Scope of Subsurface Explorations
A geotechnical investigation must be conducted on all projects involving foundation design and construction, roadway pavement, slope modifications, on significant cut and fill areas, and for assisting hydraulic engineers on erosion control measures. All geotechnical services must be conducted by engineers possessing adequate geotechnical training and experience, and also meeting all state licensing requirements.
The objective of the geotechnical services is to ensure that geotechnical investigation, design, construction, and maintenance support meet the minimum standards of LADOTD that will provide safe and cost effective solutions to the geotechnical issues. Typical geotechnical activities include the following:
- subsurface investigations,
- geological site characterization, laboratory testing,
- structural foundation design,
- retaining wall and sheetpile wall design,
- slope stability design,
- ground improvement,
- settlement mitigation,
- slope mitigation.
- construction support, and
- long term maintenance monitoring.
All geotechnical projects are site specific and should be treated as such. Extrapolation of site conditions or generalization from other projects or regional geological conditions should be verified. Therefore a preliminary investigation is very important for all projects. With the exception of insignificant structures, most geotechnical investigations shall consist of three phases: Preliminary Investigation, Primary Investigation, and Geotechnical Design. Geotechnical services during and after construction are also an integral parts of the project and should be considered as continuation of the design process. The Geotechnical Services Section will perform such tasks when the project is designed within the department. Similarly, the geotechnical consultants should include the construction support and post construction maintenance issues as part of their project.
Preliminary Investigation
During the conceptual project development phase, a preliminary geotechnical investigation shall be conducted to develop an understanding of the geology and geotechnical issues which may be used to plan for a more detailed investigation. The investigation for this phase usually consists of a field reconnaissance by the geotechnical engineer and a review of the existing records and geological maps. The preliminary investigation consists of two components: office research and field investigation.
The office research may consist of the following references:
- previous investigations within the project vicinity,
- DOTD’s Geotechnical Information System (to be available in the near future),
- USGS quadrangle maps,
- aerial photographs,
- Geological maps,
- Soil Conservation Services‘ soil survey, and
- other geological publications.
Preliminary field investigation at a minimum will consist of a site visit by an experienced engineer or technician to determine site access, potential features that may impact the project such as signs of instability, debris, vegetation, environmental contaminations, and historical structures. In addition to the site visit, preliminary soil borings or CPT soundings may be made as the baseline for future investigations.
A summary report shall be made to report the findings and to determine the scope of a more detailed geotechnical investigation program. This preliminary report shall identify the geologic unit, geological irregularities (salt dome, faulting, sinkholes… etc.), soil properties, potential construction difficulties, potential obstructions, and potential foundation types. This report shall address issues identified during site reconnaissance.
The information obtained in this phase of study may be incorporated into Load and Resistance Factor Design (LRFD) following the procedure established by Grigoriu et. al. (1987). Combining with the Primary Investigation described in the next section, one can provide a better estimate of design parameters and variability.
Primary Investigation
This is the phase of design that refines the project details such as alignment, structure types, loads and embankment heights. The purpose of this phase of geotechnical investigation is to obtain geotechnical information for foundation or pavement design. The method of investigation should be designed to reflect the nature of the project. Specialized testing, if needed, should be considered. The locations, number and depths of soil borings/CPT soundings or other exploration techniques shall be determined based on the preliminary investigation phase, anticipated foundation types as well as structural loads. In some instances, pile load tests may be conducted during this phase of study. The typical scope of the geotechnical services during this phase of investigation consists of the following:
- soil borings,
- CPT soundings,
- in-situ vane shear tests,
- piezometers,
- inclinometers,
- standard penetration tests,
- dissipation tests,
- test holes,
- pile or shaft load tests,
- shear strength tests (Q, R, R_ , S),
- pH and resistivity,
- classification tests (Atterberg limits, grain size distributions, moisture contents, and specific gravities),
- compressibility (consolidation tests),
- other appropriate sampling and testing.
The most important part for this phase of investigation is to estimate soil parameters to be used in design. The report should include recommended soil parameters, variability of the parameters in addition to the items described in the preceding section. Soil property profiles and cross sections shall be made graphically. All analyses should be included in the design memorandum or report along with the assumptions made for the analyses. Resistance factors selected for foundation design shall be justified and properly documented. The report shall be reviewed by a peer reviewer and sealed by a professional engineer registered in Louisiana. If a supplemental investigation is needed, this recommendation and the justification should be addressed in the report or a separate memorandum. Soil improvement methods may be considered on a case-by-case basis.