Matakuliah: S2094 / Rekayasa Pondasi

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Pertemuan 03Penyelidikan Tanah

• Matakuliah S2094 / Rekayasa Pondasi
• Tahun 2005
• Versi 1.1

• Media referensi video DrillingAndSampling.wmv
  minicone.wmv

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Site Exploration and Characterization

• Subsurface material properties cannot be
  specified they must be deduced through
  exploration.
• Charles Dowding (1979)

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Objectives

• 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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Site Exploration General Strategy

• Project Assessment
• Literature Search
• Field Reconnaissance
• Subsurface Drilling and Sampling
• Laboratory Testing of Soil Samples

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Project Assessment

• Type, location and approximate dimensions of the
  proposed development
• Existing topography and any proposed grading
• Any previous developments

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Literature Search

• Geologic Maps
• Soil Survey Reports
• Geotechnical Reports
• Historic Groundwater Data

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• Geologic Maps

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Remote Sensing

• Conventional Aerial Photographs
• Geologic features (landslides, faults),
  topography, drainage patterns
• Site history
• Infrared Aerial Photographs
• Springs, seepage zones
• Useful in slope stability investigations

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Field Reconnaissance

• Any previous developments, grading etc.
• Site topography, any signs of slope instability
  (landslides, soil creep)
• Site drainage conditions
• Rock outcrops
• Site access

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Field Exploration

• Site Boring Layout
• Test Borings or Test Pits?
• It depends on the type of materials, and what you
  want to know.
• Number and Frequency of Borings
• Depth, Sampling Methods and Field Testing

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Field Exploration
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III. Subsurface Exploration/Sampling

• Borehole Spacing
• Site conditions/uniformity
• type of structure (bridge, building, landfill)
• typically one borehole/2500 ft2
• Also see Table 4.1 (p.108)
• Borehole Depths
• Magnitude of loading/soil conditions
• Also see Table 4.3 (p.109)

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How Many Borings?
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How Deep?
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Look Up and Live!

• Safety Awareness
• Regular Emphasis

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Drilling and Sampling of Soils

• Subsurface Drilling
• Auger Drilling
• Solid Stem Auger
• Hollow Stem Auger
• Rotary Drilling
• Bucket Auger
• Percussion (or Cable Tool) Drilling

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Auger Drilling
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Auger Drilling

• Hollow Stem Auger
• Casing with outer spiral
• Inner rod with plug/or pilot assembly
• For sampling, remove pilot assembly and insert
  sampler
• Typically 5ft sections, keyed, box pin
  connections
• Maximum depth 60-150ft

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Hollow-Stem Augers
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Air or Mud Rotary Drilling

• http//www.redi-drill.com/ms-index.htm

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Rotary Drilling

• Bit at the end of drill rod rotated and advanced
• Soil/rock cuttings removed by circulating
  drilling fluid
• Common drilling fluid bentonite in water with
  slurry density 68-72pcf
• Air may be used as drilling fluid

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Cable Tool Drilling

• Not common in US in geotechnical explorations
• Heavy impact drilling tools lifted and dropped
• Impact loosens soil and rock
• Cuttings removed with a bailer
• Slow process Used in environmental explorations
  where drilling fluid is not permitted

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Rock Drilling, Coring, Augering

• http//www.globaldrilsup.com/cat-index.html

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Soil and Rock Sampling

• Disturbed samples
• In-place structure is not preserved
• Okay for determination of soil index properties
• Estimating the nature of the formation from the
  cuttings is like identifying the cow from the
  hamburgers.
• G.F. Sowers

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Soil and Rock Sampling

• Undisturbed samples
• Minimizes effects from potential disturbance
• Needed for determination of in-situ density,
  in-situ permeability, soil shear strength and
  compressibility

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Soil Samplers

• Standard Split Spoon Samplers
• Shelby Tube Samplers
• Piston Samplers
• Heavy Wall Samplers

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Standard Split Spoon Samplers

• Thick wall (0.25in) cylinder
• Sampling tube is split along the length
• Hammered into the ground

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Shelby Tube (Thin-wall) Samplers

• Thin wall (1/16in) sampling tube
• Sampler pushed into the ground hydraulically
• Sample extruded from tube

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Piston Samplers

• Minimizes sample disturbance caused by
  back-pressure

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Heavy-Wall Samplers

• Thicker walls provide better strength
  durability
• However, it creates more disturbance
• Sampler pounded into the ground

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Groundwater Monitoring

• Groundwater level must be determined during
  geotechnical investigation
• Can be accomplished by leaving selected soil
  borings open

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In-situ Testing

• When it is difficult to obtain undisturbed
  samples
• Cohesionless soils, Sensitive clays
• In-situ Test Methods
• Standard Penetration Test (SPT)
• Cone Penetration Test (CPT)
• Pressuremeter Test
• Flat Plate Dilatometer Test

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Standard Penetration Test (SPT)

• 140 lb (63.5 kg) Hammer
• 30in (76 cm) free fall
• Drive sampler over 18 inches
• Record no. of blows per each 6 inch penetration
• SPT blow countblows for 2nd 6 inch penetration
  blows for 3rd 6inch penetration

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Standard Penetration Test (SPT)
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Standard Penetration Test (SPT)
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Types of SPT Hammers
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Corrections to SPT blow Counts

• Factors affecting SPT blow count
• Hammer Efficiency (See Table 4.3)
• Borehole diameter (See Table 4.4)
• Type of sampler (See Table 4.4)
• Rod length (See Table 4.4)

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SPT Correction Factors
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SPT Overburden Correction
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Use of SPT Data

• To Determine Relative Density, Dr
• From AASHTO Chart
• From Eq. (4.3) p.122
• To determine ?
• From Figure 4.11 (p.123)
• To determine C
• From AASHTO Chart