September 12, 2007

Geotechnical engineering gets a tech boost

  • New innovations, such as GIS and LiDAR, have made the discipline more precise and efficient.

    Image courtesy of GeoEngineers [enlarge]
    LiDAR can accurately detect ground anomalies such as this landslide.

    What a difference a few decades makes! In the early 1980s, word processing had just become available, we relied on mainframe computers, cell phones and e-mail didn’t exist, and field data acquisition (topography and geology) relied primarily on boots-on-the-ground techniques. Since then, geotechnical engineering has evolved significantly in response to new technology.

    While some basic practices such as exploratory drilling have changed little over the years, other aspects of our business have evolved tremendously — resulting in greater value for clients.

    Three examples of technological change that have affected our industry include digital terrain analysis, GIS-based data management and advanced analysis of soil-structure interaction.

    Digital terrain analysis

    Remote sensing data from satellites and aircraft have revolutionized the study of the land surface. One example is the use of airborne LiDAR technology. This process uses an airplane or helicopter to “paint” the ground surface with up to 150,000 pulses of laser light per second. Knowing the exact position of the airborne laser source using digital global positioning systems, and the travel time to and from the ground, the horizontal and vertical positions of points on the ground can be determined with accuracies of 6 inches or less. The result is a digital terrain model (DTM) with hundreds or thousands of ground surface elevation points per acre.

    Typical study areas may comprise a few acres or include hundreds of square miles. LiDAR systems can obtain data in steep and heavily wooded areas — a huge benefit over past decades. The DTM can be processed to produce elevation contour maps and ground surface profiles with the push of a button.

    Elevation data can be manipulated on desktop computers to produce shaded relief maps of the terrain and landforms. Using LiDAR-based shaded relief maps, earth scientists have been able to identify and map geologic hazards such as faults and landslides with much greater accuracy than earlier methods allowed. With advanced terrain analysis methods, we are now able to evaluate natural hazards with greater precision and to recommend safer locations for buildings, pipelines, highways and almost any other type of important structure.

    GIS data management

    Today’s projects often involve the collection, analysis and presentation of many types of spatial information. In the 1980s, almost every engineering firm had a light table — where they overlaid maps containing different information — looking for relationships and connections between differing sets of data.

    Today’s digital advancement of the light table is geographic information systems (GIS). Land ownership, zoning, geology, topography, vegetation, wetland boundaries, transportation systems, building locations, drainage courses, subsurface bore hole locations, groundwater levels, soil contaminant concentrations and other important information can be compiled and shown in relationship to each other using GIS. It is now possible to show each of these data sets independently or in any combination with other information.

    But beyond mapping, GIS systems are excellent platforms for managing and analyzing complex data that may vary over time. Important information can be collected in the field with portable devices that allow for nearly instant integration of the data with evolving site information. The true power of this technology is in the information behind the graphics on the screen. With a database linked to GIS map information, queries of the data can be made to provide rapid answers to critical issues or concerns. For instance, the database may be queried to determine the locations of water supply wells within 1,500 feet of a proposed construction site. Or, a project member may inquire: “Where are the historical earthquake epicenters within 50 miles of this pipeline corridor?”

    This information can also be shared interactively with clients and team members in real time through secure Internet connections. In the past, many hours of manual map preparation with different overlay layers may have been necessary to show important project information to clients, team members, regulators or the public. With Web-based GIS, this information is now available with the click of a mouse.

    Soil-structure interaction

    The underground is a complex environment with variable geology and groundwater conditions. Today, high-performance personal computers combined with advanced finite element computing software enable us to model complex structure and soil interaction in two or three dimensions. Strength and elastic properties of soils can be modified readily to evaluate the sensitivity of soil characteristics to structure performance. Modifications to structure design can be made and analyzed in minutes.

    Underground conditions are even more complex in urban environments. Humans have modified the subsurface with below-grade retaining walls, basements, tunnels, foundations and utilities. All of these variables need to be considered in the design of new underground structures (for instance, a deep underground parking structure adjacent to an existing basement with intervening active utilities). Not long ago, many simplifying assumptions had to be made in order to analyze the feasibility of a deep retaining wall with variable soil conditions and complex configurations of adjacent infrastructure. The necessary assumptions would typically involve multiple levels of conservatism. Consequently, conventional analyses may conclude that a particular project or construction method was infeasible.

    With today’s computing power and analytical software, it is now possible to evaluate soil-structure interactions for very complex subsurface conditions. The result is more economical designs that are adapted to the unique set of conditions that exist at a specific project site. Our Olive 8 shoring project is an example of a shoring design that would not have been feasible without the advent of technology that supports the advanced analysis of soil-structure interaction.

    Innovations keep coming

    New advances in engineering technology are occurring every day. Each year brings new approaches and technology that benefit our projects and clients. Keeping up with technological change is challenging, but falling behind is out of the question. Stay tuned and hold on!

    Jim Miller is a registered engineer and a licensed geologist with more than 30 years of experience. He is a senior principal at GeoEngineers and the firm’s former CEO.

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