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Some of the more commonly-used geophysical tests are surface resistivity (SR), ground penetrating radar
(GPR), and electromagnetic conductivity (EM) that are effective in establishing ground stratigraphy,
detecting sudden changes in subsurface formations, locating underground cavities in karst formations, or
identifying underground utilities and/or obstructions. Mechanical waves include the compression (P-wave)
and shear (S-wave) wave types that are measured by the methods of seismic refraction, crosshole, and
downhole seismic tests and these can provide information on the dynamic elastic properties of the soil and
rock for a variety of purposes. In particular, the profile of shear wave velocity is required for seismic site
amplification studies of ground shaking, as well as useful for soil liquefaction evaluations.
Disturbed samples are obtained to determine the soil type, gradation, classification, consistency, density,
presence of contaminants, stratification, etc. Disturbed samples may be obtained by hand excavating methods
by picks and shovels, or by truck-mounted augers and other rotary drilling techniques. These samples are
considered “disturbed” since the sampling process modifies their natural structure.
In-situ testing and geophysical methods can be used to supplement soil borings. Certain tests, such as the
electronic cone penetrometer test (CPT), provide information on subsurface soils without sampling
disturbance effects with data collected continuously on a real time basis. Stratigraphy and strength
characteristics are obtained as the CPT progresses in the field. Since all measurements are taken during the
field operations and there are no laboratory samples to be tested, considerable time and cost savings may be
appreciated. In-situ methods can be particularly effective when they are used in conjunction with
conventional sampling to reduce the cost and the time for field work. These tests provide a host of subsurface
information in addition to developing more refined correlations between conventional sampling, testing and
in-situ soil parameters.
Undisturbed samples are used to determine the in place strength, compressibility (settlement), natural
moisture content, unit weight, permeability, discontinuities, fractures and fissures of subsurface formations.
Even though such samples are designated as “undisturbed,” in reality they are disturbed to varying degrees.
The degree of disturbance depends on the type of subsurface materials, type and condition of the sampling
equipment used, the skill of the drillers, and the storage and transportation methods used. As will be
discussed later, serious and costly inaccuracies may be introduced into the design if proper protocol
and care is not exercised during recovery, transporting or storing of the samples.
Frequency and Depth of Borings
The location and frequency of sampling depends on the type and critical nature of the structure, the soil and
rock formations, the known variability in stratification, and the foundation loads. While the rehabilitation
of an existing pavement may require 4 m deep borings only at locations showing signs of distress, the design
and construction of a major bridge may require borings often in excess of 30 m. Table 2-2 provides
guidelines for selecting minimum boring depths, frequency and spacing for various geotechnical features.
Frequently, it may be necessary or desirable to extend borings beyond the minimum depths to better define
the geologic setting at a project site, to determine the depth and engineering characteristics of soft underlying
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REMENTS FOR BOR
Areas of Investigation
Recommended Boring Depth
1. Spread Footings
2. Deep Foundations
For isolated footings of breadth L
, where L
, borings shall
extend a minimum of two footing widths below the bearing level.
For isolated footings where L
, borings shall extend a minimum of four
footing widths below the bearing level.
, minimum boring length shall be determined by linear
interpolation between depths of 2B
below the bearing level.
In soil, borings shall extend below the anticipated pile or shaft tip elevation a
minimum of 6 m, or a minimum of two times the maximum pile group dimension,
whichever is deeper.
For piles bearing on rock, a minimum of 3 m of rock core shall be obtained at
each boring location to verify that the boring has not terminated on a boulder.
For shafts supported on or extending into rock, a minimum of 3 m of rock core,
or a length of rock core equal to at least three times the shaft diameter for isolated
shafts or two times the maximum shaft group dimension, whichever is greater,
shall be extended below the anticipated shaft tip elevation to determine the
physical characteristics of rock within the zone of foundation influence.
Extend borings to depth below final ground line between 0.75 and 1.5 times the
height of the wall. Where stratification indicates possible deep stability or
settlement problem, borings should extend to hard stratum.
For deep foundations use criteria presented above for bridge foundations.
Extend borings a minimum of 2 m below the proposed subgrade level.
Borings should extend a minimum of 5 m below the anticipated depth of the cut
at the ditch line. Borings depths should be increased in locations where base
stability is a concern due to the presence of soft soils, or in locations where the
base of the cut is below groundwater level to determine the depth of the
underlying pervious strata.
Extend borings a minimum depth equal to twice the embankment height unless a
hard stratum is encountered above this depth. Where soft strata are encountered
which may present stability or settlement concerns the borings should extend to
Use criteria presented above for embankments.
*Note: Taken from AASHTO Standard Specifications for Design of Highway Bridges
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soil strata, or to assure that sufficient information is obtained for cases when the structure requirements are
not clearly defined at the time of drilling. Generally it should be assumed that the structure may have an
influence on the supporting subgrade soils down to a depth of twice the foundation width for static loads and
four times the foundation width for seismic loads. Where borings are drilled to rock and this rock will impact
foundation performance, it is generally recommended that a minimum 1.5-m length of rock core be obtained
to verify that the boring has indeed reached bedrock and not terminated on the surface of a boulder. Where
structures are to be founded directly on rock, the length of rock core should be not less than 3 m, and
extended further if the use of socketed piles or drilled shafts are anticipated. Selection of boring depths at
river and stream crossings must consider the potential scour depth of the stream bed.
The frequency and spacing of borings will depend on the variability of subsurface conditions, type of facility
to be designed, and the investigative phase being performed. For conceptual design or route selection studies,
very wide boring spacing (up to 300 m, or more) may be acceptable particularly in areas of generally uniform
or simple subsurface conditions. For preliminary design purposes a closer spacing is generally necessary,
but the number of borings would be limited to that necessary for making basic design decisions. For final
design, however, relatively close spacings of borings may be required, as suggested in Table 2-3.
Subsurface investigation programs, regardless to how well they may be planned, must be flexible to adjust
to variations in subsurface conditions encountered during drilling. The project geotechnical engineer should
at all times be available to confer with the field inspector. On critical projects, the geotechnical engineer
should be present during the field investigation. He/she should also establish communication with the design
engineer to discuss unusual field observations and changes to be made in the investigation plans.
Boring Locations and Elevations
It is generally recommended that a licensed surveyor be used to establish all planned drilling locations and
elevations. For cases where a surveyor cannot be provided, the field inspector has the responsibility to locate
the borings and to determine ground surface elevations at an accuracy appropriate to the project needs.
Boring locations should be taped from known site features to an accuracy of about ±1.0 m for most projects.
Portable global positioning systems (GPS) are also of value in documenting locations. When a topographic
survey is provided, boring elevations can be established by interpolation between contours. This method of
establishing boring elevations is commonly acceptable, but the field inspector must recognize that the
elevation measurement is sensitive to the horizontal position of the boring. Where contour intervals change
rapidly, the boring elevations should be determined by optical survey.
A reference benchmark (BM) should be indicated on the site plans and topographic survey. If a BM is not
shown, a temporary benchmark (TBM) should be established on a permanent feature (e.g., manhole,
intersection of two streets, fire hydrant, or existing building). A TBM should be a feature that will remain
intact during future construction operations. Typically, the TBM is set up as an arbitrary elevation (unless
the local ground elevation is uniform). Field inspectors should always indicate the BM and/or TBM that was
used on the site plan.
An engineer’s level may be used to determine elevations. The level survey should be closed to confirm the
accuracy of the survey. Elevations should be reported on the logs to the nearest tenth of a meter unless other
directions are received from the designers. In all instances, the elevation datum must be identified and
recorded. Throughout the boring program the datum selected should remain unchanged.
A list of equipment commonly needed for field explorations is presented in Table 2-4.
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NES FOR BOR
For piers or abutments over 30 m wide, provide a minimum of
For piers or abutments less than 30 m wide, provide a minimum
of one boring.
Additional borings should be provided in areas of erratic
A minimum of one boring should be performed for each retaining
wall. For retaining walls more than 30 m in length, the spacing
between borings should be no greater than 60 m. Additional
borings inboard and outboard of the wall line to define
conditions at the toe of the wall and in the zone behind the wall
to estimate lateral loads and anchorage capacities should be
The spacing of borings along the roadway alignment generally
should not exceed 60 m. The spacing and location of the borings
should be selected considering the geologic complexity and
soil/rock strata continuity within the project area, with the
objective of defining the vertical and horizontal boundaries of
distinct soil and rock units within the project limits.
A minimum of one boring should be performed for each cut
slope. For cuts more than 60 m in length, the spacing between
borings along the length of the cut should generally be between
60 and 120 m.
At critical locations and high cuts, provide a minimum of three
borings in the transverse direction to define the existing
geological conditions for stability analyses. For an active slide,
place at least one boring upslope of the sliding area.
Use criteria presented above for Cuts.
A minimum of one boring at each major culvert. Additional
borings should be provided for long culverts or in areas of erratic
*Also see FHWA Geotechnical Checklist and Guidelines; FHWA-ED-88-053
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ST OF EQU
PMENT FOR F
Field Instructions Sheet(s)
Daily field memorandum forms
Blank boring log forms
Forms for special tests (vane shear, permeability tests, etc.)
Blank sample labels or white tape
Copies of required permits
Field book (moisture proof)
Health and Safety plan
Subcontractor expense forms
Samplers and blank tubes etc.
Knife (to trim samples)
Folding rule (measured in 1 cm increments)
25 m tape with a flat-bottomed float attached to its end so that
it can also be used for water level measurements
Hand level (in some instances, an engineer’s level is needed)
Jars and core boxes
Sample boxes for shipping (if needed)
Buckets (empty) with lid if bulk samples required
Safety glasses (when working with hammer or chisel)
Rubber boots (in some instances)
Rain gear (in some instances)
Pencils, felt markers, grease pencils
Scale and straight edge
Wash bottle or test tube
Pocket Penetrometer and/or Torvane
Communication Equipment (two-way radio, cellular phone)
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Personnel and Personal Behavior
The field crew is a visible link to the public. The public's perception of the reputation and credibility of the
agency represented by the field crew may be determined by the appearance and behavior of the personnel
and field equipment. It is the drilling supervisor’s duty to maintain a positive image of field exploration
activities, including the appearance of equipment and personnel and the respectful behavior of all personnel.
In addition, the drilling supervisor is responsible for maintaining the safety of drilling operations and related
work, and for the personal safety of all field personnel and the public. The designated Health and Safety
Officer is responsible for verifying compliance of all field personnel with established health and safety
procedures related to contaminated soils or groundwater. Appendix A presents typical safety guidelines for
drilling into soil and rock and health and safety procedures for entry into borings.
The field inspector may occasionally be asked about site activities. The field inspector should always identify
the questioner. It is generally appropriate policy not to provide any detailed project-related information, since
at that stage the project is normally not finalized, there may still be on going discussions, negotiations, right-
of-way acquisitions and even litigation. An innocent statement or a statement based on one’s perception of
the project details may result in misunderstandings or potentially serious problems. In these situations it is
best to refer questions to a designated officer of the agency familiar with all aspects of the project.
Plans and Specifications
Each subsurface investigation program must include a location plan and technical specifications to define and
communicate the work to be performed.
The project location plan(s) should include as a minimum: a project location map; general surface features
such as existing roadways, streams, structures, and vegetation; north arrow and selected coordinate grid
points; ground surface contours at an appropriate elevation interval; and locations of proposed structures and
alignment of proposed roadways, including ramps. On these plans, the proposed boring, piezometer, and in-
situ test locations should be shown. A table which presents the proposed depths of each boring and sounding,
as well as the required depths for piezometer screens should be given.
The technical specifications should clearly describe the work to be performed including the materials,
equipment and procedures to be used for drilling and sampling, for performing in situ tests, and for installing
piezometers. In addition, it is particularly important that the specifications clearly define the method of
measurement and the payment provisions for all work items.
STANDARDS AND GUIDELINES
Field exploration by borings should be guided by local practice, by applicable FHWA and state DOTs
procedures, and by the AASHTO and ASTM standards listed in Table 2-5.
Current copies of these standards and manuals should be maintained in the engineer’s office for ready
reference. The geotechnical engineer and field inspector should be thoroughly familiar with the contents of
these documents, and should consult them whenever unusual subsurface situations arise during the field
investigation. The standard procedures should always be followed; improvisation of investigative techniques
may result in erroneous or misleading results which may have serious consequences on the interpretation of
the field data.
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FREQUENTLY-USED STANDARDS FOR F
Descriptive Nomenclature for Constituents of Natural Mineral Aggregates
Guide for Investigating and Sampling Soil and Rock
Test Method for Bearing Capacity of Soil for Static Load on Spread
Test Method for Repetitive Static Plate Load Tests of Soils and Flexible
Pavement Components, for Airport and Highway Pavements
Test Method for Nonrepetitive Static Plate Load Tests of Soils and
Flexible Pavement Components, for Use in Evaluation and Design of
Airport and Highway Pavements
Practice for Soil Investigation and Sampling by Auger Borings
Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils
Practice for Thin-Walled Tube Sampling of Soils
Practice for Diamond Core Drilling for Site Investigation
Test Method for Classification of Soils for Engineering Purposes
Practice for Description and Identification of Soils (Visual-Manual
Test Method for Field Vane Shear Test (VST) in Cohesive Soil
Practice for Ring-Lined Barrel Sampling of Soils
Practice for Preserving and Transporting Soil Samples
Test Method for Crosshole Seismic Test (CHT)
Practice for Estimating Peat Deposit Thickness
General Methods of Augering, Drilling, & Site Investigation
Test Method for Pressuremeter Testing (PMT) in Soils
Test Method for Determining Subsurface Liquid Levels in a Borehole or
Monitoring Well (Observation Well)
Practices for Preserving and Transporting Rock Core Samples
Design and Installation of Ground Water Monitoring Wells in Aquifers
Guide for Seismic Refraction Method for Subsurface Investigation
Test Method for Electronic Cone Penetration Testing (CPT) of Soils
Procedures for Flat Plate Dilatometer Testing (DMT) in Soils
Field Measurement of Soil Resistivity (Wenner Array)
Documents you may be interested
Documents you may be interested