Presentation on theme: "Site Investigations Associate Professor John Worden DEC"— Presentation transcript:
1Site Investigations Associate Professor John Worden DEC University of Southern Queensland
2Site 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 conditionsRock 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.
3Site 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, andPermission to access property.If necessary, drill site to assess parameters.All factors influence site selection/ rejection.
4Site 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 characteristicsDisturbed soils have lost soil structure- limited useUndisturbed soils used for wet/dry density, triaxial shear, & compression, permeability, consolidation testsSoil samples change on exposure to air- extended exposure reduces their suitability for testing.In-situ measurements give superior physical determinations.
5Site Investigations Rock properties: 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 ,etcCore 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.
6Site 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.
7Site Investigations Planes of Weakness: Foundations: 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.
8Site Investigations Dam Foundations: 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 gullyLarge 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.
9Site 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.
10Site Investigations Geophysical techniques: Advantages- Limitations- Relatively low cost;Obtain results quickly;Can be undertaken in rough , inhospitable terrains by small teams; andCan 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);
11Site InvestigationsGravity & Magnetic Surveys, andDownhole techniques.Seismic Methods involve propagation of waves through earth materialsElectrical methods involve measurement of electrical properties of earth materials either-measurement of natural earth currents, orthe 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.
12Site 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.
13Site 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).
14Site InvestigationsTheory 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 the Time 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 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;
15Site Investigations Seismic reflection: 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;
16Site Investigations Electrical Resistivity: 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:
17Site InvestigationsWater-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;
18Site Investigations Ground Penetrating Radar: 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:
19Site Investigations Magnetic & Gravity Methods: 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;
20Site Investigations Summary: 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.