Monitoring building and ground movementby precise levellingGround movement can cause damage to buildings in a number of ways: forexample, by shrinkage or heave of clay soils, landslip, mining instability andsettlement of filled sites. Movement may need to be monitored to confirm thatground movement is the cause of damage, to determine the mechanism and todecide on how and when to take appropriate remedial measures. Long-termmonitoring of building and civil engineering structures may also be required tomonitor safety or study design-behaviour.This Digest discusses precise levelling techniques and equipment which canmonitor vertical movements with an accuracy consistently better than ±0.5 mm. Itis intended for surveyors, engineers and others who are conversant with normallevelling techniques but who may wish to monitor movement in structures.

Fig 1 A precise levelThis accurately measures vertical movement of a building.

Digest 343 deals with direct measurement and monitoring of cracks inbuildings and Digest 344 describes how to determine existingdifferential vertical movements.

(*) A separate list of specifications and suppliers of items marked with this symbol is included with this Digest.

Precise levelling can reveal the amount and direction ofvertical movement at various points of a structure or site. Itcan indicate the rates of movement and angular strain in thevertical plane.

Typical examples of its use are:

● Buildings on clay soil with visible damage suspected ofresulting from foundation movement. Cyclic seasonalmovements with progressive summer settlementsindicate movement of shallow foundations due to nearbytrees. Progressive heave probably results from treeremoval. Reference 1 describes a typical case historywhere precise levelling was used to monitor groundheave over a 25-year period.

● Where movement is anticipated from landslip, mining,nearby excavation, tunnelling or other engineering work.Records showing how movement started and progressedcan be invaluable in deciding control measures or inattribution of the cause.

● For measuring settlement where buildings or test loadsare set on filled sites.

● Where settlement records are required for monitoringsafety and performance of earth dams, nuclear powerstations and other major structures.

Digest 386August 1993

CI/SfB (F47)

BBRREE DDiiggeessttConcise reviews of building technology

Building Research Establishment

Technical enquiries to:BRE Advisory ServiceGarston, Watford, WD2 7JRTel: 01923 664664 Fax: 01923 664098

PRECISE LEVELLING EQUIPMENTThe precise level*The precise level – Fig 1 – is used with an Invar staff whichis graduated at 1 cm intervals – Fig 2. The observer doesnot have to estimate the fraction of the staff intervalbecause the precise level incorporates an optical

micrometer; this is either built-in or used as an attachmentover the telescope objective. This micrometer is used todisplace the line of sight vertically and parallel to itself andis calibrated over a range of exactly one staff interval. Thesurveyor visually straddles the staff graduation line with thewedge-shaped lines of the telescope reticule – Fig 3 – byturning the micrometer knob. The micrometer scale is thenread giving the fraction of staff interval directly in mm and0.1 mm and, by estimation, to 0.01 mm. This figure is thenadded to the multiple of 10 mm indicated on the staff, thusgiving the true staff reading.

Although not essential, an automatic precise level isrecommended; it is quicker to use than a non-automatic onewhich requires tedious setting of a split bubble on the tiltingtelescope for each observation. Automatic levels areslightly less accurate but this is insignificant at therelatively short sights used.

The level is used on a sturdy tripod* which must havetelescopic legs to cope with uneven or sloping ground.

The precise staff*The scale is marked on a thermally-stable Invar metal stripwith graduations every 1 cm. The strip is attached to thestaff base but is held only loosely in slots extending thelength of the staff so that it is not affected by thermalstrains of the staff casing. The staff has a bubble so that itcan be held vertical: it is not swung in precise levelling.

The standard length is 1.82 m but a shorter one (0.92 m) isavailable for low headrooms.

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Fig 2 A precise Invar staff positioned on a levelling stationThe numbers represent centimetres.

ACCURACY REQUIREDMonitoring involves more than just ascertainingmovement to date. To find out what is happening, ratesof movement must be determined together with anychanges in these rates. Usually, the rates will be only afew millimetres a month, often less. Small seasonalmovements must also be detected. An accuracy betterthan ±0.5 mm is desirable.

This accuracy is not possible with normal site surveyingequipment. A precise level and staff, purpose-madelevelling stations and stable datums must be used.

The accuracy of a levelling instrument is defined as thestandard deviation for 1 km double-run levelling. Forthe recommended automatic level, used with an opticalmicrometer, it is ±0.3 mm; for the non-automatic levelwith built-in optical micrometer it is ±0.2 mm. The non-automatic level is more than double the price of theautomatic one and takes more time to use.

Electronic automatic levels are now available. Theyinterpret a bar-coded staff without the need for manualstaff reading. Readings are displayed locally and arestored for transfer to a computer. The most accuratemodel currently available (±0.4 mm) costs about 40%more than the recommended equipment.

Fig 3 Levelling staff seen through precise level with graduation straddledby cross hairs

Staff reading 230 mmMicrometer reading 5.56 mm

Complete reading 235.56 mm

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LEVELLING STATIONSLevelling stations must allow accurate staff positioning andmust not be liable to accidental damage or vandalism. Theyshould last for many years without corroding and beaesthetically acceptable to client and architect. The BRElevelling station has been designed to meet theserequirements. Thousands have been used satisfactorily on avariety of buildings and civil engineering structures.

BRE levelling station*The BRE Levelling Station – Fig 4 – consists of a stainlesssteel socket, 65 mm long and 22 mm diameter, set in a holedrilled in the structure; it has a removable ball-endedlevelling plug, of about the same dimensions, on which theflat base of a staff is held when levels are taken. When notin use the socket is sealed from dirt by a screw-in perspexbung. The face of the bung is flush with the structural finishand is barely visible from a few metres.

The levelling station is so designed that the plug willposition with repeatable accuracy. The thread is a loose fitand is used only to pull the plug into the socket. The plug ispositioned radially to an accuracy of 0.03 mm by aprecisely machined spigot and socket turned concentricallywith the threads on the plug and socket. As the plug isfinally tightened into the socket, their mating faces pull thetwo components coaxial. The plug is tightened firmly byhand. Spanner flats are provided for inserting and releasingthe plug if necessary; the spanner should be loaded onlylightly.

Installation in prepared holesFor most buildings, a satisfactory height above the groundis between 300 and 600 mm. Drill a horizontal hole, 32 to38 mm diameter, about 75 mm deep. It is better to drill intoa brick, even if this involves breaking into the frog, ratherthan between bricks where a socket is more likely tobecome loose as building movement progresses.

De-grease the socket with a suitable solvent and fix with astrong mortar mix of 1:21⁄ 2 ordinary Portland cement:sand.A plasticiser will help and accelerators may be used. Washout the hole; part fill the hole (and the frog if it has beenbroken into) with mortar. Using a loosely inserted bung as ahand-hold, fill the keyway of the socket with mortar andposition it in the hole so that the front face is recessed atleast 6 mm into the wall. Tamp mortar firmly all round thesocket. When the hole is filled to within about 2 mm of thesocket face, carefully unscrew the bung and ‘wet-finger’finish the mortar so that it is visibly clear of the socketmating face – Fig 5. The socket needs to be only roughlyhorizontal because the ball end of the levelling plug onwhich the staff rests will accommodate off-levelinstallation. Indeed, a levelling station may be installed atany angle up to the vertical but fit a rubber washer (27 mmoutside diameter) to the bung if cement wash is expectedwhere stations are fitted in a floor.

Hard-setting epoxy resin with sand filler can be usedinstead of mortar to install the socket in dry bricks andconcrete but the essential removal of resin from the matingface is tedious.

Fig 4 Components of the BRE Levelling StationThe socket (left) is grouted into the structure; above the screw-inplug are the plastic bung and bung installation tool.

Fig 5 Cross-section of an installed socketThis shows the recess below the wall surface to accommodate thebung. The mortar finish ensures there is no interference to theprecise mating of the levelling plug and socket.

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SURVEY DATUMSChoice of datumIdeally, each site should have a purpose-installed datumand two check datums, all separate from the structure understudy. If successive levellings show that one of the threedatums is unstable, it can be identified by the constant levelrelationship of the other two. If there is only one datum,continuity of observations can be lost if it is damaged.

It is expensive to install deep datums, and it is not alwaysnecessary. What is important is that any datum should bereliably stable. Frequently, nearby structures can be fittedwith a levelling station and used as main and check datums.The choice of datum structure involves several factors. If itis more than 30 m from the structure and requires the use ofchange points, there will be a risk to survey accuracy. If itis less than 20 years old and is on clay, it may still besettling under its own weight. The deeper the foundations,the better; if they are at least 1.5 m, seasonal movement canusually be ignored if there are no nearby trees. An oldbuilding with a basement is a good choice; a new one witha basement may be heaving if it is founded on over-consolidated clay, eg London Clay. There should be no signof movement damage and no tree involvement; this isloosely defined as the presence on shrinkable soil of onetree within a distance equal to its height, a group or row oftrees within a distance of 1.5 times their height, or treeswithin those limits that have been felled within the previous30 years.

More information is in Digest 298, and in Building neartrees (2) and BS 5837.

If there is no external datum, differential movementsbetween parts of the study building can still be measured tothe same accuracy. Designate as Datum the levelling stationthat you judge to be moving least. Proceed as usual withlevelling calculations and tabulation and record the nature ofthe assumed datum. This is called a precise differentialmovement study.

Purpose installed deep datumOn shrinkable soil, a purpose-made deep datum must befounded at least 6 m deep and, to be reasonably sure that itis free from soil movements, at least 15 m from large treesor from the site of removed trees. If this cannot beachieved, the datum should be 12 m deep.

A deep datum is a buried vertical steel rod; its base isanchored and its top is near ground level. Levels taken onthe top of the rod reflect the stability of the ground at thebase. So that the rod is not influenced by soil movement, itis sheathed with a rigid plastics or steel guard tube. Theguard tube is not anchored at its base nor allowed to bear onthe datum anchorage; this ensures that any verticalmovements of the guard tube are not transmitted to thedatum rod.

Groundwater or flood water from the top or sides of thehole are prevented from reaching the datum base by animpervious non-rigid seal between the lowest part of theguard tube and the borehole wall. The top of the rod isfitted with a stainless steel ball end fitting to reproduciblyregister with the flat staff-foot.

Reference 3 describes a deep datum designed by BRE to bedriven into clay at the base of a borehole. This system isvery reliable but demanding to install. Another design, inwhich the foot of the datum rod is embedded in mortar, isshown in Fig 6. The datum rod is 3⁄ 8 BSP galvanised pipeand couplings, with a purpose-made stainless steel foot andlevelling dome*.

To install the datum, bore a hole 100 to 150 mm diameter atleast 6 m deep and support the top with a 500 mm length ofPVC pipe flush to the surface. Place about 300 mm ofsloppy sand/cement mortar at the bottom of the hole andembed the datum rod centrally in it with its levelling dome50 to 75 mm below the ground surface. Tamp the mortardown with another rod (eg a spare datum rod). Using theguard tube, push down the hole a 150 mm annular plug ofsoft compressible material (eg polyurethane foam or lowdensity polystyrene). Maintaining light downward pressureon the guard tube with the levelling dome protruding 10 to20 mm, drop bentonite pellets and a small quantity of waterdown the hole to form a 200 mm annular plug. Leave thebentonite to gel for about an hour. Temporarily seal the gapbetween the datum rod and the top of the guard tube andbackfill the borehole with cement/PFA/bentonite grout*. Ifthe borehole is dry, the grout can be poured in at the top;otherwise it must be poured down a tube or pumped to thebottom so that the grout displaces the water. Leave thegrout for at least a day to set; top it up as necessary andthen cap the PVC ground support pipe with a vandal-proofcover*.

Fig 6 A BRE deep datum bedded in mortar

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PLANNING THE LEVELLING CIRCUITThe number and position of levelling stations will varyfrom site to site. As a rough guide, eight to 12 are oftensufficient to cover the behaviour of a house. Try to cover allthe building, not just the damaged part, so that any smallmovements in parts not yet visibly damaged will berevealed. This adds useful information when planningremedial measures.

When deciding station positions, bear in mind that levellingsights should not exceed 15 m. Try to equalise back andforesights although site features will often not allow this,nor can intermediate sights always be equal to the back orforesight.

Try to arrange for some of the stations themselves to beused as change points. If this is not possible, use a well-trodden-in, heavy-duty survey ground plate*.

If possible, traverse right round the building, taking asecond reading on the first station. This provides a buildingclosure before levelling to the original datum to give thewhole round closure.

Allocate suitable code names to the levelling points; forexample:D1, D2, ... for datums;S1, S2, ... for stations on buildings under study;C1, C2, ... for temporary change points.

Mark clearly the position of each point on a site plan,including depth of datums – see Fig 7. On clay sites, markalso the position of existing and, if known, that of removedtrees, with species and height.

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Fig 7 Site plan showing position of levelling points and datums

LEVELLING TECHNIQUEStudy the instrument instruction book carefully. Since it isoften not possible in settlement work to equalise back andforesights, the horizontality of line of sight (collimation) ofthe instrument must be maintained accurately. Howeveraccurately collimation is set in manufacture, it may havebeen disturbed in transit.

Collimate the level on receipt and then at about three-monthly intervals, according to the amount of use, andwhenever it has been subjected to rough handling. Aim toset the collimation to 0.5" of arc (0.1 mm in 40 m). At thesame time, check the bubble of the staff by comparing itsedge with a plumb line. Some staff bubbles are adjustable;fixed ones need specialist service if they are out of true.

Before each levelling session, leave the level and tripod toacclimatise for 15 minutes, or longer if the instrumenttemperature is very different from ambient. If possible,protect the level and the tripod from direct sunlightthroughout the operation. Levelling is best done in overcastweather, preferably with a slight breeze. Stability of thetripod is very important; its feet should be well trodden inon normal ground. Avoid soft, yielding ground and wobblypaving slabs; take care on tarmac because it can yield whennew and in warm weather. A tripod base* with rubber padsis available for surfaces that are slippery, hard or easilydamaged.

Maintaining the staff bubble central for long periods istiring for the staff holder. A routine of commands from thelevel operator of “bubble” when the micrometer is actuallybeing used, and “OK” will relieve fatigue and allow thestaff holder to concentrate on keeping the bubble central atthe right time. A bipod* held in contact with the staffproduces a tripod effect to help steady the staff – Fig 8.

The following procedure differs slightly from the normalmethods of focusing the eyepiece but is useful for operatorswith uncorrected astigmatism. De-focus the telescope.Against a light background, focus the eyepiece to thereticule concentrating on attaining equal clarity of the twowedge lines. Off-set this focus and re-set it. Now focus thetelescope on a staff interval and move the eye up and downslightly. Re-focus the telescope until no apparent relativemovement takes place between reticule and staff.Regardless of the apparent absolute clarity of the staff,there is no parallax and the staff is correctly focused in theplane of the reticule. Re-focus the eyepiece to obtainmaximum comparative sharpness of the reticule wedge andthe staff interval. Note the reading of the dioptric scale,engraved on the side of the eyepiece, for use as a constantfor that observer using that instrument.

When focusing on the staff to take site observations, use the‘up and down eye method’ to eliminate parallax. Read thestaff to 0.01 mm, using the micrometer several times asrequired to achieve repeatability. Record the mostrepeatable reading in a surveyor’s levelling book* or on apro-forma from the instrument instruction book.

On-site calculations and achievable accuracyIt is better to take one high-quality round of levelling thanseveral indifferent ones. Always aim at the highestaccuracy. BRE experience shows that a careful observerwill attain closures of 0.2 to 0.3 mm by followingrecommended procedures. Calculate reduced levels beforeleaving the site to check that closing errors are less than0.5 mm and there are no apparent gross errors in reducedlevels compared to previous observations. A staff readingerror at a levelling station observed as an intermediate sightwill not affect the closing error. A round that produces aclosing error greater than 0.5 mm should normally berepeated. If the level and staff are properly adjusted but tworounds do not produce the usual accuracy, the fault isprobably due to atmospheric refraction. This can produceconsiderable errors in certain conditions so accept the bestround and re-level another day.

The levelling sequenceThe datum judged the most reliable is called Main Datum(D1). Its level is designated 0.00 mm, or 1000.00 mm etcas necessary to avoid negative reduced levels. Startlevelling at D1 and then go to the nearest levelling station(S1) on the building and proceed round the building,levelling each station and any subsidiary datums, closing toS1 and then to D1.

Establishing initial ‘zero’ levelsAt the start of a precise levelling project, take two completesurveys and compute them. Building and main datumclosures should be better than 0.5 mm. If a survey is out oflimits, do another complete one so that two surveys withclosures better than 0.5 mm are available to confirm theinitial zero levels.

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Fig 8 A bipod gripped to the staff provides bubble stability

PROCESSING RESULTSRecording resultsKeep up-to-date, tabular records of the results of surveys,as well as results in the original field survey book. Figure 9contains in a concise form all information that is needed forstudy of progressive movement. It also gives sufficientinformation (together with a site ‘station’ plan) forcontinuity with future surveys even if field survey booksare lost or a different organisation takes over the project.

Tabulating resultsEach monitoring station (S1, S2, ...) has a two-columnentry, the reduced level on the left and movement to date –Fig 9. Any subsidiary datum is treated in the same way(D2, ...). Closing errors are not distributed but are recordedin two-column entries headed Closure (S1, D1). Here thesecond reduced level is recorded on the left, with theclosure on the right.

For S1, this closure will be the difference between this leveland the previous one for S1 in the same survey, alreadyrecorded at the left-hand side of the table. For D1, theclosure is the deviation from its assigned level, eg 0.00 or1 000.00. The results table should include a column forweather, any nearby works affecting the structure understudy and other relevant comments.

The first entry in the table is the one judged to be the betterof the two accepted rounds of zero levels. The secondaccepted round should also be entered as permanentconfirmation of zero levels. In later surveys, only the bestround of the day need be entered in the table but the bestshould be entered even if it has a bad closure for reasons ofweather or shortage of time on site. Results from everyround can be entered if desired.

Graphical presentation of resultsGraphical presentation is necessary for interpretation. Plotsof movement against time should be drawn as described onpage 8. A contour plan of ‘total movement to date’ may alsobe informative. Tabulation and graphical presentation* canbe carried out by computer from entry data consisting of thereduced levels previously calculated on site. Manualpresentation is acceptable but takes longer.

Figure 10 shows computer-plotted contours of themovement of a building on clay soil. The curved contoursof heave are centred on a position just north-east of S7,confirming this as the location of former large trees.

Reduced levels in mm D1 = 1000.00 Movements in mm + = upwards Declared initial levels underlined

Date S1 S2 S8 D2 Closure S1 Closure D1 Remarks and observer

15/9/79 1218.81 0.00 1223.38 0.00 1222.98 0.00 481.48 0.00 1218.98 + 0.17 1000.21 +0.21 Cloud/sun, gusty wind JC/PG

15/9/79 1219.19 +0.38 1223.81 +0.43 1223.26 +0.28 481.86 +0.38 1219.40 + 0.21 1000.28 +0.28 Cloud/sun, gusty wind DB/JC

8/10/79 1218.83 +0.02 1223.67 +0.29 1223.37 +0.39 481.81 +0.33 1218.86 + 0.03 999.93 -0.07 Calm, sun front, shade rear JC/PG

8/4/80 1220.00 +1.19 1225.54 +2.16 1225.87 +2.89 481.88 +0.40 1219.73 -0.27 999.58 -0.42 Cloud/sun, slight wind JC/RD

12/9/80 1220.95 +2.14 1227.09 +3.71 1229.40 +6.42 482.68 +1.20 1221.41 +0.46 1000.39 +0.39 Cloud/sun, calm IL/PG

31/7/81 1220.43 +3.62 1228.80 +5.42 1231.85 +8.87 481.89 +0.41 1220.91 -1.52 998.35 -1.65 Cloud/sun, calm IL/JC

16/11/81 1222.98 +4.17 1229.73 +6.35 1235.13 +12.15 482.81 +1.33 1222.94 -0.04 999.71 -0.29 Cool, overcast, calm PG/DB

Fig 9 Suggested form of tabulation of survey results

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Fig 10 Plan of site with computer-plotted contours of movementMovement in mm – positive indicates heave.

Plots of movement against timeSeparate graphs should be prepared for each buildingstation (S1, S2, ...), any subsidiary datums (D2, ...) and forclosures of S1 and D1. Use only the best results of aparticular day. Plot movements to date at 2 x natural scalevertically. A suitable horizontal time scale is 2 mm perweek. Do not use ‘best fit’ methods when presenting graphlines, because actual movements are not always ‘smooth’.Point-to-point plots will give more useful information.

Interpreting the graphTotals to date can be visually compared on the graphtogether with rates of movement and changes in rate. Thispresentation shows as much information as possible anddoes not conceal errors. In Fig 11, D2 is lifting relative toD1, but the choice of main datum was confirmed byknowledge gained later that trees had been removed nearD2. It shows also that building stations S4 and S5 at theundamaged end of the building remained virtuallystationary, as expected, when calculated on D1.

The ‘kinks’ in some of the plots for mid-1981 are due to apoor closing error, caused probably by a mis-readingbetween S3 and S4. Generally, closing errors are shown assatisfactory, except as discussed above, and on oneoccasion when the weather was poor.

REFERENCES AND FURTHER READING1 Cheney, JE. 25 years heave of a building constructed on clay

after tree removal.Ground Engineering, 1988, Vol 21, No 5, 13 – 27.

2 National House Building Council. Building near trees. StandardsChapter 4.2. Amersham, NHBC , 1992.

3 Cheney, JE. Techniques and equipment using the surveyor’s levelfor accurate measurement of building movement. Proc BGS Symp on Field Instrumentation. London. BritGeotechnical Soc, 1973, 85 – 99.

4 Cheney, JE. Some requirements and developments in surveyinginstrumentation for civil engineering monitoring.Proc Conf Industrial and Engineering Surveying. London, 1980,2.3/1 – 2.3/10.

British Standards Institution

BS 5837: 1991 Guide for for trees in relation to construction

Other BRE Digests

202 Site use of the theodolite and surveyor’s level

234 Accuracy in setting out

240 Low-rise buildings on shrinkable clay soils: Part 1

241 Low-rise buildings on shrinkable clay soils: Part 2

242 Low-rise buildings on shrinkable clay soils: Part 3

251 Assessment of damage in low-rise buildings

298 The influence of trees on house foundations in clay soils

318 Site investigation for low-rise building: desk studies

322 Site investigation for low-rise building: procurement

343 Simple measurement and monitoring of movement in low-rise buildings. Part 1: cracks

344 Simple measurement and monitoring of movement in low-rise buildings. Part 2: settlement, heave and out-of-plumb

359 Repairing brick and block masonry

361 Why do buildings crack?

381 Site investigation for low-rise building: trial pits

383 Site investigation for low-rise building: soil description

386

The BRE Levelling Station can also receive several types oftarget for use with theodolites, electronic distance measuringequipment and tensioned tapes. Their use is outside the scope ofthis Digest but more details are given in ref 4.

ISBN 0 85125 608 2© Copyright BRE 1993Republished on CD-ROM 1997, with permission of BuildingResearch Establishment Ltd,

by Construction ResearchCommunications Ltd, 151 Rosebery AvenueLondon, EC1R 4QX

Applications to republish all orany part of this publication shouldbe made to ConstructionResearch Communications Ltd, PO Box 202, Watford, Herts, WD2 7QG

Anyone wishing to use theinformation given in thispublication should satisfythemselves that it is not out ofdate, for example with referenceto the Building Regulations

Technical enquiries to: BRE Advisory ServiceGarston, Watford, WD2 7JR Telephone 01923 664664 Facsimile 01923 664098

Fig 11 Computer-plotted graph showing movements with time

8

Monitoring building and ground movement by preciselevelling

Supplement: specifications and suppliers of equipment

Digest 386SupplementAugust 1993

BBRREE DDiiggeessttConcise reviews of building technology

Building Research Establishment

Technical enquiries to:BRE Advisory ServiceGarston, Watford, WD2 7JRTel: 01923 664664 Fax: 01923 664098

PRECISE LEVELLING EQUIPMENTPRECISE LEVEL‘Wild Automatic Level NA2. Stock No. 352036.’ £1172.00. Leica UK Ltd.

The non-automatic precise level is the Wild N3; the electronic one is theWILD NA3000. Both are available from Leica UK Ltd.

OPTICAL MICROMETER‘Wild Parallel Plate Micrometer GPM3 (10 mm by 0.1 mm).Stock No. 356121.’ £1207.00.‘Combined Carry Case for NA2 and GPM3. Stock No. 321365.’ £275.15.The combined carry case is recommended. The quoted price will bediscounted if it is ordered ‘in lieu of separate cases’ at the same time asordering the NA2 & GPM3. Leica UK Ltd.

PRECISE STAFF‘Invar Industrial Staff 1.82 m length. GWL182N. Stock No. 559616’. £557.50.‘Special Rectangular Base for GWL type staff. Stock No. 236951’. £148.00.Leica UK Ltd.The special (detachable) rectangular base must be used on the staff for thework described in the Digest.The latest version of the staff varies slightly from the one shown in theDigest. The bubble on this staff is at the rear, about 600mm from the top.This is often not the best position for the work described in this Digest. Itis usually more convenient to have it on one side about 300 mm from thebottom. If your site experience suggests that it would be better re-positioned, consult Leica UK Ltd.

Some sites may have headroom problems. A short staff is available whichshould be used with the special rectangular base: ‘Invar Industrial Staff0.92 m length, GWL 92N. Stock No. 559615’. £427.00. Leica UK Ltd.

TRIPOD‘Wild Telescopic Tripod GST 20. Stock No. 296632.’ £235.50.‘Wild Shoulder Strap for tripod. Stock No. 561731.’ £5.00.Leica UK Ltd.

TRIPOD BASE‘Wild Base Support (for tripod). GST4. Stock No. 332200’. £127.00.Leica UK Ltd.

GROUND PLATE‘Wild Heavy Duty Ground Plate. Stock No. 197000’. £40.50. Leica UK Ltd.

BIPOD‘Aluminium Bipod for use with levelling staff’. POA.MAS Engineering.

BRE LEVELLING STATION COMPONENTS‘BRE Monitoring Socket’. (1 for each position to be levelled). £6.40.‘Perspex Bung’. (1 for each socket + 10% spares). £2.40.‘Wrench for Bung’. (1 only required). £6.85.‘Levelling Plug’. (1 only required). £23.00.MAS Engineering.These prices are for unit supply; enquire for quantity discounts.Longer monitoring sockets, longer levelling plugs, and male/female threadextension pieces can be supplied for special applications. Sockets are madefrom stainless steel to BS 970:Part 3:316S31; this has a very highcorrosion resistance, even to sea and road salts. Levelling plugs are ofstainless steel to BS 970:Part 3:303S31.

DATUM COMPONENTSBRE DATUM‘Datum Levelling Dome’. £18.50.‘Datum Foot’. £25.00.Both items are of stainless steel to BS 970:Part 3:303S31.MAS Engineering. This manufacturer can also supply the othermechanical parts of the datums.

GROUT‘Grout mix – 1 OPC:6 pfa:1 bentonite supplied in 25kg bags’.Approx £60.00/t collected, £110.00/t delivered. Pozament Cement Ltd.

Each 25kg bag should be mixed with 37 litres of water to give a grout mixby weight of 1 OPC : 6 pfa : 1 bentonite : 12 water. This has a set strengthof about 400 kN/m2 and is suitable for datums set in stiff clay. Cementcontent should be doubled for datums in harder strata.

BENTONITE PELLETS‘Bentonite Pellets supplied in 25kg bags’. £8.00.Marton Geotechnical Services or Bourne Geotechnical Supplies.

SECURE COVER‘Stop Cock Cover: C.I. 9" by 9" lockable’. £ 16.00.Marton Geotechnical Services.

Alternatively, use any appropriately sized, cast-iron stop cock cover,adapted so that the lid is secured by stainless steel socket-headed capscrews. The cover should be securely bedded in mortar and its lid setslightly above ground level to keep out surface water.

SURVEYOR’S LEVELLING BOOKCollimation ruling: ‘Chartwell Ref 2426’. £6.56.Rise & Fall ruling: ‘Chartwell Ref 2416’. £6.56.Wightman Mountain Ltd, or local survey equipment suppliers.

COMPUTER GRAPHICSComputer tabulation and line plots can be carried out with one of thewidely-available spreadsheet software packages, such as Excel and Lotus1-2-3. Computer contour plotting requires specialist software. A suitablecontour package for personal computers is ‘Surfer’, produced by GoldenSoftware Inc, Colorado, USA, and available from Geomem Software.£330.00.

This list covers all the items marked * in the Digest. Prices exclude VATand should be checked with the manufacturer or supplier before purchase.Delivery charges may be added. Sufficient details of each item for orderingpurposes are contained within quote marks ‘- - - -’.

Addresses, with telephone and fax numbers, are listed on page 2.

ADDRESSES AND TELEPHONE NUMBERS

BOURNE GEOTECHNICAL SUPPLIESRushlake Green, Heathfield, East Sussex, TN21 9BQ.Tel: 0435 865402. Fax: 0435 868286.

GEOMEM SOFTWARE1 High Street, Blairgowrie, Tayside, Scotland, PH10 6ET.Tel: 0250 872284. Fax: 0250 873290.

LEICA UK Ltd.Davy Avenue, Knowlhill, Milton Keynes, MK5 8LB.Tel: 0908 666663. Fax: 0908 609992.

MARTON GEOTECHNICAL SERVICESGeotechnical Centre, Rougham Ind. Estate, Rougham, Bury St. Edmunds, Suffolk, IP30 9ND.Tel: 0359 71167. Fax: 0359 71168.

MAS ENGINEERINGRectory Buildings, The Stocks, Cosgrove,Milton Keynes, MK19 7JD.Tel: 0908 566340. Fax: 0908 260539.

POZAMENT CEMENT Ltd.Swains Park Industrial Estate,Burton-upon-Trent, Staffs, DE12 6JN.Tel: 0283 213636. Fax: 0283 550302.

WIGHTMAN MOUNTAIN Ltd.1c Lyon Way, Greenford,Middlesex, UB6 0BN.Tel: 081 575 2302. Fax: 081 578 0439.

2

Supplement to Digest 386

© Copyright BRE 1993Republished on CD-ROM 1997, with permission of BuildingResearch Establishment Ltd,

by Construction ResearchCommunications Ltd, 151 Rosebery AvenueLondon, EC1R 4QX

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Anyone wishing to use theinformation given in thispublication should satisfythemselves that it is not out ofdate, for example with referenceto the Building Regulations

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