Coastal Structure Foundations Steven A

US Army Corps Design of Maritime Structures of Engineers 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 Coastal Structure Foundations of Engineers 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 Coastal Structure Foundations of Engineers 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 Overview of Foundation Design of Engineers 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 Overview of Foundation Design of Engineers 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 Overview of Foundation Design of Engineers 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 Coastal Structure Foundations of Engineers 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 Site-Specific Geotechnical US Army Corps of Engineers 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 Site-Specific Geotechnical US Army Corps of Engineers 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 Site-Specific Geotechnical US Army Corps of Engineers 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 Site-Specific Geotechnical US Army Corps of Engineers 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 Site-Specific Geotechnical US Army Corps of Engineers 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 Site-Specific Geotechnical US Army Corps of Engineers Site Investigations

Preliminary Exploration Phase Investigative Methods:

• Continuous seismic reflection surveys (soil types and strata depth) • Side-scan sonar images (surface soil characteristics, relic structures) • Dry-land methods (electro-resistivity/magnetic, seismic refraction) • 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 Site-Specific Geotechnical US Army Corps of Engineers 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 Site-Specific Geotechnical US Army Corps of Engineers 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 Common Soil Properties of Engineers

Soil Classification by Grain-Size

CHL: Steven Hughes, PhD US Army Corps Common Soil Properties of Engineers Soil Bulk Density

CHL: Steven Hughes, PhD US Army Corps Common Soil Properties of Engineers Volume of Voids

CHL: Steven Hughes, PhD US Army Corps Common Soil Properties of Engineers

Relative Density

CHL: Steven Hughes, PhD US Army Corps Common Soil Properties of Engineers 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 Common Soil Properties of Engineers Soil Deformation Moduli

Note: Typical values are given in tables in the CEM

CHL: Steven Hughes, PhD US Army Corps Soil Strength Properties of Engineers

Soil Stress Definition

CHL: Steven Hughes, PhD US Army Corps Soil Strength Properties of Engineers 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 Soil Strength Properties of Engineers

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 Soil Strength Properties of Engineers

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 Foundation Loads of Engineers 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 Foundation Loads of Engineers

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 Foundation Loads of Engineers 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 Foundation Loads of Engineers 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 Foundation Failure Modes of Engineers Slip Surface and Zone Failure Rubble-Mound Structures and Dikes

CHL: Steven Hughes, PhD US Army Corps Foundation Failure Modes of Engineers Slip Surface and Zone Failure Monolithic Structures

CHL: Steven Hughes, PhD US Army Corps Foundation Failure Modes of Engineers Slip Surface and Zone Failure Tied Wall Structures

CHL: Steven Hughes, PhD US Army Corps Foundation Failure Modes of Engineers Excess Settlement (Including Differential Settlement)

CHL: Steven Hughes, PhD US Army Corps Geotechnical Design Criteria of Engineers 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 Geotechnical Design Criteria of Engineers

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 Geotechnical Design Criteria of Engineers

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 Geotechnical Design Criteria of Engineers 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 Geotechnical Design Criteria of Engineers

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 Geotechnical Design Criteria of Engineers

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