SOILS: SURVEYING AND EXPLORATION/CLASSIFICATION/FIELD IDENTIFICATION

Much of the discussion in this chapter assumes that you are by now knowledgeable of the physical properties of soils and that you are experienced with laboratory testing procedures, such as mechanical analysis and Atterberg limits, that are necessary for accurate identification and classification of soils. Should it be necessary, you may find it helpful to review chapter 15 of the EA3 TRAMAN and chapter 13 in Part 1 of this TRAMAN before beginning your study of this chapter.

SURVEY SUPPORT FOR GEOLOGY AND PEDOLOGY

In this section you will be provided a brief familiarization with the topics of geological and pedological surveying and mapping. Although these topics could have been included in a separate discussion of topographic surveying, they have been included in this chapter since both are related to soil exploration and investigation.

In essence, surveys in support of geology are topographic surveys; however, you must be aware of the other specialized data that may be included as required by the geologist or the soil engineer when you are collecting data for engineering studies for naval construction projects.

The end product of most topographic surveys is a topographic map. In geology or other related sciences, the topographic survey is the first part of a series of interrelated surveys. The end product is a map containing not only topographical information but also other specialized data keyed to it. In geologic surveys, a geologist makes systematic observations of the physical characteristics, distribution, geologic age, and structure of the rocks as well as the groundwater and mineral resources that the rocks contain. These observations are expressed in finished form as geologic maps and texts. The objective of the geological survey is to portray, in plan or in cross section, geological data required for subsequent constructions or for other uses. Pure geological data has little direct application to naval problems; however, if the field information is interpreted into specialized lines, it is of considerable use in Naval Construction Force (NCF) planning and operations. NCF requirements may necessitate regional geological study and mapping, surveys of more limited areas, or development of detailed geological data at a construction site.

In the field, the geologist conducts his survey by examining the rock. He looks to see if it is exposed at the surface and not covered by soil or other material. At such exposures, called outcrops, he systematically records the physical characteristics of the rock, thickness of exposure, inclination of the rock, inclination of rock bedding, and development of joints or fractures. In addition, he deter-mines the age of the rock from fossils or the sequence of rock units. Rock investigations are not confined to surface exposures, as the deeper seated rocks are examined by using samples obtained from auger or boreholes. The information gathered by the geolo-gist is placed on a map base by plotting the rock types in color with other data incorporated as symbols or annotations. As an amplification of the map data, more complete descriptions of outcrops are entered in notebooks with the entries keyed to the field map. Surveyors support the geologist by preparing basic topographic maps on which they plot the results of geological investigations and then make such tie measurements to geological features as the geologist may require.

The geologist uses simple survey methods in plotting geological features on a field map. Where an outcrop can be located with reference to a cultural or relief feature, it is generally plotted on a map by spot recognition. In other cases, the relationship of a geological feature to a recognizable topographic feature is established by using a magnetic compass to determine direction and by pacing or taping to measure distance. Slope or small differences in elevation are measured by using a clinometer or hand level, while an altimeter is used where there are large differences in elevation. When the geological survey is keyed to a large-scale plan, the geologist generally uses a plane table and plots data with accuracy commensurate with the accuracy of the base plan.

 The survey for the base map should normally take place before the geological survey, because the geologist uses the map in the field to plot his data and to determine his position by identification of topographic details. If aerial photographs are available, the base map need not be made before the geological survey since the geologist can use the aerial photograph as a plotting base and later transfer the data to a base map. However, if possible, the base map should be prepared in advance, even in this case, as the number of aerial photographs needed to cover an area is generally too large to be handled in the field.

Plane table topography is the method best suited to relatively open country. In the absence of detailed instructions, the following specifications are generally satisfactory:

1. BASE DIRECTION. To determine a base direction, take from a known base a side in a triangulation net or a course of a basic control traverse.

2. LOCAL HORIZONTAL CONTROL. Use plane table traverses run in closed circuits or between known control stations of a higher order of accuracy or locate plane table stations by graphical triangulation.

7. ACCURACY. Distance measurements by stadia should be accurate to 1 part in 500. Side-shot points located by pacing or other rough measurements should be accurate to within 25 feet. Take sights for traverse lines or graphical triangulation with care to obtain the maximum accuracy inherent in the telescopic alidade.

The error in the elevation of any point, as read from the finished map, should not exceed one half of the contour interval.

Topography may be located more conveniently in heavily timbered country by stadia measurements from transit-stadia traverse than by the use of the plane table, although the time required for plotting will be increased.

The specifications listed above are generally applicable. Read horizontal angles on traverses to 1 minute and horizontal angles for side shots that will be plotted by protractor to the nearest quarter of a degree. Read vertical angles for elevation determination to 1 minute or use the stadia arc. Keep complete and carefully prepared stadia notes and sketches to assure correct plotting.

When the geologist indicates that a map of a lower order of accuracy will fulfill his needs, plane table or compass traverses are suitable.

If aerial photographs are available, the geologist generally uses them instead of a map. The most satisfactory results are obtained from large-scale photographs, 1:15,000 or larger. Some topographic features, such as some ravines, rocky knobs, or sinkholes, are too small to be shown on maps. These features, as well as the larger topographic fares, such as stream channels and swamps, can be observed directly from aerial photographs. The photos also can be used to prepare a base map for portrayal of the field data by tracing planimetric details from an uncontrolled mosaic with spot elevations added from field surveys. The geologist may satisfactorily use contact prints of aerial photographs in place of the base map except where large-scale plans for engineering purposes are to be the base. In such a case the distortion within an aerial photograph does not permit plotting of geological data commensurate with the accuracy of the final plan