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 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 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 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