coastal structure foundations

US Army Corpsof Engineers

CHL: Steven Hughes, PhD

Design of Maritime Structures

Steven A. Hughes, PhD, PE

Coastal and Hydraulics LaboratoryUS Army Engineer Research and Development Center

Waterways Experiment Station3909 Halls Ferry Road

Vicksburg, Mississippi 39180-6199

Email: [email protected]

Coastal StructureFoundations



Coastal Structure Foundations

Contents• Overview of Foundation Design

• Site-Specific Geotechnical Investigations

• Soil Characteristics

• Foundation Loading and Response

• Foundation Related Failure Modes

• Geotechnical Design Criteria

• Slip Surface and Zone FailuresCEM Chapter VI-3-1 (Author: Steven A. Hughes)CEM Chapter VI-5-5 (Author: Hans F. Burcharth)



Lecture Caveats

Not Covered: Details of foundation design techniques.This should be done by trained geotechnical engineers.

Objective: Present and discuss those aspects of foundation design that distinguish coastal structure foundations from conventional foundation design.

Emphasis: Primarily gravity structures that restdirectly on the sea bed.




Foundation Design Objectives

For structures built or placed directly on top ofsoil, sand, or other bottom material thefoundation must...

Overview of Foundation Design

• Support the structure dead weight

• Resist applied loads that are transferred to thefoundation

• Minimize foundation deformation or settlement

• Maintain sufficient reserve strength throughout thestructure service life



Added Geotechnical Factors


Major differences between geotechnical stability ofcoastal structures and land-based structures includethe following:• Wave action on the structure and foundation

• Wave-induced pore pressure variation in porous structuresand sea bed soils

Waves induce stress variations in soils which cause:• Soil strength degradation

• Pore pressure build up



Design EngineerResponsibilities

* Predict short- and long-term stress and strength developmentin the soils


• Estimate (within reason) expected loading conditions• Determine appropriate site-specific foundation soil

engineering properties and site variations• Reasonably understand the structure/soil interaction

and failure modes• Determine that applied soil stresses will not exceed soil

strength* during project lifetime



Acquire data to assess the nature and extent offoundation soil properties at the project site

Guiding Criterion: Gather sufficient data andperform necessary lab tests and analysis toassure project design adequacy andconstructability

Site-Specific Geotechnical Site Investigations

Purpose of Site Investigation



Typical Questions


• Soil types and strata at the site

• Soil mechanical properties and capability towithstand loads

• Range of environmental conditions (e.g.,freeze/thaw, wet/dry)

• Potential soil degration with time

• Signs of soil fissuring or weathering



Study Sequence

• Site Reconnaissance Phase• Preliminary Exploration Phase• Detailed Design Exploration Phase

• Three Investigative Phases overlap

• Planning specifics of latter phases depend on earlier results

• Level of detail is dictated by project scope, importance and cost

Notes:




Site Reconnaissance Phase

• Primarily a desk study to assemble existing geological data for site

• Results help establish data collection requirements of next phase

• Site visit to reconcile data and observe surface evidence and site condition

• Information sources include:

• Topographic and geologic maps

• Aerial photography

• Groundwater maps

• Past historical records and geotechnical studiesat nearby locations

• Any published studies or local descriptions

Goal: Glean from available data a feel for the project sitegeology (stratification, formation, history, groundwater, etc.)




Goals:• Recognize potential geotechnical problems• Obtain sufficient data to finalize site selection• Determine geotech parameters needed for preliminary design

• Approximate depth, thickness of strata• Depth to bedrock• Groundwater variations• Estimates of critical soil parameters• Potential sources of construction materials

Results go into Survey Report used for project authorization


Preliminary Exploration Phase



Investigative Methods:

Bottom Line: Gathered information should be sufficient toselect site and complete preliminary design


Preliminary Exploration Phase

• Continuous seismic reflection surveys (soil types and strata depth)

• Side-scan sonar images (surface soil characteristics, relicstructures)

• Dry-land methods (electro-resistivity/magnetic, seismic refraction)

• Small number of in-situ borings (when feasible) to calibrate/verifysurvey data



Detailed Design Exploration Phase

Bottom Line: Realistic soil parameters can savemore than cost of investigation, whereasuncertainties in soil strength can lead to over-design.

Purpose: Collect and analyze specific soil data to determinegeotechnical parameters needed for final design

• Specify which soil parameters are needed• At which locations• Best methods/instruments/analyses for time and budget constraints




Elements of Typical Field Study of In-Situ Soils:

• Penetration and vane shear devices to measure soil strength

• Pressure meters to estimate load-deformation characteristics

• Nuclear densimeters and sand cone devices to measure density

• Equipment to measure permeability and pore pressure

• Test loading of piles

• Instrumentation of embankments and foundations

• Monitoring during vibratory and impulse loading


Detailed Design Exploration Phase



Soil Classification by Grain-Size

Common Soil Properties



Soil Bulk Density




Volume of Voids




Relative Density




Other Soil Parameters


• Unit volume weight (based on bulk density)

• Plasticity index (water content range over which cohesivesoils remain plastic)

• Geostatic stress (soil weight on horizontal surface)

• Coefficient of lateral stress (ratio of horizontal to verticalstress

• Overconsolidation ratio (ratio between maximum andactual pore pressure)

• Normal Consolidation: Equilibrium at maximum stress ever applied

• Overconsolidation: Equilibrium at stress less than maximum everapplied



Soil Deformation Moduli

Note: Typicalvalues are given intables in the CEM




Soil Stress Definition

Soil Strength Properties



Mohr Failure Circle

• Failure occurs on the Mohr circle envelope• Determined from drained triaxial tests• Generally the failure curve is not a straight line




Mohr Circle Straight-Line Approximations

For Drained Soils

Approximation for noncohesivesoils. Valid only close tofailure load of interest




Failure Criterion for Cohesive Soils


• Strength due to friction between particles and cohesion forces

• Undrained shear strength determined by tests with monotonicload increase to failure

• For a specific clay cu depends solely on initial stress condition



Foundation Loads

Static Force Loads• Structure and foundation self weight• Relatively constant over structure lifetime

• Buoyancy effects cause cyclic variation instructure weight with tide

• Be aware of weight distribution and differentialloading

• Be aware of spanning different soil types• Lateral forces due to imbalanced hydrodynamic

pressure• Construction loads?



Dynamic Force Loads

• Wave, currents, tides, storm surges, and wind

• Earthquake ground motions in some regions

• Loads vary in time, duration and intensity

• Examine the worst likely combination

Foundation Loads



Impact Force Loads

Foundation Loads

• Ship or ice collisions, partial structure failure,slamming waves

• Importance depends on magnitude andstructure type

• Rubble-mounds can absorb a portion of impactload

• Monolithic structures transmit more load tofoundation, but large mass and naturalfrequency help reduce that load



Foundation Soil ResponseSoil Consolidation:• Reduction in soil voids over time by squeezing out water• Results in denser soil and increased soil strength• May result in unacceptable settlement

Soil Loss by Scour and Erosion

Foundation Loads

Soil Shear Stresses:

• Induced when lateral forces and overturning moments aretransmitted to the foundation soil...may lead to damage

• Excess pore pressure• Caused by rapid loading and results in decreased soil strength

• Also caused by cyclic loading of sand

• Both cases may cause liquefaction (earthquake accelerations)



Slip Surface and Zone FailureRubble-Mound Structures and Dikes

Foundation Failure Modes



Monolithic Structures


Slip Surface and Zone Failure



Tied Wall Structures


Slip Surface and Zone Failure



Excess Settlement(Including Differential Settlement)




Geotechnical Design Criteria

Foundation TypesShallow Foundations

• Load is supported by soil just beneath the sea bottom

• Most coastal structures use shallow foundations

• Foundation often serves to widen load bearing surface

Deep Foundations

• Load is supported throughout a substantial depth of soil

• Examples are pile-supported structures and piers



Shallow Foundation Environmental Factors

Factors• Currents• Tides/storm surges• Waves• Seismic activity

Site Specific Considerations• Soil type and strength• Topography• Water depth• Structure positioning




Shallow Foundation Design Considerations

Noncohesive Soils• Ultimate bearing capacity for sand is very high• Design is usually based on expected foundation settlement• Must check differential settlement• Settlement is rapid• Rubble-mounds more tolerant of differential settlements

Cohesive Soils• Must check both bearing capacity and settlement• Settlement is time-dependent




Three Phases of Settlement in Cohesive Soils

Immediate Settlement is soil distortion thatoccurs concurrently with loading

Primary Consolidation occurs over time aswater is pushed for voids

Secondary Compression occurs as the soilskeleton adjusts to the applied load afterconsolidation




Sloping Structure Design Considerations


• Slopes and embankments susceptible to slip-surfacefailure

• Pertains to bulkheads, seawalls, revetments, and dikes

• Pore pressure distribution is a necessary designparameter

• Problem increased by saturated backfill material causedby overtopping, rain, etc.

• Usually not a problem for submerged foundations exceptfor structures built on weak soils



Seismic Design Considerations


• Evaluate liquefaction potential for high seismic riskareas

• Rubble-mound structure damage usually notcatastrophic

• Slender concrete armor units may sustainsignificant breakage

• May need to include repair costs in projecteconomics



Slip Surfaces

Slope Instability

• Rarely occurs in conventional rubble-mound structures

• Problems can occur if placed on weak soils or strata

• Large breakwaters with steep slopes may have problems

• Slipping between armor layer and underlayers

• Direct wave action on permeable slopes:

• Creates extra loads by run-up

• Creates pore pressure fluctuations

• Creates hydraulic gradients



Rubble-Mound Flow Nets

Slip Surfaces



Simple Slope Analysis

Procedure: Minimize F by varying location and radius of failure circle

Slip Surfaces



Noncircular Slip Failure Surfaces

Slip Surfaces



Method of Slices

Various methodsproposed to sum upresultants of individualslices

Equations given in CEM

Slip Surfaces



Three-Dimensional Slope Failure

Slip Surfaces



Other Geotechnical AspectsPhenomena Associated With Coastal Structures


• Hydraulic gradients in porous structures

• Wave-induced internal set-up in rubble-

mound structures

• Cyclic loading of soils by waves and tides

• Dynamic loading of monolithic structures



Conclusions


• Geotechnical aspects of coastal

structures are important and should not

be overlooked

• Foundation failure modes can be critical

• Foundation design is very site-specific

coastal structure foundations

Documents