Post on 03-Nov-2018
Sinking a Caisson
by Scott Ludlow, Ph.D., P.E.
S.J. Ludlow Consulting Engineers, Inc.
Workshop on “Learning from Successes and Failures in Geotechnical Engineering”
Purdue University, West Lafayette, Indiana
April 15, 2016
Outline
Brief Background
Site
Ground conditions
Project requirements
What went wrong?
Analysis and Repairs
Concluding Remarks
Ground Conditions 40-ft thick layer of deltaic deposits consisting of loose, fine- to
medium-grained sand, and organic clayey silts and clays
Cooper Marl (indurated sandy clay and silt; highly
calcareous)
Groundwater within 5 ft of the ground surface
Project Requirements 140-ft deep, 60-ft diameter shaft for pump station
Provide temporary support of ground conditions
Serve as a final liner for pump station
Serve as a tunnel access shaft (West Ashley and Harbor
View Runs) with a portal access/chamber located outside of
the shaft
8-ft thick base slab with a lip
House an elevator, stairwell, 20-ft diameter wet well and
miscellaneous equipment
What went wrong? In any field of scientific endeavor, anything that can go
wrong, will go wrong.
Let to themselves, things always go from bad to worse.
If there is a possibility of several things going wrong, the one that will go wrong, is the one that will do the most damage.
Nothing is as easy as it looks.
Everything takes longer than you think it will.
If everything seems to be going well, you have obviously overlooked something.
Nature always sides with the hidden flaw.
No matter what occurs, someone believes it happened according to their pet theory.
Nothing is ever a complete failure; it can always serve as a bad example.
What went wrong? Uncontrolled sinking (in the upper 40 ft)
Overly-optimistic/aggressive schedule
Lack of proper planning/equipment/tooling
Analysis (FLAC3D) Static simulation (steps)
Setup properties, initialization of pore pressures and static stress
(Ko)
Excavate in 18 steps and progressively reduce the stresses and
pore pressure of the zones under excavation
Sink the shaft using the as-built geometry
Allow pore pressures to dissipate for only a brief period to keep
and undrained response of the surrounding soil and develop
equilibrium before excavating the next phase
Analysis (FLAC3D) Dynamic simulation
Soil properties from static analysis
Damping (no data; based on available literature)
Explicit crack representation (i.e., residual v. intact)
Structural concrete (moment-curvature diagram for various levels
of axial thrust)
Three different time histories (applied in two different directions
and a vertical component of 2/3 horizontal)
Sensitivity analysis (zone size, mesh density)
Water levels in the wet well (no water, operational and maximum)
Repairs Stress resultants (review)
12-in. thick relined section
Revisions to elevator, stairwell and wet well sections
Develop a positive connection with the existing shaft wall