Foundation investigation

Foundations on Expansive Clay Soil Investigation of
Part 3 - Investigation of Foundation Failure
Failed Foundations • Is structure moving?
• Where is movement occurring?
• Is structure experiencing heave,
settlement or a combination?
• Is movement excessive?
Presented by: • Why is movement occurring?
Eric Green, P.E.
Structural Engineer
• What (if any) remedial measures are
required?
Slide 1 Copyright Eric Green 2005 Slide 2 Copyright Eric Green 2005

Investigation Steps Step 1 - Data Collection

• Data Collection
• Testing
• Interpretation
• Recommendations


Review of Construction Field Investigation
Documents • Elevation survey of slab
• Civil, structural and landscaping plans • Distress survey (interior and exterior)
• Original geotechnical report • Leak testing of plumbing lines
• Compaction reports • As-built structural documentation
• Fill testing • Geotechnical investigation
• Time of construction • Topographic/drainage survey
• Reports by other consultants • Vegetation survey
• Owner interview

Elevation Survey
• Most important investigation tool.
• Normally done with digital water level.
Elevation • Adjust for thickness of floor coverings.
Survey • Floors are not constructed flat.
Differential elevation does not equal
differential movement.
• Multiple surveys over time are useful
where possible.


Distress Survey - Interior Finish crack
• Map locations and type of interior
distress.
– Separation vs. crack
– Orientation & locations
– Width & pattern
– Photos
• Door alignment in frame & adjusted
door strikes.
• Previous repairs to structure

Finish Cracks Finish Separations

Separation


Door Alignment Door Alignment

Door-Frame
Misalignment


Prior Repairs to Finishes Interior Underpinning


Slab & Tile Cracks

Interior
Distress
Map


Distress Survey - Exterior Finish Cracks
• Map location and type of exterior
distress.
– Crack vs separation
– Orientation
– Width & pattern
– Photos
• Movement in adjacent flat work.
• Age


Finish Separations Previous Repairs to Finishes

Separation
s


Differential Movement
Previous Repairs to Finishes
at Pier Caps


Movement of Flat Work Movement of Flat Work

Ramp


Previous Underpinning A/C Condensate Drain


Cracking of Exposed Slab

Exterior
Distress
Map


Sewer Line Leak Testing Hydrostatic Leak Testing
• Tested for leaks under hydrostatic
pressure using inflatable plugs to isolate
lines. Water level is observed from
toilet or other fixture.
• Leakage under operational conditions
can be determined using flow test.
• Video cameras can be used to examine
condition of sewer lines and geometry
of leaks.


Leak Test Limitations Leak Test Limitations
• Systems which leak under hydrostatic • Small leaks may not be detected in flow
pressure may not leak in operation. testing.
• Testing cannot reliably locate multiple • Flow testing may not replicate actual
leaks in the same branch. Some areas flow conditions during operation.
cannot be tested due to lack of access. • Soil saturation from hydrostatic test
• Flow test results may not match may affect flow test results.
hydrostatic test results.


Multiple Leaks As-Built Construction
• Check for piers using soil probe. If piers
are present, excavate to determine if
piers are original or remedial.
• Check pier depth and bell size.
• Check for voids and void boxes under
grade beams and slab.
• Check depth of grade beam.
• Locations of expansion joints.


Pier Excavation
As-Built Construction
• Concrete compressive strength
• Slab reinforcing (size, spacing and
depth).

Bell


Gap at Pier Cap The Wrong Way

Dead Man
Walking

Gap between Pier
and Grade Beam


Test Pits
Geotechnical Investigation
Test pits dug along grade beams can
reveal significant information: • Soil moisture profile (water content,
Depth of grade beam.
liquidity index and/or suction).
•
• Presence/absence of void boxes. • Soil classification.
• Voids grade beams. • Depth and type of fill.
• Presence of piers. • Swell potential/plasticity index.
• Separation between pier and grade beam. • Depth to water table.
• Water under slab. • Interior, adjacent and remote (baseline)
• Type of fill at perimeter (clay vs loam). borings.

Geotechnical Investigation Petrography
• Design data • Testing conducted on cores recovered in
– Allowable bearing pressure for piers interior geotechnical borings.
– Potential vertical rise • Shrinkage vs stress crack
– Expected consolidation under design loads. • Plastic shrinkage cracking
• w/c ratio


Test Pits Drainage Survey
• Depth of grade beams. • Micro slope adjacent to building.
• Presence of voids or void boxed under • Locations of downspouts and splash
grade beams. blocks.
• Water under foundation. • Drainage of flower and shrub beds,
• Presence of piers. including drainage plane.
• Separations between piers and grade • Areas of negative drainage.
beams.


Improper Surface Drains Trapped Drainage

Trapped by
Sidewalk

Gap between
downspout and drain


Trapped Drainage Pool MEP

Flowerbed Traps
Surface Drainage


Perched Drainage Plane
Landscaping Survey
• Location, type and sizes of large trees.
• Extent of tree canopies.
• Gaps between soil and grade beams.
• Locations of shrub and flower beds.
• Locations of sprinkler heads.
• Location of concrete and impermeable
flatwork.


Soil Gaps at Grade Beam Gap Under Grade Beam

Repair Pier


Dessicated Soil Dessicated Soil


Trees
Owner Interview
• History of foundation movement.
• Timing of distress.
• Sequence of distress.
• Cyclical/seasonal movement of cracks.
• Plumbing problems.
• Irrigation practices.


Structural Monitoring Crack Gage
• Monitoring over time can help to
determine cause of distress and
appropriate repair methodology.
• Crack widths can be monitored with
crack gage.
• Ensure elevation measurements are
taken at the same locations.
• Use permanent external benchmark if
possible

Crack Measurement Step 2 - Data Analysis

• The information gathered in the data
collection phase must tell a consistent
story.
• No single source of data can be used to
determine magnitude, mode or cause of
movement.


Failure Patterns Types of Movement
• Center heave (also called doming,
hogging or center lift).
• Edge settlement
• Center settlement
• Edge heave (also called edge lift)
• Slope creep (typically on slopes greater
than 5%)


Center Heave/Edge Settlement Edge Settlement


Edge Heave/Center Settlement Causes of Foundation Movement
• Variation in climate.
• Change in depth of water table.
• Removal of trees.
• Planting of trees
• Inadequate drainage.
• Seepage along construction interfaces.
• Irrigation


Causes of Foundation Movement Interpretation of Differential
Elevation Survey
• Leaking utility lines
• Slabs are not poured perfectly level.
• Drying of soil below heated rooms
• Is differential elevation movement of
• Seepage of moisture along utility as-built condition?
trenches
• ACI tolerance for slab on grade is ±¾”
• Poor compaction of fill. from nominal elevation.
• Improper fill material. • F-number gives allowable tilt. For bull-
floated slab, maximimum local tilt
ranges from 0.9% to 1.0%.

Geotechnical Interpretation Interpretation of Field Data

• Existing vs pre-construction MC and LI . • Are voids
• MC and LI at high and low areas.
• Existing vs pre-construction bearing
strength.
• MC and LI at leak and remote.
• MC and LI at interior and perimeter.


Design Review Analysis of Cracks
• Not all cracks are caused by foundation
• Soil shrink-swell or soil failure under movement.
loads? – Differential Movement (shear & flexural
• Design analysis vs. failure analysis. stress).
• Service loads on piers vs. allowable – Thermal cracking (axial stress).
loads. – Moisture changes (axial stress).
• Bearing pressure under grade beams. • Rigid finishes are more susceptible to
• Bearing pressure under full foundation movement.
area. • Distress must be consistent with
differential movement.

Shear Cracks due to Shear Cracks due to
Differential Movement Differential Movement
– Diagonal cracks in walls are
generally due to differential
movement.
– Diagonal cracks form at
location of maximum
principle stress (tension).
– Vertical cracks in walls are
generally due to thermal
movement..
– Cracks due to expansive soil
move generally show
cyclical movement to some
extent.
– Direction of diagonal crack
can show direction of
relative motion but not
direction of absolute
motion.


Flexural Cracks due to Thermal Cracking
Differential Movement
• Can be mistaken for cracking caused by
• Flexure of the slab can result in differential movement.
vertically oriented cracking of finishes. • Thermal movement in masonry (Brick
• These cracks will be tapered (wider at and CMU) results in uniform width
the top or bottom) depending on the vertical cracks due to axial stresses.
mode of movement (center lift or edge • Cracks tend to occur at openings and
lift). changes in wall alignment.
• Absence of expansion joints will
increase probability of thermal cracks.

Thermal Crack

Thermal
Cracking
Rates
( No Differential
Movement)


Thermal Crack Thermal Crack


General Analysis of Cracks Cracking of Slab
• Slab cracking without vertical
• All structures have cracks. displacement across the crack is rarely
• Distress should be worse in area a result of differential movement.
exhibiting movement. • Common causes of slab cracks:
• Direction of relative movement should – Drying shrinkage
be consistent with differential elevation Plastic shrinkage
–
survey and cause of movement.
– Restraint to shrinkage
– Excess w/c ratio
– Inadequate control joints or slab steel

Drying Separations Drying Separations

• Separations in wood trim and minor
cracking of drywall finishes can be a
result of drying of wood framing and/or
trim.


Timing of Failure Climate Review
The probability of foundation failure • A review of the climate records is useful
increases with time: in determining the mode of movement
• Vegetation increases in size. (heave vs. settlement).
• Probability of experiencing extreme • Heave can be associate with periods
drought or extreme precipitation with above average rainfall and/or
increases. below average temperatures.
• Probability of a failure in utility systems • Settlement tends to be associated with
increases (water and sewer lines). periods of below average rainfall and
above average temperature


Plumbing Leaks
• Sewer leaks can result in localized
heave of the foundation.
• Sewer leaks rarely cause significant
movement.
– Impermeability of clay prevents water
migration over wide area.
– Leaks are not pressurized.
• Leaks can be caused by movement;
chicken and egg problem.

Plumbing Leaks and
Plumbing Leaks
Settlement
• Leaks in riser can migrate through sand • Plumbing leaks can cause settlement via
bed and/or plumbing trench. two mechanisms:
• Grade beams act as barrier to migration – Liquification of soil (loss of bearing
through sand bed. strength)
– Erosion.
• These phenomena are rarely seen in
practice.


Need for Repairs Need for Repairs
• There is no generally accepted criteria • Considerations for recommending
to determine when repairs to a repairs include:
foundation are needed.
– The owners performance expectations.
• All slab-on-ground structures will – The owners economic sensitivity (cost of
experience some movement. repairs vs. cost of maintenance).
• All slab-on-ground structures will – Potential for future movement.
experience some cosmetic cracking of – Life safety (rarely an issue).
finishes.
– Effectiveness of repairs.


Foundation investigation

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