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THE UNIVERSITY OF HONG KONGLIBRARIES

Hong Kong Collectiongift from

Hong Kong (China). Geotechnical EngineeringOffice, Civil Engineering Dept.

Geotechnical Engineering Office, Civil Engineering DepartmentThe Government of the Hong Kong Special Administrative Region

GEO Technical Guidance Note No, 19 (TGN19)Installation of Soil Nails and Control of Grouting

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1. SCOPE

1.1 This Technical Guidance Note (TGN) provides guidelines on the installation, testingand acceptance of soil nailing works during construction to ensure the quality of theworks. It is also intended to be a quick reference for contract management andsupervisory staff involved in the supervision of soil nailing works as well as staffcarrying out on-site audits of such work.

1.2 Any feedback on this TGN should be referred to Chief Geotechnical Engineer/Works ofthe GEO.

2. TECHNICAL POLICY

2.1 The technical recommendations promulgated in this TGN were agreed by GEO'sGeotechnical Control Conference on 13 April 2004.

3. RELATED DOCUMENTS

3.1 Geotechnical Engineering Office (2003). Non-destructive Tests for Determining theLengths of Installed Steel Soil Nails, GEO Report No, 133. Geotechnical EngineeringOffice, Hong Kong, 54p.

3.2 Hong Kong Government (1995). Construction Standard - Carbon Steel Bars for theReinforcement of Concrete CS2:1995. Hong Kong Government, 35p.

3.3 Hong Kong Government (1993). Project Administration Handbook for CivilEngineering Works, Volume 3. Hong Kong Government.

3.4 Buildings Department (2003). Practice Note for Authorized Persons and RegisteredStructural Engineers No. 284, Quality Supervision of Soil Nailing Works. BuildingsDepartment, Hong Kong, 9p.

4. DEFINITIONS

4.1 The PE shall mean the project engineer responsible for administering the contract andshall include personnel within his/her site supervisory team.

5. BACKGROUND

5.1 Soil nailing is currently the predominant method used in the upgrading of sub-standardslopes for the vast majority of Landslip Preventive Measures works. It is a relativelysimple and fast method of working and, if properly carried out, should pose the least

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GEO Technical Guidance Note No. 19 (TGN 19)Installation of Soil Nails and Control of Grouting

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disturbance to the public at large. However, once the soil nails have been installed intothe ground, it generally becomes difficult to prove the quality of the built product.Proper supervision and documentation of the works are therefore of paramountimportance.

5.2 Reference can be made to the sample Particular Specifications and standard drawingsfor soil nailing works as well as guidance notes and checklists used by the GEO underits Landslip Preventive Measures programme in CED's web sitehttp://wwwxed.gov.hk/eng/generalinfor/generalinfo f.htm.

6. TECHNICAL RECOMMENDATIONS

6.1 BUILDABILITY OF SOIL NAILS

6.1.1 In designing soil nails, the designer should give due consideration to the buildability ofthe soil nails to ensure that the design is practical and buildable.

6.1.2. Drilling for long soil nails, say with length exceeding 20m, stands a higher chance ofhitting groundwater tables and other adverse geological features, e.g. local weakgeological zones and seams or dykes of high contrast in hydraulic conductivity. Thismay lead to difficulties in construction such as collapses of soil or rock fragments alongthe drillhole (which may obstruct the insertion of the nail reinforcement assembly), andhigh grout loss. Grout quality may also be difficult to ensure as the length of nailincreases. Soil nails at close spacing may intersect due to hole deviations duringdrilling. For cases where long or closely spaced nails are proposed or where theground or groundwater conditions are likely to be adverse to soil nail construction, a sitetrial may be required to assess the buildability and the effects of soil nail installation onthe existing ground and groundwater conditions, prior to carrying out the soil nailingworks. Details of the trial including its locations, potential problems and contingencymeasures if the trial nail failed should be included in the designer's requirements underthe contract. The site trial may be carried out in parallel with the installation of soilnails for the pullout tests.

6.1.3 Examples of ground conditions that may present difficulties for soil nail constructioninclude loose fill or colluvium, bouldery ground, rockfill, presence of soil pipes, buriedstream course, rock mass with open discontinuities and fractures, alternating zones ofsoil and rock, etc. The installation of soil nails in areas with groundwater will poseparticular difficulties. The hole drilled may be more susceptible to collapse than in dryground and the quality of the grout may also be in doubt, because of grout lossassociated with groundwater flow. Suitable measures, e.g. groundwater drainage, mayhave to be incorporated to facilitate installation of soil nails.




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6.2 SUPERVISION OF SOIL NAILING WORKS

6.2.1 All soil nailing works should be supervised by personnel with suitable qualificationsand experience. For Government projects, the Project Administration Handbook givesguidance on supervision requirements for soil nailing works. For private sectorprojects, Practice Note for Authorized Persons and Registered Structural Engineers(PNAP) 284 issued by the Buildings Department outlines the requirements for qualitysupervision of soil nailing works.

6.2.2 In general, full-time supervision should be accorded to all soil nailing works with someaspects such as pullout tests, insertion of steel bars and grouting of the soil nails to beindividually inspected and checked by the PE. PNAP 284 gives additionalrequirements for inspection and checking by the contractor's personnel in privatedevelopments.

6.3 PULLOUT TESTS

6.3.1 Pullout tests are principally used by the designer to verify the assumptions made duringdesign of the soil nails. The test requires the application of a tensile load on the steelbar to determine the pullout resistance of the soil in the anchorage zone of the soil nail.

6.3.2 In general, the number of pullout tests to be carried out should be in the region of 2% ofthe total number of soil nails specified but this can be reduced for slopes with a largenumber of nails. During drilling, information such as the drill rate, type of materialencountered, the presence of groundwater etc, should be collected for the designer toinfer the actual ground conditions.

6.3.3 Grouting for pullout tests should be carried out slowly and carefully to preventover-grouting. During grouting, air can be blown manually through the return pipe asa means to monitor the grout take.

6.3.4 When setting up the pullout test apparatus, the steel bearing plate to be used for the testshould not be allowed to bear down onto the steel bar as this will deflect the bar therebygiving incorrect readings during the test.

6.4 CONTRACTOR' s SUBMISSION

6.4.1 The PE should check the contractor's submissions against the contract requirements andin particular ensure that the contractor's proposals for measurement of the groutvolumes in individual soil nails are clearly outlined and considered practicable.




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6.4.2 For soil nails installed into poor quality ground where there is a likelihood that holecollapse may take place, the PE should alert the contractor and ask him to submit in hisproposal the details and method to overcome such ground conditions.

6.4.3 Once a trial mix has been successfully carried out the contractor should not be allowedto make any changes without prior approval and subject to any further successful tests.The contractor should be instructed to clearly display the mix proportion on or near thegrout mixer for workers5 reference during grouting. The PE should also bear in mindthat wide variations in ambient temperature will affect the acceptability of grout andfurther grout mix tests should be instructed when such temperature changes occur.

6.5 DRILLING EQUIPMENT FOR SOIL NAILING WORK

6.5.1 The PE should ensure that the total length of drill rods available at each drilling locationis sufficient for the length of the soil nails to be constructed. He should also ensurethat the diameter of the drill bits is appropriate for the specified diameter of the drillholes.

6.6 SETTING OUT OF SOIL NAILS

6.6.1 The positions and reference numbers of the soil nails should be clearly marked on theslope face by the contractor for easy identification.

6.6.2 The positions of the soil nails should be checked to see whether any existing utilities,channels, surface boulders, trees, foundations and other structures etc. would be affected.Their positions should also be checked to see whether they will obstruct any otherproposed work such as surface channels and sub-surface drains. If so, their positionsshould be shifted to minimize all such obstructions. The MTRC/KCRC should havebeen consulted and their agreement obtained on the proposed positions and lengths ofsoil nails if the works are within their protective boundaries.

6.7 DRILLING FOR SOIL NAILS

6.7.1 In order to avoid causing nuisance to the environment, it is important that appropriatemeasures are implemented to minimize the spread of dust during drilling. Where it isconsidered likely that bedrock or boulders may be encountered, water sprays and dusttraps at the mouth of the drillholes should be provided. Drilling works on workingplatforms that may be visible to nearby residents should be shielded from view bytarpaulin sheets. Adequate noise reduction measures such as designed noise barriersshould also be incorporated when drilling in areas with very sensitive receivers.

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6.7.2 Where close spacing and long nails are specified, more care should be taken to ensurethat the orientation in both the vertical and horizontal planes are as close as possible tothat specified in order to minimize the risk of clashing of nails at depth.

6.7.3 For drilling long soil nails, the drill rate should be suitably reduced to minimize theeccentricity produced by the dip of the drill rods, which may unduly enlarge thediameter of the drillhole and cause hole collapse.

6.7.4 During drilling, random checks should be carried out on the correctness of theorientation and inclination of the drill hole. Normally this will be carried out by usinggeological compasses (provided under the contract) and measuring the inclination of thedrillrods.

FABRICATION AND INSERTION OF STEEL BARS

The total length of each set of steel bars should be measured against the specified lengthof the soil nails. It is important that different lengths of steel bars are stored separatelyto minimize the risk of wrong lengths being used in the soil nail.

6.8.2 On completion of assembling of all components, the PE should check that the spacingof the centralizers is adequate for supporting the steel bar (generally between 1 and 1.5m depending on the size of bar) and that no side holes are cut along the length of thegrout pipe except within 150 mm from the end of the pipe,

6.8.3 During insertion of the steel bar, the PE should ensure that the bar is not pushedcompletely into the bottom of the hole but that at least 50mm clearance is maintainedbetween the end of the bar and the bottom of the drilled hole.

6.8.4 If obstructions are encountered during insertion of steel bar into the drillhole, the barshould be withdrawn, the obstruction removed and the bar reinserted.

6.9 GROUTING OF SOIL NAILS

6.9.1 Depending on the cement brand and admixture used, the acceptable water-cement ratiofor cement grout normally ranges from 0.38 to 0,42 with Flow Cone Test results fallingwithin the range of 15 to 30 seconds. Batching and mixing of cement grout should beproperly controlled. A significant change in the results of the Flow Cone and/orBleeding Tests could indicate poor control in batching of the materials or freshness ofthe cement used. Any batch of cement with air-set lumps should not be used in thework.

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6.9.2 Where high groundwater level is encountered, the grouting operation should not becarried out until the water level has dropped to an acceptable level. If the level ofgroundwater persists, raking drains or other form of dewatering will need to beimplemented prior to insertion and grouting of the soil nail. The effects of anydewatering should be duly assessed and mitigation measures taken as appropriate.

6.9.3 Packers used for sealing of soil nail holes are normally made from foam wrappedaround the steel bar. However, this may not form an effective seal and other formssuch as mechanical plugs, which can be screwed tight against the face of the drillhole,may need to be used.

6.9.4 The contractor is required to carry out Flow Cone and Bleeding Tests before grouting ofthe soil nails. However, due to the long duration of the Bleeding Test, the contractormay wish to commence the grouting operation before the test is fully completed. Hemay do so at his own risk. The PE should also instruct the contractor to make groutcubes for strength tests during the grouting operation on randomly selected batches ofgrout. During the course of grouting, random Flow Cone tests should also be carriedout to ensure the consistency of the grout used.

6.9.5 All grout must be introduced into the soil nail hole using the grout pipe such that anyair/ground water is completely displaced out of the drillhole.

6.9.6 In case of excessive grout loss, the PE should be notified immediately for him todetermine the action to be taken. The volume of any additional grout used should berecorded. Cases of excessive grout loss should be promptly reported to the designerfor a review of the assumed geological model and for determination of any changes tothe design required.

6.9.7 Where soil nails are to be installed through loose material or there is a high chance thatthe ground may induce excessive grout loss, the designer should specify casings to beinstalled within the loose material. In such cases, permanent metal casings should beused which should not be withdrawn during grouting of the hole. Where such casingis specified, the ER should ensure that the internal diameter of the casing is not less thanthe specified diameter of the soil nail. If necessary, the drillhole will need to bereamed by using an ODEX-type drilling method or similarly equipped machine to allowinsertion of the casing into the hole.

6.10 CONSTRUCTION AND TESTING OF SOIL NAIL HEADS

6.10.1 Concrete of a certain grade is normally specified in the contract. However, due to thedifficulty of transport and placement of concrete, contractors invariably propose the useof shotcrete of an equivalent strength. In such cases, it is important that the soil nail




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heads are constructed without honeycombing beneath the bearing plate. One suchmethod would be for the shotcrete to be applied in two stages. The first-stageshotcrete should be applied to approximately 25mm above the bottom of the steel plate.The bearing plate should then be hammered into place and the nut tightened onto thesteel bar before application of the second stage of shotcrete.

6.10.2 It may be prudent to include provision in the contract to require the uncovering of asmall percentage (generally up to 3% with a minimum of 3) of the soil nail heads caston the same day as a quality check. If inferior workmanship is identified in theuncovered soil nail heads, the PE should take appropriate action against the contractorin accordance with the Contract. This would normally take the form of a detailedinvestigation undertaken by the contractor on the batch of nail heads constructed on thatday and proposals for remedial action if defective work is uncovered.

6.11 NON-DESTRUCTIVE TESTING OF INSTALLED SOIL NAILS

6.11.1 On completion of installation of soil nails, non-destructive testing (NDT) (at least 1 % ofsoil nails with a minimum of 2 nails per slope or wall should be tested) may be requiredto verify the length of the installed soil nails. As a guide, several NDT methods forchecking the length of installed soil nails are described in GEO (2003). The morepopular methods include the Time Domain Reflectometry (TDK) test which requires theinstallation of an electrically conducting wire alongside the steel reinforcement, and themagnetometer test which requires drilling of a hole parallel to the soil nail to be tested.Other methods may be proposed prior to the test. A report with the test results andtheir interpretations together with re-assessment of the adequacy of the installed soilnails (if needed) shall be prepared for checking of compliance against the designrequirements,

7. ANNEXES

7.1 Nil

(R K S Chan)Head, Geotechnical Engineering Office

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