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G I N Dec08


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Geotechnical Instrumentation News
John Dunnicliff

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G I N Dec08

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  2. 2. Geotechnical Instrumentation News John Dunnicliff Introduction MEMS ing. Open boreholes are drilled This is the fifty-seventh episode of GIN. In GIN-54 (March 2008) we had four throughout the profile at close centers. Three articles this time, all following up articles about MEMS (Micro-Electri- There’s no need for casing to support on previous GIN topics. cal-Mechanical Systems), two of which the boreholes, and drilling mud would told us about the ShapeAccelArray be environmentally unacceptable. A Fiber Optic Sensing (SAA). Erik Mikkelsen and I have put brown volatile and aromatic liquid is In GIN- 52 (September 2007) we had a together some of our views on this in- then poured into the boreholes and al- two-part article by Daniele Inaudi and strument. lowed to permeate the matrix under a Branko Glisic about this subject, in falling head. This ground treatment is which they described the basics and told GIN Available on the Web repeated until saturation takes place. us about the four main types: point sen- Starting with GIN-55 (June 2008), epi- (If you don’t know what this is all sors, multiplexed sensors, long-base sodes of GIN can be accessed on about, ask someone from the Mother sensors and distributed sensors. The BiTech’s website Click Country, perhaps the boss of the jet current article by Peter Bennett tells us on the link “Geotechnical News”. grouting crew, Irene Dunnicliff). more about distributed sensors—these Next Instrumentation Course in are clearly powerful tools to have in our Florida Closure tool box. The next course will be on 15-17 Please send contributions to this col- March, 2009 at Cocoa Beach Florida. umn, or an article for GIN, to me as an See page 30 for more information. De- e-mail attachment in MSWord, to tails are o n h ttp ://co n f er-, or by fax or mail: Little Leat, Whisselwell, GIN Bovey Tracey, Devon TQ13 9LA, Eng- can be accessed on Soil Profile for December 25 land. Tel. and fax +44-1626-832919. BiTech’s website The figure on this page depicts a classic soil profile. There’s a hard white desic- Ooogy Wawa! (Zulu drinking toast) cated crust overlying a yellow-orange stiff silty clay, and below this a compact a n d h e t e r o g e- neous mix of cob- Monitoring by Manual and bles and boulders Automated Optical Survey in a matrix of dark We’ve had five previous articles on this brown CL mate- subject, which are listed at the begin- rial. ning of the current article by Joel W h e n s a m- Volterra. There’s a very strong consen- pled, the matrix sus that this technology is not being clearly lacks an used to our full benefit, primarily be- essential property, cause of poor specifications and the fact but this can be that the field work is awarded on a low overcome by a bid basis. Read and learn! form of jet grout- 22 Geotechnical News, December 2008
  3. 3. GEOTECHNICAL INSTRUMENTATION NEWS Distributed Optical Fibre Strain Measurements in Civil Engineering Peter Bennett Two articles in the fifty-second episode described in the previous GIN articles. electrical impedance by deformation of of GIN (Vol. 25, No. 3, September However there is growing interest in us- a coaxial cable. 2007) by Inaudi and Glisic gave an in- ing this technique on all structures Table 1 shows a comparison of the troduction to optical fibre strain sen- where a high density of measurement performance among distributed optical sors, particularly distributed strain sen- points is required. This is particularly fibre strain sensors based on Brillouin sors. Unlike conventional strain gauges the case in geotechnical applications, optical time domain reflectometry which can be used to measure the strain because soil loading is non-uniformly (BOTDR), conventional vibrating wire only at a single point, distributed strain distributed and can change its magni- strain gauges (VWSG) and fibre Bragg sensors allow strain measurement con- tude in short distance due to soil layer- grating sensors (FBG). VWSGs are typ- tinuously along a cable. A suitably in- ing. This technology is also of interest ically preferred to resistance gauges in stalled optical fibre cable can give the for increasingly complex structures be- most civil engineering applications be- full strain profile of a structure. This ar- cause soil loading patterns are more dif- cause they have a much better long term ticle describes some of the applications ficult to predict. A continuous strain performance. The FBG sensors are also of this technology. profile can be easier for field engineers point sensors, but allow more than one to interpret, and has the advantage that sensor per cable. They are described in Introduction – When to Use local features, e.g. cracks, can be de- more detail in the previous GIN articles. Distributed Optical Fibre Strain tected. When a large number of measure- Sensors It is important for readers to appreci- ments are required, the high cost of indi- Since the range over which the strain ate that the BOTDR optical technique vidual point sensors can be prohibitive. profile can be measured is very large, should not be confused with time do- In contrast, the cost of the optical fibre potentially up to tens of kilometres, this main reflectometry (TDR) techniques can be very low. The cost of the analyser technique is attractive for large scale that are based on detecting changes in is higher than for VWSGs and FBGs, structures such as dams and pipes, as but the analyser can easily be moved be- tween locations (no need for Table 1. Comparison of strain monitoring technologies recalibration) to spread the cost. This is particularly advantageous if the sam- Method Vibrating Wire FBG BOTDR pling frequency varies over the project as the capital investment is not locked to Sensor Vibrating wire Fibre Bragg Grating Optical fibre a particular location, as it generally is Measurement Discrete Discrete Distributed with the other technologies. Strain resolu- 0.5-1με 0.1-10με 2-30με Optical Fibres/Cables tion A simple optical fibre is shown in Fig- Limit of spa- 50-250mm ~2-40mm (length of ~1m ure 1A. This fibre costs ~20 cents per tial resolution grating meter, but is fragile and care must be No. of mea- 1 per copper Typical 40 sensors 20,000-100,000 taken when installing it. Extra layers of surements cable (up to every protection are often placed around more 50mm) than one fibre to form a cable. Special strain sensing optical fibre cables are Measure- Several cycles Capable of acoustic 4-25min available. These are more robust, but ments time (of freq. (up to 5MHz) still transmit the strain applied through 600Hz-3KHz) to the glass optical fibre and allow the Maximum 3000με ~10,000με ~10,000με strain to be measured. As these are not strain currently produced in large quantities, they can cost up to ~$20 per meter. Al- Analyser cost $2,000-20,000 $20,000-100,000 $100,000-200,000 though this is considerably more expen- Sensor cost Sensor $150-500 Gratings ~$50-500 Fibre ~$0.2-20 per sive, these are likely to be faster to in- metre stall as they do not require such gentle handling. Examples of these fibres are Feature Established High strain accu- Distributed shown in Figures 1B and 1C. More ro- technique racy, fast response measurements Geotechnical News, December 2008 23
  4. 4. GEOTECHNICAL INSTRUMENTATION NEWS give a good signal to noise ratio, al- though the measurement time should be limited to avoid thermal effects during the reading. The required measurement time is expected to reduce with contin- ued improvements in optical technology, particularly detectors (laboratory mea- surements at 1KHz have recently been reported). The analysers currently com- mercially available are more suitable for long term structural health monitoring. Analysers BOTDR distributed strain analysers have been commercially available for over ten years. But with increasing in- Figure 1. Types of optical fibre and cables. terest in the area there has also been sev- eral new analysers launched by differ- bust forms of standard telecom cables the speed at which the samples can be ent manufacturers. There are important have thick plastic coatings, sometimes taken. The speed at which a VWSG can differences between the analysers cur- reinforced with steel, around a be sampled is limited as several oscilla- rently available as they perform the gel-filled tube containing the optical tions at the resonant frequency are re- measurement in different ways, which fibres (as shown in Figure 1D). This quired to make a measurement (typi- may have a have a significant affect on makes these cables unsuitable for strain cally 600Hz-3KHz, depending on the the performance and suitability for a sensing as the optical fibres move inside pre-tension). FBGs have the advantage particular application. The most estab- rather than carry strain. However, this that they can be sampled at very high lished is analyser is the Yokogawa type of cable can be used to carry the op- frequencies, including acoustic and ul- AQ6803. This is a compact single unit tical signal between the sensing cable trasonic frequencies. This could have with built-in screen for viewing the and the analyser. This is particularly applications in dynamic and Statnamic d ata. Th e latest an aly s er fro m useful for connecting a remote monitor- load testing. In contrast the distributed Advantest, the N8510, is currently only ing location to the site office as the cable strain measurement based on BOTDR available in Japan and is undergoing is very robust and still inexpensive (~$1 takes much longer for a single measure- safety certification for other markets. per meter). ment (typically 4-25 minutes). This Unlike the AQ6803 this is run with a technique is based on detecting the very separate computer. This means that up- Time Response weak backscattered signal (a more de- grading, e.g. to a larger hard disk, is Apart from taking a distributed mea- tailed explanation can be found in the possible. Both these models are based surement rather than point measure- previous GIN articles). The analyser on spontaneous Brillouin scattering as ments, the other major difference be- needs to average the signal in order to described in the previous GIN article. A tween the techniques listed in Table 1 is Figure 2. Strain profile measured in a pile group with Figure 3. Strain profile measured with BOTDR in a pile after BOTDR and VWSGs during a static load test at 95% of curing, during which one level of basement was excavated. failure. Peaks are due to cracks up to ~0.3 mm width. 24 Geotechnical News, December 2008
  5. 5. GEOTECHNICAL INSTRUMENTATION NEWS different measurement system is used in by the Cambridge geotechnical group break. By installing the optical fibre the OmniSens STA100/200 and Oz using a Yokogawa AQ6803. down one side of a pile and back up the Opitcs ‘Foresight’. These models en- other, the pile can be tested individually hance the optical signal by using a Strain Profiles in Piles – and later connected to adjacent instru- counter-propagating pump pulse of Installation Techniques mented piles to allow several to be mea- light. This boosts the signal to noise ra- The optical fibre is typically installed sured at once. tio, improving the strain resolution or under a pre-tension so that if the struc- An installation of cables on both reducing the measurement time, but has ture being monitored goes into com- sides of the pile can also be used to mea- the disadvantage that access to both pression the cable does not go slack. sure lateral movements in addition to ends of the fibre are required, therefore The area of interest is very easy to iden- the axial movements. This technique a break in the cable means that measure- tify from the measured strain profile. has been tested on secant pile walls to ments can no longer be made anywhere Changes in strain are then observed by monitor the lateral movement during along the cable. It also means that for subtracting the initial strain profile from construction of a large basement in some installations it may not be possi- new measurements. In the case of a pile London. As the wall bends one side ble to make any measurements until the the pre-tensioned optical fibre can be at- goes into compression and the other in fibre installation is complete. The tached to the rebars with clamps or ep- tension. The advantage of the BOTDR Sensornet DTSS also uses simulated oxy. Figure 2 shows a comparison be- technique over a conventional incli- Brillouin scattering but in a reflective tween the strain profile measured in a nometer is that the optical fibre cable configuration, so that it can measure up pile group with VWSGs and BOTDR. can be routed through any structure to a break in the cable. This model also The agreement is very good. An addi- built on top of the wall so that the mea- varies the power injected into the opti- tional unstrained fibre may be used for surements can be performed throughout cal fibre so it can independently mea- temperature compensation (this may be the life of the building, without requir- sure strain and temperature from the a different fibre contained in the same ing direct access to the top of the wall. same optical fibre. However it is cur- cable or a separate cable installed rently only capable of taking a reading nearby). For analysers requiring access Crack Detection – Spatial every 1m, so it may not be suitable for to both ends of the cable it must be in- Resolution all applications. stalled in a loop. This is also the pre- One of the perceived limitations of The following sections give some ferred configuration for reflective BOTDR for strain sensing is that the examples of applications of BOTDR analysers as in the case of a break they spatial resolution is normally quoted as measurements which were conducted can still obtain the full strain profile by 1m. This limitation comes from the measuring each direction up to the physical length of the pulse of light in the optical fibre, a 10 nanosecond pulse is ~1 metre long. However BOTDR can still be used to measure localized fea- tures such as cracks. BOTDR gives a centre weighted average over ~1m, so a very short (less than 5cm) region of strain such as a crack will be detected as a sharp spike (in fact a Gaussian curve with a width of ~30cm). The height of the spike can then be used to estimate the crack width. Figure 3 shows tension cracks developed during the curing of a pile and subsequent heaving of the ground on a basement and building con- struction site in London. These cracks have a width of up to ~0.3mm. Measur- ing the strain profile all the way along the pile means cracks are easy to detect. Conventional point gauges may not be located exactly on the crack and there- Figure 4. Strain developed around first of twin tunnels during the construction of fore may be unable to detect it. If there the second tunnel in close proximity. Solid lines when face of 2nd TBM is level are more than one crack within a very with monitoring location, dotted lines when 2nd TBM is more than two tunnel short distance (less than 4 measurement diameters past. Thin lines are measured data. Bold lines are strains steps), they will not be individually re- calculated,knowing attachment points. Inset is a schematic of the movement solved, but the combined crack width observed. would be measured. Geotechnical News, December 2008 25
  6. 6. GEOTECHNICAL INSTRUMENTATION NEWS Tunnelling – Point Attachment monitoring in the invert). This was re- measurements are required to obtain In some circumstances it is not possible peated every 7 rings (a spacing of 9.8m) strain profiles for accurate monitoring to bond the fibre continuously to the with a total of 14 rings being monitored. of geotechnical construction processes. structure. Point attachment can still be Figure 4 shows the strain developed at With the recent launch of several new fi- used to monitor multiple points along two times during the tunnelling. From bre optics analysers there is more the structure; the movement being the these strains the relative movement of choice of equipment that can provide strain measured multiplied by the dis- the anchor points can be estimated. The such measurements. However, as with tance between the attachment points. strain profile is smoothed because of the any form of monitoring, the limitations An example is the use of BOTDR to ~1m gauge length. However, because need to be understood and the equip- monitor the first of twin tunnels during the position of the attachment points is ment and sensors must be installed ap- the construction of the second tunnel in known, the exact strain profile can be propriately to obtain good information Singapore. The two tunnels are in close recovered and is shown in bold (this from the system (and of course a good proximity (minimum clear separation process may be used even if the attach- understanding of the geotechnical pro- being 2.3m or 0.4 times the tunnel di- ment points are separated by less than cesses to make sure that you are mea- ameter). The tunnel is part of the new the gauge length). suring the right thing!). Circle Line Stage 3 , b etween Serangoon and Bartley stations, com- Conclusion Peter Bennett, Cambridge University, missioned by the Land Transport Au- There is increasing interest in the use of Engineering Department, Trumpington thority. The optical fibre is attached at distributed strain measurement based Street, Cambridge CB2 1PZ, England. 11 locations around the section of the on BOTDR technology. It can have Tel: + 4 4 1 2 2 3 - 3 3 2 6 0 0 , e m a il: tunnel, monitoring ~ 2/3 of the ring considerable performance and financial (track and TBM supply pipes prevented advantages when a large number of Monitoring by Manual and/or Automated Optical Survey Joel L. Volterra The following articles about manual Rutledge, D. “Discussions of Cook’s can attest, from personal experience on and/or automated optical survey have GIN-49 Article”, GIN-50, March projects awarded to low bidders, to the been published in previous episodes of 2007, pp 33-38. Also reply by Cook. lack of accuracy generally obtained by GIN: • Hope, C. and Chaqui, M., “Manual manual surveys and also improperly in- • Cook, D. “Robotic Total Stations Total Station Monitoring”, GIN-56, stalled or maintained automated optical and Remote Data Capture: Chal- September 2008, pp 28-30. survey in the New York City market, lenges in Construction”, GIN- 49, • Marr, W.A., “Monitoring Deforma- where reports of regular fluctuations of December 2006, pp 42-45. tions with Automated Total Sta- 0.25 inch horizontal or vertical are as • Kontogianni, V., Kornarou, S., and tions”, GIN-56, September 2008, pp common as reported changes of 0.000 S. Stiros. “Monitoring with Elec- 30-33. feet, both of which are equally concern- tronic Total Stations: Performance I applaud John Dunnicliff for his ing. Low bid procedures simply do not and Accuracy of Prismatic and persistence in soliciting these articles, allow the monitoring programs to reach Non-prismatic Reflectors”, GIN-50, and the authors who have provided les- their fullest potential. March 2007, pp 30-33. sons for the rest of us. The articles ad- Often raw data become the end prod- • Beth, M., Dorwart, B., Flanagan, R., dress specific issues which make or uct, without temperature corrections Greening, T., Roy, D., Jensen, N., break an optical monitoring program. I and without accompanying information 26 Geotechnical News, December 2008
  7. 7. GEOTECHNICAL INSTRUMENTATION NEWS necessary to allow for temperature cor- manual system in order to obtain im- missions of unnecessary back- rections by third parties. This issue is proved results at a more cost effective ground data. wider than survey data alone, and in this and less labor intensive effort. The word A qualified engineering team with writer’s opinion, it plagues the instru- is out however, and as a result, owners, adequate resources and an understand- mentation community. The inclusion of architects and engineers have learned ing of anticipated deformations and the thermal corrections on the instruments that they can easily obtain sufficient in- consequences of such deformations themselves and of the structures upon formation (from manufactures or col- (even if they have little direct instru- which they monitor requires judgment, leagues or publications) to include such mentation experience) may be better interpretation, quality assurance and requirements in project specifications. suited than an experienced surveyor un- time. Adequate time is not usually Unfortunately they may do this without dertaking the work with technicians. available if instrumentation work is in- possessing the direct experience or Surveyors generally lack a comprehen- knowledge to appreciate the nuances of sive understanding of the larger picture such a system, nuances that are touched and as a consequence large amounts of upon in the above articles. The result is unnecessary data will usually be gener- Improved often an inability on behalf of owners ated, submitted and/or made available communication ... and their project teams to evaluate suffi- online, with little or no emphasis placed ciently the qualifications of the moni- on that relatively small percentage of should not be toring personnel, the performance of data which are relevant and critical to interchanged with the monitoring program, and/or to en- the active construction-related activi- simply force or obtain the quality of informa- ties. This small percentage are the data increased tion specified and ultimately strived for that are likely to result in significant communication ... and purchased. short-term deformations and which are I agree that more emphasis should be worthy of regular examination by quali- placed on: fied professionals. • properly written and enforced cluded in the general construction con- specifications tract, because construction work may • less low bid awards, because these preclude comparing similar scopes ... instrumentation cause deformation of adjacent struc- tures before adequate baseline data and abilities, and hamper the ability programs lose out have been documented. This limitation to collect adequate baseline data on reaching their can be overcome by the owner entering well in advance of the construction fullest potential. into a specialty contract directly with an contract instrumentation consultant during the • increased input and involvement design phase, so that adequate baseline from qualified engineers to interpret data can be established before construc- collected data Therein is the missing link in many tion can cause any deformation of adja- • improved communication between programs. In many programs the instru- cent structures. parties. Improved communication mentation data are provided separately Further publications and open dis- differs from, and should not be inter- without interpretation. In others, even cussions can only result in indus- changed with simply increased com- more frequently, vital construction re- try-wide advancement. As stated by the munication, which often results in cords are not available—records that above authors, the use of total stations too frequently scheduled and are essential for comprehending, vali- for optical survey is not new. What over-attended meetings and/or too dating or writing-off the observed would appear new is the gaining or frequent often daily hard copy sub- trends or spikes. In these situations, wider acceptance of the use of auto- knowing when to sound an alarm or mated motorized total stations (AMTS) change construction procedures be- (also referred to as robotic total stations comes increasingly difficult, and instru- – RTS) to monitor building deforma- mentation programs lose out on tions adjacent to active construction. ... less inclusion in reaching their fullest potential. Increased efforts are being made by de- low bid construction signers on behalf of owners to incorpo- contracts, because Jo el L. Vo l t er ra, A sso c i a t e, r a te th e s e an d o th e r im p r oved Geotechnical Engineer, Mueser technologies in project specifications this hampers Rutledge Consulting Engineers, 225 where they are deemed appropriate. the ability to collect West 34th Street – 14 Penn Plaza, New I n g e n e r a l th e p r a c tice h a s adequate York, NY 10122, Tel. (917) 339-9363, previously been limited to specialty baseline data. email: consultants bidding an alternative to a Geotechnical News, December 2008 27
  8. 8. GEOTECHNICAL INSTRUMENTATION NEWS Some Views on a Recent Addition to our Instrumentation Tool Box—the ShapeAccelArray (SAA) P. Erik Mikkelsen John Dunnicliff The March 2008 episode of GIN in- When considering the selection of eters, the inclinometer system capabil- c lu d e d two a r ticles ab o u t th e IPIs as opposed to conventional probe ity, precision, and reliability have not ShapeAccelArray (SAA) instrument, a inclinometers, the higher hardware cost been independently evaluated and dem- wireless MEMS-based system for must be balanced against the much onstrated—note that this is the same real-time deformation monitoring. The lesser labor cost. And are real-time data Bennett of RPI who played a major part first was by Tarek Abdoun and Victoria truly needed? In our experience, fully in the development of the SAA. Bennett of Rensselaer Polytechnic In- automated, full profile, real-time incli- It is important to understand that the stitute (RPI), who played a major part in nometer data are not needed for the major- reported +/- 7.6mm per 30m accuracy the development of the instrument. The ity of applications. However, the for probe inclinometers includes a cor- second was a case history by Matthew development of innovative sensors over rectable allowance for systematic error Barendse of New York State Depart- the last 10 to 15 years has substantially of +/-6.3 mm, plus a random error of ment of Transportation. The instrument lowered costs of IPI systems, making their +/-1.3 mm (Mikkelsen, 2003). The sys- is manufactured by Measurand Inc. application more feasible and attractive. tematic error is proportional to installa- ( The SAA development is a welcome addi- tion properties such as verticality, and The same episode of GIN included tion. the +/- 6.3 mm tolerance is for an article on MEMS basics by Barrie less-than-ideal installations. When “di- Sellers and Robert Taylor, a description rectly compared” the probe inclinome- of performance testing of MEMS-based ter can achieve an accuracy equal to or tilt sensors by Thomas Sheehan, David better than the SAA. Mazzei and John McRae, and a ques- When “directly It is also important to understand that tion and answer (Q&A) exchange be- compared” the probe with any type of IPI there is a potential tw een th e G IN ed ito r an d th e inclinometer can for reduced accuracy because of sensor developers of the SAA. The Q&A was achieve an accuracy drift, whereas with conventional probe an attempt by the editor to clarify some inclinometers any drift is removed from of the characteristics the SAA, but sev- equal to or better the determination of deformation by the eral readers were not satisfied with the than the SAA. A0 - A180 procedure. But Abdoun and exchange. Bennett state, The use of MEMS accel- Without doubt the SAA is a valuable erometers virtually eliminates concerns addition to our instrumentation tool box. Accuracy of long-term drift in the SAA. This view It typically provides deformation data at Abdoun and Bennett state, The accu- is supported by Sellers (2008), who re- ten times the detail provided by tradi- racy of deformation measurement of the ports on long-term MEMS tiltmeter tional in-place inclinometer (IPI) installa- SAA is +/- 1.5 mm per 30m. This figure zero stability tests, which have been tions, i.e. 3 m (10 ft) typical gage length can be directly compared to the re- running for eight months. The drift is of for an IPI versus 0.33 m (1 ft.) for the ported system accuracy of traditional the order of 0.1 mm/meter per year. SAA. The data acquisition and graphical probe inclinometers, +/- 7.6mm per Abdoun and Bennett also state that presentations are much better integrated 30m. However, it seems to us that this The SAA system accuracy specification than other more modular systems such as has an apple/orange flavor, because a was derived empirically from thou- Campbell CR1000, with many options to primary reason for this increased accu- sands of frames of wireless data over a present data from Microsoft Excel to web period of 1.5 years, from three different racy results from the SAA system, like accessible SQL databases. However, as field locations. In order to know what all IPI systems, having no placement good as these improvements are, various the true deformation is, it is necessary errors associated with moving an incli- characteristics need to be taken into ac- to compare the data with an absolute nometer probe up and down the count if this instrument is to be chosen in standard. Perhaps there is some confu- grooved casing. Assuming no sensor preference to conventional probe incli- sion in terminology here, such that the drift, similar accuracies can be obtained nometers or other types of IPI. The pur- statement refers to precision (repeat- with other types of IPI. pose of this article is to make an attempt at ability) rather than accuracy (closeness Machan and Bennett (October 2008) putting some of these characteristics in to truth). say, for MEMS-based probe inclinom- perspective. 28 Geotechnical News, December 2008
  9. 9. GEOTECHNICAL INSTRUMENTATION NEWS 3D or 2D for Static earthquake acceleration, and therefore if Measurements? used in an earthquake-prone location it The concluding words by Abdoun and would be an added benefit to have all dy- Bennett in the Q&A exchange are, Sand is not namic components measured for a com- These are true 3D devices. Machan and plete seismic record. But how good a suitable backfill Bennett (October 2008) repeat the would such dynamic records be? To ob- material in any claim: The sensor array is capable of tain representative dynamic records it is circumstances. measuring 3-D ground deformations at essential to ensure a solid connection be- 1-ft (30-cm) intervals up to depths of tween sensors and ground, and this is un- 330 ft (100 m). These statements need likely to happen if the SAA is installed us- explanation. Neither pair of authors ex- ing sand backfill or loosely inside PVC plains, but perhaps all they mean is that access pipe. tions is relatively recent, since 2005. MEMS are omni-directional sensors. There are limitations to this technology, The SAA is not compressible axially Method of Installation including temperature sensitivity and and it cannot be used to monitor settle- Abdoun and Bennett describe early in- related effects. ment in a near-vertical borehole. Con- stallations in which inclinometer casing We agree that these sensor calibra- was grouted into a borehole, the SAA tions are sufficient for typical under- lowered into the casing and backfilled ground applications where temperature with sand, to allow for retrievability by variations are small, but for applications jetting. They accept the concern about where a significant temperature gradi- ... the SAA provides incomplete sand backfilling and de- ent is expected, such as behind and in 2D and not scribe an alternative installation proce- excavated walls, individual temperature 3D data ... dure. A 25 mm (1 in.) pipe is either sensitivity factors are needed. For ex- grouted in a borehole or is surrounded ample, at a recent lock wall improvement by sand backfill, and the SAA inserted project where vertical IPI-MEMS were in- within the pipe together with a flat web- stalled there was about 15 °C variation finement by the surrounding soil would bing to allow for retrievability. prevent the formation of any significant from spring to fall, causing a significant Sand is not a suitable backfill mate- change in sensor output. In the specifica- zig-zagging S and C shapes caused by rial in any circumstances. buckling of the axially-compressed tions for the SAA listed by Bennett et al For dynamic measurements, neither (2007), they state: Effect of temperature pipe in which the SAA segments are in- of the above methods is suitable, and the stalled. In all likelihood the pipe would after compensation: < 0.1 degree per C SAA must be grouted and non-retriev- (preliminary). This is < 360 arc-seconds either push out of the ground as the soil able. settled past it, or it would fail by shear- per °C, a level that may be unacceptable if ing. The same issue would arise if the Sensor Alignment a significant temperature gradient is ex- SAA is installed without a casing. For It is claimed by Bennett et al (2007) that pected. Sellers and Taylor say, for this reason, when inclinometer casings the SAA uses “fiber optic orientation sens- MEMS, They have low drift and ther- are subjected to large amounts of settle- ing”, but we see no evidence of any sensor mal coefficients, about one arc second ment, it is necessary either to use tele- in the SAA system to measure orientation per degree C. But test results by scoping couplings or to surround the (azimuth). This aspect needs to be ex- Sheahan, Mazzie and McRae show that, casing with an axially-compressible plained. The array has a tough external if subjected to significant temperature pipe. anti-torque jacket, but we have no infor- changes, MEMS are temperature sensi- Therefore, when the SAA is in- mation about how resistant this is to tive enough to warrant individual char- stalled in a near-vertical borehole and torque. The array has to be manipulated acterization of temperature response, there is vertical compression, the SAA into the correct orientation and any together with sensors to measure tem- provides 2D and not 3D data. Examples down-hole spiral would not be known. perature. are monitoring stability of a cut or natu- ral slope where there may or may not be Temperature Sensitivity References vertical compression, and monitoring Abdoun and Bennett say, under their Bennett, V.G., Abdoun, T., Danisch, L., horizontal deformation at the toe of an heading Temperature Sensitivity, A dig- Shantz, T. and Jang, D. (2007), “Un- embankment on soft ground where ital temperature sensor is included stable slope monitoring with a wire- there is vertical compression. within the SAA near each microproces- less Shape-Acceleration Array sor. Thus, each temperature sensor cali- system”, Proc. Symp. Field Mea- Dynamic Measurements brates the MEMS sensors in the eight s u r e m e n t s in G e o m e c h a n i c s It is claimed that the SAA can measure segments surrounding it. Machan and (FMGM), Boston. both statically and dynamically, i.e. that it Bennett (October 2008) say, The use of Machan, G. and Bennett, V.G. (October has the capability to record vibration and MEMS sensors in inclinometer applica- 2008), “Use of inclinometers for Geotechnical News, December 2008 29
  10. 10. GEOTECHNICAL INSTRUMENTATION NEWS geotechnical instrumentation on way, pp 555-567. John Dunnicliff, Geotechnical transportation projects: state of the Sellers, J.B. (2008). Personal commu- Instrumentation Consultant, practice”, Transportation Research nication, May. Little Leat, Whisselwell, Bovey Tracey, Board, Transportation Research Devon TQ13 9LA, England, Circular No. E-C129, 79 pp. P. Erik Mikkelsen, Consulting Engineer, Tel: +44-1626-832919, Mikkelsen, P.E. (2003), “Advances in Geometron Inc PS, 16483 SE 57 th email: inclinometer data analysis”, Proc. Place, Bellevue, WA 98006, Symp. Field Measurements in Tel: (425) 746-9577, Geomechanics (FMGM), Oslo, Nor- email: 30 Geotechnical News, December 2008