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

Charleston, SC

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

Thank you!