KARST IN HONG KONG - AGS (HK

1 Typical Required DataMarble Quality Designation (MQD)MQD is a risk based approach to interpret GI data and classify marble masses into 5 zones reflecting foundation = Average RQD x Marble Rock Recovery Ratio (MR)Full definitions and guidelines are given in GEO Report No. depths of drillholes should relate to the depth of marble bedrock and the magnitude of the load to be applied by the structure. If marble is encountered, a minimumpenetration of 20m into sound marble rock is recommended in order to reduce the risk of existing cavities not being INVESTIGATION KARST IN hong KONGW ebsite: do we need to know?Ground InvestigationTypical Properties to be determinedGeneral InformationWhat is to be built and where (roads, tunnels, reservoirs, utilities, foundations, sensitivestructures - tolerance to movement, extent of stress bulbs and likelihood of waterleakages causing dissolution)?

2 Will the development impact upon the stability of existing structures?Existing (damaged/disrupted) utilities actively reactivating dissolution? Evidence ofnatural ground or structural subsidence/damage due to subsurface dissolution? (crownhole development, swallow holes, caves, cracking or repointing of mortar/masonry,window or door frame misalignment.)The geological model is of paramount investigation should attempt to determine whether there are, or are likely to be,any significant solution features in the rock solution features are expected or identified, how large are they, how deep do theyextend and what is the nature of the infilling, if any. Inclined drillholes should also GI carefully: use of SPTs in cavities, recording depths and quantities of drillingflush loss and fluctuations in flush pressures, recording penetration rate and identifyingany significant or immediate drop in drilling rods (ensure recovery in cavity zones).

3 Geological ModelGroundwater and surface water hydrology should be at least broadly characterisedand the likely changes induced by the engineering works should be insitu permability testing that could encourage washing the infillHydrogeological ModelScheme layoutdrawingsBackground study ofavailable recordsLocal dilapidation andexisting utilities surveySite Investigation & experiencedsupervisory staff(preferably Cat. Isupervisor withexperience in KARST GI)GeotechnicalInstrumentationDrillhole SamplingSuperficial cohesivesoils: U100/76 or MazierSuperficial granularsoils: U100/76 Rock: minimum 80mm diameter, triple-tube airfoam drilling in weaker and more decomposedrockGroundwater samplingDrillhole TestingSPTs (looking for low values bounded by hardrock) permeability tests in cavity infill shouldbe considered only for special circumstanceEngineering Geophysics OptionsMicrogravity, gravityCross-hole seismicGamma loggingInstrumentationStandpipes to define the water table aboverockhead and piezometers to determinepressure levels in cavity infillWeathering GradeKnowledge of the weathering grade on a mass scale ( zonalclassification)

4 Is fundamental to understanding KARST , and identifying thepresence of Karstic featuresClassification Schemes and Weathering of CarbonatesThe six-zone classification scheme given in Geoguide 3 is intended foruse in the igneous granite and volcanic rocks found in hong kong . It is notappropriate to calcareous rocks since weathering is mainly due to dissolutionand not mineral decomposition. More detail is given in Ch14 & 16 of AGS2004 Publication: Geology of Site Investigation of superficial materials (typicallyalluvium, marine sediments, KARST surface(debris flow) deposits)Properties of underlying rock ) Identification of cavity s extent & infilling;b) Identification of floating rock lenseswithin a large cavity soil infill;c) Identification of rockhead for founding ofpilesSurface and groundwater characteristicsStrength and permeability of superficialsoil horizons Unconsolidated Undrainedtriaxial and lab K testsNature, strength and degree ofdecomposition of underlying rock, triaxial,UCS, point load properties of existing orimported fill and foundation materials andgroundwaterChemical TestsPH, SO3.

5 : Government GuidelinesKarst : Geological ImpactUseful Publications Specific toKarst in HKThe involvement of an appropriately experienced andqualified engineering geologist to consult on the scope,sufficiency and findings of ground investigations (GI) isespecially important for surface and sub-surfacedevelopments in the metasedimentary and marble-bearing rocks of hong kong . Rarely is the geologicalmodel and fullness of information more important inHong kong , YC (1994). Factors Affecting Sinkhole Formation(GEO Report No. 28). Geotechnical Engineering Office, hong , YC (1994). Classification and Zoning of MarbleSites (GEO Report No. 29). Geotechnical EngineeringOffice, hong , YC and Pun, WK (1994). KARST Morphology forFoundation Design (GEO Report No.)

6 32). GeotechnicalEngineering Office, hong , Transport and Works Bureau TechnicalCircular (Works) No. 4/2004 Checking of FoundationWorks in the Scheduled Areas of Northwest NewTerritories and Ma On Shan and the Designated Areaof Northshore Control Office (1990). FoundationProperties of Marble and Other Rocks in the Yuen Long Tuen Mun Area (GCO Publication No. 2/90).Geotechnical Control Office, hong Geology in hong kong . Geological Society ofHong kong Bulletin (1990).Chris N Fletcher (2004). Geology of Site InvestigationBoreholes from hong kong . Published by AGS hong KongBuildings Department Practice Notes: PNAP161 Albert T Yeung (Editor) 2004 Foundation Practice inHong kong - Centre for Research & ProfessionalDevelopmentKarst is a term used in hong kong , to describecarbonate rocks containing dissolution features andexhibiting highly irregular rock surfaces.

7 Themagnitude of the dissolution features is a functionof the purity and thickness of the marble, and theprevailing hydrogeological conditions over thegeological history of the rock mass. Cavities,sometimes several metres thick, with or withouttypical silty/clayey infill have been encounteredwithin the zone of influence of engineeringstructures, in identified Karstic areas. The presenceor expectancy of encountering such features hashad a major impact on the design and constructionof foundations in these areas. Marble isencountered at least 20m below ground and isusually capped by low permeability superficialdeposits and little to no dissolution occursMetasedimentary strata include marble, outcrop andsubcrop in the Northwest New Territories (Yuen Longto Lok Ma Chau, the Northshore Lantau area, nearTung Chung and Ma On Shan) areas of hong clasts, sometimes metres across, are alsopresent in the volcanic formations of the Tuen Munarea.

8 The presence of cavities, within some of themarble units, has lead to the designation of theNorthwest New Territories and Ma On Shan asScheduled Areas 2 and 4 under BD PNAP161 (aswell as the issue of the GEO TGN12 TheDesignation Area of North Lantau ), respectively,under the Buildings Ordinance. It has also focusedattention on the presence of Karstic features as aconstraint in engineering development in each ofthese GI ProblemsGeological Schematic of KARST Profile(after Fletcher 2004)AlluviumFresh graniteCompletely decomposedgraniteFaultBeddingFault materialPure marbleImpure marbleMetasiltstoneSupra- KARST depositsCavity-fill deposits, with possible voids in placesKarst deposits associatedwith collapse featureGround Investigation Interpretation IssuesKarst GI.

9 SupervisionDrillholes terminated on competent marble rock lens within a solution or collapse cavity infilled with a soilmatrix, not founded on sufficiently load bearing dipping irregular rockhead makes defining DH termination depths and therefore founding level forbored piles infill with possible voids below rockhead and base of bored pile, which may cause bearing capacity /settlement in competent bedrock may result in weak or broken ground and require deeper FillMaterialMarbleMetasiltstone1234 Notes:1