CEGR 40905090 Soil Improvement in Geotechnical and Geoenvironmental Engineering

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CEGR 4090/5090Soil Improvement in Geotechnical
and Geoenvironmental Engineering
Instructor Dr. John Daniels, P.E. Class
Time Tuesdays, 530 820 PM Class Location
Friday 005/010
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Tonights Lecture

• Introduction
• Difficult soils
• Site Investigation

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Common Applications

• Foundations
• Embankments
• Excavation
• Retaining Walls
• Earth Dams
• Geoenvironmental
• Cutoff walls, landfill covers, waste material
  utilization

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• What are the critical properties we try to change?

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Critical Properties

• Shear strength
• Volume change
• Hydraulic conductivity

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What if available soils are difficult

• Select new site
• Replace soils
• By pass soil altogether with deep foundation
• Design structure accordingly
• Soil improvement

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What if available soils are difficult

• Select new site
• Replace soils
• By pass soil altogether with deep foundation
• Design structure accordingly
• Soil improvement

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Difficult Soils or Conditions

• Compressible/soft soils
• Collapsible soils
• Expansive soils
• Frost-susceptible soils
• Liquefaction potential
• Karst geology
• Specification

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Compressible/soft soils

• Excessive Settlement
• Clay soils
• Organic soils
• Loose sand deposits
• High moisture content

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Compressible/soft soils
Source Coduto, 1999
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Need for Soil ImprovementLeaning Tower of Pisa
Soft Clay
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Collapsible soils

• Sudden Volume Change
• Granular Soils (e.g. Loess)
• Light cementation
• Apparent cohesion
• High void ratio and low water content
• Clays
• Sensitivity
• Cardhouse structure

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Sensitive Clays
Source Holtz and Kovacs, 1981
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Need for Soil ImprovementSensitive Clays
Undisturbed and remolded Leda Clay (Ottawa,
Canada) St 1500
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Expansive Soils

• Mechanisms (Clay soils)
• Negative charge
• Isomorphic substitution
• Cations and Water
• Osmotic attraction
• Moisture conditions
• GW fluctuation
• Rainwater infiltration
• Irrigation/landscaping/surface drainage
• Vegetation

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Clay building blocks

• Silica sheet (Si4O10)4-
• Bases in a single plane with tips in preferred
  direction
• Octahedral sheet Al2(OH)6 or Mg3(OH)6

G
B
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11 Clay Minerals

• Kaolinite clay constituent in Piedmont Residual
  Clay
• SSA 10 20 m2/g, CEC 3-15 meq/100g, LL ? 50

G
7.2 Å
G
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21 Clay Minerals

• Montmorillonite
• SSA 700 800 m2/g, CEC 80-150 meq/100g, LL gt
  500

G
9.6 Å - 8
G
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21 Clay Minerals

• Illite most commonly found mineral
• SSA 65-100 m2/g, CEC 10-40 meq/100g, LL ? 100

G
10.0 Å
G
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Others.

• Chlorites
• Chain structure, attapulgite
• Mixed layers

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Clay Minerals

• Classification
• Crystal structure and stacking sequence
• Unit cells consist of two to four sheets
• 11 and 21 minerals common
• Bonding between sheets strong
• Bonding between layers varies, often weak

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Isomorphous Substitution

• Ideal Sheets
• (Si4O10)4- sheet, all cations are Si4
• Al2(OH)6 sheet, all cations are Al3
• Substitutions
• Al for Si, Mg for Al, etc.
• No change in crystal structure
• Ions /- 15 in size
• Results in net negative charge

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Net negative charge

• Example montmorillonite sample
• (Al1.77Mg0.23)(Si3.74Al0.26)O10(OH)2
• Octahedral sheet should have 6 (Al2)
• Compare with 1.77 X 3 0.23 X 2 5.77
• Tetrahedral sheet should have 16 (Si4)
• Compare with 3.74 X 4 0.26 X 3 15.74
• Total net negative charge 0.23 0.26 0.49

Octahedral substitution
Tetrahedral substitution
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Charge balance

• Electroneutrality is met through
• Dissolved cations e.g., exchange capacity (CEC in
  meq/100 g)
• Water molecules
• This is met through weak bonding between and
  adjacent to unit layers
• If water is used instead of cations, greater
  expansion occurs
• AlsoOsmotic attraction-water tries to dilute
  cations

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Expansive Soils
Source Holtz, 1969 Gibbs, 1969
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Expansive Soils US 9 Billion/year in damage to
buildings, roads, airports, etc. (Jones and Jones
1987)
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Frost Susceptible Soils

• Causes
• Ice crystallization
• Ice lensing and propagation
• Variables
• Freezing temperature
• Freezing time
• Moisture conditions

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Frost Heave Ice lenses

• Critical variables
• Frost susceptible soil
• Freezing temperatures
• Freezing rate
• Supply of water

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Freeze-thaw action

• Moisture migration and redistribution

Heave
Cold
Consolidated
Warm
Soil after freezing, Closed System
Soil Prior to Freezing
Soil after freezing, Open System
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Frost Heave Mechanisms

• Conversion of Water into Ice
• Soil becomes less saturated, lower potential is
  created near ice lens, water flows toward this
• Assisted by capillarity
• Controlled by hydraulic conductivity
• As suction increases, hydraulic conductivity
  decreases
• Frost-susceptible soils-silty soils

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Thaw Consolidation

• Total settlement change
• Phase change from ice to water
• Increased moisture content where ice lenses
  formed
• Weakened soil structure (after freezing)

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Frost Susceptible Soils
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Need for Soil ImprovementFrost Heave
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Liquefaction

• Ground motion-induced build up of pore water
  pressure
• Liquefaction will occur if
• Soil is cohesionless
• Soil is loose
• Soil is saturated
• Sufficient shaking, i.e., earthquake
• Undrained conditions

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Liquefaction
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Liquefaction
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Subsurface Cavities

• Abandoned mines
• About 8000 km2 in U.S.
• Soluble bedrock
• CaCO3

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Need for Soil ImprovementSinkholes
Dissolution of carbonate bedrock
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Need for Soil ImprovementSinkholes
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Other Soils and Conditions

• Unusually large loads
• 8.8 million lb dragline/PCS Phosphate
• Strict Requirements
• Temporary excavation

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Tonights Lecture

• Introduction
• Difficult soils
• Site Investigation

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Site Investigation

• Strata thickness, areal extent, location
• Groundwater table
• Soil sample recovery and testing
• In situ or ex situ

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Site Investigation

• Project Assessment (type of structure, etc.)
• Literature search (soil surveys, etc.)
• Remote sensing (aerial photos)
• Surface exploration (site walk)
• Subsurface exploration

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Site Investigation

• Project Assessment
• Literature search
• Remote sensing
• Surface exploration
• Subsurface exploration
• unique to geotech

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Boring Logs
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(No Transcript)
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Subsurface Exploration

• Trenches
• Boreholes
• 3-24 Diameter (75-600 mm)
• 5-100 Depth (2-30 m)
• Methods
• Backhoes
• Hand augers
• Power augers
• Flight
• Bucket

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Hollow Stem Auger (HSA)
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Hollow Stem Auger (HSA)
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HSA Method

• Screw auger into ground
• Add auger sections as necessary
• Insert sampler into hollow stem
• Remove sample and continue drilling

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Rough Spacing Guidelines
Source Coduto, 1999
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Rough Depth Guidelines
Source Coduto, 1999
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Example
A three story steel frame office building is to
be built on a marginal site where the soils are
of questionable quality and uniformity. The
building will have a 30 x 40 m footprint and is
expected to be supported on spread footing
foundations located 1 m below the ground surface
(bgs). Bedrock is 100 m bgs How many borings
should be drilled and to what depth?
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Soil Sampling

• Ex situ
• Split Spoon/SPT sampler
• Thin-wall tube/Shelby tube
• In situ
• SPT
• CPT

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Soil Sampling

• Disturbed
• In situ structure not retained
• Water content, classification, compaction
• Undisturbed
• Less disturbed
• Shear strength, consolidation, permeability

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Soil Sampling

• Disturbances
• Shearing and compression
• In situ stress release
• Drying
• Vibrations
• Categories
• Area Ratio
• Recovery Ratio

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SPT Sampler
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Standard Penetration Test (SPT)

• In situ test (Note ASTM standard)
• Drill boring (perhaps with HSA)
• Insert split barrel sampler
• Raise hammer 30 and allow it to fall
• Repeat process until sampler driven in 18
• Record of blows for each 6 of penetration
• Last 12 constitutes blow number
• Extract sample and continue

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Standard Penetration Test (SPT)

• Refusal
• 50 blows or more per 6 increment
• 100 or more total blows
• 10 successive blows produce no advance
• Corrections
• to enhance repeatability
• See Table 3-3

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Example

• A sample was collected in the field with an
  automatic trip safety hammer, a rod length of 8
  m, hole diameter of 150 mm, overburden pressure
  of 205 kPa with a liner through a clay strata. A
  blow count of 12 was observed.
• What is the corrected N70?

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SPT Correlations

• Empirical Relationships
• See Tables 3-4 and 3-5

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CPT

• In situ test (Note ASTM standard)
• Widely used in lieu of SPT for soft clays
• Cone driven into subsurface
• Cone and frictional resistance measured
• Many adapters
• Pore pressure
• Chemical sensors
• Moisture content
• Visual

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CPT
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Visual CPT Liquefaction potential
Lower liquefaction potential
Higher liquefaction potential
Source University of Michigan
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CPT Correlations

• Use directly
• Friction angle
• Relative density
• Undrained shear strength
• Soil classification
• SPT N-values

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Example
Classify the soil in Figure 3-15 (b) at the 10-12
m depth. Estimate the undrained shear strength,
Su, if the unit weight is 19.65 kN/m3 for the
entire depth of interest.
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Quick Comparison SPT/CPT