GEOLOGIC, SOIL, AND GROUNDWATER INVESTIGATIONS

3-1. General. Before selecting or designing a sys­tem for dewatering an excavation, it is necessary to consider or investigate subsurface soils, groundwater conditions, power availability, and other factors as listed in table 3-1. The extent and detail of these in­vestigations will depend on the effect groundwater and hydrostatic pressure will have on the construction of the project and the complexity of the dewatering problem.

1- 2. Geologic and soil conditions. An un­derstanding of the geology of the area is necessary to plan any investigation of subsurface soil conditions. Information obtained from the geologic and soil in­vestigations as outlined in TM 5-818-1/AFM 88-3, Chapter 7 or NAVFAC DM7.1, should he used in evaluating a dewatering or groundwater control prob lem. Depending on the completeness of information available, it may be possible to postulate the general

Table 3-1. Preliminary Investigations


Item

 

Investigate

 

Reference

 

Geologic and soil conditions

 

Type, stratification, and thickness of soil involved in excavation and dewatering

 

Para 3-2; TM 5-818-1/

AFM 88-3, Chapter 7 NAVFAC DM7.1

 

Criticality Reliability of power system,

damage to excavation or foundation in event of failure, rate of rebound, etc.

 

Groundwater table or hydro – Para 2-3 and 3-3 static pressure in area and its source. Variation with river stage, season of year, etc. Type of seepage (arte­sian, gravity, combined).

Chemical characteristics and temperature of groundwa ter.

 

Groundwater or
piezometric

pressure

characteristics

 

Permeability Determine permeability from Para 3-4; Appendix C

visual, field, or labora­tory tests, preferably by field tests.

 

Power Availability, reliability, Para 3-5

and capacity of power at site.

 

Degree of possible Rainfall in area. Runoff Para 3-6

flooding characteristics. High-

water levels in nearby bodies of water.

 

characteristics and stratification of the soil and rock formations in the area. With this information and the size of and depth of the excavation to be dewatered, the remainder of the geologic and soil investigations can be planned. Seismic or resistivity surveys (as well as logged core and soil borings) may be useful in deline­ating the thickness and boundaries of major geologic and soil formations and will often show irregularities in the geologic profile that might otherwise go unde – tected(fig. 3-1).

a. Borings.

(1) A thorough knowledge of the extent, thick­ness, stratification, andseepage characteristics ofthe subsurface soil or rock adjacent to and beneath an ex­cavation is required to analyze and design a dewater­ing system. These factors are generally determined during the normal field exploration that is required for most structures. Samples of the soil or rock formation obtained from these borings should be suitable for classifying and testing for grain size and permeability, if the complexity of the project warrants. All of the in­formation gatheredin theinvestigation shouldbe pre­sented on soil or geologic profiles ofthe site. For large, complex dewatering or drainage projects, it may be de­sirable to construct a three-dimensional model ofcol – ored pegs or transparent plastic to depict the different geologic or soil formations at the site.

(2) The depth and spacing of borings (and sam­ples) depend on the character of the materials and on thetypeandconfigurationoftheformationsordepos – its as discussed in TM 5-818-1/AFM 88-3, Chapter 7. Care must be taken that the borings accomplish the following:

(a) Completely penetrate and sample all aquifers that may have a bearing on dewatering an ex­cavation and controlhng artesian pressures.

(b) Identify (and sample) all soils or rocks that would affect or be affected by seepage or hydrostatic pressure.