Geotechnical engineering often is a key connection between a civil engineer’s design and its successful construction and operation. “Any time infrastructure is going to be constructed, geotechnical problems must be considered in the design process,” said Scott L. Deaton, Ph.D., president and chief software architect, Dataforensics. “Whether it is a levee-related issue in New Orleans; a landfill in Nebraska; or retaining walls, cut slopes, and fill embankments for highway designs in Nevada, site investigations are always performed to ensure the infrastructure can be constructed and [to determine] the required design and construction techniques.”
In simple terms, problems commonly addressed by geotechnical investigations and tools are attributable to water; movement of soil or rock, including settlement; loads on structures or foundations; or a combination of these conditions. “Soils can offer unsuitable bearing capacities or excessive settlement from foundation loads, or be susceptible to liquefaction during a seismic event,” said James Hussin, director, Hayward Baker Inc. “Geotechnical construction techniques can improve subsurface soils to increase bearing capacity, reduce settlement, add shear resistance, and reduce the liquefaction potential for planned or existing foundations.”
Additionally, Hussin said, specialty underpinning techniques can solve settlement issues related to increased loading or inadequate subsurface soils and, depending on the rate of movement and site logistics, active landslides can be stabilized.
However, settlement and landslides may be symptoms of another problem. “Although we often refer to these as soil problems, I think they are more often water problems,” said Frank Callanan, P.E., vice president, Geotechnology Inc. “Unanticipated or excessive quantities of water are often the cause of slope failures, block retaining wall collapse, premature pavement cracking, and building distress (settlement and heaving). Geotechnical engineers have a reputation for being conservative and using cautious language in our reports. When you have experienced the range of soil conditions and water-related problems, in particular, you start to appreciate why we need to err on the side of caution.”
Nevertheless, monitoring soils and structures can help avert many of these problems, or at least prevent loss of life in critical situations. “People have utilized various sensors to monitor critical slopes in rainstorms to determine if a mudslide might occur or a hillside might wash away,” said Ken Stevens, manager, Geotechnical & Structural Group, Campbell Scientific Inc. “People monitor ground saturation and can predict if flooding may occur in time to have people evacuate threatened areas. When it comes to foundations, geotechnical solutions include monitoring the movement of cracks in the foundation, an increase in load or stress on the foundation, earthquake damage, or erosion of soil surrounding the foundation.”
Foundations are particularly challenging, according to Gina Beim, P.E., marketing department head, Pile Dynamics Inc. “They are structural elements that not only cannot be visually inspected after installation, but once installed become part of a foundation-soil system in which one of the components (the soil) is non-homogenous and changes with time.” Although engineers have developed creative solutions to analyze and monitor foundations, the field of foundation QA/QC is still evolving, Beim said. “There [now] is more incentive than ever to develop foundation testing solutions that are not only efficient and accurate, but also fast and economically viable,” she said. “On one hand, new LRFD codes give incentive to projects in which more foundation testing is performed (by allowing a leaner design). On the other hand, there is less money for testing. The new products — or updates to existing products — that are popping up these days are coming from this mindset. The civil engineering projects that are early adopters are definitely reaping the benefits.”
However, foundation problems often develop when changes occur in the surrounding soil caused by adjacent construction or demolition activities such as excavation, dewatering, and vibration, said Christopher Kavars, president and CEO, SENSR. He recommended that prior to construction, design professionals should survey the condition of properties adjacent to the construction site to understand their present condition and fragility, establish acceptable response limits, conduct soil-structure analyses of various earth-support systems, develop limits on their respective movements, and develop a monitoring strategy.
Dynamic monitoring capabilities have extended to three areas, according to Kavars: pre-construction and post-construction surveys, building-performance monitoring, and construction diagnostic monitoring. “All of these systems provide better analysis up front, better assessment during the project, and better mitigation plans, reducing potential problems,” Kavars said. “Whether used to measure the sway of a building, or the vibration caused by construction, or the angle of a bridge pier during a scour event, it provides the solution to assist an engineer’s decision-making, ensuring safety in a structure and protecting all involved in a construction project.”
During construction, computerized data acquisition systems (DAQ) can help ensure that product quality is maintained and specifications are met, Hussin said. “Historically, these systems have been used simply to record,” he said. “Today, DAQ systems allow the operator to monitor parameters in real time, and the computer takes control of the equipment based on inputted parameters to increase consistency, efficiency, and quality of the work.”
Geotechnology impacts design
Modern geotechnology not only helps protect adjacent structures and enhances safety during construction through use of pre- and post-construction surveys and continuous-monitoring systems, but also can provide civil engineers with greater design flexibility by supporting leaner, less-expensive foundation designs and allowing construction in less suitable areas, including on slopes and in poor soils.
“New technologies are allowing engineers to design and build in areas that were not suitable for structures previously,” said Garrett Krapf, engineer/sales representative, Pearson Pilings LLC. “By better understanding soils and the makeup of what is beneath a structure, engineers can avoid the issues [discussed above].”
Geotechnology’s Callanan noted that many solutions now exist for supporting intermediate foundation loads — two- to six-story structures — on poor ground. The range of generic and proprietary systems include vibrocompaction, vibroreplacement/stone columns, Rammed Aggregate Piers, Controlled Modulus Columns, Impact Piers, vibro concrete columns, dynamic compactions, grouting (jet, permeation, compaction, compensation), soil mixing, and more. “Although having all the competing systems is good,” he said, “it requires more effort to evaluate these systems because the design process may be unique to a particular foundation contractor. An unfortunate consequence is the occasional oversight in design. We’ve been called in to evaluate projects (by others) following excessive settlement. Errors can often be tracked back to the fundamentals. There should always be a good site investigation appropriate to the range of foundation systems that will be considered (borings should be deep enough). The geotechnical design recommendations should always be peer-reviewed. The solution should be the best combination of calculation and experience.”
Real-time, remote monitoring based on better sensors and user-friendly software is providing civil engineers with greater flexibility in completing projects successfully, said Campbell Scientific’s Stevens.
Fiber optics and web-based monitoring are both improving data acquisition, said Jeff Keller, P.G., marketing and sales manager, Applied Geomechanics. Fiber optics technology, in particular, is fairly new in the civil engineering arena, he said, but is being used increasingly in bridge, pipeline, and mining projects around the world.
“Many projects these days are using loggers, cell modems, and web-based monitoring to retrieve instrumentation data and visualize without going in the field,” Keller said. “Today it is not a question of getting data, but what to do with all the data.”
However, that’s not a new problem, noted Dataforensics’ Deaton. “Most departments of transportation and other government organizations involved in infrastructure projects have large repositories of geotechnical information including boreholes, lab testing, and in-situ testing,” he said. “Unfortunately, these repositories are largely inaccessible and unusable due to them being stored in inconsistent digital formats, or on paper in filing rooms or warehouses, or on microfilm. Geotechnical and geo-environmental data is a valuable asset and should be treated as such.”
Contributors to this article highlighted the following solutions to a range of geotechnical issues:
Applied Geomechanics (www.geomechanics.com) developed a program called the Instrument Service (IS) that can connect, interpret data formatting, and control many types of sensors — such as fiber optic, GPS, voltage, and vibrating wire — using the same timestamp to synchronize them all. IS contains a data collection scheduler and FTP capabilities to send formatted data to the company’s ATLAS web-based software. Applied Geomechanics also developed a Digital Borehole Dipper (DBD) that can be installed in an open borehole (without casing). The DBD, rated for 5,000 psi, can be installed in series (as many as 100 sensors) and grouted in place. It has an on-board compass and gyro to correct for positioning.
Campbell Scientific (www.campbellsci.com/geotechnical) dataloggers can process various measurements and send an e-mail to appropriate personnel when a predetermined threshold is reached. The same datalogger can turn on alarms or trigger a camera to take a picture or stream video. The company recently released a vibrating wire interface (AVW200-series) based on using spectral analysis to determine the resonant frequency for a variety of vibrating wire sensors. Because spectral analysis can distinguish signal from noise on the basis of frequency content, this method offers improved immunity to competing noise.
Dataforensics’ (www.dataforensics.net) PLog Enterprise is a GIS-based geotechnical data-management system that allows organizations to have all of their data in a single repository. PLog Enterprise facilitates finding and reporting data via a map-based interface. Users can query the data spatially or non-spatially using flexible query tools.
Geotechnology (www.geotechnology.com) added piezo-cone penetration testing (CPTu) to its list of capabilities. CPTu allows the firm to collect subsurface data over large areas cost effectively for projects such as wind farms, levees, and transportation projects. Geotechnology, which started providing geophysical survey services 24 years ago, has expanded its range of services to include seismic reflection, refraction, tomography, resistivity, radar, and electro magnetic. These techniques are used to identify subsurface voids and utilities, as well as collect seismic and soil resistivity data.
Hayward Baker (www.haywardbaker.com) is developing product-specific data acquisition systems to incorporate into its work to maximize quality and efficiency. For example, when soil mixing, a computer can read the penetration and withdrawal rates and deliver the precise amount of binder needed to stabilize the soil. When grouting is involved, grout injection rate, pressure, and quantity are remotely controlled by a grouting specialist.
Pearson Pilings’ (www.pearsonpilings.com) fiber-reinforced polymer composite piles do not contain leachates or preservatives and, according to the company, will not rot, rust, or deteriorate, providing a long-lasting foundation and structure.
Pile Dynamics’ (www.pile.com) Pile Driving Analyzer, used for dynamic foundation testing, now uses wireless sensors and allows an engineer to observe testing remotely using SiteLink technology. The company also developed a Thermal Integrity Profiler to test drilled shaft integrity and expects the method to answer some questions that other testing methods leave open.
SENSR (www.sensr.com) recently launched SENSRnet, an Internet-enabled assessment and monitoring system for measuring changes in a structure. The system allows engineers to use smartphones to monitor critical changes on a jobsite and allows entire project teams to stay continuously aware of pertinent data that may affect a project’s safety and efficiency.