AND ENGINEERING GEOLOGY REPORTS
J. David Rogers, Ph.D., P.E., R.G., C.E.G.
Karl F. Hasselmann Chair in Geological Engineering
Department of Geological Engineering
University of Missouri-Rolla
129 McNutt Hall, 1870 Miner Circle
Rolla, MO 65409-0230
When required, a geotechnical engineering report shall be prepared by a professional engineering firm under the direction of a geotechnical engineer. For hillside subdivisions, an engineering geology report, prepared under the direction of an engineering geologist, is also required, unless the geotechnical engineering report meets the requirements of these guidelines. Both requirements may be met by a single report.
A. Geological information in the report(s) shall be as follows:
1. Maps depicting regional geologic setting and site-specific geologic formations and structure.
2. Review of generally-accepted previously published geologic reports pertaining to the area.
3. An adequate description of the geology of the site, including description of bedrock lithology, structure, and physical characteristics (strength, jointing, weathering, hardness, special physical or chemical features), and discussion of how the geologic structure affects the site.
4. Identification of active and dormant landslides, based on both site analysis and published geologic maps. Geologic cross-sections where appropriate to convey information on geologic relationships.
5. Discussion of geomorphology and relevant hillslope and stream processes. Identification and evaluation of any faults on or adjacent to the site, including their degree of activity and their possible engineering significance.
6. Description and evaluation of offsite geologic hazards that may affect the graded site.
7. Conclusions and recommendations regarding the effect of geologic conditions on the proposed development.
8. These studies shall be performed for the entire parcel, and shall not be limited only to those portions to be graded.
B. Geotechnical reports shall include the following:
1. Subsurface exploration and testing sufficient to represent conditions of the entire parcel. At a minimum all reports should address the following:a. Description of the soil or bedrock encountered.
b. The in situ dry density and moisture content of each soil or bedrock unit.
c. Presence and depth of groundwater encountered.
d. Analysis and recommendations appropriate to the project, consistent with these guidelines.
2. Laboratory tests, which should include the following, as appropriate:a. Classification tests, using the Unified Soil Classification System. These consist of sieve analysis and plasticity testing.
b. Strength tests. Either triaxial or direct shear tests may be used. Saturated, drained tests should be used for evaluating long-term problems, such as slope stability.
c. Consolidation tests, if loads are to be placed upon clay soils.
d. Expansion tests, such as UBC Standard 29-2, when clays are expected to be within 5 feet of finished grade.
e. Compaction tests, performed in accordance with ASTM D1557, for soils to be used in engineered fills or backfills of retaining walls.
f. Permeability testing, when criteria to prevent piping into subsurface drains are required.
g. Stabilometer tests (R-value), performed in accordance with California Test Method 301, for areas to be paved.
h. Resistivity and pH testing for buried metal pipes or piles.
3. Analysis of areas to be graded.
a. Field and laboratory tests of the land to be covered with fill, to determine bearing value of the land and consolidation potential.
b. Field and laboratory soil analysis of the material proposed for the fill, including its source and expansive quality and statement as to its suitability. The analysis shall also specify the maximum density and optimum moisture content in accordance with ASTM D-1557 or an approved equal test.
c. Field and laboratory soil analysis of existing soil conditions in proposed cut locations, including saturated strength, expansive qualities and bearing values. Sampling and testing shall be sufficient to evaluate the stability of cut slopes, and should include strength testing of saturated soil materials and evaluation of bedrock jointing and weathering processes. Quantitative stability analyses generally should be performed for any proposed slopes steeper than 3:1.
d. Field and laboratory analysis of all identified active and dormant landslides as well as significant bodies of colluvium on and immediately adjacent to the parcel. Sampling and testing shall be sufficient to evaluate saturated strength of materials, depth to slide surface, potential for future movement, and risks to proposed structures or roadways. The evaluation should include a numerical slope stability analysis.
4. Groundwater levels and evaluation of any potential ground water condition that may affect soil strength, consolidation, or slope stability.
5. Appropriate laboratory analyses and associated data to support any proposals to replace, rework, or blend or to stabilize or modify with additives either the natural site soils or the proposed fill materials.
6. The location of and effects of active faults which may affect the proposed development. This analysis would normally include a tabulation of active faults, their distance from the site, their expected seismicity (magnitude and recurrence interval), and expected level of groundshaking (peak accelerations) at the site. Hazards of groundshaking, surface fault rupture, liquefaction, and seismically-induced landsliding shall be considered and addressed as applicable. Reports for public occupancy buildings should analyze site period and ground response and should provide design parameters. Critical facilities may require a dynamic site response analysis. Recommendations shall be made regarding building setback distances from nearby active faults and foundation and grading design required due to expected seismic activity.
7. Professional conclusions and recommendations for grading and foundations as appropriate to the site, including :a. Foundation type and capacity.
b. Short and long-term settlement potential, including expected magnitude of settlement and time variation.
c. Mitigation measures for expansive soils.
d. Lateral loads (passive, active, at rest) for current and proposed site conditions.
e. Slope stability. This should include site-specific specifications for any proposed repairs.
f. Shrinkage or settlement of engineered fills.
g. The maximum acceptable stable slope inclinations for proposed cut and fill slopes, assuming adequate terracing, drainage, and erosion control planting.
h. Surface and subsurface drainage necessary to ensure longterm stability of cut slopes and bodies of fill.
8. A complete and detailed specification for clearing, grubbing, and all aspects of grading, including utility trench backfill and retaining wall backfill, with special emphasis on the depth of fill layers, benching into native materials, preparation of areas to receive fill, compaction methods, moisture content, frequency of field density tests, and minimum density to be obtained in the field as related to laboratory tests.
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