Transcript Foundation Notes - Civil and Environmental Engineering

Foundations
Foundations
• Feature common to
bridge and building
structures
• Deep vs. shallow
• Safely transfer dead
and live loads with
acceptable levels of
settlement
Foundations
Shallow Foundations
•
•
•
•
– spread footings: column,
combined, mat
– strip footings: walls
placed on undisturbed or
well-compacted soil
below frost line
consider effects of
groundwater
typically reinforced or plain
concrete
Foundations
Deep Foundations
• Piles
– driven or drilled
– bearing capacity from end-bearing or skin
friction: usually combination of both
– driven plum or battered (1:6 to 1:3)
– driven to attain minimum tip elevation or
required resistance in blows per foot (bpf)
Foundations
Piles
• Types
– timber piles
– precast/prestressed
concrete piles
– cast-in-place concrete
– steel H-piles
– steel pipe piles
– composite
– minipiles or micro piles
Foundations
Pile Driving
• Equipment
– mobile crane (usually crawler)
– leads: fixed or swinging
– hammer, helmet, & cushion
– compressor (if air driven)
– hydraulic unit (if vibratory)
– steam plant (becoming
obsolete)
Foundations
Pile Driving
• Equipment
– follower
– mandrel
• Pile accessories
– point attachments
– splices
– pile to cap connections
Foundations
Pile Driving
• Hammers
– steam or air: singleacting, double-acting,
differential
– diesel hammer: singleacting (open end) or
double-acting (closed
end)
– vibratory
Pile Driving
• Dynamic analysis - wave equation “WEAP”
– based on wave propagation theory with input
from the static analysis
– provides relationship between pile capacity
and driving resistance i.e.: equates capacity
to blow counts
– predicts pile driving stresses
Pile Driving
• Benefits of using wave equation
– verify piles can be driven to required depth &
capacity
– determine minimum required pile section
– avoid overstressing pile during driving
– optimize cost
Pile Driving
• Dynamic testing - Pile
Driving Analyzers (PDA)
and CAPWAP
– uses strain gauge &
accelerometers to
measure force &
velocity (converts
strain to force and
acceleration to
velocity)
– field tests the driving
hammer and system
Pile Driving
• Dynamic testing - Pile Driving
Analyzers (PDA) and
CAPWAP
– cost effective method of
optimizing the driving
system and estimating
static pile capacity
– using CAPWAP, hammer
blows can be analyzed to
estimate skin friction, end
bearing, dynamic damping
factors, and soil stiffness
Pile Driving
Pile Load Test (static)
• many tests have been used over the years to
evaluate or confirm axial and lateral capacities
• Quick pile load tests (QPLT) are most common
• QPLT consists of constructing a reaction frame
around the test pile
• the frame is anchored to reaction piles
• pile is incrementally loaded with an hydraulic jack
Pile Driving
Pile Driving
Pile Load Test (static)
• the test load is measured by a load cell
• deformation is measured with dial gauges, wiremirror-scale, and surveying instruments
• apply load in 10 to 15% increments, maintaining
a 2½ minute interval
• check deformation readings immediately before
and after applying load
Pile Driving
Pile Load Test (static)
• incrementally increase load until reaching 200%
of the design load or until continuos jacking is
required to maintain the test load
• remove the load in 4 equal decrements
• plot the load-settlement data using the Davisson
method
• the point where the load-settlement curve
crosses the failure criterion is the failure load (Qf)
Pile Driving
Other Factors
• soil setup or “freeze” - common in clays
• relaxation - much less common, found in
some silts or fine sand
• restrike pile to verify strength gain or loss
• 120 bpf is practical, safe, upper limit driving
blow count
• 200 to 240 bpf is practical refusal
Pile Driving
Inspection
• position, plumb, or batter
• check cushion; change as necessary
• record blow counts
• verify correct operation of hammer
• check splices
• check piles for damage
• check for heave
Pile Driving
Driving Records
• prepare a sketch of foundation to identify
piles and order of installation
• Drive records include:
– pile ID and order of installation
– ground elevation
– cut-off elevation
– tip elevation
– final length
– blows per foot (final foot recorded in bpi)
– driving duration
– location of splices
Foundations
Retaining Walls
• gravity or cantilever
• components
– stem wall
– horizontal base
includes:
toe, heel, & key
– weep holes for
drainage
Foundations
Temporary Support
Structures and
• sheeting w/wales and
tie-backs
• soldier beams (piles)
w/lagging
• pipe sleeves
• tremie concrete
Foundations
Underpinning
• remedial or
precautionary
• remedial adds
foundation capacity to
inadequately supported
structures
• precautionary - higher
loads or changes in
ground conditions
Foundations
Underpinning
• Temporary supports
– shores
– needle beams
– grillage
• Pit underpinning - used
for installing or
extending foundation
walls or footings
• Pile underpinning &
various other methods