US Army Corps of Engineers
Design of Maritime Structures
Coastal Structure Foundations Steven A. Hughes, PhD, PE Coastal and Hydraulics Laboratory US Army Engineer Research and Development Center Waterways Experiment Station 3909 Halls Ferry Road Vicksburg, Mississippi 39180-6199 Email:
[email protected] CHL: Steven Hughes, PhD
US Army Corps of Engineers
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 Failures CEM Chapter VI-3-1 (Author: Steven A. Hughes) CEM Chapter VI-5-5 (Author: Hans F. Burcharth) CHL: Steven Hughes, PhD
US Army Corps of Engineers
Coastal Structure Foundations
Lecture Caveats Objective: Present and discuss those aspects of foundation design that distinguish coastal structure foundations from conventional foundation design. Not Covered: Details of foundation design techniques. This should be done by trained geotechnical engineers. Emphasis: Primarily gravity structures that rest directly on the sea bed. CHL: Steven Hughes, PhD
US Army Corps of Engineers
Overview of Foundation Design
Foundation Design Objectives For structures built or placed directly on top of soil, sand, or other bottom material the foundation must...
• Support the structure dead weight • Resist applied loads that are transferred to the • •
foundation Minimize foundation deformation or settlement Maintain sufficient reserve strength throughout the structure service life CHL: Steven Hughes, PhD
US Army Corps of Engineers
Overview of Foundation Design
Added Geotechnical Factors Major differences between geotechnical stability of coastal structures and land-based structures include the following:
• •
Wave action on the structure and foundation Wave-induced pore pressure variation in porous structures and sea bed soils
Waves induce stress variations in soils which cause:
• •
Soil strength degradation Pore pressure build up CHL: Steven Hughes, PhD
US Army Corps of Engineers
Overview of Foundation Design
Design Engineer Responsibilities • 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
* Predict short- and long-term stress and strength development in the soils CHL: Steven Hughes, PhD
US Army Corps of Engineers
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 Failures CEM Chapter VI-3-1 (Author: Steven A. Hughes) CEM Chapter VI-5-5 (Author: Hans F. Burcharth) CHL: Steven Hughes, PhD
US Army Corps of Engineers
Site-Specific Geotechnical Site Investigations
Purpose of Site Investigation Acquire data to assess the nature and extent of foundation soil properties at the project site Guiding Criterion: Gather sufficient data and perform necessary lab tests and analysis to assure project design adequacy and constructability CHL: Steven Hughes, PhD
US Army Corps of Engineers
Site-Specific Geotechnical Site Investigations
Typical Questions • • • • •
Soil types and strata at the site Soil mechanical properties and capability to withstand loads Range of environmental conditions (e.g., freeze/thaw, wet/dry) Potential soil degration with time Signs of soil fissuring or weathering CHL: Steven Hughes, PhD
US Army Corps of Engineers
Site-Specific Geotechnical Site Investigations
Study Sequence • Site Reconnaissance Phase
• Preliminary Exploration Phase • Detailed Design Exploration Phase Notes: • Three Investigative Phases overlap • Planning specifics of latter phases depend on earlier results • Level of detail is dictated by project scope, importance and cost CHL: Steven Hughes, PhD
US Army Corps of Engineers
Site-Specific Geotechnical Site Investigations
Site Reconnaissance Phase Goal: Glean from available data a feel for the project site geology (stratification, formation, history, groundwater, etc.)
• 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 CHL: Steven Hughes, PhD
US Army Corps of Engineers
Site-Specific Geotechnical Site Investigations
Preliminary Exploration Phase 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 CHL: Steven Hughes, PhD
US Army Corps of Engineers
Site-Specific Geotechnical Site Investigations
Preliminary Exploration Phase Investigative Methods: • •
Continuous seismic reflection surveys (soil types and strata depth)
• •
Dry-land methods (electro-resistivity/magnetic, seismic refraction)
Side-scan sonar images (surface soil characteristics, relic structures) Small number of in-situ borings (when feasible) to calibrate/verify survey data
Bottom Line: Gathered information should be sufficient to select site and complete preliminary design CHL: Steven Hughes, PhD
US Army Corps of Engineers
Site-Specific Geotechnical Site Investigations
Detailed Design Exploration Phase Purpose: Collect and analyze specific soil data to determine geotechnical parameters needed for final design
• Specify which soil parameters are needed • At which locations • Best methods/instruments/analyses for time and budget constraints
Bottom Line: Realistic soil parameters can save more than cost of investigation, whereas uncertainties in soil strength can lead to over-design. CHL: Steven Hughes, PhD
US Army Corps of Engineers
Site-Specific Geotechnical Site Investigations
Detailed Design Exploration Phase 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 CHL: Steven Hughes, PhD
US Army Corps of Engineers
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 Failures CEM Chapter VI-3-1 (Author: Steven A. Hughes) CEM Chapter VI-5-5 (Author: Hans F. Burcharth) CHL: Steven Hughes, PhD
US Army Corps of Engineers
Common Soil Properties
Soil Classification by Grain-Size
CHL: Steven Hughes, PhD
US Army Corps of Engineers
Common Soil Properties
Soil Bulk Density
CHL: Steven Hughes, PhD
US Army Corps of Engineers
Common Soil Properties
Volume of Voids
CHL: Steven Hughes, PhD
US Army Corps of Engineers
Common Soil Properties
Relative Density
CHL: Steven Hughes, PhD
US Army Corps of Engineers
Common Soil Properties
Other Soil Parameters • • • • •
Unit volume weight (based on bulk density) Plasticity index (water content range over which cohesive soils remain plastic) Geostatic stress (soil weight on horizontal surface) Coefficient of lateral stress (ratio of horizontal to vertical stress Overconsolidation ratio (ratio between maximum and actual pore pressure) • Normal Consolidation: Equilibrium at maximum stress ever applied • Overconsolidation: Equilibrium at stress less than maximum ever applied CHL: Steven Hughes, PhD
US Army Corps of Engineers
Common Soil Properties
Soil Deformation Moduli
Note: Typical values are given in tables in the CEM
CHL: Steven Hughes, PhD
US Army Corps of Engineers
Soil Strength Properties
Soil Stress Definition
CHL: Steven Hughes, PhD
US Army Corps of Engineers
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 CHL: Steven Hughes, PhD
US Army Corps of Engineers
Soil Strength Properties
Mohr Circle Straight-Line Approximations For Drained Soils
Approximation for noncohesive soils. Valid only close to failure load of interest CHL: Steven Hughes, PhD
US Army Corps of Engineers
Soil Strength Properties
Failure Criterion for Cohesive Soils
• • •
Strength due to friction between particles and cohesion forces Undrained shear strength determined by tests with monotonic load increase to failure For a specific clay cu depends solely on initial stress condition CHL: Steven Hughes, PhD
US Army Corps of Engineers
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 Failures CEM Chapter VI-3-1 (Author: Steven A. Hughes) CEM Chapter VI-5-5 (Author: Hans F. Burcharth) CHL: Steven Hughes, PhD
US Army Corps of Engineers
Foundation Loads
Static Force Loads • Structure and foundation self weight • Relatively constant over structure lifetime • Buoyancy effects cause cyclic variation in
• •
structure weight with tide • Be aware of weight distribution and differential loading • Be aware of spanning different soil types Lateral forces due to imbalanced hydrodynamic pressure Construction loads? CHL: Steven Hughes, PhD
US Army Corps of Engineers
Foundation 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 CHL: Steven Hughes, PhD
US Army Corps of Engineers
Foundation Loads
Impact Force Loads • Ship or ice collisions, partial structure failure, •
slamming waves Importance depends on magnitude and structure type • Rubble-mounds can absorb a portion of impact load • Monolithic structures transmit more load to foundation, but large mass and natural frequency help reduce that load CHL: Steven Hughes, PhD
US Army Corps of Engineers
Foundation Loads
Foundation Soil Response Soil 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 Shear Stresses: • Induced when lateral forces and overturning moments are transmitted 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) Soil Loss by Scour and Erosion CHL: Steven Hughes, PhD
US Army Corps of Engineers
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 Failures CEM Chapter VI-3-1 (Author: Steven A. Hughes) CEM Chapter VI-5-5 (Author: Hans F. Burcharth) CHL: Steven Hughes, PhD
US Army Corps of Engineers
Foundation Failure Modes
Slip Surface and Zone Failure Rubble-Mound Structures and Dikes
CHL: Steven Hughes, PhD
US Army Corps of Engineers
Foundation Failure Modes
Slip Surface and Zone Failure Monolithic Structures
CHL: Steven Hughes, PhD
US Army Corps of Engineers
Foundation Failure Modes
Slip Surface and Zone Failure Tied Wall Structures
CHL: Steven Hughes, PhD
US Army Corps of Engineers
Foundation Failure Modes
Excess Settlement (Including Differential Settlement)
CHL: Steven Hughes, PhD
US Army Corps of Engineers
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 Failures CEM Chapter VI-3-1 (Author: Steven A. Hughes) CEM Chapter VI-5-5 (Author: Hans F. Burcharth) CHL: Steven Hughes, PhD
US Army Corps of Engineers
Geotechnical Design Criteria
Foundation Types Shallow 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 CHL: Steven Hughes, PhD
US Army Corps of Engineers
Geotechnical Design Criteria
Shallow Foundation Environmental Factors Factors
• Currents • Tides/storm surges • Waves • Seismic activity Site Specific Considerations
• Soil type and strength • Topography • Water depth • Structure positioning
CHL: Steven Hughes, PhD
US Army Corps of Engineers
Geotechnical Design Criteria
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
CHL: Steven Hughes, PhD
US Army Corps of Engineers
Geotechnical Design Criteria
Three Phases of Settlement in Cohesive Soils Immediate Settlement is soil distortion that occurs concurrently with loading Primary Consolidation occurs over time as water is pushed for voids Secondary Compression occurs as the soil skeleton adjusts to the applied load after consolidation CHL: Steven Hughes, PhD
US Army Corps of Engineers
Geotechnical Design Criteria
Sloping Structure Design Considerations • Slopes and embankments susceptible to slip-surface failure
• Pertains to bulkheads, seawalls, revetments, and dikes • Pore pressure distribution is a necessary design parameter
• Problem increased by saturated backfill material caused by overtopping, rain, etc.
• Usually not a problem for submerged foundations except for structures built on weak soils CHL: Steven Hughes, PhD
US Army Corps of Engineers
Geotechnical Design Criteria
Seismic Design Considerations • Evaluate liquefaction potential for high seismic risk areas
• Rubble-mound structure damage usually not catastrophic
• Slender concrete armor units may sustain significant breakage
• May need to include repair costs in project economics CHL: Steven Hughes, PhD
US Army Corps of Engineers
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 Failures CEM Chapter VI-3-1 (Author: Steven A. Hughes) CEM Chapter VI-5-5 (Author: Hans F. Burcharth) CHL: Steven Hughes, PhD
Slip Surfaces
US Army Corps of Engineers
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 CHL: Steven Hughes, PhD
US Army Corps of Engineers
Slip Surfaces
Rubble-Mound Flow Nets
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US Army Corps of Engineers
Slip Surfaces
Simple Slope Analysis
Procedure: Minimize F by varying location and radius of failure circle CHL: Steven Hughes, PhD
US Army Corps of Engineers
Slip Surfaces
Noncircular Slip Failure Surfaces
CHL: Steven Hughes, PhD
US Army Corps of Engineers
Slip Surfaces Method of Slices
Various methods proposed to sum up resultants of individual slices Equations given in CEM CHL: Steven Hughes, PhD
US Army Corps of Engineers
Slip Surfaces
Three-Dimensional Slope Failure
CHL: Steven Hughes, PhD
US Army Corps of Engineers
Coastal Structure Foundations
Other Geotechnical Aspects Phenomena Associated With Coastal Structures
• •
Hydraulic gradients in porous structures
• •
Cyclic loading of soils by waves and tides
Wave-induced internal set-up in rubblemound structures Dynamic loading of monolithic structures CHL: Steven Hughes, PhD
US Army Corps of Engineers
Coastal Structure Foundations
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 CHL: Steven Hughes, PhD