{ CE 443 Foundation Design Fall 2022 Week 1 { Subsurface Investigations Learning Objective In this lecture we will introduce the concepts and strategies employed in executing a Subsurface Investigation. The Subsurface Investigation generally consists of the following four stages: 1. 2. 3. 4. Desk Study (Collection of Preliminary Information and Project Assessment) Site Reconnaissance Site Investigation Laboratory Testing Program It is imperative that the engineer have an understanding of the regional geology, surficial geology, groundwater conditions and depositional characteristics of the project site. This understanding can not be understated and is critical in the successful design of a cost efficient and constructible foundation which is your goal as foundation engineer. Introduction • All infrastructure is built either on earth, in earth, and/or with earth. • Unlike manufactured construction materials, the properties of soil a rock are the results of the natural processes that have formed them, and natural or man-made events following their formation. The replacement of inferior foundation materials often is impractical and uneconomical. • The large volume of soil and rock needed for construction of bridges, buildings and the like, as a rule, makes it prohibitive to manufacture and transport pre-engineered materials. • The foundations engineer in designing and constructing these facilities is faced with the challenge of using the foundation and construction materials available on or near the project site. • Therefore, the designing and building of such structures requires a thorough understanding of properties of available soils and rocks that will constitute the foundation and other components of the structures. Purpose of a Subsurface Investigation Subsurface Investigation: The process of identifying layers and deposits that underlie a proposed structure and their physical characteristics. The purpose of the investigation is to obtain information that will aid the geotechnical engineer in: • Identifying the location and thickness of soil and rock. • Selecting type and depth of foundations. • Evaluating load bearing capacity. • Estimating settlement. • Identifying potential foundation challenges and issues. • Locating the water table. • Predicting lateral earth pressures. • Establish construction methods for changing soil conditions. Desk Study / Project Assessment Before locating borings or performing a single test, gather the following information (if available): • Type of proposed development or remediation (building/bridge type, location/alignment, dimensions, use, etc…) • Magnitude of loads and allowable displacement tolerances (settlements). • Existing topography (survey) and proposed grading. • Previous developments at proposed location. • Design Criteria required for design (AASHTO, NCY Building Code, AREMA, IBC, NYSDOT, NJDOT, etc…) Desk Study: Literature Search Prior to development of a field investigation program a literature search of all available data should be conducted. This can include: • Utility Maps • Aerial Photographs, Satellite Imagery, Orthophotography • Topographic Maps • Existing Subsurface Investigation Reports • Geologic Reports and Maps • Water/Brine Well Logs • Flood Insurance Maps • Soil Survey • Sanborn Fire Insurance Maps Desk Study: Literature Search Review Available Information From FHWA Geotechnical Circular No. 5, Evaluation of Soil and Rock Properties, FHWA-IF-02-034 Desk Study: Literature Search Review Available Information From FHWA Geotechnical Circular No. 5, Evaluation of Soil and Rock Properties, FHWA-IF-02-034 Desk Study: Literature Search Resources available in for the State of NJ to aid in the Literature Search http://elibrary.rutgers.edu/quadpage/ https://njgin.state.nj.us/DOT_GDMS/ http://www.state.nj.us/dep/njgs/geodata/ http://www.state.nj.us/dep/njgs/pricelst/geolmapquad.htm http://www.state.nj.us/dep/njgs/pricelst/ofmaps.htm Lets look at some examples…. Desk Study: Literature Search Virtual Site Visit: Google Earth Explore Google Earth. • Visit your site without leaving our desk. Desk Study: Literature Search Topography: Rutgers Geologic Topographic Map Download Depot http://elibrary.rutgers.edu/quadpage/ • • • • Provides index map of site area Allows for estimation of sit topography Identifies physical features in the site area Can be used to assess access restrictions Desk Study: Literature Search Existing Subsurface Data: Geotechnical Data Management System (GDMS) Viewer https://geoapps.nj.gov/dot_gdms/ GDMS (nj.gov) May provide information on nearby soil/rock type; strength parameters; hydrological issues; environmental concerns. • May provide information on layer thickness and location of rock. Desk Study: Literature Search Surficial Geology: New Jersey Geological Survey http://www.state.nj.us/dep/njgs/geodata/ http://www.state.nj.us/dep/njgs/pricelst/g eolmapquad.htm United States Geologic Survey (USGS) https://www.usgs.gov/products/maps/ge ologic-maps • • • • • • Soil Characteristics Hydrological Characteristics Geologic Formation Existing Borings and Wells Environmental Concerns May provide information on layer thickness and location of rock. • Subsurface Profiles Desk Study: Literature Search Bedrock Geology: New Jersey Geological Survey http://www.state.nj.us/dep/njgs/geodata/ http://www.state.nj.us/dep/njgs/pricelst/g eolmapquad.htm United States Geologic Survey (USGS) https://www.usgs.gov/products/maps/ge ologic-maps • • • • • • Rock Characteristics Geomorphology Existing Faults Strength Characteristics Chemical Composition Subsurface Profiles Desk Study: Literature Search Soil Survey Reports: USDA Natural Resources Conservation Service (NRCS) http://websoilsurvey.nrcs.usda.gov/ap p/WebSoilSurvey.aspx • • • • Identifies site soil type Permeability of site soils Climatic and geologic information Environmental Concerns Desk Study: Literature Search Sanborn Maps: The maps include: • Outlines of each building and outbuilding; • Street names; • Street and sidewalk widths; • Property boundaries; • Natural features (rivers, canals, etc.); • Railroad corridors; • House and block number; • Composition of building materials including the framing, flooring, and roofing materials; • Location of water and gas mains; • Names of most public buildings, churches and businesses. Source: Wikipedia, Accessed on 9/10/16 Site Reconnaissance After review of the available data and prior to mobilizing investigation equipment to the project site, a site visit should be performed. During the site visit, the engineer should carefully observe all relevant physical features of the area and record detailed notes. These observations should include: • Utility locations (overhead and underground); • Access issues (e.g., location, width, and condition of all potential) • Access roads; trees; power lines; buildings; right-of-way); • Conditions of nearby structures (record location, type, and depth of existing structures and foundations); • Geologic constraints (e.g., rivers, streams, bluffs, outcrops); • Exposed rock outcrops, locations and orientation; Source: FHWA CEC No. 5, Evaluation of Soil and Rock Properties, FHWA-IF-02-034 Site Reconnaissance Cont. • • • • • • • • Distance to Railroads. Topographic conditions (e.g., ditches, hills, valleys); Soil/rock type (e.g., clay, sand, rock outcrops, conditions when wet); Surface conditions (e.g., desiccated surface, lack of vegetation, debris, ponded water, deposits of colluvium or talus, evidence of rock/soil slope failures); Geomorphic controls (e.g., landslides, floodplains, karst, erosional/depositional conditions); Flood levels/drainage issues; Adjacent property use and potential performance issues; Potential borrow source areas (if applicable). Source: FHWA CEC No. 5, Evaluation of Soil and Rock Properties, FHWA-IF-02-034 Site Investigation This phase of the subsurface investigation consists of planning and implementing the site investigation. Identify Required Material Properties: • Identify the specific goals of the investigation related to design and construction issues (i.e., performance requirements); • Identify the engineering properties that are needed; • Identify the type of structure that is to be constructed and the required foundation type or alternatives if known. Source: FHWA CEC No. 5, Evaluation of Soil and Rock Properties, FHWA-IF-02-034 Site Investigation Plan Site Investigation: Historical information gather during the Desk Study phase, coupled with knowledge of the specific design will allow an efficient site specific investigation strategy to be developed. • Contingency plans should be considered based on anticipated variability in subsurface conditions. • Sampling intervals should be identified and an in situ testing program should be developed that meet the goals of the investigation. • Generally, there are five types of field subsurface investigation methods: 1. Remote sensing 2. Geophysical investigations 3. Disturbed sampling 4. In-situ testing 5. Undisturbed sampling Source: FHWA CEC No. 5, Evaluation of Soil and Rock Properties, FHWA-IF-02-034 Site Investigation Specific tools for sampling and testing include: Non-Intrusive Testing • Geophysical and Remote Sensing Methods Disturbed Sampling • Standard Penetration Test (SPT), augering, non-core rock drilling (by means of borings) In-Situ Soil and Rock Testing • Cone Penetrometer Tests (CPT), Flat-plate Dilatometer Test (DMT), Vane Shear Test (VST), Pressuremeter Test (PMT), Borehole Dilatometer, In-situ Direct Shear Test, etc. Undisturbed Sampling • Thin Walled (Shelby) Tubes, Block Samples, Rock Core (by means of Source: FHWA CEC No. 5, Evaluation of Soil and borings) Rock Properties, FHWA-IF-02-034 Site Investigation Source: FHWA CEC No. 5, Evaluation of Soil and Rock Properties, FHWA-IF-02-034 Site Investigation: Borings Geotechnical borings are a critical component of any subsurface exploration program. They are performed to satisfy several objectives including those listed below: • Identification of the subsurface distribution of materials with distinctive properties, including the presence and geometry of distinct layers; • Determination of data on the characteristics of each layer by retrieving samples for use in evaluating engineering properties; • Acquisition of groundwater data; and • Provide access for disturbed sampling, undisturbed sampling and introduction of in-situ testing tools. Source: FHWA CEC No. 5, Evaluation of Soil and Rock Properties, FHWA-IF-02-034 Site Investigation: Borings Frequency and Depth of Borings The location and frequency of sampling depends on: • Type and critical nature of the structure; • Types of soil and rock formations, expected; • Variability in stratification; • Foundation loads; • Type of foundation (shallow or deep); • Size of site; • Site Access; • Published Design Guidelines and Criteria While the rehabilitation of an existing pavement may require 12 ft deep borings only at locations showing signs of distress, the design and construction of a major bridge may require borings often in excess of 100 ft. Site Investigation: Borings Example Guidelines for Depths of Explanatory Borings • AASHTO LRFD Bridge Design Specifications Table 10.4.2-1 – Minimum Number of Exploratory Points and Depth of Exploration (modified after Sabatini et al., 2002) Source: AASHTO LRFD Bridge Design Specifications 6th Ed. Site Investigation: Borings Example Guidelines for Depths of Explanatory Borings • AASHTO LRFD Bridge Design Specifications Table 10.4.2-1 – Minimum Number of Exploratory Points and Depth of Exploration Source: AASHTO LRFD Bridge Design (modified after Sabatini et al., 2002) th Specifications 6 Ed. Site Investigation: Borings Example Guidelines for Depths of Explanatory Borings • AASHTO LRFD Bridge Design Specifications Table 10.4.2-1 – Minimum Number of Exploratory Points and Depth of Exploration Source: AASHTO LRFD Bridge Design (modified after Sabatini et al., 2002) th Specifications 6 Ed. Site Investigation: Borings Example Guidelines for Depths of Explanatory Borings • NYC Building Code – Section 1802.0 Foundation Investigations Source: NYC Building Code Site Investigation: Borings Example Guidelines for Depths of Explanatory Borings Source: Foundation Design, Principles and Practices, 3rd Edition, Coduto, Kitch, & Yeung. Pearson, 2016 Site Investigation: Borings Example Guidelines for Depths of Explanatory Borings Source: Foundation Design, Principles and Practices, 3rd Edition, Coduto, Kitch, & Yeung. Pearson, 2016 Site Investigation: Borings Example Guidelines for Depths of Explanatory Borings Determining the depth relies on rules established by the American Society of Civil Engineers. 1. Determine the net increase in effective stress (Δσ’) under the foundation with specific depth. 2. Estimate the variation of vertical effective stress with depth (Δσo’) . 3. Determine the depth D = D1 where effective stress is equal to 0.1q where q is the estimated net stress on foundation. 4. Determine the depth D = D2 where effective stress divided by vertical effective stress is Δσ’/ Δσo’ < 0.05 Source: Das, Principles of Foundation Engineering, 8th edition, 2016. Site Investigation: Borings A wide variety of equipment is available for performing borings and obtaining soil samples. The method used to advance the boring should be compatible with the soil and groundwater conditions. Below the groundwater level, the sidewalls and bottom of the boring in soft clays or cohesionless soils often need to be stabilized. In most geotechnical explorations, borings are usually advanced with one of the following methods: • Solid stem continuous flight, • Hollow-stem augers, • Rotary wash boring methods. Source: FHWA CEC No. 5, Evaluation of Soil and Rock Properties, FHWA-IF-02-034 Site Investigation: Borings Solid Stem Continuous Flight Auger • For use in Firm and Dense Soil • At desired depths flights are removed and sampling equipment is inserted. • Can meet refusal with large boulders Site Investigation: Borings Methods of dealing with caving or squeezing soils: Casing Hollow Stem Auger Rotary Wash Boring Source: Foundation Design, Principles and Practices, 3rd Edition, Coduto, Kitch, & Yeung. Pearson, 2016 Site Investigation: Borings Hollow Stem Auger • For use in soils prone to caving or squeezing • Sampling is performed through the auger • Eight inch diameter augers are common Site Investigation: Borings Rotary Wash Drilling • For use in soils prone to caving or squeezing • Roller or chopping bit is attached to drill rods • Three to six inch diameter borings are common Site Investigation: Borings Rock Core • Collects undisturbed rock samples • Single core or double core barrels available. • Quality of rock is assessed by Rock Quality Designation (RQD) Site Investigation: Borings Rock Quality Designation The RQD is a modified core recovery percentage in which the lengths of all pieces of sound core over 100 mm (4 in) long are summed and divided by the length of the core run. The RQD is an index of rock quality in that problematic rock that is highly weathered, soft, fractured, sheared, and jointed typically yields lower RQD values. Thus, RQD is simply a measurement of the percentage of "good" rock recovered from an interval of a borehole. Source: NHI Course No 132031 Subsurface Investigations – Geotechnical Site Characterization, FHWA NHI-01-031, May 2002 Site Investigation: Water Table Observation of Water Table • A water table near a foundation significantly affects the foundation’s load-bearing capacity and settlement. • The water level will change seasonally, and after precipitation events • It is important to establish the highest and lowest possible levels of water during the life of a project. • Groundwater level is the level to which water fills an open boring. • Can install piezometer (slotted PVC pipe) into borehole to collect water table measurement (natural, artesian, depressed) • Use an electronic probe to measure depth of groundwater. Site Investigation: Sampling and Testing Insitu or Exitu • Insitu tests are performed in the field with minimum disturbance to the soils natural state. • Exitu tests are conducted in a laboratory, with disturbed, remolded or undisturbed soil samples from the field. Disturbed or Undisturbed • Disturbed samples: • Bulk Samples. • SPT Samples. • Undisturbed sample: • Thin wall sampler (Shelby tube) “relatively undisturbed” • Piston Samplers • Ground Freezing. Site Investigation: Sampling and Testing Sample Disturbance Sand Silt Clay Grab Samples D D D Split Spoon Samples D D D/U Thin Wall Samples D D/U U Piston Samples D/U U U Frozen/Block Samples U U U D- Disturbed U-Undisturbed Site Investigation: Sampling and Testing Thin Walled Tube (Shelby Tube) • Made of seamless steel. • Area ratio < 10% disturbance (“undisturbed”) • Tubes can be attached to drill rods. • Samples may be used for consolidation or shear tests. • Moisture needs to be sealed and avoid damage! (don’t throw in trunk Source: Das, Principles of Foundation Engineering, 8th edition, 2016. Site Investigation: Boring Logs Site Investigation: Boring Logs Site Investigation: In-Situ Testing Site Investigation: In-Situ Testing Assessment of In-Situ Test Methods Source: Foundation Design, Principles and Practices, 3rd Edition, Coduto, Kitch, & Yeung. Pearson, 2016 Site Investigation: In-Situ Testing Standard Penetration Test (SPT) • Developed in 1920’s in the UK. • Widely accepted and available which makes it inexpensive. • Consists of driving a sampler “spoon” a total of 18 inches (sometimes 24 inches) by dropping a 140 pound weight a distance of 30 in. Site Investigation: In-Situ Testing Standard Penetration Test (SPT) Procedure: • Drill a 2.5 to 8 in hole to the required depth of testing. • Insert SPT sampler. • Use a rope and cathead, or an automatic tripping hammer, to drop a 140 lb hammer a distance of 30 inches. • Record how many blows to penetrate the sampler a distance of 6 inches, for a total of 18 inches (sometimes 24 inches). • Compute N-value (sum the blow counts for the last 12 inches or middle 12 when 24 inches is samples). • Extract the sampler and remove soil sample. • Drill to the next depth and repeat. Site Investigation: In-Situ Testing Types of SPT hammers Source: Das, Principles of Foundation Engineering, 8th edition, 2016. Site Investigation: In-Situ Testing Donut Hammers Site Investigation: In-Situ Testing Safety Hammers Site Investigation: In-Situ Testing Auto hammers Site Investigation: In-Situ Testing Correction to SPT test results Standard practice dictates expressing the N-value to an average energy ratio of 60% (N60) • N60 = Corrected SPT for field procedures • nH = Hammer efficiency • nB = Borehole diameter correction • nS = Sampler correction • nR = Rod length correction • N = SPT N-value from field Other Corrections: Overburden Source: Das, Principles of Foundation Engineering, 8th edition, 2016. Site Investigation: In-Situ Testing Correction to SPT test results (cont.) Source: Das, Principles of Foundation Engineering, 8th edition, 2016. Site Investigation: In-Situ Testing Correction to SPT test results (cont.) In granular soils, the value of N60 is affected by the effective overburden pressure. The value of N60 obtained from field exploration under granular soil effective overburden pressures should be changed to correspond to a standard value of σ’o and is notated as (N1)60. (N1)60 = CNN60 Liao and Whitman’s relationship for correction factor CN Where: Pa = atmospheric pressure (2000 psf) σ’o = effective overburden pressure Source: Das, Principles of Foundation Engineering, 8th edition, 2016. Site Investigation: In-Situ Testing Cone Penetration Test (CPT) Source: NHI Course No 132031 Subsurface Investigations – Geotechnical Site Characterization, FHWA NHI-01-031, May 2002 Site Investigation: In-Situ Testing Cone Penetration Test (CPT) • Developed in 1930’s, 1950’s and recent decades. • Originally known as the Dutch cone. • Consists of a 35.7 mm dia. cone shaped tip (60° apex) with a 35.7 mm dia. by 133.7 mm long sleeve. • Follows ASTM D 5778 Procedures • Hydraulic Push at 20 mm/s • No Boring, No Samples, No Cuttings, No Spoil • Continuous readings of stress, friction, pressure • Two types of cones: • Mechanical cone • Electronic cone • Piezocones: Have an embedded pressure transducer. Site Investigation: In-Situ Testing Cone Penetration Test (CPT) Advantages • Fast and continuous profiling; • Economical and productive; • Results not operator dependent; • Strong Theoretical basis in interpretation; • Particularly suited for soft soils Disadvantages • High Capital Investment • Requires a skilled operator to run • Electronic drift, noise and calibration • No soil samples are obtained • Unsuitable for gravel of boulder deposits. Source: NHI Course No 132031 Subsurface Investigations – Geotechnical Site Characterization, FHWA NHI-01-031, May 2002 Site Investigation: In-Situ Testing Cone Penetration Test (CPT) SPT-N (bpf) and qc (MPa) 0 20 40 60 80 Soil Profile 100 0 Results can be used to estimate: 4 1982-B3 • Relative density • N60 • Soil types • Undrained shear strength 8 D e p th (m e te rs) • Drained angle of internal friction 12 Fill Silty Sand 1982-B5 CPT-qc (MPa) Sandy Silt 16 Gravelly Sand 20 Desiccated 24 Clayey Sand 28 OC Clay Gravelly Sand • OCR Source: NHI Course No 132031 Subsurface Investigations – Geotechnical Site Characterization, FHWA NHI-01-031, May 2002 1982 B1 Site Investigation: In-Situ Testing Vane Shear Test (VST) Source: NHI Course No 132031 Subsurface Investigations – Geotechnical Site Characterization, FHWA NHI-01-031, May 2002 Site Investigation: In-Situ Testing Vane Shear Test (VST) • Used to determine the insitu undrained shear strength (cu) of clay soils. • Vane consists of four blades fastened to rods. At the test depth rotate and record torque and rotation. • Can only be used in very soft to soft clays and silt. 6 = 7 Where; • Tf = torque at failure • d = diameter of vane • λ = empirical correction factor cu can also be used to estimate preconsolidation pressure ( ′ ) and over consolidation ratio (OCR). Site Investigation: In-Situ Testing Vane Shear Test (VST) Advantages • Assessment of undrained shear strength • Simple test and equipment • Measure in-situ clay sensitivity • Long history of use in practice Disadvantages • Limited application to stiff clays • Slow and time consuming • Raw suv needs empirical correction • Can be affected by sand lenses and seams Source: NHI Course No 132031 Subsurface Investigations – Geotechnical Site Characterization, FHWA NHI-01-031, May 2002 Site Investigation: In-Situ Testing Pressuremeter Test (PMT) Source: NHI Course No 132031 Subsurface Investigations – Geotechnical Site Characterization, FHWA NHI-01-031, May 2002 Site Investigation: In-Situ Testing Pressuremeter Test (PMT) • An in situ test conducted in a borehole. • Developed by Louis Menard • Consists primarily of a probe with three cells. • Top and bottom ones are guard cells. • The middle cell is the measuring cell. • The measuring cell volume (Vo), is measured; probe inserted into the borehole. • Soil is considered to have failed when the total volume of the expanded cavity (V) is about twice the volume of the original cavity. Used to evaluate: • In-situ stress. • Compressibility and strength of soils (i.e., undrained shear strength and preconsolidation pressure) • P-Y Curves for lateral design. Source: Das, Principles of Foundation Engineering, 8th edition, 2016. Site Investigation: In-Situ Testing Pressuremeter Test (PMT) Advantages • Theoretically sound in detemination of soil parameters • Tests larger zone of soil mass than other in-situ tests]] Disadvantages • Complicated procedures; requires high level of expertise in the field • Time consuming and expensive (good day gives 6 to 8 tests) • Delicate and easily damaged Source: NHI Course No 132031 Subsurface Investigations – Geotechnical Site Characterization, FHWA NHI-01-031, May 2002 Site Investigation: In-Situ Testing Dilatometer Test (DMT) Source: NHI Course No 132031 Subsurface Investigations – Geotechnical Site Characterization, FHWA NHI-01-031, May 2002 Site Investigation: In-Situ Testing Dilatometer Test (DMT) • Developed by Silvano Marchetti (1970’s) • Consists of a 95mm wide, 15mm thick metal blade with a thin flat circular steel membrane on one side. Procedure: • Press dilatometer into soil. • Apply nitrogen gas pressure to membrane. Record pressure to move center of membrane 0.05mm, and the required pressure to move it 1.10mm. • Depressurize and record pressure (measure of Pore water pressure). Benefit: • Lateral stress conditions. • Compressibility. • P-Y Curves. • Undrained shear strength • Friction angle Site Investigation: In-Situ Testing Dilatometer Test (DMT) Advantages • Simple and robust. • Repeatable and operator independent. • Quick and economical. Disadvantages • Difficult to push in dense and hard materials. • Primarily relies on correlative relationships. • Need calibrations for local geologies. Source: NHI Course No 132031 Subsurface Investigations – Geotechnical Site Characterization, FHWA NHI-01-031, May 2002 Site Investigation: Test Pits Exploratory Trenches and Test Pits • Excavation typically performed with a backhoe. • Do not enter unless test pits are adequately shored or are sloped following OSHA excavation requirements. • Test pits should be properly backfilled when completed. NO!!! Site Investigation: Remote Sensing Remote Sensing is the process of detecting features on the earths surface from a remote location Aerial Photography • Overlapping of aerial photography for 3-D imaging. Infrared Aerial Photography • Detects presence of water • Health of vegetation • Health of wetlands LIDAR (Light Detection And Ranging) • Measures distance by illuminating a target with a laser light Site Investigation: Geophysical Exploration Several types of geophysical exploration techniques permit a rapid evaluation of sub-soil characteristics. Three types of geophysical exploration technique • Seismic refraction survey • Cross-hole seismic survey • Resistivity survey Geophysical Exploration Techniques: • Also allow rapid coverage of large areas and are less expensive than conventional exploration by drilling. • However, definitive interpretation of the results is difficult. Source: Das, Principles of Foundation Engineering, 8th edition, 2016. Site Investigation: Geophysical Exploration Seismic Refraction Survey • Used to determine the thickness of the layering of various soils • Used to determine the depth to rock or hard soil. • -Conducted by impacting the surface, such as at point A and observing the first arrival of the disturbance (stress waves) at several other points (e.g., B, C, D,…). Source: Das, Principles of Foundation Engineering, 8th edition, 2016. Site Investigation: Geophysical Exploration Seismic Refraction Survey The velocities of P waves in various layers indicate the types of soil or rock present below the ground surface. Source: Das, Principles of Foundation Engineering, 8th edition, 2016. Site Investigation: Geophysical Exploration Cross-Hole Seismic Survey Determines the velocity shear waves created as the result of an impact to a given layer of soil. Source: Das, Principles of Foundation Engineering, 8th edition, 2016. Site Investigation: Geophysical Exploration Electrical Resistivity Survey Most common procedure • Four electrodes driven into the ground and spaced equally along a straight line (Wenner method). • The two outside electrodes are used to send an electrical current I into the ground. • The voltage drop, V, is measured between the two inside electrodes. Source: Das, Principles of Foundation Engineering, 8th edition, 2016. Site Investigation: Geophysical Exploration Electrical Resistivity Survey The resistivity of soils depends primarily on their moisture content and concentration of dissolved ions. • Saturated clays have a very low resistivity. • Dry soils and rocks have a high resistivity. Source: Das, Principles of Foundation Engineering, 8th edition, 2016. Site Investigation: Subsurface Profiles Borings can be compiled to create a subsurface profile Site Investigation: Laboratory Testing Laboratory Tests Soil Identification Tests • ASTM D2216 – Water Content • ASTM D422 - Sieve analysis including hydrometer • ASTM 4318 - Atterberg Limits Shear strength Tests • ASTM D3080 - Direct Shear • ASTM D2850 – Triaxial Unconsolidated Undrained (UU) • ASTM D4767 – Triaxial Consolidated Undrained (CIU) • Elastic Modulus Consolidation Tests • ASTM D2435 – Incremental Consolidation Site Investigation: Laboratory Testing Laboratory Tests – Triaxial Testing Site Investigation: Laboratory Testing Laboratory Tests – Consolidation Site Investigation: Laboratory Testing Laboratory Tests – Direct Shear Site Investigation To complete the site investigation phase and prepare for design the following general steps should be followed: 1. Conduct Site Investigation and Field Testing 2. Describe, Collect and Log Samples 3. Review Design Objectives 4. Develop Subsurface Profile 5. Select Samples for Performance Testing 6. Conduct Laboratory Testing 7. Perform Design Source: FHWA CEC No. 5, Evaluation of Soil and Rock Properties, FHWA-IF-02-034 Week 1 Assignments • • • • Complete Assigned HW Review Chapter 2 Review Chapter 3.11 to 3.29 Read Chapter 6.1 to 6.5
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