Download - Basic Geotechnical Engineering
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Geotechnical Engineering Geotechnical Engineering deals with the
application of Civil Engineering Technology to some aspects of earth
Geotechnical Commision of Swedish State Railways (1914-1922) was the first to use the word Geotechnical in the sense that we know it today: the combination of Civil Engineering technology and Geology
Geotechnical Engineering deals with; Design of Foundation Stability of Slopes and Cuts Design of Earth Structures Design of Roads and Airfield
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Soil Descriptions Soil is an unconsolidated agglomerate of
minerals with or without organic matter found at or near the surface of the earth crust, with which or upon which civil engineers build their structures
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Soil Formation Mechanical Weathering (Physical
Weathering Chemical Weathering Oxidation Carbonation Hydration Desilication Leaching
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WeatheringPhysical processes of weathering
Unloading○ e.g. uplift, erosion, or change in fluid pressure.
Thermal expansion and contractionAlternate wetting and dryingCrystal growth, including frost actionOrganic activity
○e.g. the growth of plant roots.
Chemical Process of weatheringHydrolysis
○ is the reaction with water○will not continue in the static water.○involves solubility of silica and alumina
Chelation○Involves the complexing and removal of metal ions .
Cation exchange ○ is important to the formation of clay minerals
Oxidation and reduction.Carbonation
○is the combination of carbonate ions such as the reaction with CO2
Factors affect weatheringMany factors can affect the weathering process such as climate, topography, features of parent rocks, biological reactions, and others.
Climate determines the amount of water and the temperature.
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Transportation of Weathering Products
Residual soils- to remain at the original place
Engineering properties of residual soils are different with those of transported soils
The knowledge of "classical" geotechnical engineering is mostly based on behavior of transported soils. The understanding of residual soils is insufficient in general.
Transported soils-
to be moved and deposited to other places.
The particle sizes of transported soils are selected by the transportation agents such as streams, wind, etc.
The transported soils can be categorize based on the mode of transportation and deposition (six types).
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Transported Soils (Cont.)(1) Glacial soils: formed by transportation and deposition
of glaciers.(2) Alluvial soils: transported by running water and
deposited along streams.(3) Lacustrine soils: formed by deposition in quiet lakes
(e.g. soils in Taipei basin).(4) Marine soils: formed by deposition in the seas (Hong
Kong).(5) Aeolian soils: transported and deposited by the wind
(e.g. soils in the loess plateau, China).(6) Colluvial soils: formed by movement of soil from its
original place by gravity, such as during landslide
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Uses of Soil As a supporting material to bear the loads
of structures resting on earth As a raw constructional material for
construction of earth structures (Dams, levees, roads)
As a processed material (Burnt bricks, concrete mix etc.)
In Pottery (Kaolinite) Kaolinite is also used in paper paint and
pharmaceuticals Bentonite is used in drilling
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Principle Soil Types Loam Silt Mud Caliche Marl Hardpan Peat Clays Drift Shale Black Cotton Soil
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Three Volumetric Ratios (1) Void ratio e (given in decimal, 0.65)
(2) Porosity n (given in percent 100%, 65%)
(3) Degree of Saturation S (given in percent 100%, 65%)
)V(solidsofVolume
)V(voidsofVolumee
s
v
)V(samplesoilofvolumeTotal
)V(voidsofVolumen
t
v
%100)V(voidsofvolumeTotal
)V(watercontainsvoidsofvolumeTotalS
v
w
e1
e
)e1(V
eVn
s
s
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Typical Values of Specific Gravity
Material Gs
Quartz sand
Cacareous sand
Clay minerals
2.65
2.64 – 2.71
2.67 – 2.73
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Types of Soil in Pakistan
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The Objectives of Soil Investigations Identify & describe pertinent surface conditions
Determine location and thickness of soil and rock strata (subsurface soil profile)
Determine location of groundwater table
Recover samples for laboratory testing
Conduct lab and/or field testing
Identify special problems and concerns
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Desk Study To determine the available information and become familiar with
the project Geological maps, topographical maps and other records from
Library Remote sensing data, Satellite imageries, Aerial Photographs Local authorities, statutory bodies, the geological survey, mining
companies Experience of local contractors Maps are used to identify natural features such as streams,
springs, scraps, landslides and vegetation etc. Maps are also used to identify man made surface features such as
fences, power lines, roads and buildings
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Site Reconnaissance
To confirm the findings of the desk study Current maps should be marked with site boundaries
and major structures Visually check the ground conditions Look for settlement cracks on existing structures Record the location of transmission lines, trees,
manholes or any other possible obstruction High water marks on old buildings, bridge abutments
etc. Interviews of the local people
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Preliminary Design Data
Soil exploration and preliminary design should be started about the same time
For buildingsType, Size and height, depth of basementApproximate arrangement of columns and bearing
wallsApproximate range of column and wall loads
For bridgesType and length of bridge spanApproximate loads on piers and abutments
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Steps in Soil Exploration
Boring Sampling Testing
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Common Types of Boring
Test (Trial) Pits Auger Percussion Drill Rotary Drill
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Test Pits
For shallow visual investigations and to take sample from top few meters of soil
Economical and rapid Excavation with hand tools, backhoe, bull dozer or
caisson drilling equipment 1 to 3 m deep, can be extended to 6m Retaining structure my be used if collapse is
expected Unsuitable for granular soil and below water level Some times trenches are dug
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Auger
To drill holes to a shallow depth
(up to 6m) Operated by hand or by power For disturbed or undisturbed
samples Up to 200 mm dia hole Difficult in water bearing sand and
gravels
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Percussion Drill
Common method for advancing test holes in all soils and weak rocks for shallow and deep exploration
Holes advanced by jack hammer and chopping bits 150 to 300 mm dia holes can be drilled from 1 to 80 m,
most common depth is up to 30 m Shells, bailers, clay cutters and chisels are boring tools Hole is advanced by repeatedly raising and dropping
the boring tool Casing may be required in unstable soils and deep
holes
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Rotary Drilling
To drill holes in almost all ground conditions for sampling and installation of instruments
75 mm to 150mm dia boreholes can be drilled from 1 to 250 mm (most common depth 1 to 30 m)
Hole is advanced either by rotating bit or downhole hammer
Cutting is removed by the flushing medium which is pumped down to the drill bit with a pump
Casing is usually required in unstable soils and deep holes
Common drilling fluid; bentonite in water with slurry density 68-72pcf
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Types of Bits
Core bits (for sample recovery)
Diamond or tungsten Drag (to create open holes) Rock rollers Percussion bits
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Flushing Medium
Purpose is to remove particles and to cool the drill bits Air flush (operates at 1000 m3/min) Water flush (operates at 24-50 m3/min) Mud flush include bentonite and polymer based muds Mist and foam flushes (mixture of air, water and mud)
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Sampling
To obtain specimens for determining the physical, chemical and mechanical characteristics of the soil in laboratory
Samples can be either jar, bag, tube, continuous, core or block
Ideal sampler must be thin walled (less area ratio), have smaller taper angle and smooth walls
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Classification of SamplesClass Properties Symbols Technique
1 Classification
Strength
Deformation
Permeability
w, γ, SG, PI, PSD
C, Φ, Cu
E, G, mv
K, Cv
Pushed thin wall sampler
Some thick wall samplers
Some core barrels
2 Classification w, γ, SG, PI, PSD
Thin and thick wall samplers and core barrels
3 Classification W, PI, PSD Clay cutter or auger (dry)
4 Classification PSD, PI Clay cutter or auger (wet)
5 None Cutting and flushing return
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Sample Application
Type Size Installation Class Ground
Jar Small Cuttings 5 All
Bag Large Shell, Core 3,4,5 Soil
U100 100 mm dia
0.5 m long
Pushed
jacked
1,2 Clay
Piston 75 – 200 mm dia
0.5 – 1 m long
Jacked 1,2 Clay
Tube 75 – 100 mm dia
0.5 – 1 m long
Jacked 1,2 Clay
Core NX – SX Drilled 2 All
Block Up to 1 m Cut 1 All
Continuous 50 – 100 mm dia Jacked 2 Soil
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Sample Disturbance
Should be minimum so that the parameters obtained should be representative
Caused during drilling due to stress relief, swelling, compaction, piping and collapse of the borehole
Caused during sampling by stress relief, remoulding, compaction, segregation, fracture, loss, friction
Caused during transport and storage
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Minimizing the Sample Disturbance
SwellingBy minimizing the time of sampling
CompactionBy keeping the casing above the base of borehole
PipingBy maintaining a water balance
CollapseBy using casing or mud or by keeping the hole dry
DistortionBy using smooth clean tubes and thin walled sampler
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Common Sampling Methods
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Types of Samples
Disturbed Samples
Disturbed samples are those obtained using equipment that destroy the macro structure of the soil but do not alter its mineralogical composition. For determining grain size, Atterberg limits, and compaction characteristics of soils disturbed samples are taken
“…Estimating the nature of the formation from the cuttings is like identifying the cow from the hamburgers.” G.F. Sowers
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Types of Samples (Continued)
Undisturbed samples
Undisturbed samples are obtained in clay soil strata for use in laboratory testing to determine the engineering properties of those soils. Undisturbed samples of granular soils can be obtained, but often specialized procedures are required such as freezing or resin impregnation and block or core type sampling Specimens obtained by undisturbed sampling methods are used to determine the strength, stratification, permeability, density, consolidation, dynamic properties, and other engineering characteristics of soils
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Testing
Laboratory Testing Field Testing
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Laboratory Testing
Property of Soil Type of Test
Grain size distribution Mechanical analysis
Consistency Liquid limit (LL)
Plastic limit (PL)
Plasticity Index (PI)
Unit weight Specific gravity
Moisture Natural moisture content
Field moisture equivalent
Compressibility Consolidation
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Laboratory Testing (Continued)
Properties of Soil Type of Test
Shear Strength Cohesive soils Non cohesive soils General
Unconfined compressionDirect ShearTriaxial
Volume Change Shrinkage factorsVolume changeExpansion pressure
Permeability Constant headFalling head
Compaction characteristics Standard proctorModified proctor
California bearing ratio CBR
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Field Testing
Purpose of Test Type of Test
Compaction Control Moisture density relation
In-place density
Shear strength (soft clay) Vane test
Relative density (granular soil) Penetration test
Permeability Pumping test
Bearing capacity Pavement
Footings Piles
CBRPlate bearingPlate bearingLoad test
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Standard Penetration Test (SPT)
SPT are used for most of the soil types A cheap, simple and rapid test Most suitable for granular soils A split spoon sampler is used which is an open
ended steel cylinder splitting longitudinally Most common size of sampler are
I.D = 1.4 – 1.5 inO.D = 2.0 inLength = 2 ft
Hammer wt. = 140 lbs Free drop = 30 in
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Standard Penetration Test (SPT) (continued)
Penetration resistance is reported in no. of blows per feet
The spoon is withdrawn and a representative sample is secured
Samples are kept in airtight jars
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Correlations of SPTRelative Density of Sand
N (blows/ft) Relative Density
0-4 Very Loose
4-10 Loose
10-30 Medium
30-50 Dense
>50 Very Dense
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Correlations of SPTStrength of Clay
N (blows/ft)
Unconfined Compressive Strength (tons/ft2)
Consistency
<2 <0.25 Very soft
2 - 4 0.25 - 0.50 Soft
4 - 8 0.50 - 1.00 Medium
8 - 15 1.00 - 2.00 Stiff
15 - 30 2.00 - 4.00 Very Stiff
>30 >4.00 Hard
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Cone Penetration Test (CPT)
A cone is connected with standard rods and an electric cable
The cone is pushed continuously into the ground and automatic measurements are taken of the force on the tip and sleeves
Depth is also recorded Cone resistance qc and
friction resistance fs are
used together with charts
to obtain soil parameters
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Cone Penetration Test (CPT)(continued)
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Cone Penetration Test (CPT)(continued) Rod dimensions are 36 mm dia, 600 mm long Cone has 600 angle, surface area = 10 cm2
Mostly used for fine to medium grained soils
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Types of Pressuremeter
Pre-bored Pressuremeter (PBP)Menard Pressuremeter (MPM)Elastometer (OYO)High Pressure Dilatometer (HPD)
Self-bored Pressuremeter (SBP)Cambridge self-boring pressuremeter (CSBP)Pressuremeter AutoforuerPushed-in PressuremeterCone PressuremeterStress Probe
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Pressuremeters
Pressuremeter are probes that are installed into the ground below the borehole
An expanding section is inflated and the displacement of the expanding section and the pressure required to cause that displacement are measured
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Dilatometers•The flat plate dilatometer (DMT) is an in-situ test device with specific advantages that can useful on certain projects. However, it is not yet used as frequently as the cone penetration test (CPT) in engineering practice.
•A 60-mm diameter circular membrane on the flat face (95 mm wide by 220 mm long). This membrane is expanded after the DMT has been pushed to the desired depth.
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Other Penetration Tests
DPT (Dynamic Probe) Mackintosh Probe Static Probes
Friction Cone (Dutch Cone)
Piezocone (CPTU)
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Geophysical Tests
Ground Penetrating Radar Frequency Domain Electromagnetics Time Domain ElectromagneticsVery Low Frequency EMResistivity Spontaneous PotentialSeismic RefractionSeismic ReflectionMagneticGravityThermalRadioactiveMetal Detectors
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Vane Shear Test
Consists of a Vane connected to the surface with rods. The vane is turned by a torque
Used in soft to firm clays Rotation is used to measure the sensitivity of clay A rapid and simple method
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Vane Shear Test (continued)
T = cЛ[d2h/2 + d3/6]
where
T = Torque required to shear
the soil
c = cohesion of clay
d = dia
h = height of vane
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Ground Water Measurement
In sand or gravels
- Clean the hole by horizontal jets
- Use steel tape coated with chalk
In silt or silty sand
- Can be measured in several days
In clays
- Piezometer is used
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How Many Borings & How Deep?
“No hard-and-fast rule exists for determining the number of borings or the depth to which borings are to be advanced.”
But guidelines exist in –
• Textbooks
• Design manuals
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How Many Borings? Conventional Wisdom
The number (density) of borings will increase:○ As soil variability increases○ As the loads increase○ For more critical/significant structures
Rules of Thumb:Soft soils, critical structures – 50'Soft Soils - Space 100' to 200'As soils become harder, spacing may be
increased up to 500’
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No. of Borings
Source: Sowers 1979
Structure or Project
Subsurface Variability
Spacing of Borings (ft)
Highway Subgrade
Irregular 100-1000 (200, typical)
Average 200-2000 (500, typical)
Uniform 400-4000 (1000, typical)
Multistory Building
Irregular 25-75
Average 50-150
Uniform 100-300
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No. Of Borings (continued)
Project
Distance Between Borings (ft.)
Horizontal stratification of soil
Uniform Average Erratic
Min. No. of Borings
1 or 2 story building200 100 50 3
Multi-story buildings 150 100 50 4
Bridge,Pier,Abutment 100 25 1 – 2
Highways 1000 500 100
Borrow pits 1000-500 500-200 100-50
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No. Of Borings (continued)
Area (acre) Total No. of Borings Deep Boring
< 10 4 1
10 – 49 8 2
50 – 99 14 4
100 – 200 20 5
> 20024 plus 1 boring each additional 10 acre
6 plus 1 boring each additional 10 acre
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No. Of Borings (continued)
Soil ConditionFoundation Footprint Area / borehole
(m2) (ft2)
Poor 100 – 300 1000 – 3000
Average 200 – 400 2000 – 4000
Good 300 – 1000 3000 – 10000
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No. Of Borings
Project Spacing (m)
One story house 25 – 30
Multistory buildings 15 – 25
Highways/Railways 250 – 500
Earth Dams 25 – 50
Residential sub-division 60 - 100
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Depth of Boring
Highway and airfieldMin. depth 5 ftShould extend below organic soil, muck, fill, or
compressible layers Retaining walls
Deeper than possible surface of slidingDeeper than width of base of wall
Embankment and cutsDeeper than possible surface of slidingEqual to the width at bottom of cuts
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How Deep (Bridges)?
Boring depth is governed by various factors, including:Foundation typeFoundation loadLowering of grade line at underpass?Channel relocation, widening, dredging?Scour?
Rules of Thumb Generally speaking, 50’- 80’ is reasonableLocal experience is helpfulLook at nearby structures if availableIf no experience or other info available, plan for long
first hole, then adjust.
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How Deep (Retaining Walls)?
Boring depth is governed by various factors, including:Wall type (Fill vs. Cut)Lowering of grade line at wall?Scour?
Rules of Thumb (TxDOT):Fill Walls: Depth = Wall Height +/-Soil Nailed Walls: Depth = Through Nailed Area,
plus 10’Drilled Shaft Walls: Depth = Exposed Wall Height plus
150% of Wall Height
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Depth of Boring (continued)
Soil Condition Minimum Depth of Boring (m)
Poor 6 x S0.7 + Df
Average5 x S0.7 + Df
Good3 x S0.7 + Df
S = No. of stories
Df = Tentative depth of footing (m)
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Depth of Boring (continued)
For embankments1.5 to 2.0 times the height of embankmentShould be sufficient to check possible shear failure
and probable settlement For dams
Should be such to explore all starta through which piping and seepage may occur
Some borings up to rock bed with minimum 3 m into the rock
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Depth of Boring (continued)
For roads and airfieldsUp to depth of 2 – 3 mFor embankment roads up to depth = 1.5 – 2.0 x
height of embankment + 2 – 3 ft. For Pipelines
1 – 2 m below invert level For structural foundation
1.5 x width or up to good soil10 % of the contact pressure
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Rocks Rocks are described from observation of natural
outcrops, quarries, cuttings, excavations and rock cores.
Description of rock material for engineering purposes generally follow the sequence below:
Colour Grain Size Texture and Structure State of Weathering Rock Name Strength Other Characteristics or Parameters
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Types of Rocks
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Spacing of Discontinuities in Rocks
Spacing Structural Features Discontinuities in One Dimension
Discontinuities in Three Dimensions
>2m
600mm-2m
200-600mm
60-200mm
20-60mm
6-20mm
<6mm
Very thick
Thick
Medium
Thin
Very thin
Thickly laminated
Thinly laminated
Very widely spaced
Widely spaced
Medium spaced
Closely spaced
Very closely spaced
Extremely closely spaced
Very large
Large
Medium
Small
Very small
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Compressive Strength of Rock Material
Compressive Strength (MN/m2) Description
<1.25
1.25 – 5.0
5.0 – 12.5
12.5 – 50.0
50 – 100
100 – 200
>200
Very weak
Weak
Moderately weak
Moderately strong
Strong
Very strong
Extremely strong
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Rock Mass Classification
Terzaghi’s descriptionIntact rockStratified rockModerately jointed rockBlock and seamy rockCrushed but chemically intact rockSqueezing rockSwelling rock
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Rock CoringDouble-tube core barrel is typicalDiamond or tungsten-carbide tooth bitSize of core samples varies (NX, NQ, HQ,
etc.)
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Rock Quality Designation (RQD)
RQD = ΣLength of core pieces>10 cm long X 100
The core should be at least NW size (2.15 in dia) Should be drilled with double-tube core barrel
Total length of core run
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Rock Quality Designation (RQD)(continued)
RQD (%) Rock Quality
< 25 Very poor
25 – 50 Poor
50 – 75 Fair
75 – 90 Good
90 - 100 Excellent
RQD = 115 – 3.3 Jv
Where Jv is sum of the No. of joints per unit length of all joints (Volumetric joint count)
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Plotting of RQD Values
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QUESTIONS