Post on 18-Dec-2015
transcript
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: single-acting, double-acting, differential
– diesel hammer: single-acting (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 Load Test (static)• the test load is measured by a load cell• deformation is measured with dial gauges, wire-
mirror-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 DrivingDriving 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