OBJECTIVES OF A SOIL SURVEY The overall objective of a soil survey is to gather (explore)  as  much  information  of  engineering significance as possible pertaining to the subsurface conditions in a specified area. Soil samples are collected for laboratory tests to determine if the existing soil conditions could support the type of structure planned for  construction  without  adding  other  material  for stabilization. The exploration is conducted in a specific manner to determine the following information: 1. Location, nature, and classification of soil layers 2. Condition of soils in place (density and moisture content) 3.  Drainage  characteristics 4.  Groundwater  and  bedrock 5. Development of a soil profile. Location, Nature and Classification of Soil Layers Adequate and economic earthwork and foundation design of a structure can be done only when the types and depths of soil are known. By the classification of the soils (discussed later in this chapter), you can predict the extent of problems concerning drainage, frost action, settlement,  stability,  and  similar  factors.  While  you  can estimate  the  soil  characteristics  by  field  observations, for  laboratory  testing,  you  should  obtain  samples  of  the major soil types as well as less extensive deposits that may  conversely  influence  design. Condition of Natural Soils The moisture content and density of a soil in its natural state plays an important part in design and construction. The moisture content of a soil in place may be so high as to require the selection of a different site. If  the  natural  soil  is  sufficiently  dense  and  meets  the required specifications, no compaction of subgrade is required.  On  the  other  hand,  extremely  dense  soil  lying in cut sections maybe difficult to excavate with ordinary tractor-scraper units. Such dense soil often needs to be scarified  or  rooted  before  excavation. Drainage  Characteristics Drainage characteristics, both surface and sub- surface, of a soil greatly affect the strength of the soil. This characteristic is controlled by a combination of factors.  Some  of  these  factors  are  void  ratio,  soil structure and stratification, temperature of soil, depth to water table, and the extent of local disturbance by roots and  worms.  Coarse-grained  soils  have  better  internal drainage  than  fine-grained  soils. Groundwater  and  Bedrock All structures must be constructed at an elevation that ensures they will not be adversely affected by the groundwater table. If a proposed grade line lies below the  elevation  of  the  water  table,  either  the  grade  line must be raised or the water table must be lowered by artificial  drainage. The unexpected discovery of bedrock within the limits of an excavation greatly increases the time and equipment required to excavate. If the amount of rock is extensive, a change in grade or even a change of site may be the only way out. Field Notes and Soil Profile The engineer or  EA in charge of the soil survey must keep  accurate  field  notes  and  logs.  This  person  is responsible for surveying, numbering, and recording each  boring,  test  pit,  or  other  exploration  investigation. A log is kept of each test hole. It should show the depth  below  the  surface  (or  the  top  and  bottom elevations) of each soil layer, the field identification of each soil present at the site, and the number and type of each sample taken. Other items of information you need to include in the log are the density of each soil, changes in moisture content, depth to groundwater, and depth to rock. Keep a detailed field log of each auger boring or test pit made during the soil survey. A typical boring log is shown in figure 16-1. When you complete the survey, consolidate the information contained in the separate logs. Classify and show the depth of soil layers in each log. It is also helpful for the log keeper to show the natural water contents of fine-grained soils, when possible. Record this along the side of each log. Note the elevation of the groundwater table. This elevation is simply that of any free water standing in the test hole. To permit the water to reach maximum  elevation,  the  engineer  or  EA  should  allow 24 hours to elapse before measuring it. This gives a more accurate  measurement. 16-5