GeO - Equipment

Geotechnical Observations specialise in monitoring and understanding the behaviour of slopes. To do this we use a range of routine and specialist equipment for measuring displacements and pore water pressures. On this page you will find information about some of this equipment.

Measuring displacements

Inclinometers

Inclinometers are used to detect lateral displacements and shear planes in excavations and slopes. A sacrificial casing with orthogonal grooves is installed in a grouted borehole with one set of grooves aligned in the direction (A) of principal displacement. A wheeled probe with orthogonal tilt sensors is placed inside the casing with the wheels in the (A) groove. The tilt sensors record the angle of inclination at 0.5m intervals and the results are summed from the bottom to calculate the profile of the casing. Subsequent readings of the casing are compared with the initial profile to calculate the relative displacement.


Grooved inclinometer casing	Inclinometer probe
(Pictures courtesy of Slope Indicator Inc. Seattle)

Geotechnical Observations has inclinometer probes for use in vertical and horizontal inclinometer casings. All our probes are calibrated in-house using our UKAS accredited calibration frame. We also have a probe for measuring the amount of twist in long inclinometer casings.

At the start of monitoring GeO routinely reads each casing twice with two independent inclinometer probes. The readings for each probe are compared to confirm that they do not differ by more than the random error associated with the casing. One probe is assigned to the routine monitoring programme and the other is used as a backup in the event of the routine probe being unavailable. The backup probe can also be used to check the results of the routine probe if large displacements are encountered. This ensures continuity, confidence and accuracy in the monitoring programme.

ShapeAccelArray (SAA)

SAA or ShapeAccelArray (manufactured by Measurand Inc. of Fredericton Canada) is relatively new equipment for measuring ground and structural deformations. An SAA is an array (or string) of rigid segments separated by joints (see picture). SAAs can be used in vertical, inclined and horizontal orientations. MEMS gravity sensors measure tilt in two directions and processors within each array transform the displacements to represent the shape of the array. For inclined and near vertical arrays the result is an X,Y,Z transformation and for horizontal arrays the transformation represents settlement (e.g. X-Z).

Each SAA comes wrapped around a drum (see picture) and for the detection of ground displacements can be fed into a small (27mm) diameter tube that is grouted into a borehole (i.e. there is no joining of individual sensors to be done in the field). Structural displacements can be measured by fixing the SAA directly to a structure (e.g. to the reinforcement in reinforced concrete) or placing it into a predrilled hole or casing.

Displacements and vibration data can be recorded onto a datalogger or directly onto a computer using a USB interface, for which software is freely available over the internet.

SAAs come in lengths up to 70m and are waterproof to approximately 100m of water. Long-term tests have shown that SAAs are accurate to ±1.5mm for a 32m array (i.e. comparable with standard torpedo inclinometers) in a grouted borehole. Better accuracies have been achieved when the SAA is fully grouted (i.e. without the use of a plastic casing) into a concrete structure.

SAAs have been extensively used to detect displacements in slopes and have also been used in bridges, tunnels, dams and piles. SAAs can also be retro-fitted inside standard inclinometer casings where displacements have been detected using a probe and can even be installed into casings that have been excessively distorted and are no longer useable with a probe inclinometer.

In recent tests SAAs have proven comparable to probe and other forms of in-place inclinometer systems

Magnet Extensometers

Magnet extensometers are used for measuring vertical displacements in boreholes. They consist of a donut shaped section (known as a spider) housing typically three equally spaced magnets. Each section also has sprung legs and typically relies on these sprung legs embedding themselves into the soil when the spider is threaded over a central hollow tube inside a vertical borehole. This is probably the case in soft soils but should not be relied upon in stiff soils, such as those found in embankments and slopes.

GeO’s magnet extensometer uses a unique active embedment procedure to ensure that the sprung legs are firmly fixed in the soil and not simply resting on the wall of the borehole. Moreover GeO’s active embedment procedure guarantees that each magnet is securely placed at the required initial depth.


Spider magnet and insertion tool	Insertion procedure

A borehole with a diameter between 100mm and 200mm is required for the installation of a GeO spider magnet extensometer.

Sufficient head room must be available to feed 3m lengths of extensometer access pipe into the borehole.

Each spider magnet is pushed along the access pipe to a position approximately 5cm below the required depth using an insertion tool (NB. magnets are inserted with the sprung legs pointing upwards).

On reaching the required depth the magnet is pulled back up the borehole by approximately 5cm, causing the sprung legs to penetrate into the soil.

The position of each magnet can be read using a reed switch, which is lowered inside the access pipe.

The image above shows vertical displacments recorded by a GeO Magnet Extensometer in a vegetated railway embankment where the vegetation was substantially removed after one season of monitoring. During the initial season vertical shrinkage and swelling was detected to 6m. After removal of the vegetation (which was mostly mature oak trees) swelling ws detected to about 3m and evetutally seasonal shrinkage and swelling was confined to the upper 1m.

Insitu measurements of pore water pressures

Geotechnical Observations has a range of equipment for measuring soil suction and pore water pressures. This includes our unique flushable piezometer and our versitile water level recorder.

GeO flushable piezometer

GeO flushable piezometers are designed for monitoring positive or negative pore water pressures in earth structures (e.g. embankments, cuttings, natural slopes, excavations and behind retaining walls). GeO flushable piezometers are only available for rent. Geotechnical Observations will install and commission each piezometer. Site visits to recover data will be made at regular intervals by a representative of Geotechnical Observations. Data will be supplied in spreadsheet or report format.

Each piezometer consists of a 50mm diameter permanent plastic stem with a porous filter at the bottom. Piezometer stem is sold by the metre.

Each stem can be installed into a borehole with a minimum open diameter of 70mm (ideal for window sampling).

Up to 3 stems can be installed into a 150mm diameter borehole and up to 5 stems will fit into a 200mm diameter borehole (such as formed by cable percussion and rotary drilling techniques).

The borehole is filled with a calibrated cement-bentonite grout that has been designed to act as a supplementary porous filter (for transmitting the pore water pressure from the ground to the piezometer) and to seal the borehole from the effects of vertical flow.

Each piezometer stem has a dedicated pore pressure sensor, valve, flushing tubes and data logger. These items are available for rent on a weekly basis.

The pressure sensor and valve lock into the piezometer stem just above the porous filter.

Water can be flushed into the filter section, removing air or gas, which may be present there when negative pore water pressures are being measured.

When all of the air has been removed from the piezometer the valve is closed, thereby isolating the pore pressure sensor with the porous filter. The pressure that is measured is that in the ground at the depth of the porous filter (positive or negative).

A qualified GeO engineer/technician will visit the site to recover data and reset the datalogger.

GeO flushable piezometers are very well suited to short or long-term monitoring. For short term monitoring you only pay for the period that the piezometer is installed. Piezometer rental is charged at weekly intervals. For long-term monitoring regular insitu calibration checks can be made and if erros are detected the piezometer and datalogger can be removed, repaired and replaced. Piezometer rental includes for replacement piezometers and dataloggers.

GeO Water Level Recorder

The recording of the water level within an open standpipe is a time consuming operation. Sites where large fluctuations of ground water level are encountered require intensive monitoring programmes. Furthermore a response test to establish the in situ permeability and equilibrium conditions requires the prolonged presence of personnel engaged in the frequent measurement of the distance from ground level to water level.

The GeO standpipe water level recorder will automatically record the level of water inside an open standpipe and store the data for collection and analysis.

The standpipe water level recorder is the ideal instrument for undertaking falling or rising head response tests. These tests are useful for (i) assessing the integrity and operation of a standpipe following installation, (ii) estimating the equilibrium ground water level and (iii) estimating the permeability of the ground.

The figure below shows the results of a falling head test from a full standpipe. To improve the clarity the quantity of recorded data has been reduced by 90%. Also indicated are the theoretical relationship and the equation used to estimate permeability from the results.

In some situations it may be necessary to monitor the seasonal variation of the ground water level. The standpipe water level recorder is the ideal instrument for this. Following a falling head test the instrument can be left in the standpipe and with the recording interval lengthened the ground water level can be monitored over an extended period. The water level recorder is also useful for recording data during a pumping test, where a large amount of data may be required.

Laboratory measurements

In addition to making field measurements of soil behaviour Geotechnical Observations also has a range of specialist laboratory equipment for measuring soil suction and stiffness on samples recovered from the ground.

Suction probes

Direct observations of large soil suctions can be made using suction probes. Suction probes are unique in their ability to make direct measurements over a wide range of soil suctions (e.g. up to 1800 kPa) and have been used extensively in both laboratory and field applications for a variety of clients and on a wide range of soil types throughout the world. Suction probes are widely acknowledged as a milestone in the measurement of soil suction and are actively contributing to a better understanding of the influence of suction on soil behaviour.

Laboratory measurements of soil suction can be used to assess sample quality, desiccation and in situ stress. Soil suction can also be used to assess the effectiveness of different in situ compaction procedures.

Measurements can be made on extruded samples or on soil whilst it is still inside a sample tube. In situ measurements can be made inside boreholes at shallow depths.

Soil samples are prepared and placed on a pedestal in which a suction probe is embedded. Each pedestal is then covered to prevent the sample drying.

Measurements can also be made whilst the soil is still inside the sample tube. This is useful for loose soils that are likely to lose their structure during extrusion.

Suction probes respond rapidly, with full equilibrium normally being reached within 24 hours. All of the recorded data is stored on a computer. Suction probes are calibrated using a UKAS accredited dead weight calibrator. Suction probes give better repeatability and are less operator dependent than other measurement techniques.

Soil Stiffness from bender elements

Routine estimations of soil stiffness have traditionally been made in a stress path triaxial apparatus using local displacement transducers fixed directly on the sample. Recent research has brought about the development of dynamic methods for the measurement of soil stiffness at very small strains using piezo-ceramic plates (bender elements).

Bender elements can be used to measure the shear wave velocity in the hh, hv and vh directions. The sample bulk density can be used to transform the shear wave velocity to the shear modulus. If the soil suction is also measured the insitu stress state of the soil can be estimated.

Bender elements provide a reliable, cost effective alternative to undertaking locally instrumented stress path triaxial tests and can be readily performed on unconfined samples in the laboratory. The value of bender element testing and its application to geotechnical design was highlighted by Prof. J.Atkinson in his 2000 Rankine Lecture and more recently by Prof. C. Clayton in his 2010 Rankine Lecture.

Piezo-ceramic elements distort or bend when subjected to a change in voltage. Two such elements are placed opposite one another and inserted a small distance into a soil sample (typically 3mm). The voltage in one element is varied creating shear waves through the sample, which are received by the opposite element. The input voltage, (created using a function generator) and the received signal are recorded continuously using an oscilloscope, allowing the travel time of the shear waves to be measured from which the dynamic elastic shear modulus Gmax can be determined.