Geotechnical Testing, Observation, and Documentation

Accurate classification of soil in the field is the most basic, and yet the most important, procedure performed during field sampling and observation. The Unified Soil Classification System is the most widely adopted and recognized classification system used for engineering purposes. When classifying soil in the field, it is important to closely observe the material and then give as accurate a written description as possible. This chapter presents some simple field tests to help in the visual classification of fine-grained material. These tests include dry strength, stickiness test, dilatancy, and field dispersion test; the color of natural, near-surface soil varies from dark brown in vegetated moist climates to light brown or near-white in an arid desert terrain.

This geotechnical report describes the existing surface and subsurface conditions of grading sites and provides recommendations on such aspects as removal depths of unsuitable soil, compaction, and moisture conditioning requirements. Exploration methods used to obtain material for lab testing and subsurface profiling information are examined; these may include borehole drilling and sampling, backhoe trenching, cone penetration test, seismic refraction, or other more complex geophysical methods. Use of boreholes for sampling and logging soil as drilling progresses from near-surface elevation to hundreds of feet or more is discussed. One specific type of borehole sampler described is the standard split barrel penetration test. Tube sampling disturbs the sample less than drive sampling methods. Often, an investigation is performed to help determine the hardness of volcanic flow, bedrock, or cemented formations.

Laboratory testing determines soil attributes by approved test methods. Standardized test methods are a consistent way to characterize in-situ and imported soil properties, including particle size, plasticity, expansion, moisture content, and strength, as well as to obtain a myriad of other important data. This chapter presents some more commonly used laboratory tests; some include general test outlines, while a few are more detailed procedures. Engineers use the cumulative results of these laboratory tests to help define how a soil will act under specific construction or natural conditions. The modified Proctor test determines the moisture content at which a given soil type will compact best. Hydrometer analysis uses a sedimentation process to determine the particle size distribution of material that is finer than the No 200 sieve.

Sand cone and nuclear field density (compaction) tests) are introduced. During backfill or grading operations, both the sand cone and nuclear gauge tests measure the moisture content and degree of compaction, which are expressed as a percentage of the optimum moisture and maximum density, respectively. Density tests must be taken at a number of different locations and depths within the trench to help confirm that the trench soils have been sufficiently compacted. The sand cone has long been widely used to determine in-place field density. Many geotechnical firms use the sand cone test as the preferred method for forensic studies, as a referee method, or simply because the equipment is less expensive. The wide acceptance of the nuclear gauge as an accurate and rapid method for taking moisture and density tests in the field is analyzed.

This chapter reviews the roles and responsibilities of the soil technician, field staff, and inspector in jobsite soil construction. One principal responsibility of the soil technician or inspector is to help confirm that the recommendations presented in the geotechnical report are implemented in the field during grading and other construction processes. To properly interpret such recommendations, field staff must be familiar with soil engineering terminology. Common grading requirements in the geotechnical report are discussed. It is important that the technician coordinate hillside testing and inspection with the project geologist or geotechnical engineer. The geotechnical report will indicate a criterion for the matrix soil, such as sand equivalency. Also critical is accurate logging of soil and rock strata during the drilling of shafts for deeper foundations.

This chapter explores some geologic factors that should be considered in hillside areas in addition to the standard issues of grading in flat lands. Specific emphasis is given to the geologic map, which indicates the location of surficial materials such as alluvium and the different rock types that are anticipated, as well as review of the boring and trench logs. Recognizing potential geologic problems depends on careful attention to the soil and bedrock, whether pre-exposed or exposed during grading. A comprehensive list is provided of features of concern to geologists in hillside grading projects, including potentially unstable bedrock; exposed clay seams, seeps, or springs; and heavy organics or uncontrolled fill.

Technicians and Inspectors must be fully committed to each project in which they become involved. Drawing from typical recommendations, as well as newer and unique construction methods, this chapter discusses the symbiosis of both office and field personnel, particularly inspectors and technicians, that is necessary for a project to succeed. Also evaluated is the role of low-impact development (LID) systems, which often include underground stormwater capture, storage, and infiltration systems, in offsetting the occluded absorption areas resulting from grading and construction. Progress involving geotechnical recommendations, implementation, and materials used is assessed. Applications of lime treatment may be used in stabilization of soft, wet, and pumping road subgrade or simply to dry back wet soil prior to placement as trench backfill.

This chapter focuses on the critical role of technicians and inspectors, specifically, observing and documenting the work performed by the grading contractor on a project. The work of a technician during projects is divided into multiple phases: project preparation, observation, communication, testing, and the documentation of data for the final report. Important factors to consider are the project duration and the estimated budget. Also highlighted are some principal tools for technicians and inspectors; these include the daily field report, which is the written record of meetings, discussions, issues of concern, locations of removals and fills, equipment utilized, and any other important developments that transpire; and the personal daily diary in which general overviews of discussions and activities for each project are recorded.

This chapter presents an example geotechnical report in which a local developer asks a geotechnical firm to perform a geotechnical investigation on a nearby 14-acre plot of land. Information obtained from the site investigation will be used by the project engineer to develop recommendations for earthwork and specifications for foundations, paved parking areas and driveways, concrete flatwork, and utility placement. During exploration, lab work, grading, and construction, technicians from the firm may be involved, and their preparation prior to beginning a project is paramount. The following important tasks should be addressed: site investigation, office prejob meetings, project organization, and on-site pregrade meetings. A technician may be asked to do a site reconnaissance during the planning process. Some of the site information necessary to gather for the project engineer include locating all existing underground utilities, ascertaining site accessibility, potential hazards to avoid during site exploration, and creating a site sketch.

This chapter provides trench safety guidelines. These include all excavations must be examined by a competent person; ensure safe access and egress; avoid hazardous atmospheres; always wear a hardhat in excavations; watch for any overhead hazards; and understand sloping, benching, and bracing. The four basic soil classification categories are detailed: stable rock, type A, type B, and type C; some of the compliance methods permitted under the Excavation Standards require a competent person to classify the four designations of soil rock deposits. In addition, all federal and/or state guidelines should be followed, as well as those originating from local authorities and any project-specific safety requirements.