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Site Investigations Associate Professor John Worden Associate Professor John Worden DEC DEC University of Southern Queensland University of Southern Queensland.

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Presentation on theme: "Site Investigations Associate Professor John Worden Associate Professor John Worden DEC DEC University of Southern Queensland University of Southern Queensland."— Presentation transcript:

1 Site Investigations Associate Professor John Worden Associate Professor John Worden DEC DEC University of Southern Queensland University of Southern Queensland

2 Site Investigations lOverview: vKnowledge of the sub-surface is imperative for design & construction of foundations for engineering structures. vCollate all existing information from previous exploration, drilling or geophysical exploration. vTypical information sought: Depth to Bedrock; Topography;Depth to Bedrock; Topography; Soil types;Water table & Ground water conditionsSoil types;Water table & Ground water conditions Rock types:Climate.Rock types:Climate. vReview all existing information. vAssess magnitude of loads to be transmitted to foundation. vEnd result is that rocks & soils underlying proposed structure influence foundation design.

3 Site Investigations vOn-site investigations must include: Nature of immediately underlying soil + rock;Nature of immediately underlying soil + rock; Geology of the project & adjacent areas;Geology of the project & adjacent areas; Topography & Vegetation;Topography & Vegetation; Ground water situations, ie seepage,springs, etc;Ground water situations, ie seepage,springs, etc; Gullying & Natural slopes;Gullying & Natural slopes; Depth to Bedrock;Depth to Bedrock; Types of materials to be excavated;Types of materials to be excavated; Stability of any excavations;Stability of any excavations; Absence of toxic wastes;Absence of toxic wastes; Position of any Utilities, andPosition of any Utilities, and Permission to access property.Permission to access property. vIf necessary, drill site to assess parameters. vAll factors influence site selection/ rejection.

4 Site Investigations lFoundation Design Preparation: vCalculate proposed loads to be transmitted by foundation to underlying rocks & soils. vIncorporate all requirements into design of foundations. vObtain data on soils & rock properties. vSoil investigations- Check for natural exposures in road cuts or gullies, or auger drill for samples;Check for natural exposures in road cuts or gullies, or auger drill for samples; Laboratory test soils for shear strength, compressibility, swelling characteristicsLaboratory test soils for shear strength, compressibility, swelling characteristics Disturbed soils have lost soil structure- limited useDisturbed soils have lost soil structure- limited use Undisturbed soils used for wet/dry density, triaxial shear, & compression, permeability, consolidation testsUndisturbed soils used for wet/dry density, triaxial shear, & compression, permeability, consolidation tests Soil samples change on exposure to air- extended exposure reduces their suitability for testing.Soil samples change on exposure to air- extended exposure reduces their suitability for testing. In-situ measurements give superior physical determinations.In-situ measurements give superior physical determinations.

5 Site Investigations Alternatively Trenches & Pits can expose Soil profile.Alternatively Trenches & Pits can expose Soil profile. Standard Penetration Test (SPT). A split spoon sampler hammered into ground for set number of blows/ 150 mm increments. ie, 6/7/5. This equates to 6 blows for first 150 mm, 7 blows for the second & 5 for the third. The last two increments used ie, = 12. Indicates density/ consistency of soil.Standard Penetration Test (SPT). A split spoon sampler hammered into ground for set number of blows/ 150 mm increments. ie, 6/7/5. This equates to 6 blows for first 150 mm, 7 blows for the second & 5 for the third. The last two increments used ie, = 12. Indicates density/ consistency of soil. vRock properties: Deduced from Exposures or Drilling;Deduced from Exposures or Drilling; Drilling types- Auger, Churn, Percussion, Rotary or Diamond;Drilling types- Auger, Churn, Percussion, Rotary or Diamond; Diamond drilling only way to obtain undisturbed rock samples revealing dip orientation, bedding, foliations, joints,etcDiamond drilling only way to obtain undisturbed rock samples revealing dip orientation, bedding, foliations, joints,etc Core loss reflects closely-spaced fractures, weak rock,Core loss reflects closely-spaced fractures, weak rock, Diamond drilling uses circular, cylindrical, diamond- impregnated bit. As bit proceeds through rock, a central section remains stationary. This is broken off & retrieved periodically. Wire-line or triple core barrel technology speeds process.Diamond drilling uses circular, cylindrical, diamond- impregnated bit. As bit proceeds through rock, a central section remains stationary. This is broken off & retrieved periodically. Wire-line or triple core barrel technology speeds process.

6 Site Investigations lRock Variability: vRocks & lithologies highly variable in all three dimensions. vMust assess this variability/ anisotropy. vNot all rocks outcrop equally- some more resistant to weathering. vSurface outcrops can yield biased data, if considered solely. vSoft & less resistant rocks may not outcrop at all. vAll rocks can be obscured by thin sheets of younger sediments. vDeformation features such as folds, faults, fractures, shear zones must be identified. vThese are frequently preferentially weathered & infilled by secondary materials. vFolds alter orientations of planes of weakness. vWeathering depths may vary considerably over different rock types - affect rock strengths.

7 Site Investigations lPlanes of Weakness: vDiscontinuities in rocks have > effect on rock properties than lithology. vInclude - bedding planes, joints, foliations, cleavage, faults, etc. vAll influence Foundation design. vRemember- compressive strength is > perpendicular to discontinuity than // to it. vAll discontinuities evaluated for character, orientation, frequency, etc. vData best determined on rock exposures- more difficult on core. vTrenches & costeans very useful. lFoundations: vThree main types- Solid rock - rock strength & discontinuities identified.Solid rock - rock strength & discontinuities identified. Soil & solid rock at accessible depths - establish depth to Bedrock & as above.Soil & solid rock at accessible depths - establish depth to Bedrock & as above.

8 Site Investigations No Solid Rock - must place foundations in unconsolidated materials. Must also allow for rate of anticipated settling.No Solid Rock - must place foundations in unconsolidated materials. Must also allow for rate of anticipated settling. lDam Foundations: vSmall dams for rural purposes- based on soil mechanics & sited in gully vLarge Dams- must investigate underlying rocks of dam area: Check rock strength is adequate to support water load;Check rock strength is adequate to support water load; Check for weakness planes & potential slippage;Check for weakness planes & potential slippage; Establish orientation of any weakness planes;Establish orientation of any weakness planes; Depth of weathering- removal prior to dam construction;Depth of weathering- removal prior to dam construction; Determine Durability of rock to water exposure;Determine Durability of rock to water exposure; Measure rock permeability;Measure rock permeability; Identify any seismic record of earthquake activity;Identify any seismic record of earthquake activity; Dam type chosen based on availability of materials;Dam type chosen based on availability of materials; Establish risk of siltation reducing dam capacity, before construction.Establish risk of siltation reducing dam capacity, before construction.

9 Site Investigations lGeneral Procedures: vCollect & assess all published accessible data on soils & rocks. vConduct detailed geological study of project areas. vUse combination of drilling & geophysical surveys to complete geology, and confirm interpreted geology from surface outcrops. vField test sub surface materials to determine engineering properties. vDetailed laboratory tests on sub surface materials to determine physical properties. vAll data reviewed, assessed, & recommendations made on site suitability for project. vContinued investigations of sub surface materials while project constructed -confirms earlier interpretations or leads to modifications of plans/ construction methods.

10 Site Investigations lGeophysical techniques: vAdvantages- Relatively low cost;Relatively low cost; Obtain results quickly;Obtain results quickly; Can be undertaken in rough, inhospitable terrains by small teams; andCan be undertaken in rough, inhospitable terrains by small teams; and Can assist planning of expensive drilling programs.Can assist planning of expensive drilling programs. vLimitations- Techniques all identify boundaries between two layers with appreciably different properties. Little contrast - poor definition of layers.Techniques all identify boundaries between two layers with appreciably different properties. Little contrast - poor definition of layers. Requires confirmation by independent means.Requires confirmation by independent means. vTechniques- Seismic reflection & refraction;Seismic reflection & refraction; Electrical resistivity (ER);Electrical resistivity (ER); Ground Penetrating Radar (GPR);Ground Penetrating Radar (GPR);

11 Site Investigations Gravity & Magnetic Surveys, andGravity & Magnetic Surveys, and Downhole techniques.Downhole techniques. vSeismic Methods involve propagation of waves through earth materials vElectrical methods involve measurement of electrical properties of earth materials either- measurement of natural earth currents, ormeasurement of natural earth currents, or the resistance to induced electrical flow.the resistance to induced electrical flow. vNatural earth current flow generated under geological conditions in which anode & cathode develop naturally. Measurement of strength & extent of current helps establish geologic conditions. vElectrical resistivity is resistance to electrical flow through earth materials. Current induced & resistivity measured- identifies basic property of earth material.

12 Site Investigations vGround-penetrating Radar: Essentially the same as Reflection Seismology:Essentially the same as Reflection Seismology: Radar impulse is energy source & receiver used to detect reflections;Radar impulse is energy source & receiver used to detect reflections; Strength of reflections depends on the electromagnetic properties;Strength of reflections depends on the electromagnetic properties; vGravity & Magnetic Methods: Deal with strength of the fields of gravity & magnetism generated between a mass of rock and the Earth;Deal with strength of the fields of gravity & magnetism generated between a mass of rock and the Earth; Measure Earths surface gravity or magnetic field & compare with that of an adjacent area. Changes known as anomalies that imply the size, nature & location of a high or low gravity/magnetic source;Measure Earths surface gravity or magnetic field & compare with that of an adjacent area. Changes known as anomalies that imply the size, nature & location of a high or low gravity/magnetic source; Used for specialised engineering applications only.Used for specialised engineering applications only. vWell logging methods; A variety of techniques that involve lowering instruments down a drill hole & generating data on sub- surface rock types as the instrument traverses the hole.A variety of techniques that involve lowering instruments down a drill hole & generating data on sub- surface rock types as the instrument traverses the hole.

13 Site Investigations lSeismic refraction: vTheoretical treatments of theory of Elasticity,& wavelength of seismic waves confirms that velocity of P waves > than S waves; vAlso velocities of seismic waves dependent on rock density & Youngs modulus. Both increase with depth, so wave velocity also increases; vP waves behave like visible spectrum waves- obey Snells law; vMan-made seismic wave created& times of arrival of P waves sensed by regularly spaced geophones; vBoth refracted & reflected events measured on same seismic signal; vEngineering relies on P wave ( rock strength) vCannot rip apart material whose seismic velocity exceeds 2,500 m/sec.(compacted sand with 40% porosity has P wave velocity = 1800 m/sec).

14 Site Investigations vTheory of refraction derived from behaviour of rays that bend on entering a different velocity medium; vThe larger the velocity difference between two media, the larger the refraction; vBy plotting the Time of arrival of rays versus distance of geophones, can establish critical distance, and thus the depth. vSeismic lines should be repeated in reverse order to enable dipping interfaces to be determined; vSeismic velocities can indicate rock type; vUsed for depths of metres; vSoils below ground water table can be distinguished from unsaturated soils above water table; vPorous, poorly cemented sandstones have low velocities, strongly cemented ones high velocities;

15 Site Investigations vChanges in jointing, dipping beds & cementation changes will affect seismic velocity; vUsed for many years to predict ease of excavation of earth materials; vLimitation- geologic units must increase in velocity with depth to ensure that refracted ray can return to the Earths surface. lSeismic reflection: vprovides a detailed picture of sub surface structure & interfaces; vdepths determined by observing travel times of P waves generated near surface & reflected back from deep formations; vcomparable to echo sounding of water depths. vAdvantages- permits mapping of many horizons for each shot; vcan determine depths to dipping interfaces, as well as angle of dip;

16 Site Investigations vNot used as much as refraction, but refraction will not work where a high velocity layer overlies a low one; vReflection profiling in permafrost areas is not affected by the high velocity permafrost, whereas refraction techniques can be nullified completely. lElectrical Resistivity: vThe range of resistivities( ) of rocks is enormous: vAmount of ground water& dissolved ions in rocks & water of great importance; vDry rock has virtually no electrical conductance:

17 Site Investigations vWater-bearing rock resistivity is function of amount of groundwater present & salinity ; vResistivity of saturated fine-grained sedimentary rocks tends to be lower than coarse-grained sedimentary rocks because of greater porosity; vGravels have more ground water recharge & less total dissolved solids (TDS) than fine-grained material such as colluvium or till; vResistivity used to map overburden thickness, faults, fractures, specific rock units, etc; vDifficult to relate resistivity value directly to rock type. Fortunately show profound anomalies; vTherefore should be compared to drill log data; vIn practice use Werner array- constant spacing, and moving whole array- This is resistivity profiling;

18 Site Investigations vWerner array is arrangement of four electrodes equally spaced along a line, with the two outside ones being current electrodes, & the two inner ones being potential electrodes. The distance between electrodes is designated a; vAlternatively, a can be progressively increased thereby probing deeper and conducting Resistivity Sounding; lGround Penetrating Radar: vCan identify sinkholes at depths of 25 m; vSubsurface anomalies were identified as voids in old earth-filled dam in Michigan -locations aided in a grouting program to fill voids; vGaining rapid acceptance in environmental engineering applications; vCan be used for non-destructive testing of highways:

19 Site Investigations vCommon application is detection of buried pipes & tanks; vPerformance is affected by electrical properties of rocks; vRadar & acoustic pulse propagation depend on the material properties; vDisadvantages include limited depths in clay, but up to 15 m in sands. lMagnetic & Gravity Methods: vRarely used in engineering geology; vMany new geophysical devices that utilise these properties of rocks; vMost commonly used in delineation of buried valleys or basin fills. lWell logging techniques: vRecord generated by lowering probe into an uncased drill hole; vSpontaneous potential (SP), & resistivity logging are most common; vAlso Gamma ray- all rocks & soils are naturally radioactive;

20 Site Investigations vUseful to confirm rock type boundaries; vGamma ray logging indicates clay versus sand units in soils & clayey & shaly beds versus limestone or sandstone in bedrock. lSummary: vRefraction seismic & electrical resistivity are two most applicable techniques to engineering geology; vBest methods practicable for exploring shallow depths of the sub- surface to 30 m. vIf there is a low velocity layer below a high velocity layer, refraction seismic will not work properly. vThis occurs where sand & gravel layer lies below a clay layer; vElectrical resistivity method would work well here; vRefraction seismic works well where soil overlies bedrock.


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