Ground Movements

The measurement of foundation and ground movements is concerned with the accurate

Determination of settlements,

Lateral movements

Change of position,

Change of dimensions

Change in length of foundation elements

The most conventional survey techniques are used for the determination of

Elevation

Change in Elevation

Determination of lateral displacement by offset measurement from a line of sight

Determination of distance and distance change by tape measurement between observation pillars

Determination of change of position by triangulation

Elevation determination

Survey accuracy depends upon the type of reference points and the datum to which the measurements are referred.

For absolute movement determination it is essential that the datum benchmark be located well away from the zone of ground movement.

A permanent bench mark is used.

Where this is not available a bench mark or a number of bench mark depending on the size of the project are formed.

Degree of sophistication of benchmark depends on the accuracy required, the ground and environmental conditions and the permanency of the installation.

Rock benchmark installation

Benchmark may be used for direct settlement readings.

In this the steel casing is connected to a settlement plate and the differential movement between the plate and the bench mark is recorded by a dial gauge.

Settlement gauge and benchmark

Settlement points may be classified into two groupsThose for ground surface settlementThose attached to foundation structures

The ground settlement device a block of concrete surrounding a reinforcing bar.Though the device is simple and cheap it is susceptible to damage from the construction activity and frost heave. This limitations may be overcome by isolation of the measuring reference bar by means of concentric tube.The more common approach is to install a pipe or rod in a small diameter hole, the rod being isolated from the ground by means of casing. eg Borros point

Borros point

Extension of the point is the settlement plate used with an anchor post.

The plate is placed on top of the compressible layer, and fill placed on top up to the ground surface.

The anchor post is then installed.

The movement of the plate relative to the top of the anchor rod is measured by dial gauge thus eliminating the need for a survey crew.

Settlement platform with anchor post

The reference points on foundation structures are either rigidly attached to the structure by bolting or welding or special demountable points are employed. Eg a pin and socket commonly used . Comprises a steel or brass socket grouted into a hole in the side of the foundations. The reference pin screws into the socket. When not in use the socket is protected with a cover plate. The levelling staff is placed on top of the reference pin

Pin and socket for foundation settlement measurement

For survey of roads it is impossible to maintain the reference points specially if they are projecting above the ground surface.

In such cases special reference head are used.

They consist of a steel tube about 25 mm diameter which is placed in the ground so that the top end with the special fitting plug is about 20 mm below ground surface.

The plug has a central hole about 15 mm deep into which an extension rod about 150mm in length fits.

The top end of the extension rod is cone shaped

Heave measurement

The heave point are made from four 6mm thick steel fins welded together to form a vane 90mm in diameter and 0.3 m in length.

A plate 19mm thick, welded to the top ends of the point and with a 12 mm diameter tapped hole in the centre aids installation.

To install a heave gauge a 100mm diameter hole is drilled to the required depth .

The heave gauge is lowered on a string of drill rods and forced into the bottom of the borehole until the top plate is flush with the bottom of the borehole.

The drill rods are then removed and the hole backfilled with a red bentonite slurry.

The bentonite is placed at a water content of about 900 % and coloured with Erythrosine dye.

The bentonite protects the borehole from cave in and the red dye facilitates location of boreholes while excavation is in progress.

By sounding through the bentonite with stainless steel flush jointed rods of about 10mm diameter , the top plate is located and elevation is determined by levelling techniques

Heave gauge

Lateral movement from offsets

By locating permanent targets at the two ends of a line of sight the line of sight is defined.

When the movement reference points have been established either in the ground or on the structure foundation.

The line of sight defined by the theodolite and the permanent foresight target is dropped in turn onto a steel tape or scale held at right angles across the line of sight and zeroed on the reference mark on top of the reference point.

In some cases permanent scales are attached to the reference points.

Wherever possible the reference point should be set in line between the stable end references

Tape measurement between observation points

Triangulation

Electronic distance measurement

Laser

Photogrammetric method of movement measurement

Single point settlement Gauge

There are many situations where it is not practicable to carry a vertical reference rod through the ground surface eg vertical settlement of a point beneath a road.

Terzaghi introduced a hose level manometer and this has lead to a number of more accurate and more versatile instrument which rely on the manometer principle .

All remote settlement gauges rely on the determination of the elevation of one point with respect to a bench mark by optical leveling.

The elevation of other point is obtained by relative elevation addition to the known elevation.

Thus the accuracy of measurement is controlled by the accuracy of optical levelling as well as the factors common to the manometer system namely temperature differential along with water filled tube and the presence of air bubbles in the manometer tube.

Water manometer tube gauge

A measuring cell is buried in the fill and connected to a remote reading unit comprising a drain tube and air tube connected to the cell and a vertical scale alongside a stand pipe.

The cell is at the same elevation as the read out unit.

In use de aired water is pumped into the vertical standpipe which causes overflow in the cell buried in the fill.

The overflow returns from the cell via the drain tube.

The elevation of the overflow level in the cell is then read on the scale attached to the vertical standpipe.

The height of the standpipe must be sufficient to cover the elevation of the cell unit and its anticipated settlement.

Water manometer tube gauge

Mercury settlement gauge

Nitrogen pressure is applied to the cell unit until an electrical contact between the base of the cell and mercury is made .

The elevation of the contact is determined from the measured nitrogen pressure with knowledge of the elevation of the mercury surface at the read out unit

Mercury settlement gauge

Multipoint settlement gauges

The principal of the multipoint gauge is that several addition cells are installed in the manometer tube.

The gauge consists of a series of interconnected water pots.

Each pot is a brass cylinder 28 mm i.d and sealed at top with a brass lid.

A flexible hose connects the bottom of the pots to water and top of the pots to an air supply .

The pots are mounted on three foot screws for levelling purpose.

A hollow stainless steel float with conical shaped top bears on a lever which actuates a displacement transducer.

Relative movement of the brass pots changes the elevation of the water surface and hence the transducer readings.

Multi point settlement gauge

High Sensitivity Settlement SensorSettlement Monitoring System consists of a series of vessels containing liquid level sensors interconnected by a liquid filled tube.

A reference vessel is positioned at a stable bench mark location with additional vessels positioned at different locations at approximately the same elevation.

Differential settlement or heave between any vessel and the reference vessel results in an apparent rise or fall of the liquid level in that vessel.

This system is particularly suitable for critical situations where high resolutions are required.

Elevation changes of as little as ±0.02mm (±0.001in.) are detectable.

Mercury settlement gauge can also be used as a multi point instrument.

Additional cells each with a separate electric contact and cable are installed in the manometer tube.

Readings of the elevation are taken by increasing the nitrogen pressure to lift the mercury column and break contact between the cell base and mercury column in each cell in turn

Multi point mercury settlement gauge

Vertical tube settlement gauge.

All gauges rely on a string of telescopic tubes to which a series of plates are attached.

The settlement of these plates are measured and hence average strain in the ground can be determined.

In building research station electrical gauge rigid P.V.C telescopic tubing is placed in the fill during construction.

The measurement points comprise 0.3m square steel plates placed around the plastic tube at about 3m vertical interval.

The steel plates can move independent of the tube.

A probe which consists of a coil forming the active arm of a parallel resonance bridge is lowered on the end of a graduated cable down the inside of the plastic pipe.

When the coil is central in each steel plate a maximum imbalance is read .

Vertical tube settlement gauge

The cross arm type gauge is the oldest being developed by the U.S bureau of reclamation .

A 38 mm pipe is attached to a short length of channel .

a spacer of 50mm pipe is carried up to the next channel section cross arm.

The cross arm ensures that the 38mm dia pipe moves together a distance equal to the compression of the soil between the cross arms.

When each cross arms installed the elevation of the reference point on top pipe section is determined by levelling.

Depths to the measuring point at the lower end of the 38 mm pipe are sounded by a special torpedo.

Cross arm vertical settlement gauge

Full profile gauges

The principal of operation of these devices is that the cell unit is pulled along a buried flexible pipe laid in a trench, usually horizontal and the distance from the fixed measuring unit is measured by a tape.

Th removable cell unit is referred to as a torpedo.

The elevation of the torpedo may be made at any position within the pipe thus giving a vertical profile of the pipe with respect to the fixed measuring unit.

In the overflow type the torpedo consists of an overflow gauge.

With this set up an accuracy of 2.5 mm was reported at a dam site.

The set up was used to measure both vertical and horizontal movements and comprised rigid p.v.c pipes laid in the fill to a fall of 1in 40 from the core of the dam to the gauge house on the downstream face.

Steel plates with central holes which fits over the pipe were spaced at 51m intervals and their horizontal movements were related to reference plate fixed to the instrument house.

This was achieved by use of an induction coil which can be passed through the plate.

The water level device as also pulled through the duct by the motor and the elevation of the pipe at predetermined positions determined with respect to the elevation of the instrument house.

In the full profile gauge described the torpedo consists of a strain gauge pressure transducer.

With this instrument the pressure transducer measures the liquid head between the torpedo and the free liquid surface.

The gauge is particularly useful for monitoring of vertical settlement profiles beneath foundations on compressible soils

Settlement gauge

Settlement Cell Slope Indicator’s settlement cells have three main components: a pressure transducer, a liquid-filled tube, and a reservoir.

Settlement cells are available in pneumatic and VS versions.The pressure transducer, with liquid-filled tube attached, is embedded in fill.

The other end of the tube is terminated at the reservoir.

The reservoir should be located at a higher elevation on stable ground.

The tube acts as a column of liquid, and the transducer at the bottom of the tube measures the pressure created by the height of the column.

The transducer settles with the surrounding soil, effectively increasing the height of the column of liquid and the pressure on the transducer.

Settlement is calculated by finding the change in pressure and converting it to feet or meters of liquid head.

Horizontal movement gauges

The measurement of lateral movement within a soil mass requires the installation of devices such as telescoping tubes, tensioned wires and transverse extensometers as well as the conventional surface monuments.

Telescoping tubes

The technique is also used for vertical movement recording and the principle of operation is that a telescoping tube which has steel plates attached at predetermined interval has an elevated torpedo passed through it to locate the position of the plate with respect to on end.

The torpedo is pulled through either by a pulley and wire system or by a special motor.

Tensioned wire devices

The tensioned wire devices consists of steel wires or cables attached to anchor zones and conducted through guide pipes to a measuring point where the movement of a point on a cable is observed relative to a fixed reference e, the tension in the wire being constant.

The multiple point extensometer measures the relative displacement of the wires which are fixed in the ground along the axis of a bore hole. During installation each measuring wire is tensioned by spring cantilevers in the measuring head.

As the ground adjacent to the borehole deforms, the distance between the sensor head and fixed point changes resulting in positive or negative cantilever deflections which are sensed by a manual or automatic transducer.

This may be used for vertical as well as in inclined directions.

The interfels extensometer depends on a similar principle of operation but either a tensioned wire or a rigid rod is used to measure relative movement from one measuring point to another.

The rods are assembled on site to the required length and at the end of an installation most parts of the extensometer can be recovered for use elsewhere.

The measuring device is a special dial gauge which fits into an opening on the sensing head and allows the plunger to bear on the end of the protruding movement rod.

For remote reading the extensometer are equipped with electric measuring transmitters. An alarm system can be built into the system to go off when a pre set displacement occurs.

Other instruments used for horizontal movements

Transverse extensometers

Deflectometer

Strain meters

Surface Extensometers

Tape Extensometers

Borehole extensometer

Soil extensometer

Magnet extensometer

Inclinometers