Planning Subsurface Investigations
John Poullain, P.E.
This three-hour online course discusses the procedures used for planning subsurface investigations and provides guidelines for site reconnaissance and sources of existing geological and historic data. Also presented are guidelines for determining subsurface information by visual examination during drilling and the basic operations for boring and sampling equipment. The student will gain an understanding of the types of subsurface investigation and requirements for the minimum number of borings and the depths and spacing for boring layouts. Care and accurate identification of subsurface materials are stressed. The AASHTO and ASTM designations for frequently used drilling and sampling equipment and tests are provided.
This course includes
a multiple-choice quiz at the end, which is designed to enhance the understanding
of the course materials.
At the conclusion of this course, the student will:
This course is intended for civil engineers and project engineers.
The course emphasizes the importance of subsurface investigation and presents guidelines for planning subsurface investigations. Additional costs and construction failures are frequently caused from inadequate planning and incorrect interpretation of subsurface data. The design of building foundations, roadways, excavations, and fills requires an understanding of soil strength; soil characteristics, problem soil and how soil behaves under imposed loads. Drilling and coring provide the necessary samples for laboratory soil and rock tests, for in-situ field-testing and for the detailed subsurface record, the boring log.
From the initiation of a project, subsurface investigations progress through phases from the concept to construction and to operation and maintenance. Initially the investigations are general and cover larger areas such as for road studies. As the project develops from the planning and concept phases to the plans and specifications for construction investigations become more detailed. It is important to follow established criteria and guidelines in order to select the appropriate drilling methods especially since subsurface exploration is expensive but not nearly as expensive as for a project failure caused by inaccurate or incomplete boring data.
Because of the large variety of soils and applied soil mechanics problems there is also a large variety of soil and rock exploration and sampling methods for determining the engineering properties of soils. Before the boring program can be prepared the design engineer must define the purpose for the exploration program and the testing program for his self and for field and laboratory personnel. Accurate observations and operations of the exploration equipment are very important to provide the required data.
Planning for subsurface investigations can be broken into these steps:
a. Historical and
Data is gathered and geological maps, topographical maps, aerial photographs, site histories and adjacent property developments are reviewed. Required proposed construction features are studied for access and to plan investigations. Land use, problems areas, subsoil information, terrain conditions will indicate required subsurface investigations.
b. Site Reconnaissance.
Comparisons are made with the existing surface features and topography of the site with the data gathered from prior historical and geological studies. Telltale signs of problem areas, presence of soft terrain, wet areas, outcrops and changes in topography like cut or fill developments will detected.
c. Subsurface Investigations
Extent of investigations will depend on available subsurface data, geological data, groundwater variability, adjacent development and the proposed construction. Investigations progress from generalized and large areas to more specific and detailed studies. Conceptual or preliminary subsurface investigations require limited laboratory and sampling work and are used to prepare possible routes or structures layout for evaluation. More detailed subsurface investigations including borings, samplings and testing are performed in order to design for the selected route or structure location.
Boring logs are prepared from subsurface information encountered while drilling and sampling. Information is derived from measurements such as the energy required for drilling per foot of borehole, monitoring rock and soil debris, drilling mud and return water pumped from the borehole, sample recovery percentages and loss of drilling mud, to mention a few. It is very important to complete the logs in the field and not the laboratory and that the field observations and lab test results are differentiated to make clear the source of information. When more comprehensive information is desired, downhole logging may be performed.
Exploratory personnel must be well trained and conscientious; the drilling and sampling equipment must provide accurate samples and data for a variety of tests. Soil and rock samples must be handled and stored with care following established standards. Samples should be inventoried, examined and tested as soon as they are received. Sometimes, especially for large testing programs, it may become necessary to store the samples for days or weeks, but should not be longer than 15 days if possible. If samples are stored for a longer time the undisturbed samples should be protected against damage or changes in water content by maintaining at temperatures close to those required for the project. Rewaxing and relabeling may be required. Nevertheless the stored samples may undergo physical and chemical changes when stored too long no matter how carefully stored and resealed.
Downhole logging uses tools, such as wireline logging, where electronic instruments are lowered down the borehole or logging-while-drilling (LWD) where the instruments are in the drill pipe behind the drill bit itself. Other tools take measurements while drilling and measure inclinations, tool temperatures and optimize casing selection. Although they may be used to complement standard boring measurements and sampling methods, they have these advantages:
a. downhole logs
give a continuous record of subsurface formations, especially if sample recovery
b. downhole logs can reveal a more realistic stratigraphy of fractures, fissures etc and in real time
c. LWD's tools record the logs in memory devices which are downloaded for assessment when returned to the surface after a drilling run
d. downhole log measurements present in-situ conditions as compared to recovered cores
e. material cored may physically swell or slake when no longer under pressure at depth and give an erroneous presentation of the actual underground conditions
Other devises such as borehole cameras and TV cameras are used to prepare logs and serve to record subsurface conditions that may not have been obvious while boring and therefore not recorded on the log. They can be reviewed many times to assure the subsurface conditions are as presented on the original exploration.
Soil particles are rearranged and densified to improve the soils' engineering properties of strength, permeability and compressibility. The existing subgrade may have poor strength or instability due to excess clay, expansive clays, silts, fine sands, voids, collapsing soils or high watertables. The existing soil properties must be determined to protect against potential settlement from the design bearing capacities. There are problem soils such as loess, hydraulic fills and tailings, which have collapsing or low-density structures, and when saturated have large decreases in volume and loss of strength. Other soils, which contain clays such as bentonite, can expand and increase in volume when exposed to water. Expansive soils can shrink or decrease in volume when water is not present. There are also dispersive clays so named because the soil particles are not structurally sound which allows them to easily disperse or detach and erode in still water.
Using chemical or bitumen additives to improve a soil may be used but handling and excavation of the existing soil would also have to be considered. Certain soils because of their chemical nature, organic or high acid compounds may not be responsive to these stabilization methods and may be corrosive to steel reinforcement. Often the soils are not readily distinguished by their classification or physical properties. Corrositivity and pH tests will determine the chemical and organic content of the soil if these are suspected problems.
The course is primarily based on Chapter 2 of the US Dept of Transportation
FHWA publication FHWA NHI-01-031, "Subsurface Investigation-Geotechnical
Site Characterization", (2001 Edition, 16 pages), PDF file. The course
is also based on Chapter 5, Sections I - IV, of the US Army Corps of Engineers
Engineer Manual EM 1110-1-1804, "Geotechnical Investigations", (2001
Edition, 17 pages), PDF file. The course includes Figures 3-2 through 3-10 and Figures 7-1 through 7-10 from the USCOE EM 1110-1-1804, 'Geotechnical Investigations', (2001 Edition, 14 pages), PDF file.
The links to the parts of both documents are as follows:
Please click on
the above underlined hypertext to view, download or print the document for your
study. Because of the large file size, we recommend that you first save the
file to your computer by right clicking the mouse and choosing "Save Target
As ...", and then open the file in Adobe Acrobat Reader. If you still experience
any difficulty in downloading or opening this file, you may need to close some
applications or reboot your computer to free up some memory.
This course should serve as a guide for planning soil boring and rock coring operations for subsurface investigations. The course provides basics for selection of spacing of drilling and the depths needed to determine the condition of subgrade soil and rock formations. Guidelines for site reconnaissance and sources of existing geological and historical data are presented. Basic concepts for types of subsurface investigations and boring and sampling methods are discussed to better understand planning subsurface investigations. The importance of quality assurance for subsurface exploration, sampling and for recording of data in the field is stressed.
technical information related to this subject, please refer to:
Information and applications describing subsurface investigations, boring, sampling methods, soil identification, coring rock, preparation of logs and borehole viewing means.
Logging while drilling provides real time information while drilling by using various resistivity, nuclear and acoustic means to define the subsurface conditions.
Lists over 50 software packages, shareware or commercial as noted, for soil boring logs and subsurface profiling.