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Site Investigations Associate Professor John Worden DEC University of Southern Queensland
Site Investigations • Overview: • Knowledge of the sub-surface is imperative for design & construction of foundations for engineering structures. • Collate all existing information from previous exploration, drilling or geophysical exploration. • Typical information sought: • Depth to Bedrock; Topography; • Soil types; Water table & Ground water conditions • Rock types: Climate. • Review all existing information. • Assess magnitude of loads to be transmitted to foundation. • End result is that rocks & soils underlying proposed structure influence foundation design.
Site Investigations • On-site investigations must include: • Nature of immediately underlying soil + rock; • Geology of the project & adjacent areas; • Topography & Vegetation; • Ground water situations, ie seepage ,springs, etc; • Gullying & Natural slopes; • Depth to Bedrock; • Types of materials to be excavated; • Stability of any excavations; • Absence of toxic wastes; • Position of any Utilities, and • Permission to access property. • If necessary, drill site to assess parameters. • All factors influence site selection/ rejection.
Site Investigations • Foundation Design Preparation: • Calculate proposed loads to be transmitted by foundation to underlying rocks & soils. • Incorporate all requirements into design of foundations. • Obtain data on soils & rock properties. • Soil investigations- • Check for natural exposures in road cuts or gullies, or auger drill for samples; • Laboratory test soils for shear strength, compressibility, swelling characteristics • Disturbed soils have lost soil structure- limited use • Undisturbed 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. • In-situ measurements give superior physical determinations.
Site Investigations • 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. • Rock properties: • Deduced from Exposures or Drilling; • Drilling types- Auger, Churn, Percussion, Rotary or Diamond; • Diamond drilling only way to obtain undisturbed rock samples revealing dip orientation, bedding, foliations, joints ,etc • 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.
Site Investigations • Rock Variability: • Rocks & lithologies highly variable in all three dimensions. • Must assess this variability/ anisotropy. • Not all rocks outcrop equally- some more resistant to weathering. • Surface outcrops can yield biased data, if considered solely. • Soft & less resistant rocks may not outcrop at all. • All rocks can be obscured by thin sheets of younger sediments. • Deformation features such as folds, faults, fractures, shear zones must be identified. • These are frequently preferentially weathered & infilled by secondary materials. • Folds alter orientations of planes of weakness. • Weathering depths may vary considerably over different rock types - affect rock strengths.
Site Investigations • Planes of Weakness: • Discontinuities in rocks have > effect on rock properties than lithology. • Include - bedding planes, joints, foliations, cleavage, faults, etc. • All influence Foundation design. • Remember- compressive strength is > perpendicular to discontinuity than // to it. • All discontinuities evaluated for character, orientation, frequency, etc. • Data best determined on rock exposures- more difficult on core. • Trenches & costeans very useful. • Foundations: • Three main types- • Solid rock - rock strength & discontinuities identified. • Soil & solid rock at accessible depths - establish depth to Bedrock & as above.
Site Investigations • No Solid Rock - must place foundations in unconsolidated materials . Must also allow for rate of anticipated settling. • Dam Foundations: • Small dams for rural purposes- based on soil mechanics & sited in gully • Large Dams- must investigate underlying rocks of dam area: • Check rock strength is adequate to support water load; • Check for weakness planes & potential slippage; • Establish orientation of any weakness planes; • Depth of weathering- removal prior to dam construction; • Determine Durability of rock to water exposure; • Measure rock permeability; • Identify any seismic record of earthquake activity; • Dam type chosen based on availability of materials; • Establish risk of siltation reducing dam capacity, before construction.
Site Investigations • General Procedures: • Collect & assess all published accessible data on soils & rocks. • Conduct detailed geological study of project areas. • Use combination of drilling & geophysical surveys to complete geology, and confirm interpreted geology from surface outcrops. • Field test sub surface materials to determine engineering properties. • Detailed laboratory tests on sub surface materials to determine physical properties. • All data reviewed, assessed, & recommendations made on site suitability for project. • Continued investigations of sub surface materials while project constructed -confirms earlier interpretations or leads to modifications of plans/construction methods.
Site Investigations • Geophysical techniques: • Advantages- • Relatively low cost; • Obtain results quickly; • Can be undertaken in rough , inhospitable terrains by small teams; and • Can assist planning of expensive drilling programs. • Limitations- • Techniques all identify boundaries between two layers with appreciably different properties. Little contrast - poor definition of layers. • Requires confirmation by independent means. • Techniques- • Seismic reflection & refraction; • Electrical resistivity (ER); • Ground Penetrating Radar (GPR);
Site Investigations • Gravity & Magnetic Surveys, and • Downhole techniques. • Seismic Methods involve propagation of waves through earth materials • Electrical methods involve measurement of electrical properties of earth materials either- • measurement of natural earth currents, or • the resistance to induced electrical flow. • Natural earth current flow generated under geological conditions in which anode & cathode develop naturally. Measurement of strength & extent of current helps establish geologic conditions. • Electrical resistivity is resistance to electrical flow through earth materials. Current induced & resistivity measured- identifies basic property of earth material.
Site Investigations • Ground-penetrating Radar: • Essentially the same as Reflection Seismology: • Radar impulse is energy source & receiver used to detect reflections; • Strength of reflections depends on the electromagnetic properties; • Gravity & Magnetic Methods: • Deal with strength of the fields of gravity & magnetism generated between a mass of rock and the Earth; • Measure Earth’s 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. • Well 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.
Site Investigations • Seismic refraction: • Theoretical treatments of theory of Elasticity,& wavelength of seismic waves confirms that velocity of P waves > than S waves; • Also velocities of seismic waves dependent on rock density & Young’s modulus. Both increase with depth, so wave velocity also increases; • P waves behave like visible spectrum waves- obey Snell’s law; • Man-made seismic wave created& times of arrival of P waves sensed by regularly spaced geophones; • Both refracted & reflected events measured on same seismic signal; • Engineering relies on P wave( rock strength) • Cannot rip apart material whose seismic velocity exceeds 2,500 m/sec.(compacted sand with 40% porosity has P wave velocity = 1800 m/sec).
Site Investigations • Theory of refraction derived from behaviour of rays that bend on entering a different velocity medium; • The larger the velocity difference between two media, the larger the refraction; • By plotting theTime of arrival of rays versus distance of geophones, can establish critical distance, and thus the depth. • Seismic lines should be repeated in reverse order to enable dipping interfaces to be determined; • Seismic velocities can indicate rock type; • Used for depths of 30-60 metres; • Soils below ground water table can be distinguished from unsaturated soils above water table; • Porous, poorly cemented sandstones have low velocities, strongly cemented ones high velocities;
Site Investigations • Changes in jointing, dipping beds & cementation changes will affect seismic velocity; • Used for many years to predict ease of excavation of earth materials; • Limitation- geologic units must increase in velocity with depth to ensure that refracted ray can return to the Earth’s surface. • Seismic reflection: • provides a detailed picture of sub surface structure & interfaces; • depths determined by observing travel times of P waves generated near surface & reflected back from deep formations; • comparable to echo sounding of water depths. • Advantages- permits mapping of many horizons for each shot; • can determine depths to dipping interfaces, as well as angle of dip;
Site Investigations • Not used as much as refraction, but refraction will not work where a high velocity layer overlies a low one; • Reflection profiling in permafrost areas is not affected by the high velocity permafrost, whereas refraction techniques can be nullified completely. • Electrical Resistivity: • The range of resistivities( ) of rocks is enormous: • Amount of ground water& dissolved ions in rocks & water of great importance; • Dry rock has virtually no electrical conductance:
Site Investigations • Water-bearing rock resistivity is function of amount of groundwater present & salinity; • Resistivity of saturated fine-grained sedimentary rocks tends to be lower than coarse-grained sedimentary rocks because of greater porosity; • Gravels have more ground water recharge & less total dissolved solids (TDS) than fine-grained material such as colluvium or till; • Resistivity used to map overburden thickness, faults, fractures, specific rock units, etc; • Difficult to relate resistivity value directly to rock type. Fortunately show profound anomalies; • Therefore should be compared to drill log data; • In practice use Werner array- constant spacing, and moving whole array- This is resistivity profiling;
Site Investigations • Werner 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”; • Alternatively, “a” can be progressively increased thereby probing deeper and conducting Resistivity Sounding; • Ground Penetrating Radar: • Can identify sinkholes at depths of 25 m; • Subsurface anomalies were identified as voids in old earth-filled dam in Michigan -locations aided in a grouting program to fill voids; • Gaining rapid acceptance in environmental engineering applications; • Can be used for non-destructive testing of highways:
Site Investigations • Common application is detection of buried pipes & tanks; • Performance is affected by electrical properties of rocks; • Radar & acoustic pulse propagation depend on the material properties; • Disadvantages include limited depths in clay, but up to 15 m in sands. • Magnetic & Gravity Methods: • Rarely used in engineering geology; • Many new geophysical devices that utilise these properties of rocks; • Most commonly used in delineation of buried valleys or basin fills. • Well logging techniques: • Record generated by lowering probe into an uncased drill hole; • Spontaneous potential (SP), & resistivity logging are most common; • Also Gamma ray- all rocks & soils are naturally radioactive;
Site Investigations • Useful to confirm rock type boundaries; • Gamma ray logging indicates clay versus sand units in soils & clayey & shaly beds versus limestone or sandstone in bedrock. • Summary: • Refraction seismic & electrical resistivity are two most applicable techniques to engineering geology; • Best methods practicable for exploring shallow depths of the sub-surface to 30 m. • If there is a low velocity layer below a high velocity layer, refraction seismic will not work properly. • This occurs where sand & gravel layer lies below a clay layer; • Electrical resistivity method would work well here; • Refraction seismic works well where soil overlies bedrock.