instrumentation be included in the contract specifications to ensure their protection at

Field measurements should be processed as they are obtained and the data made available to those responsible for the project to make optimum use of the results. This also provides a check on the performance of the instrumentation while the project is under way. Far too often field measurements are

accumulated, stored and analysed only after the construction is completed. At this time questionable measurements cannot be checked so that the analysis of the results is also questionable.

CONCLUSIONS

Field instrumentation to measure the performance of a building and its foundations and the engineering properties of foundation soils, have been described. These measurements are useful to obtain design values, to check engineering predictions, to check the stability of temporary and permanent structures and to control construction. All instrumentation should be checked and calibrated before installation. A stable bench mark is required for all measurements. The observations should be processed as soon as possible to make optimum use of the instrumentation.

ACKNOWLEDGMENTS

This work is based upon the experience of many members of the

Geotechnical Section, Division of Building Research, National Research Council of Canada. Thanks are due to Mr. A.W. Dalgliesh and to Dr. G. Pernica, also members of the Division, for their assistance in describing measurements of wind and vibrations respectively. This paper is a contribution from the Division of Building Research, National Research Council of Canada, and is published with the approval of the Director of the Division.

REFERENCES

Agarwal, S.L., and Venkatesan, S., 1965. An instrument to measure skin friction and normal earth pressure on deep foundations. American Society for Testing and Materials, STP No. 392, Instruments and Apparatus for Soil and Rock Mechanics, p. 152-169.

Arthur, J.R.F., and Roscoe, K.H., 1961. An earth pressure cell for the measurement of normal and shear stresses. Civil Engineering and Public Works Review, Vol. 56, No. 659, p. 765-770.

Bjerrum, L., Kenny, T.C., and Kjarensli, B., 1965. Measuring instruments for strutted excavations. Proc., American Society of Civil Engineers, Vol. 91, SM1, January, Part 1, p. 111-141.

Bozozuk, M., 1960. Description and installation of piezometers for measuring pore-water pressures in clay soils. National Research Council of Canada, Division of Building Research, Building Research Note NO. 37, Ottawa, 18 p.

Bozozuk, M., 1963. The modulus of elasticity of Leda clay from field

measurements. Canadian Geotechnical Journal, Vol. 1, No. 1, p.43-51. Bozozuk, M., 1968. The spiral-foot settlement gauge. Canadian Geotechnical

Journal, Vol. V, No. 2, p. 123-125.

Bozozuk, M., 1970. Field instrumentation of soil. Proc. Conference on Design and Installation of Pile Foundations and Cellular Structures, Lehigh University, Bethlehem, P.A., Envo Publishing Co., p. 145-157.

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