WEEK 9 ACTIVITY. Lecture (3 hours) Self Assessment. 34 slides

Transcription

1 WEEK 9 ACTIVITY Lecture (3 hours) 34 slides Self Assessment Site Investigation (ECG513) ARM

2 LEARNING OUTCOMES Week 9 : (3H) Coverage : Geophysical Methods, Permeability and Ground Stress measurement. Learning outcomes: At the end of this lecture/week the students would be able to: discuss different in situ tests : geophysical methods, field permeability and pore pressure measurements and ground stress measurements Explain the usage of the respective tests and the associate parameters obtained. Site Investigation (ECG513) ARM

3 IN SITU TESTING & ANALYSIS OUTLINE of PRESENTATION 4.2 Geophysical Methods of Ground Investigation 4.3 Permeability and Pore water Pressure 4.4 In situ ground stress measurement Site Investigation (ECG513) ARM

4 IN SITU TESTING & ANALYSIS 4.2 Geophysical Methods for Ground Investigation Involve the techniques of determining underground materials by measuring some physical property of the material and, through some correlations, using the obtained values for identifications. Most methods determine conditions over a sizable distance. The methods do not actually measure engineering properties. Several types can be utilised, namely: Seismic refraction method Electrical resistivity method Ground-penetrating radar Site Investigation (ECG513) ARM

5 IN SITU TESTING & ANALYSIS Seismic Refraction Method Based on the seismic waves travelling through the surrounding soil and rock at speeds relating to the density and bonding characteristics of the material. The velocity of the seismic waves passing through subsurface soil or rock materials is determined, and the magnitude of the velocity is then utilised to identify the material. Site Investigation (ECG513) ARM

6 Seismic Refraction Method The mechanical properties of the rocks through which the seismic waves travel quickly organize the waves into two types. Compressional waves, also known as primary or P-waves, travel fastest, at speeds between 1.5 and 8 kilometers per second in the Earth's crust. Shear waves, also known as secondary or S- waves, travel more slowly, usually at 60% to 70% of the speed of P-waves.

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8 IN SITU TESTING & ANALYSIS Representative Seismic values Soil unconsolidated material m/sec Most unconsolidated materials Below 900 Soil normal - hard-packed Water 1500 Loose sand above water table - below water table Loose mixed sand and gravel, wet Loose gravel, wet Hard clay Site Investigation (ECG513) ARM

9 Representative Seismic values Rock consolidated material m/sec Most hard rocks Above 2400 Shale soft - hard Sandstone soft - hard Limestone weathered - hard 1200? Basalt Granite and unweathered gneiss Compacted glacial tills, hardpan, cemented gravels Frozen soil Pure ice Site Investigation (ECG513) ARM

10 IN SITU TESTING & ANALYSIS Electrical Resistivity Method Resistivity is a property possessed by all materials. The method for determining subsurface conditions utilizes the knowledge that in soil and rock materials, the resistance values differ sufficiently to permit that property to be used for identification purposes Site Investigation (ECG513) ARM

11 IN SITU TESTING & ANALYSIS Electrical Resistivity Method cont. Two different field procedures are used : Electrical profiling used for establishing boundaries between different materials and has practical application in prospecting for sand and gravel deposits or ore deposits. Electrical sounding used to provide information on the variation of subsurface conditions with depth and has practical application in indicating layered conditions and approximate thicknesses. Site Investigation (ECG513) ARM

12 Concept on electrical resistivity Ohm s Law, V=IR, R=V/I The geometrically independent quantity is called resistivity. I Ammet er R Batte ry Resistivity is a fundamental parameter of a material and describes how easily a wire or the material can transmit an electrical current. Resistance is a characteristic of a particular path of an electrical current whereas resistivity is a physical property of a material. Resistance = R Length = L Area=A RA L

13 Electrical Resistivity Method Significant in investigating subsurface profile Image need to be interpreted Geo-material index required

14 FIELD MEASUREMENT Electrical Resistivity on Marine Clay Deposit. 2.3 m Filled Material Black soil Grey soil

15 JPS ground water survey for projek tanaman cili Diraja Kelantan

16 Representative Resistivity values Types of Materials Resistivity (ohm-ft) Wet-to-moist clayey soils 5-10 Wet-to-moist silty clay and silty soils Wet-to-moist silty and sandy soils Well-fractured to slightly fractured bedrock with moist soil filled cracks Sand and gravel with silt 1000 Slightly fractured bedrock with dry soilfilled cracks; sand and gravel with layers of silt Massive bedded and hard bedrock; coarse dry sand and gravel deposits Site Investigation (ECG513) ARM

17 IN SITU TESTING & ANALYSIS Ground-penetrating Radar Also identified as ground-probing radar. Capable of defining the shallow zones of soil and rock materials that underlie an area. The method relies on the penetration and reflection of high frequency radio waves. Site Investigation (ECG513) ARM

18 FLOW OF WATER IN SOILS Field Permeability 4.3 Permeability & Pore Water Pressure Field permeability test Pore Pressure measurement

19 FLOW OF WATER IN SOILS Field Permeability Field determination of permeability Pumping out test confined aquifer unconfined aquifer

20 FLOW OF WATER IN SOILS Field Permeability Confined Aquifer Pumping out test Confined aquifer Site Investigation (ECG513) ARM 2008

21 FLOW OF WATER IN SOILS Field Permeability confined aquifer Pumping test in a confined aquifer In a confined aquifer, the pumping rate must not be high enough to reduce the lever in the pumping well below the top of the aquifer. In a steady state conditions, the flow is considered through an elemental cylinder having radius r, thickness dr and height h. Hydraulic gradient (outside to inside) Area through which flow takes place, dh i dr A 2rD Starting with Darcy s equation : q Aki dh 2rDk d r Site Investigation (ECG513) ARM 2008

22 FLOW OF WATER IN SOILS Field Permeability confined aquifer or dr r 2 Dkdh q Integrating 2Dk ln( r2 / r1 ) ( h2 - h1 ) q Giving k q ln( r2 / r1 ) ( h - h ) 2D 2 1 or k 2.3q 2D log ( h 10 2 ( r2 / r - h ) 1 1 ) Site Investigation (ECG513) ARM 2008

23 FLOW OF WATER IN SOILS Field Permeability unconfined aquifer Pumping out test Unconfined aquifer Site Investigation (ECG513) ARM 2008

24 FLOW OF WATER IN SOILS Field Permeability unconfined aquifer Pumping test in an unconfined aquifer An unconfined aquifer is a free-draining surface layer underlain by an impervious base. Under conditions of steady state pumping the hydraulic gradient at a given radius is assumed to be constant in a homogeneous medium. Consider an inflow through an elemental cylinder having radius r, thickness dr and height h. Hydraulic gradient (outside to inside) Area through which flow takes place, Starting with Darcy s equation : i q dh dr A 2rh Aki dh 2rhk d r Site Investigation (ECG513) ARM 2008

25 FLOW OF WATER IN SOILS Field Permeability unconfined aquifer Integrating Giving or h kh q r r d 2 d ) - ( ) / ln( h h k q r r ) - ( ) / ln( h h r r q k ) - ( ) / ( log h h r r q k or Site Investigation (ECG513) ARM 2008

26 IN SITU TESTING & ANALYSIS Standpipe and Piezometer Standpipe/open tube is the simplest form of a piezometer used to measure the pore water pressure. Piezometer is inserted into the soil to monitor water level which would indicate the pore water pressure in the soil especially during construction process. However this equipment is limited only in soils of relatively high permeability. For low permeability soils more sophisticated variants need to be used to measure the pore pressure development. Site Investigation (ECG513) ARM 2008

27 IN SITU TESTING & ANALYSIS Piezometer variants are : Pneumatic piezometer is a flexible diaphragm in the barrel that separates the end section open to the groundwater from an interior pressurized chamber. Groudwater pressure (i.e. soil pore water pressure) exerted on one side of the diaphragm is determined as the equalizing pressure required from a compressed gas supplied to the pressure chamber section from the surface tank that is connected through the monitor control unit. Site Investigation (ECG513) ARM 2008

28 IN SITU TESTING & ANALYSIS Piezometer variants cont. The vibrating wire piezometer is based on the vibrating wire technology that relates the tensile force or strain in a taut wire to the natural vibrating (or resonant) frequency. Pore water pressure against the diaphragm causes deflection, resulting in a change to the wire length and the resonant frequency vibration. Site Investigation (ECG513) ARM 2008

29 IN SITU TESTING & ANALYSIS Piezometer variants cont. The fibre-optic piezometer uses an optical fibre within a sealed chamber of the barrel to measure the distance across a closed cavity space created between the fibre optic housing and a diaphragm in the barrel. The cavity space will change as pore pressure causes the diaphragm to deflect; the cavity separation distance measurement is relayed to a surface readout unit and translated to a value of pore water pressure. Site Investigation (ECG513) ARM 2008

30 FLOW OF WATER IN SOILS Field Permeability 4.4 In situ Ground Stress measurement Pressuremeter test Flat Plate Dilatometer (DMT)

31 IN SITU TESTING & ANALYSIS Pressuremeter test Site Investigation (ECG513) ARM

32 SITE INVESTIGATION - 2 Menard Pressuremeter Site Investigation (ECG513) ARM

33 Menard Pressuremeter Site Investigation (ECG513) ARM

34 IN SITU TESTING & ANALYSIS Flat Plate Dilatometer (DMT) Consists of tapered blade 95 mm wide and 15 mm thick and 240 mm long. On the flat face the dilatometer is a flexible steel membrane 60 mm in diameter that when inflated pushes the soil laterally. Tests are normally conducted every 200 mm. Results from the test have been related to undrained shear strength, lateral earth pressures, overconsolidation ratios and elastic modulus. Simple and quick to conduct. Provides reasonable estimates of the horizontal stress and is less costly than the pressuremeter test. Site Investigation (ECG513) ARM