Geotechnical Engineering Services Selection Guide

Image Credit: Element Materials Technology

 

Geotechnical services includes soil, rock, and subsurface testing and analysis for site feasibility, foundations, embankments, pavements, retention structures, and other earthworks projects. Geotechnical engineering services are a vital part of any construction project, from providing soil analysis for a building site to foundation design for the large caissons and footings for bridges. They also use analytical laboratory services, field studies, standardized test methods, and/or modeling in order to optimize land use and management.  

 

Geotechnical Disciplines 

Various geotechnical disciplines are used in order to provide geotechnical services. Geologists, hydrologists, and laboratory technicians may be versed in hydrology, stratigraphy, and/or sedimentology. These sciences are used collectively in order to assess, quantify, and qualify physical surroundings and material present.  

 

Hydrology

Hydrology is the study of water quality, transport, and distribution. Perc (percolation) test, water quality monitoring, and observation wells are all used to ensure that the surface water, ground water, and hydraulic conductivity meet site requirements. 

Geotechnical Engineering Services Selection Guide

Image Credit: USGS

 

 

Sedimentology

Sedimentology is the science and study of sediment transport, deposition, and classification. Sedimentologists have a deep understanding of the historical importance of how sediments have been deposited at site locations throughout long time periods. They can predict future sedimentation rates, erosion rates, expected physical characteristics of local sediments, and can classify grain size. 

 

The following table depicts how sediments are classified: by grain size; clay to pebbles, shape; angular to well rounded, and assortment; well sorted, poorly sorted, or a matrix of two distinctive grains.

 

Geotechnical Engineering Services Selection Guide

Image Credit University of Michigan

 

Stratigraphy

Stratigraphy is a branch of geology that addresses the layering of rocks, known as strata. As the type of strata formed and their arrangement shifts over time we need not only to understand what is visible on the surface but rather the underlying strata as well. The composition and orientation of all underlying strata can depict the stability of the land mass, which becomes increasingly important when planning for land use.

 

Composition

Strata are sedimentary or igneous in nature meaning they have been formed either from sediments or magma. The process by which sediments are hardened into a single rock mass is known as lithification. There are three processes by which this can happen: compaction, recrystallization, or cementation, which are all facilitated by heat, pressure, and fluid transport. Igneous rocks are either intrusive or extrusive meaning that they are both cooled and formed within the earth's crust or on the surface. 

 

Orientation

Over geological time scales, thousands if not millions of years, the local geological processes that shape and form strata changes at various geographical locations. Structural geologists study the identification and interpretation of the resultant features. They observe the orientation of the strata by the zenith by which strata intersect a flat plane and the inclination, known as strike and dip. Other features of interest may include a lineation such as a fault, intrusion, and foliation.

Orientation by Strike and Dip

 

 Image Credit: Lamont-Doherty Earth Observatory

Testing Methods 

Geotechnical service providers have a variety of methods that they may use to sample, test, analyze, and/or assess natural resources including field mapping, core sampling, site assessment, perc test, water quality monitoring, use of observation wells, the atterberg limit, and the limerock bearing ratio.

Field mapping is the production of a graphical presentation of strata and sediments. Field measurements are used to understand the relative age of exposed strata, identify faults, predict underlying strata orientation, and establish preferred locations for site development.

Core sampling is the extraction and analysis of one to several solid cores. In most cases several cores are randomly sampled to establish a detailed report. Core sampling can be used to identify soil depth, material characterization, analyze existing strata, or other material characterization services.

Site assessment may be conducted for a range of customers. Site assessment may be used for civil engineering projects, residential as well as commercial building and construction projects, environmental impact studies, brownfield site assessment, verification of remediation efforts, and to locate natural resources such as ore deposits, ground water, and oil and gas deposits.

 

Perc tests are conducted to gauge the feasibility of a septic tank or leach field. The test method is conducted by loading a soil mass with water in order to determine the absorption rate. Local authorities set benchmarks of required test duration and absorption rates.

Water quality monitoring can be conducted to assess if given water reserves need treatment or if they are potable and safe for human consumption. Water quality monitoring can also be conducted to assess environmental impact and to verify remediation efforts when an aquifer or other water reservoir has been contaminated.

Observation wells can be used to monitor ground water movement, water quality, or may be used to assess well placement and site development. Observation wells are shallow drilled wells that are used to conduct these types of tests. After testing is complete they may be capped or filled in.

 

The atterberg limit is a measure of the soil's liquid limit and plastic limit. These limits describe the mechanics of the soil, or the soil's viscosity or brittleness.

The limerock bearing ratio is another standard used by geotechnical service providers for determining the underlying stability of the substrates under a roadbed.

 

Historical Proof

Geotechnical services help optimize the use of natural resources. Stability, sustainability, and feasibility requirements are important factors to take into consideration. Historical evidence for the need of geotechnical services is portrayed by slope failure and subsidence. 

 

Slope Failure

Slope failure occurs when structures are erected on unstable land masses. Stability is dependent on the orientation of strata, porosity, saturation, material composition and the amount of vegetation present. 

Geotechnical Engineering Services Selection Guide

Image Credit: Indiana University

  

Subsidence

Subsidence is a natural process that occurs as unconsolidated sediments settle causing land masses to compress, deform, and shift downward. It can also occur due to urban development, changes in land use, loading of land masses, mining, and well pumping. Geotechnical engineers consult, model, forecast, and mitigate affected land masses when subsidence is of concern. 

Geotechnical Engineering Services Selection Guide

Image Credit: USGS

 

  

Resources:

 

Geologic and Hydrogeologic-Site Characterization

 

Geotechnical Engineering

 

International Society for Soil Mechanics and Geotechnical Engineering