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7 SOIL EXPLORATION Soil exploration is a vital part of the preliminary engineering survey for location, design, and construction of highways. Soil exploration provides information on conditions of the underlying strata that can affect the performance of pavement structures (8). The process also involves recovery of representative soil samples for classification and testing purposes. The general purpose of subsurface exploration is to: ⢠Identify and locate soil and rock strata ⢠Identify ground water table conditions ⢠Establish subsoil (moisture and density) conditions ⢠Define characteristics and relevant engineering properties of subsurface materials by sampling and in situ testing ⢠Provide preliminary assessment of the need for stabilization of sub-grade, sub-base and base materials ⢠Locate suitable materials for fills, subgrade treatment, materials for base and aggregate ⢠Identify local conditions requiring special considerations Preliminary Data Collection A close and interactive relationship exists among geology, pedology and engineering. Pedology is the study of the soil profile based on the soil forming processes and factors such as climate, age, vegetation, and drainage that have altered the parent material to form the soil. The soil profile is layered into horizons which can be used to identify the reactivity of the soil with stabilizers as a function of depth and to identify the presence of harmful minerals or compounds that may react negatively with the selected stabilizer. In addition, the mineralogical composition and chemical composition of the horizons within the profile can be used to assess whether or not the soil within the horizon will be reactive with the selected stabilizer. A pedological system of classification can be used as a basic approach for soil classification (9). The National Resources Conservation Service (NRCS) County Soil Survey is an excellent source of data that must be considered in any sampling effort in order to identify the required depth and frequency of sampling and to establish the expected results of the sampling process. Geological data can be used to interpret the impact of land forms, processes that lead to their development, their history and also to identify the sub-surface terrain features that might affect the behavior of these layers. For example sulfate bearing seams below the layer to be stabilized may provide a source for sulfate diffusion into the stabilized layer via capillary rise. Geological and pedological knowledge at the location provides the ground work by which to differentiate earth materials and identify problem zones. This relationship is especially valid for highway construction as pavements are built on and, in some cases, of earth materials. In addition to the National Resource Conservation Service (NRCS) County Soil Survey Reports, geological data and information can be obtained from the United States Department of Agriculture, and the U. S. Geological Survey Reports (2). State Geologic Survey Reports, if available, can also be used as a source for geologic information for the location. Subsurface Investigations Subsurface investigations are most often site specific and should be guided by the purpose, requirements, and geographical settings of the project location (10). Available data regarding the
8 project location should be reviewed prior to beginning the field investigation. Geologic maps, topographic maps, aerial and satellite photos, statewide or county soil surveys, and engineering maps are some of the useful sources of information regarding soil properties. Subsurface investigation reports of adjacent projects, if any, should also be studied. Field exploration methods, sampling requirements, and the type and frequency of field tests should be determined based on existing information, design requirements, availability of equipment and local practices. Subsurface exploration should be conducted in accordance with AASHTO R13. A comprehensive exploration plan should be developed to communicate the intent and level of testing required for the project (8, 10). The success of a subsurface investigation depends primarily on the effectiveness of the geotechnical engineers and technicians involved in field operations and therefore should only be performed by responsible, well-trained and experienced people. Sampling Plan A properly designed sampling plan should be developed to minimize sampling error and to optimize sampling efficiency. Samples should be taken in a manner that minimizes bias of the person taking the samples. This requires a plan to randomize sampling locations (10). However, boring and sampling programs must be planned and executed within budget constraints with appropriate consideration of other variables that can affect the site investigation. The development of a good sampling plan may include: ⢠Statement of the problem for which sampling is required ⢠Collection of available, relevant soil data ⢠Evaluation of different possible sampling plans, in terms of over-all cost, precision and difficulty Sampling of Soils Direct observation of subsurface conditions and retrieval of field samples can be achieved by examination of soil formations using accessible excavations, such as shafts, tunnels, test pits, or trenches, or by drilling and sampling to obtain cores or cuttings (10). Since stabilization operations involve mixing and compaction operations that destroy the original soil fabric, disturbance of samples during extraction does not normally compromise the quality of neither the sample nor its acceptability for testing. Hence undisturbed soil samples are not normally required for testing to evaluate the efficacy of soil stabilization. The testing involves evaluation of the soil properties including gradation, Atterberg limits, mineralogy, organic content and sulfate content. Sample units of roadway materials should be selected randomly in accordance with ASTM D 3665. The number of field samples to be collected depends on the level of confidence required by project specifications. Guidance in determining the number of samples required to obtain the desired confidence levels are detailed in ASTM test methods E 105, E 122 and E 141. Frequency and Depth of Sampling Subsurface conditions can be identified at the individual test pits, boring holes or by examining open cut sections. Soil strata can show significant spatial variability and the soil conditions can vary significantly between test pits. Therefore the continuity of soil and rock formations should be considered during the investigation. Geophysical techniques may be used to obtain general information pertaining to location of boundaries between bedrock and overlying deposits.
