This section contains information on exploration methods including use of air photos and remote sensing, geophysical methods, test pits, test borings, and penetrometers. Also presented is information on methods of sampling, measuring in situ properties of soil and rock, field measurements, and geotechnical monitoring equipment.
The initial step in any project must include consideration of the soil or rock on which the embankments and structures are to be supported. The extent of the site investigation will depend on many factors, not the least of which will be the project scheduling, general subsurface conditions, and the nature of the loads to be supported. In any event, certain basic steps should be followed before a drill rig moves onto the project. The first step in the investigation is to collect and analyse all existing data.
The area concept of site investigation allows the foundation engineer to extend the results from a limited number of explorations in a particular landform to the entire deposit. This concept is a powerful tool in reducing subsurface exploration costs and in providing the planning engineer the following useful data in the location phase:
- Design. Knowledge of the landforms and of the engineering properties of the soils enables the designer to determine the most economical location for highway alignment and grade, to evaluate design problems for each type of soil deposit, and to determine sources of granular borrow.
- Construction. The type and extent of problem soils to be encountered during construction may be predetermined, and construction cost more accurately estimated.
4.2. General Requirements of Field Investigations
The initial phase of field investigations should consist of detailed review of geological conditions at the site and in its general environs. This should include a desktop study of available data including historical data, remote sensing imagery, aerial photography, and a field reconnaissance. The information obtained should be used as a guide in planning the exploration.
To the extent possible, borings should be supplemented by lower cost exploration techniques such as test pits, probes, seismic refraction surveys, and electrical resistivity surveys. This is particularly true in the offshore environment where borings are exceptionally expensive.
The extent of the exploration will vary considerably with the nature of the project. However, the following general standards apply to all investigation programs or as appropriate for the specific project and agreed upon:
- Preliminary exploration depths should be estimated from data obtained during field reconnaissance, existing data, and local experience. The borings should penetrate unsuitable founding materials (organic soils, soft clays, loose sands, etc.) and terminate in competent material. Competent materials are those suitable for support of the foundations being considered.
- All borings shall be extended below the estimated scour depths.
- Each boring, sounding, and test pit should be given a unique identification number for easy reference.
- The ground surface elevation and actual location should be noted for each boring, sounding, and test pit. Offshore borings should be referenced to mean sea level with the aid of a tide gauge. (Note: There are two vertical data. They are the 1927 datum and the 1988 datum; ensure that the proper one is being referenced.)
- A sufficient number of samples, suitable for the types of testing intended, should be obtained within each layer of material.
- Water table observation within each boring or test pit should be recorded when first encountered, at the end of each day and after sufficient time has elapsed for the water table to stabilize. Refer to ASTM D 4750. Other groundwater observations (artesian pressure, etc.) should also be recorded.
- Unless serving as an observation well, each borehole, sounding, and test pit should be backfilled or grouted according to applicable environmental guidelines.
4.4. Guidelines for Detailed Explorations
Following is a description of the recommended minimum explorations for various types of projects. It is stressed that these guidelines represent the minimum extent of exploration and testing anticipated for most projects and must be adapted to the specific requirements of each individual project.
It is noted that the guidelines below consider the use of conventional borings only. While this is the most common type of exploration, the engineer may deem it appropriate on individual projects to include soundings, test pits, geophysical methods, or in-situ testing as supplementary explorations or as substitutes for some, but not all, of the conventional borings noted in the following sections.
4.4.1. General Scope of Program
In regard to the scope of the subsurface program for a structure, one must carefully consider the small cost of a boring in relation to the foundation cost. A 2 1/2” diameter drill hole will cost less than one 12” diameter pile. Yet, the knowledge gained from that boring would permit proper design techniques to be used that may allow elimination of all piles for that structure. Without adequate boring data, the foundation design engineer cannot utilize his technique or experience and must rely on extremely conservative designs with high safety factors.
Planning a soils or foundation exploration program should include determining the depth and location of borings, test pits, or other procedures to be used and establishing the methods of soil sampling and testing to be employed. Usually, the extent of the work is established as it progresses, unless knowledge of foundation conditions is available from geological studies, earlier investigations, or records of existing structures. The number, depth, spacing, and character of tests to be made in any individual exploration program are so dependent upon site conditions, type of structure, and its requirements, that no rigid rules may be established. However, certain general principles for the guidance of those charged with the investigation can be outlined.
Embankments are less sensitive than structures to variations in subsurface conditions. Embankment loads are spread over a wide area while structure loads are concentrated. Designers of highways in cut sections are less concerned with deep exploration of subsurface conditions than defining the properties of the soil or rock on which the subgrade materials will be placed. The subsurface exploration program for embankments or cuts must necessarily be widely spaced, as the major portion of a highway alignment is one or the other. This section of the manual will deal primarily with approach embankments. Highway embankment and cut explorations are done using the same procedures, but the spacing and depth of borings vary, as shown below.
4.4.2. Investigation Steps
The objective of either deep or shallow borings is to obtain information and samples necessary to define soil and rock subsurface conditions. The following program will produce the minimum foundation data for a typical structure site. Soft ground conditions may require undisturbed sample explorations or in situ testing as previously mentioned.
o Physical description and extent of each stratum.
o Thickness and elevation of various locations of top and bottom of each stratum.
For cohesive soils (each stratum):
o Natural moisture contents.
o Atterberg limits.
o Presence of organic materials.
o Evidence of desiccation or previous soil disturbance, shearing, or slickensides. o Swelling characteristics.
o Shear strength
For granular soils (each stratum):
o In-situ density(average and range) typically determined from Standard Penetration Tests or Cone Tests.
o Grain-size distributions (gradation). o Presence of organic materials.
o Piezometricsurfaceoversitearea,existing,past,andprobablyrangeinfuture(observeat several times).
o Perched water table.
o Depth over entire site.
o Type of rock.
o Extent and character of weathering.
o Joints, including distribution, spacing, whether open or closed, and joint infilling.
o Solution effects in limestone or other soluble rocks.
o Core recovery and soundness (RQD).
The reasons for obtaining this minimum data are clear; the engineer must have adequate data to determine the soil type and relative compactness, and the position of the static water level. Methods such as driving open-end rod without obtaining soil samples or water level readings taken after the last soil sample was removed must be discouraged. Good communication between the driller and the foundation engineer is essential during all phases of the subsurface investigation program.
When soft ground is encountered, field (in situ) testing and/or undisturbed sample explorations should be done.
4.4.3. Field Boring Procedures
The importance of good logging and field notes cannot be overemphasized. It is most necessary for the logger to realize that a good field description must be recorded. The field-boring log is the major portion of the factual data used in the analysis of foundation conditions.
The log is a record that should contain all of the information obtained from a boring whether or not it may seem important at the time of drilling. It is important to record the maximum amount of accurate information. This record is the “field” boring log, as opposed to the “finished” boring log used in the preparation of the final report made to the designer. The finished log is drawn from the data given in the field log supplemented by the results of lab visual identification of samples and lab classification tests.
The person who actually logs the field information will vary from organization to organization. Some will have an engineering geologist, or trained technician accompany the drill crew, while others may train the drill crew supervisor to log the borehole. In order to obtain the maximum amount of accurate data, the logger should work closely with the driller and consult with him as to changes in materials and operations while drilling.
Generally, the logger should be responsible for recording the following information:
- General description of each rock and soil stratum, and the depth to the top and bottom of each stratum.
- The depth at which each sample is taken the type of sample taken, its number, and any loss of samples taken during extraction from the hole.
- The depths at which field tests are made and the results of the test.
- Information generally required by the log format, such as:
- Boring number and location.
- Date of start and finish of the hole.
- Name of driller (and of logger, if applicable).
- Elevation at top of hole.
- Depth of hole and reason for termination.
- Diameter of any casing used.
- Size of hammer and free fall used on casing (if driven).
- Blows per foot to advance casing (if driven).
- Description and size of sampler.
- Size of drive hammer and free fall used on sampler in dynamic field tests.
- Blow count for each 6 inches to drive sampler. (Sampler should be driven three 6” increments or to 100 blows).
- Type of drilling machine used.
- Type and size of core barrel used.
- Length of time to drill each core run or foot of core run.
- Length of each core run and amount of core per run.
- Recovery of sample in inches and RQD of rock core.
- Project identification.
5. Notes encountered, such as:
regarding any other pertinent information and remarks on miscellaneous conditions
- Depth of observed groundwater, elapsed time from completion of drilling, conditions under which observations were made, and comparison with the elevation noted during reconnaissance (if any).
- Artesian water pressure.
- Obstructions encountered.
- Difficulties in drilling (caving, coring boulders, surging or rise of sands in casing, caverns, etc.).
- Loss of circulating water and addition of extra drilling water.
- Drilling mud and casing as needed and why.
- Odor of recovered sample.
6. Any other information the collection of which may be required by policy.
During progression of a boring, the field drilling personnel should only roughly identify and describe the soils encountered. Unfortunately the drillers are usually delegated the task of exactly identifying and describing the soil samples. This is unfair, as drillers must be concerned with many other tasks involving mechanical operation of the rig and preparation of pertinent data for the subsurface log. In addition, the visual identification test should not be done outdoors in an atmosphere subjected to the elements, as this ingle operation will provide the basis for later testing and soil profile development. Instead, the soil samples should be sent to a laboratory and visually identified by a technician experienced in soils work. This is of great importance where no laboratory testing is to be performed and design values are estimated on the visual description and SPT results.
When additional undisturbed sample borings are taken, the undisturbed samples are sent to a soils laboratory for testing. Drilling personnel should exercise great care in extracting, handling, and transporting these samples to avoid disturbing the natural soil structure. Tubes should only be pressed, not driven with a hammer. The length of press should be 4 to 6 inches less than the tube length (DO NOT OVERPRESS). A one-inch thick plug composed of a mixture of bees wax and paraffin should be poured to seal the tube against moisture loss. The void at the upper tube end should be filled with sawdust and then both ends capped and taped before transport. The most common sources of disturbance are rough, careless handling of the tube (such as dropping the tube samples in the back of a truck and driving 50 miles over a bumpy road), or temperature extremes (leaving the tube sample outside in below zero weather or storing in front of a furnace). Proper storage and transport should be done with the tube upright and encased in an insulated box partially filled with sawdust or Styrofoam to act as a cushion. Each tube should be physically separated from adjacent tubes like bottles in a case. An alternate method to ease transportation and storage problem is to extrude tube in the field. These samples should be carefully sectioned in 6 to 8 inch lengths, wrapped in wax paper and sealed in a cardboard container (such as ice cream cartons) using liquid paraffin.
4.4.4. Roadway Soil Surveys
Soil survey explorations are made along the proposed roadway alignment for defining subsurface materials. This information is used in the design of the pavement section, as well as in defining the limits of unsuitable materials and any remedial measures to be taken. Soil survey information is also used in predicting the probable stability of cut or fill slopes.
Minimum criteria for soil surveys vary substantially, depending on the location of the proposed roadway, the anticipated subsurface materials, and the type of roadway. The following are basic guidelines covering general conditions. It is important that the engineer visit the site to ensure that all features are covered. In general, if a structure boring is located in close proximity to a planned soil survey boring, the soil survey boring may be omitted.
- At least one boring shall be placed at each 100-foot (30 m) interval. Generally, borings are to be staggered left and right of the centerline to cover the entire roadway corridor. Borings may be spaced further apart if pre-existing information indicates the presence of uniform subsurface conditions. Additional borings shall be located as necessary to define the limits of any undesirable materials or to better define soils stratification.
- In areas of highly variable soil conditions, additional borings shall be located at each interval considering the following criteria.
- For interstate highways, three borings are to be placed at each interval, one within the median and one within each roadway.
- For four lane roadways, two borings are to be placed at each interval, one within each roadway.
c. For roadway widenings that provide an additional lane, one boring shall be placed within the additional lane at each interval.
- In areas of cut or fill, where stability analysis is anticipated, a minimum of two additional borings shall be placed at each interval near the outer reaches of the sloped areas.
- In all cases, at least three samples per mile (two samples per kilometer) or 3 per project whichever is greater shall be obtained for each stratum encountered. Each of the samples representing a particular stratum shall be obtained from a different location, with sampling locations spread out over each mile (kilometre). Samples should be of adequate size to permit classification and moisture content testing.
- Additional samples shall be obtained to permit LBR and corrosion testing. As a minimum, three LBR samples per mile (two samples per kilometer) or 3 per project whichever is greater per stratum of all materials shall be obtained and tested. LBR samples shall also be obtained of all strata located in excavation areas (i.e., water retention areas, ditches, cuts, etc.). Corrosion series samples shall be obtained (unless no structures are to be installed) on a frequency of at least one sample per stratum per 1,500 feet (450 m) of alignment. When a rigid pavement is being considered for design, obtain sufficient samples to perform laboratory permeability tests.
- Borings in areas of little or no grade change shall extend a minimum of 5 feet (1.5 m) below grade, drainage pipe or culvert invert level whichever is deeper. Every 500 feet (150 m), one boring shall be extended to a nominal depth of 20 feet (6 m) below grade. The 20 feet (6 m) borings apply to projects with proposed buried storm sewer systems; project specifics may dictate adjustments. Borings may or may not include Standard Penetration Tests (SPT), depending on the specific project and its location.
- In areas of cut, borings shall extend a minimum of 10 feet (3 m) below the proposed grade. If poor soil conditions are encountered at this depth, borings shall be extended to firm materials or to a depth below grade equal to the depth of cut, whichever occurs first. Bag, SPT, undisturbed and core samples shall be obtained as appropriate for analyses.
- In areas of fill, borings shall extend to firm material or to a depth of twice the embankment height. Bag, SPT, and undisturbed samples shall be obtained as appropriate.
- Areas of muck must be probed to delineate both the vertical and the horizontal extents.
18.104.22.168. General Notes
The purpose of structure borings is to provide sufficient information about the subsurface materials to permit design of the structure foundations and related geotechnical construction. The following general criteria should satisfy this purpose on most projects; however, it is the engineer’s responsibility to assure that appropriate explorations are carried out for each specific project.
The procedure for this is outlined as follows, with details for different types of structures given in the following sections:
• Progress the drill holes according to the recommendations given below for different types of structures. The drill holes may be advanced with casing, drilling mud, or continuous flight augers.
- Estimate the boring depth from existing data obtained during the terrain reconnaissance phases or, less preferred, from requested boring resistance data.
- Obtain standard split spoon samples at proper intervals or at changes in material.
- Record the standard penetration test data on each drill hole in accordance with ASTM D-1586. This is the most economical method presently available of procuring useful data regardless of the oft-cited frailties of the test.
- Instruct the drilling crew to perform a rough visual analysis of the soil samples and record all pertinent data on a standard drill log form. The disturbed spoon samples must be carefully sealed in plastic bags, placed in jars, and sent to the laboratory for analysis. Undisturbed tube samples must be sealed and stored upright in a shock proof, insulated container normally constructed from plywood and filled with cushioning material.
- Observe the water level in each boring and record the depth below top of hole and the date of the reading on the drill log for:o Waterseepageorartesianpressureencounteredduringdrilling.Artesianpressuremaybe measured by extending drill casing above the ground until flow stops. Report the pressure as the number of feet of head above ground.o Water level at the end of each day and at completion of boring.
o Water level 24 hours (minimum) after hole completion. Long-term readings may requireinstallation of a perforated plastic tube before abandoning the hole.
A false indication of water level may be obtained when water is used in drilling and adequate time is not permitted after hole completion for the water level to stabilize. In low permeability soils, such as clays, more than one week may be required to obtain accurate readings.
- Designate a unique identification number for each drill hole to prevent duplication during later exploration phases. Much confusion has resulted on projects where exploration phases. Much confusion has resulted on projects where only single numbers did exploration numbering.
Perform at least one 2.5” (63.5 mm) minimum diameter borehole at each pier or abutment location. The hole pattern should be staggered so that borings occur at the opposite ends of adjacent piers. Pier foundations or abutments over 100 feet (30 m) in plan length may require at least two borings, preferably at the extremities of the proposed substructure. For structure widenings, the total number of borings may be reduced depending on the information available for the existing structure.
- 1) If pier locations are unknown, their probable approximate locations may be deduced based on experience and a preliminary design concept for the structure. If this is not possible, place borings at no more than 100-foot (30 m) intervals along the alignment. Additionally, for projects which include a water crossing that includes a pier in the water, at least one boring should be located in the water when practical depending on the width of the crossing.
- 2) Borings shall be continued until all unsuitable foundation materials have been penetrated and the predicted stress from the foundation loading is less than 10% of the original overburden pressure (see Figure 4-2), or until a minimum of 10 feet (3 m) of competent rock has been penetrated. If no data is available for predicting the foundation stress, extend the boring until at least 20 feet (6 m) of bedrock or other competent bearing material (N-values of 50 or greater) is encountered. (Scour and lateral requirements must be taken into account.) Other possible criteria could be: “The borings for structure foundations shall be terminated when a minimum resistance criteria of 100 blows per foot on the