Getting the Best Data Starts with Proper Handling of Geotechnical Samples

As many wise geotechnical engineers have said, “Good quality test results start with a good quality sample!”

It is vital for geotechnical engineers and project managers to think twice about how the samples for their project will be handled from extraction in the field to arrival at the laboratory, as ultimately, the information from these samples will be turned into pertinent data for use in project design. The resulting quality of that data, and its impact on design and project costs, can be highly dependent on how well the samples were cared for.

Before the number crunching begins by the geotechnical design team, geotechnical exploration must be completed, and samples must be obtained for analysis. The expense and complexity of obtaining those samples increase exponentially when the site is hard to access. Drilling in dense woods requires clearing, a slow and painstaking process. Drilling in remote or limited access areas often puts the drill rig into dangerous zones. Offshore exploration can mean drilling off ships or barges where the costs and logistics to mobilize are prohibitive and “sampling do-overs” are virtually impossible. Squeezing a drill rig into a busy urban environment can be a logistical nightmare. Bottom line…it takes A LOT to gather those subsurface samples, and how they are handled from the moment they leave the ground to the time a technician extrudes them in the laboratory for processing and testing is of PARAMOUNT importance to the test results and the final design.

A lone 4-inch soil sample (never mind a box of them!) can yield a wealth of information on which a quality geotechnical design depends on. Therefore, careful planning and consideration can help deliver the samples to the laboratory in as close to “in-situ” conditions as possible. An excellent reference that is often used is the standard ASTM D4220/D4220M-14 (Standard Practices for Preserving and Transporting Soil Samples), but other resources are available. While some level of sample disturbance must be expected, the totality of it is very dependent on soil type, sampling methods, handling and storing of the samples, the know-how of the drilling crew, as well as laboratory staff. The best geotechnical teams will work in tandem with drilling crews and laboratory staff to utilize the best sampling methods appropriate for the encountered soil conditions and be prepared to properly extract samples from the ground and store them in the field utilizing containers suitable for the encountered soil types.

Crate of Shelby Tube sample slots – designed with energy absorption materials developed by NASA for minimal sample disturbance

There are a few things about proper handling of soil samples that are most important for geotechnical engineers to keep in mind:

Soil Matrix Integrity

Once a sample is extracted from the ground, it should be housed in such a container that it stays intact until extruded in the laboratory. The sample should be protected from vibration and disturbance that could potentially alter the matrix of the sample through shaking, bouncing, dropping, etc. Padded containers with bubble wrap, foam, or other means of absorbing vibration should be used for all samples but particularly on loose and sandy samples prone to disturbance.

Sample Moisture

It is just as critical to maintain the in-situ moisture content. Containers that seal the sample and prevent air from entering or escaping are recommended to maintain the sample’s state close to what it experienced underground. If the time of transport or time in storage is of concern, wax or similar seals should be implemented on the samples for added protection.

Sample Orientation

Storing and transporting soil samples to the laboratory while maintaining the sample’s true orientation is very important. Containers that store samples upright, such as the GeoTesting Express (GTX) Shelby tube shipping container, take care of this while also minimizing vibration of the sample during transport.  

Anna Kotas with a GTX Shelby tube container

A state-of-the-art geotechnical laboratory can produce some of the most valuable data about the classification, compressibility, and strength behavior of soils. Geotechnical laboratory technicians and geotechnical engineers can aid designers in the most complex geotechnical projects. However, their work depends heavily on the quality of the soil sample that arrives in the laboratory. One further step that can be considered for sensitive or limited samples is to X-ray the Shelby tube, which can be very helpful to understand the state internally to allow choosing the best candidate for testing. Every team member that takes part in the initial phases of a geotechnical program should put the highest emphasis on treating each geotechnical field sample as if they were precious, one-of-a-kind, answers to subsurface mysteries that only that sample can unlock.

Just as it matters to a new bride if her diamond measures at 1.01 or 1.11 carat (as does the cost), it matters to a geotechnical engineer if the φ angle is 30° instead of 27° . A few degrees of difference in friction angle can translate to enormous cost differences in design when hundreds of deep piles are to be installed. Be it a friction angle, φ, or cohesion value of a soil, or any of the tens of engineering parameters a laboratory delivers to a design engineer, it can be a difference in thousands of dollars and significant labor on a large construction project. This “elevated” treatment of all subsurface samples will result in the most accurate testing results and better, safer engineering design. 

Post by: Anna Kotas, PE, Geotechnical Engineer at Geocomp/GeoTesting Express. Anna has been with GTX for 7 years doing business development throughout the Mid-Atlantic region and beyond. Throughout her career, she managed geotechnical projects from the earliest phases, including site recon, drilling, lab testing, analysis, and report preparation. She believes her hands on experience in the early years of her career combined with her management roles were invaluable to her current role as a representative of a world-class geotechnical laboratory.

Geotechnical Illustrated: Eyes Wide Open Below Ground

Geophysics plays an important role in identifying subsurface conditions below ground. They act as a below-ground “camera” to capture different conditions that may be key to project designs or risk avoidance. They are non-destructive, relatively quick, and non-invasive.

Often these methods are used to define subsurface conditions, identify cavities, map depth to bedrock, or aid forensic investigations. Geophysical methods include, amongst others, active seismic methods such as multi-channel analysis of surface waves (MASW) or passive methods such as seismic interferometry (SI). Both active and passive seismic methods can measure the shear wave velocity profile within the subsurface by recording propagating surface waves and performing dispersion analysis. Dispersion is the phenomenon by which waves of different frequencies travel at different velocities through the subsurface.

Nodal setup along the rail tracks for the S-scan acquisition

S-scan, a seismic mapping system that has evolved from the oil and gas industry, is a unique hybrid solution that incorporates 3rd generation MEMS accelerometers, DAS/fiber optic sensing, and patented algorithms for passive multi-seismic processing to accurately characterize near-surface conditions. It creates high-resolution images of subsurface conditions using signals from background sources (ambient noise) such as trains, wind, rain, or vehicle traffic. S-scan can provide higher resolution information at much greater depths than other geophysical methods (e.g., GPR or MASW) because of its enhanced sensitivity performance at lower frequencies. Moreover, as a hybrid solution, it can create high-resolution images faster than other passive seismic methods because of superior algorithms. While initially developed for railway applications, this technology is now being dedicated to site characterization related to mining, transportation, and civil engineering applications to detect and monitor subsurface changes.

   S-wave velocity profiles identifying weak zones (circled in red) that were not detected by a prior Ground Penetrating Radar investigation

Geotechnical engineers use soil borings, standard penetration testing (SPT), and cone penetration testing (CPT) to characterize the subsurface. However, these tests are invasive, time-consuming, and limited to identifying a small zone around the penetration. Geophysical methods can provide a continuous 3D profile, are non-invasive, and can be made relatively quickly. However, these methods provide more valuable data for engineers when there is an element of ground truthing. A combined approach can be optimized using a geophysical scan with fewer borings and/or penetration tests. For example, a high-resolution S-scan survey (2D or 3D) over a soil-rock layering confirmed with limited borings and/or CPT soundings can delineate the top of the rock and the quality of the rock with better clarity and accuracy. This combined approach provides enhanced detail across the site with a much smaller number of borings and often with a reduced cost. The shear wave velocities measured with a passive seismic survey can also be correlated to valuable engineering parameters thereby further reducing the need for more invasive and costly subsurface investigations.

Recent advancements in fiber optics sensing and high-power computing also enable us to use existing fiber networks as an alternative sensing tool that can identify weak zones in the subsurface and monitor changes over time. One application is along highways and railways in karst-prone regions.

The S-scan seismic mapping system is analogous to an eye in the ground that can see within the subgrade using minimal effort (no need for an active signal source). This solution can often provide a more economical, more beneficial, and simpler alternative to intrusive subsurface investigations, especially when coupled with a smaller scope geotechnical investigation for validation. 

To learn more about the S-scan solution, contact us at info@geocomp.com.

Assem Elsayed, Ph.D., P.E. is the Vice President and Practice Area Leader of Geostructural Engineering at Geocomp. Assem has extensive experience with support of deep excavation, ground improvement techniques, geotechnical analytical methods including finite element analyses with Plaxis and groundwater control with GeoStudio.

Seda Gokyer Erbis, Ph.D., P.E., is a Geotechnical Engineer and Senior Project Manager at Geocomp. She has extensive experience in research & development, and consulting in geotechnical earthquake engineering in addition to seismic instrumentation and numerical modeling in the flow and transport in soils.

Antonios Vytiniotis, Ph.D., P.E., is the Director and Group Lead of Geocomp’s Massachusetts Consulting Group. Antonios has a background in structural and geotechnical earthquake engineering and numerical analysis. He has experience in using probabilities to understand seismic risk of geotechnical components and geotechnical standards for soil-structure interaction.

What do Geotechnical Forensic Engineers do?

How often do you see news about a structural collapse or massive landslide occurring somewhere in the world? What happens after such an event occurs? Once the situation is stabilized, a selected group of experts may be brought to the site to conduct an investigation and provide opinions on possible causes of failure.

Forensic engineering is the engineering practice which determines the likely cause(s) of the failure of structures. Merriam-Webster dictionary defines a failure as an unforeseen and unplanned event or circumstance. In geotechnical engineering, failures can be related to man-made structures or natural formations. Such failures can result in significant losses that may include environmental damages, economic losses, and societal impacts, among others.

What is it like working on a forensic engineering project? It feels like solving a mystery. It starts with brainstorming about potential causative factors and continues to comprehensive analyses of each contributing mechanism to establish the likely root cause of the failure. Forensic engineers have to rely on available data, new data gathered in the investigation, their background knowledge and experience, as well as industry practices and applicable standards and codes. Often, they have to devise innovative ways to process data or to acquire new information.

By studying failures, geotechnical engineers improve their practices and analysis methodologies. Multiple failures have contributed to such improvements. In 2004, the metro tunnel along Nicoll Highway in Singapore failed. The nature of this failure illustrated the need to use representative soil properties in numerical analysis and to property update analysis based on field observations. The Vajont Dam overtopping in 1963 in Italy was one of the key failures showing the importance of conspiring pore pressures in landslide triggering, and the need for a thorough geotechnical investigation to locate any potential weak failure planes. The landslide that occurred east of Oso, Washington in 2014 showed the importance of a detailed study of site history and monitoring of field condition changes.

Oblique aerial photograph of the 2014 landslide in northwest Washington. Credit: Mark Reid, USGS(Public domain.) Source: https://www.usgs.gov/news/revisiting-oso-landslide

Historic records show that dam failures occur somewhat frequently in the US. There has been an average of 10 dam failures per year since the mid-nineteenth century, many of which are small failures with limited impact to the downstream area. However, some of the largest disasters in the US like South Fork Dam in Pennsylvania (1889), St. Francis Dam Failure in California (1928) and Buffalo Creek dam failure in 1972 each took lives of hundreds of people. Through extensive review of case studies, geotechnical engineers determined that the most frequent causes of dam failures are overtopping and seepage related causes.

Experts in the forensic geotechnical engineering field use advanced geotechnical exploration, laboratory studies, instrumentation and monitoring, and advanced analyses using digital models to understand the sequence of events leading to failure. The conclusions of the investigation are usually summarized in forensic engineering reports, which may include recommendations for remedial measures. Many reports of forensic investigations have become publicly available, and the lessons learned have been very useful to improve design methodologies, construction means and methods, and rules and regulations. 

For example, the now famous case of failed apartment buildings after the Niigata earthquake in Japan in 1964 triggered much focus on geotechnical earthquake engineering. As a result, we now have much more robust methodologies to design against earthquake induced liquefaction. The Oroville spillway dam failure in 2017, triggered substantial discussion on spillway design and general industry practices in the Unites States. California state implemented new laws and regulations for classification of hazard potential of dams and emergency preparedness, which were adopted by the end of the same year.

Oroville Dam (California).
Source: https://damfailures.org/case-study/oroville-dam-california-2017/

What makes forensic geotechnical engineering unique? It is the variety of technical issues that occur under different conditions all over the world. Forensic analyses oftentimes need state-of-the-art methodologies. This is one of the reasons they can push the envelope in improving our current standards. Forensic engineers allow us to gather and share knowledge about reasons of failures and appropriate remedial measures. With the advances in science and technology and utilizing lessons learned from past performances engineers can update standards of practice and be better prepared to face challenges of the future.

Anna Suprunenko earned her Master of Science Degree in Geotechnical Engineering from the University of New Hampshire. She has over six years of experience in slope stability and settlement analysis, interpretation of in-situ measurements, geotechnical and geophysical field testing, foundation design and advanced laboratory testing.She volunteers for the MATHCOUNTS school competition and Mass STEM Hub program to promote engineering as a field of study and profession among middle school students.  She also gives back to her profession by serving on STEM Outreach Task force of ACEC-MA.

References:

Delatte, Norbert J., “Beyond Failure: Forensic Case Studies for Civil Engineers” (2009). Civil and Environmental Engineering Department Books.

https://www.ferc.gov/industries-data/hydropower/dam-safety-and-inspections/oroville-dam-service-spillway-p-2100

https://www.icold-cigb.org/GB/dams/dams_safety.asp

Geotechnical Illustrated: Triaxial Testing in the 21st Century

The triaxial test is arguably the most widely used test method in investigating soils’ stress-strain behavior in geotechnical engineering. Its popularity is partially due to the well-understood state of stress during the test, as well as its versatility. In a triaxial test, the specimen is encased in a membrane with drainage from top and bottom (and sides if filter paper is used). The specimen is confined to specific stress through a pressurized fluid (such as water) inside the test chamber. In many cases, the specimen is backpressure saturated before being consolidated to desired stresses. Pumps or other sources of pressure and volume control can be used to generate confining stress and backpressure. Once the specimen is at the target stresses, deviator stress is gradually applied, usually axially, via a load frame, until the specimen fails, or the desired strain or stress is achieved.

Various types of failure during a Triaxial test

The most common triaxial tests are Consolidated Drained (CD), Consolidated Undrained (CU), and Unconsolidated Undrained (UU) conducted in isotropic condition and with deviator stress applied with a constant rate of axial strain. Using the results of several tests at different confining stresses, one can develop shear strength parameters using a failure criterion like Mohr-Coulomb. Shear strength parameters can then be used in the process of designing geostructures.

Soils are unusual materials in that their engineering properties are stress path-dependent. This means their strength and stiffness depend on past, present, and future stress conditions. For instance, an in-place soil element will have different strength and stiffness if the stresses increase in horizontal direction compared to stresses increased in the vertical direction.

In some cases, the underground soil is not isotropically consolidated but is in at-rest (K0) condition. The soil elements change in axial and radial stresses that may be increased or decreased. The geostructure is subjected to axial or radial unloading rather than loading, or there may be a pore-pressure decrease or increase due to the changes in the hydraulic equilibrium and condition of the area such as heavy rain. These complicated stress paths and conditions have traditionally been hard to simulate in the laboratory. For example, to conduct a triaxial test under the K0 condition, one must maintain the radial strain at zero while applying the target axial stress to the specimen by constantly changing the cell pressure. This requires a fast closed-loop controller which renders manual testing impractical.

The introduction of computer-controlled equipment with high-speed closed-loop and adaptive control systems has revolutionized the quality, reliability, and speed at which stress path tests can be conducted. A modern computer-controlled triaxial testing apparatus with advanced adaptive control can:

  • Easily back-pressure saturate specimens, conduct B-checks and move on to the next step automatically when saturation is complete
  • Consolidate the specimen at any stress state (isotropic or anisotropic), automatically
  • Determine the K0 value for the applied axial stress (for K0 tests)
  • Ensure completion of the consolidation before automatically moving on to the next step (after a predefined time or number of log cycles are past the 100% primary consolidation)
  • Easily impose various stress paths on the specimen

Such advanced systems enable engineers and researchers to go beyond standard tests and study the specimen’s behavior in more realistic stress states, however complicated they might be. They can simulate different scenarios or possible failure mechanisms, with a high level of accuracy.

Another benefit of the triaxial test is that it can be fitted with additional measurement sensors to expand the capabilities of this test method further. For example, by adding piezoelectric sensors at the end caps or sides of the specimen, one can determine the S- and P- wave velocities in different directions from which the low-strain shear and P- wave modulus can be measured at any stage of the test. The resulting data can provide valuable insight into the low-strain characteristics of the specimen at any given stress state and be used to investigate possible levels of disturbance in the specimen. End caps can also be retrofitted to measure the electrical resistivity of the specimen or measure and apply matric suction for unsaturated testing.

PnS caps

With the addition of a cyclic loading unit, the cyclic strength, liquefaction susceptibility, cyclic strength, cyclic mobility characteristics, and dynamic properties of soil specimens under triaxial loading conditions can be investigated. More advanced equipment can even apply the actual Cyclic Stress Ratio (CSR) or strain time history of an earthquake or other vibration sources at any given effective stress on the specimen for a more realistic study of its behavior.

Geocomp pioneered the introduction of fully automated testing equipment in production laboratories with the 1985 installation of three fully automated consolidometers in the USACE materials testing facility in Dallas, TX. Since then, it has continuously improved and expanded its line of fully automated geotechnical testing equipment to include triaxial, consolidation, direct shear, direct simple shear, permeability, resilient modulus, resonant column torsional shear, cyclic triaxial, cyclic direct simple shear, and cyclic torsional shear tests. Nowadays, engineers have many affordable options in their tool chests thanks to the technological advances and expanded capabilities of automated testing machines. This is in a sense the golden age of geotechnical engineering. We can put aside conservative approaches and take on data-driven best practices to manage risks and save money and lives at the same time.

Kaveh Zehtab is the Director of Research, Development, and Production at Geocomp. Kaveh has a wide array of experience in investigating soil behavior, experimental geomechanics, and automated testing.

Geotechnical Illustrated: Sulfate Reducing Bacteria (SRB) – What is it?

Microbes live everywhere including the oceans. Sulfate-reducing bacteria (SRB) are considered microbes. SRB is one of many common bacteria species that live throughout our ecosystem. Seawater is a primary source of SRB where a single gallon of seawater has roughly a few billion different kinds of bacteria. While most bacteria are aerobic and require oxygen to survive, SRB is among a group of species that are anaerobic and thrive only in the absence of oxygen. While SRB exist in the wild and does not present a danger to humans or animals, its effects can cause damage to metals and concrete. Offshore structures either resting on the seabed or penetrating through, whether made of concrete or steel are exposed to those attacks by SRB. Offshore structures, such as oil and gas platforms, monopiles foundations and concrete gravity base structures for wind turbines, subsea pipelines, and subsea manifolds are potential targets for those bacteria attacks.

This attack can be described as a phenomenon called Microbiologically Induced (or Influenced) Corrosion (MIC). MIC is the degradation of structures because of the activity of various microorganisms. So, the attack comes in a form of corrosion and can be called, microbial corrosion, bacterial corrosion, biological corrosion, or concrete/metal eating bacteria.

SRB perform anaerobic respiration utilizing sulfate (SO42) as a terminal electron acceptor, reducing it to hydrogen sulfide (H2S), which is a highly corrosive compound to concrete, steel, and other elements such as copper. Another set of microbes turns hydrogen sulfide to sulfuric acid, which can cause pitting of steel or concrete and hence reducing the service life of the structures. In other words, these sulfidogenic microorganisms “breathe” sulfate rather than oxygen and the results are highly corrosive substance.

Cathodic protection (CP) is one of the corrosion control methods mostly used with satisfactory results; however, where the phenomenon is microbiologically influenced corrosion (MIC) related, the efficiency of such control methods decreases significantly, due to several factors, (e.g. MIC may increase the kinetics of the corrosion reactions, which in turn increases the CP current necessary to achieve a given level of polarization.) The microorganism can attack pipeline coating for example, increasing exposed metal surface area and further increasing the CP current required to achieve the desired polarization. Hence, the use of coatings with high resistance in the presence of bacteria is a must.

Corrosion protection of offshore structures according to several organizations such as DNV (a European based advisor for the maritime industry) must include a holistic approach including corrosion allowance, cathodic protection, corrosion protective coatings and use of corrosion resistant materials. Measurement of corrosion rate with techniques such as linear polarization resistance (LPR) may also be very useful especially both for new and old structures.

Although SRB is a very tiny organism we cannot see, it can lead to potentially catastrophic consequences and costly operational shutdowns and should be taken seriously in the design of any offshore structures.

Assem Elsayed is the Vice President and Practice Area Leader of GeoStructural Engineering at Geocomp. Assem has extensive experience with waterfront and marine structures, design of monopiles for wind farms, and support of deep excavation.

How to Assess the Strength of Coal Combustion Residuals

Stability failures of impoundments involving release of Coal Combustion Residuals (CCRs; commonly known as coal ash,) like the one that occurred in Dan River Coal Ash Spill in 2014, have recently highlighted the importance of assessing the stability of CCR impoundments. CCR materials contain pollutants like Mercury, Cadmium and Arsenic. Without proper management, these contaminants can be released, resulting in pollution of waterways, ground water, or drinking water.

Collapsed coal ash impoundment with closed power plant in the background

Failures in CCR slopes can occur suddenly with little time for corrective action to occur. It is hence crucial to assess CCR slop stability with realistic strength properties of the CCR materials to help avoid this undesirable performance.

What are these CCR materials? How are these deposited? And more importantly, how do we determine the strength of these materials to use in calculating the factor of safety for stability?

What are CCRs?

CCRs are produced from burning of coal in coal-fired power plants. They include various by-products such as fly ash, bottom ash, boiler slag or flue gas desulfurization material.

Coal Combustion Residuals:

  • Usually classify as non-plastic or very low plasticity silt, silty sand, or sandy silt
  • Are much lighter than typical soil with a specific gravity as low as 2.0
  • May have water content as low as 20% when dry stacked or as high as 80% when loosely placed

How are CCRs deposited?

CCRs are placed by various methods. The two most common ways are:

  • Ash stacks: CCRs are “dry stacked” and compacted. Dry stacks of CCR are most commonly unsaturated and compacted to some degree.
  • Ash ponds: CCRs are loosely placed by hydraulic sluicing and receive no compaction. These sluiced materials are generally very loose, saturated and therefore susceptible to liquefaction.

CCRs can exhibit considerable variability in deposition over short distances.

Example x-ray: undisturbed tube of ash

How do we determine the strength of CCRs?

Engineers typically perform stability assessments of CCR impoundments which have been in place for some time by assuming they are in a steady state seepage condition. That means they are considering a “drained” failure mode where excess pore water pressures do not develop during shear. Undrained stability with accompanying excess pore pressures generated by shear has generally not been considered a potential failure mode unless the impoundment is still under construction, or some significant alteration to its condition has been recently done.

This common approach for stability assessments overlooks the potential for an undrained failure to be triggered by an event that causes a rapid increase in mobilized shear stress or a rapid decrease in effective stress, such as and earthquake or heavy rainfall. Undrained strength of loose granular materials is less than the drained strength so the undrained failure mode can be more critical. In these instances, if a contractive saturated material (susceptible to liquefaction) is present, an undrained failure can be triggered. An undrained behavior in contractive materials can trigger liquefaction (the CCR material transitions to behave like a viscous fluid) and result in a flow slide. 

Such a failure can be evaluated by means of an undrained stability analysis using undrained shear strengths for the contractile materials. Saturated, contractive CCR materials tend to produce shear strength ratios ranging from 0.12 to 0.35. Common drained strength friction angles for CCR materials range from about 28 to 45 degrees. The associated strength ratios for these friction angles range from 0.53 to 1.00. The difference in strength ratios for an undrained contractive CCR clearly shows the undrained condition to be more concerning.

It is a challenge to obtain a meaningful assessment of the strength of materials like CCRs. They are highly variable and very sensitive to sample disturbance which alters their mechanical properties. In our experience, we find we must use both field and laboratory methods to obtain reliable strength parameters.

Field Testing

These tests require mobilization of drilling equipment to the site and monitoring of the work by trained engineering staff. In addition to the described tests, disturbed and undisturbed samples are generally recovered and returned to a laboratory for further testing. Field tests are accompanied by in-situ measurements of pore pressures in CCR stacks using vibrating wire piezometers that are installed in various depths as well as spatially. We recommend using grouted-in-place pressure transducers with several installed in each borehole to measure total head changes in the vertical and horizontal directions. Flow in most stacks is almost never hydrostatic as assumed by many engineers. Measured pore pressures can help understand the flow regime and the saturation conditions within the deposited CCR.

Standard Penetration Tests (SPTs) are the traditional tool used by geotechnical engineers to characterize granular materials. The so-called “blow count,” for blows per foot required for a 140 lb hammer dropping 30 inches to drive the sampler 12 inches into the soil, gives an indication of the relative density of the penetrated material and returns a disturbed sample. However, as typically performed by drillers, where drilling mud is not used, this measurement can give misleadingly low values. To be meaningful, the hole must be advanced with heavy drilling mud with the mud level always kept above the top of the hole. Otherwise, the SPT value can be quite low and lead to a wrong assessment of the material’s condition. SPT values obtained strictly following ASTM D1586 and D6066 can provide valuable data to help assess the strength behavior of CCR materials.  

Cone Penetration Tests (CPTs) provide the tip resistance, side shear and excess pore pressure developed when pushing a 10 cm2 probe into the material at a constant rate. It can provide lots of data on relative density of the CCR quickly and inexpensively and is a good tool to capture the material variability. However, the measurement is an index test which must be converted to strength through semi-analytical, empirical correlations. Such correlations are reasonably well understood for natural clays and sands, but for CCRs, well established correlation data are limited. Developing an understanding of the strength characteristics of these unique materials requires advanced laboratory testing.

Cross-Hole Seismic Tests can provide a profile of shear wave velocity measurements throughout the CCRs, providing information on the variability within the deposition of the CCRs and the low strain stiffness. Shear wave velocity measured in the field when compared with the corresponding laboratory shear wave velocity measurements can help with assessing the sample disturbance and guide targeted laboratory testing to determine the strength of the CCRs.

Advanced Laboratory Testing

Since disturbance alters the properties of CCR materials, laboratory tests to measure mechanical properties are generally performed on undisturbed samples recovered from the field investigation. Since no sample obtained in the field can be truly undisturbed, testing technique and test interpretation must seek to minimize the effects of sample disturbance on the test results. The advanced tests should be accompanied by index tests (e.g., material classification, moisture content, Atterberg limits, specific gravity) from the same sampling tube to support the knowledge of the tested material and interpretation of the test results.

Laboratory Shear Wave Velocity Measurements is a very useful and quick test to determine the level of sample disturbance and whether the test specimen is representative of field conditions. Piezo-ceramic plates also known as bender elements can be embedded in any type of strength test apparatus and used to measure the shear wave velocity of the specimen at stress levels similar to the in-situ stress conditions. Shear wave velocity gives an indication of the relative density of the test specimen.

Direct Simple Shear Test (DSS) measures the strength of a test specimen in a simple shear mode (distorting the specimen similar to shifting a rectangular cross section to a parallelogram).  A few advantages of DSS are:

  • It is an undrained test
  • The shearing in DSS is closer to the mode of shearing for global stability under CCR embankments which is different from the mode of shearing induced in triaxial loading
  • The size of the specimen is short (1” in height) allowing many more tests from a sampling tube
  • The total duration of the test is much shorter since there is no saturation phase for the specimen and the duration of the consolidation phase is reduced with the short height of the specimen

Closure

Dealing with unique materials like coal combustion residuals may require using a series of field and laboratory techniques to assess both their strength and variability. The combination of an appropriate field and laboratory testing program can help reduce unnecessary conservatism and accurately evaluate the stability of a CCR embankment. Actual field pore pressures are also important as they directly affect the effective stresses within the impoundment barrier which in turn directly affect the material strength.  We strongly recommend measuring actual pore pressures in the field at multiple locations vertically and horizontally rather than assume certain flow conditions.

Our practice is to use the most appropriate tools and methods for both field and laboratory testing to obtain a reliable picture of how the CCR material is likely to behave for all the potential failure modes of an impoundment. This comprehensive approach helps us avoid the unnecessary conservatism used when data quality for CCR strength are incomplete or unreliable. This approach takes more engineering effort up front, but it usually pays for itself many times over by avoiding unnecessary conservatism in the stability assessment, reducing, or removing the need for expensive corrective actions, and providing a firm basis to help manage risk.

Seda Gokyer Erbis is a Project Engineer/Assistant Project Manager for Geocomp’s Massachusetts Consulting Group. She has been with Geocomp for six years, holding a doctoral degree in Geotechnical Engineering. She has been leading the project management and technical efforts for one of Geocomp’s largest consulting projects on seismic assessment of coal ash impoundments. She has over seven years of experience in geotechnical earthquake engineering especially in advanced laboratory testing. She has authored and co-authored several publications in peer reviewed ASCE and ASTM journals and conference proceedings.

The Value of Numerical Modeling for Geo-Structural Engineering

Numerical modeling uses complex computer programs to build a digital model of a site from which to calculate how a design will perform for various loading conditions. These methods avoid some of the simplifying and conservative assumptions that get made in simpler approaches. They help improve the efficiency of a design, predict how a design is likely to perform, show how a structure’s foundation and the groundwater will interact as a unit, and provide insight into the important mechanisms controlling how the design will perform. Improved predictions, better understanding of complex behavior, and optimized designs with minimal assumptions help reduce risks and costs for many projects. 

What is Numerical Modeling? Numerical modeling uses digital computer models to analyze stress, strain, and deformations in a project with complicated soil, water and structural geometries and materials. Numerical modeling methods include the finite element method (FEM), the finite difference method (FDM), the boundary element method (BEM), and the discrete element method (DEM). Nonlinear material properties can be considered which allow designs to be optimized with some degree of non-linear behavior and yielding which results in less conservatism and less cost. Numerical modeling can be used to analyze almost any type of geostructural problem in two or three dimensions.

When is Numerical Modeling Needed? Numerical modeling is used when the structure/soil/water geometry and materials are too complicated to solve with closed-form analytical equations. The methods rely on breaking the complex geometry into smaller pieces. The behavior of each small piece is described with a stress-strain model that represents the basic understanding of each material’s behavior (bending of a beam, stress/strain behavior of a soil cube, flow of water through soil, etc.). The numerical model combines engineering mechanics equations for force equilibrium, conservation of mass, kinematic continuity, and stress-strain-strength behavior of each piece into a large set of equations describing how the pieces interact.  Matrix algebra is used to combine the pieces and equations to create the digital model that gives stresses, strains, and displacements throughout the geometry for each specific load case. Examples of different types of problems addressed with a numerical model are described herein.

Evaluating Potential to Create a Seepage Barrier with Ground Freezing

The first example shown in Figure 1 is for a client that was designing a groundwater cutoff in an area with flowing groundwater. Flowing groundwater makes it difficult to freeze the ground (a construction technique used to provide temporary earth support and groundwater control). The left figure in Figure 1 shows a plan view (looking down from the top) of frozen soil in blue around the freeze pipe shown in white. The right three figures in Figure 1 show the frozen soil (in blue) for conditions of increasing velocities of groundwater flow. The flow direction is from the bottom to the top in the figure. Numerical modeling demonstrates to the client that flowing groundwater must be taken into account in the design of the frozen groundwater cutoff. Numerical modeling of the effects of groundwater flow saved the client millions of dollars in delays and redesign.

Figure 1: Numerical modeling of the effects of groundwater flow on growth of a freeze wall

Underground Storage of Compressed Gas

In the US, compressed natural gas has been stored in solution mined caverns in salt domes since the 1960’s. Salt creep is a major factor which needs to be considered in the design and stability analysis of these underground openings. Numerical modeling is used to evaluate the stresses around the opening and predict the deformation of rock salt with use and time. When the gas pressure is cycled in an underground cavern, there are changes in temperature and stress that influence the creep rate. Under these conditions numerical modeling is the only method available to calculate the closure rates in the cavern walls. Numerical modeling provides the client with minimum and maximum cavern operating pressures and design parameters for spacing between multiple caverns. This benefits the project designers by allowing them to develop an optimized storage design based on site specific geotechnical engineering analysis and operational needs. 

Figure 2: Numerical model of a deep cavern in salt for compressed air energy storage

Helping Design for a Complex Use Case

In the final example shown in Figure 3, a client wanted to develop design charts to promote the use of a proprietary retaining wall system comprised of steel soldier beams with soil mix lagging spanning between the beams. Traditional design methods for lagging are mostly empirical built up from experience. Numerical modeling provided the client with a parametric design chart used to promote the use of composite soil mix retaining wall structures. The numerical models also helped show that part of the load of the final structure could be supported by these retaining walls resulting in further efficiency of the design.

Figure 3: 3D numerical analysis of horizontal deformations in a composite soil mix retaining wall

In summary, numerical modeling is an important tool for analyzing complex geo-structural cases. It eliminates some of the conservatism used in regular design, reduces risk, and saves the client money. When analytical methods are not available, numerical modeling offers a great alternative to help develop safe designs without unnecessary conservatism.

Martin Hawkes is a Senior Geotechnical Engineer with a master’s degree in Geotechnical Engineering from MIT. Mr. Hawkes has been with Geocomp for 27 years, involved with developing the early versions if iSiteCentral, developing the laboratory information  database management system (LIMS) for GeoTesting Express, installing instrumentation, and geotechnical consulting. Martin provides a key role in numerical modeling with a unique skillset as a mentor for younger engineers.

Geotechnical Illustrated: Good Vibrations or Not?

Heavy construction in densely populated cities can be conducted near neighboring buildings. Vibrations induced by such heavy construction are one of the most claimed causes of adjacent building damage. These vibrations can be generated by multiple types of equipment such as pavement breakers, excavators, or pile drivers. Such activities can generate short term vibrations or contribute to longer periods of increased vibration levels.

Property owners experiencing construction-induced vibrations may file claims for nuisance or cosmetic/architectural and structural damages. Since the vibration threshold for nuisance is typically lower than the threshold to cause any damage, property owners may express concerns about vibrations that are not substantial enough to damage their property. During the COVID-19 era, more residents stay and work from home, giving a rise to construction-induced vibration claims.

Construction activities cause vibrations of various amplitudes and frequencies that propagate within the subsoils. The waves propagated through the soils typically attenuate with distance. Wave attenuation is caused by geometric and material damping. Geometric damping is caused by the spreading of the wave front over an increasing area. Material damping is caused by energy converted to thermal energy within the subsoils. The attenuation of vibrations with distance is typically estimated by a straight line in a double logarithmic plot of peak particle velocity (PPV) versus distance.

Construction vibrations are typically a nuisance to residents; however, many times they can contribute to property damage. Ground vibrations may be of sufficient magnitude to cause direct damage to structures. The magnitude of vibrations that causes damage varies with the type and the vibration response of the structure. For example, generally historic buildings are more sensitive to vibrations compared to modern residential or commercial buildings. Vibration damage can vary from threshold damage such as paint loosening, or minor damage such as masonry cracks to major damage that can cause structural weakening and distress. Various levels of threshold PPVs have been evaluated in the industry for different structures and damage levels. For example, Caltrans (2004) has proposed threshold PPVs as low as 0.12 inches per second for extremely fragile historic buildings to 2 inches per second for modern industrial buildings.

Vibrations can also cause indirect damage. For example, they can cause densification of loose soils beneath buildings, resulting in differential settlements. Vibrations can also cause partial loss of strength of loose saturated subgrade soils and hence contribute to loss of bearing capacity. Both for densification and for partial loss of strength to occur, a sufficient shear strain within the subsoils should be triggered (i.e., threshold shear strain) and enough vibration cycles should occur.

Vibration levels can be measured by geophones (i.e., velocimeters) or accelerometers. Geophones have been traditionally used in construction and is a proven technology. Accelerometers historically were less effective for the lower frequencies of construction vibrations, however modern accelerometers may be competitive with geophones. Such devices can relate to data collection systems, which automatically stream data to interfaces such as Geocomp’s iSiteCentral® portal. Such systems can also automatically provide text or email alerts to all involved stakeholders.

The vibration data measured at a site can be compiled, and over-laid with a series of other data such as aerial imagery, property parcels, tax assessor information, city maps, surface elevations or others. These publicly available geospatial data and project specific data, including damage claim data, can be combined within GIS software to create an incredibly robust tool to efficiently perform sophisticated analyses of damage claims. Once the data are input in a GIS system, the user can create graphics and statistics that allow for analysis of the specific issues of each property. Moreover, the graphics produced can become very compelling exhibits in a dispute (e.g., court exhibits). 

Compiled publicly available geospatial data and project specific data, along with any pre and post construction surveys of any adjacent buildings are efficiently analyzed to answer both simple and more complicated questions such as:

  1. Are construction activities sufficiently close to cause damage to the subject property? An overlay of construction drawings, field records, along with aerial photos allows for estimation of the distance of the subject property and the limits of various construction activities such as pavement removal, excavations, and pavement compaction with private buildings and improvements.
  2. Can construction-induced vibrations contribute to the claimed conditions? A geospatial overlay of the measured peak vibration levels can indicate whether significant vibrations occurred within a construction period and at what distances from the subject properties.
  3. Are there any effects of non-construction activities that need to be considered? For example, has the damaged property ever been flooded from a large flooding event? A simple overlay of the property and an inundation map could show if that ever occurred. Can vegetation affect the measured cracking within a building? An overlay of a city-wide tree map and aerial imagery can show whether any trees exist that can contribute to this damage mechanism. 

Fortunately, many effects of construction vibrations can be mitigated. If damage in a building is claimed due to construction vibrations, then the above mechanisms can be investigated to demonstrate cause-and-effect.

Antonios Vytiniotis is the Director/Group Lead of the Massachusetts Consulting Group at Geocomp. Antonios has wide experience in evaluating and mitigating damages from machine and construction vibrations.

Geotechnical Illustrated: What is your CPT Test Really Telling You?

In many cases with offshore filling construction, such as building and forming artificial islands in the middle of a body of water, borrow areas of suitable seabed materials must be first identified. A comprehensive soil investigation will subsequently help assess the suitability of the potential seabed borrow material to be dredged and used for engineering purposes.  Some of the seabed in areas around the world consist of carbonate sand that may have shell fragments within its matrix composition. Shell fragments contribute to carbonate content that may be in the order of 90% or more. This specific soil composition can influence the measurements obtained by in-situ soil testing techniques.

One of the most commonly used in-situ techniques to assess soil strength, especially in offshore applications, is cone penetration tests (CPT). The most important measurement obtained from CPT is the cone penetration resistance, which is the force required to push the tip of the cone through the soil of interest and is defined in ASTM D: 5778.  However, most of the standardized in-situ testing techniques, such as CPT, have been commonly calibrated to interpret soil properties for soils found onshore, such as silica sands.

  • Ariel view of The Palm on Dubai’s man-made Palm Jumeirah artificial island; REUTERS, August 31, 2011

After reclamation of an artificial island using carbonate sands, CPT can be used to assess its strength to carry loads for engineering purposes. However, a correction factor is required to adjust the measured tip resistance so that correlations developed for silica sands can be used for the carbonate deposits. This correction factor is called the Shell Correction Factor (F shell) and has been extensively studied by researchers around the world.  The Shell Correction Factor is the ratio between cone tip resistance of silica sand to cone tip resistance of carbonate sand. F shell commonly varies from 1.2 to 1.6 depending on the relative density of the sand.

So, don’t just pick up some standard correlations to interpret your CPT results. Make sure you understand the soil’s composition and its effects on engineering properties. 

Assem Elsayed is the Vice President and Practice Area Leader of GeoStructural Engineering at Geocomp. Assem has extensive experience with waterfront and marine structures, design of monopiles for wind farms, and support of deep excavation.

Geotechnical Illustrated: Risks of Spudcan Punch-through

Jack up rigs are used widely in offshore drilling and for offshore wind turbine installation. Such rigs can be often times supported by spudcan footings such as the ones shown below. However, there are multiple challenges in evaluating the capacity of such footings.

Punch-through of spudcan legs for offshore barges is defined as rapid uncontrolled barge leg penetration into the seabed.  Such an event could result in catastrophic damage and even loss of lives such as the one shown in the figure below. 

Accident Aboard Pemex Jack-Up, Bay of Campeche, Mexico, Picture Credit: Reuters, May 5, 2015

Spudcan legs jacked against seabed that is made of interbedded soil layers creates a recipe for potential punch-through.  In addition, the presence of a strong soil layer on top of a weak layer can contribute to sometimes unaccounted for punch-through.  These risks can be quantified using a probabilistic approach as a decision-making tool.

Barges are typically proof-loaded by vertical preloading through water ballasting prior to being operational, in order to obtain a safety margin against extreme storm design events.  It is a common practice to assess the potential for barge leg penetration into the seabed using both lower and upper bound soil parameters. This analysis requires engineering judgment to choose the right soil parameters based on the available soil information using a deterministic approach.  A more informative methodology is to perform a quantitative probabilistic study taking into account all possible variable soil data as well as other uncertainties. Insurance companies may be more willing to insure a project once the risks of failure are properly quantified.

A probabilistic analysis can be performed easily when the variability of soil properties or other input parameters is known. The most common approach is to assume that a normal distribution with a specified mean value and a standard deviation of a parameter, based on the available geotechnical information, can accurately represent the “randomness” of such input parameters. The parameters can be soil properties such as undrained shear strength and friction angle, or geometry parameters (e.g., soil layer thickness).

Such analyses can be commonly performed with simple random number generator tools, two way data tables (What-if Analysis) in tools like Excel. A common way to incorporate probabilistic data from multiple variables is a Monte Carlo simulation. Via this simulation, a cumulative probability distribution function of the spudcan footing’s factor of safety can be determined. An example of such a cumulative probability curve is shown in the figure below. An engineer, a project owner, or an insurance agent can then be able to better represent the uncertainty in the factor of safety calculation. For example, in the schematic below, there is only a 20% probability for the factor of safety to be less than 2, and a 90% probability that the factor of safety is less than 3.

This simple probabilistic approach can be applied to any engineering equation to quantify the associated results in a simple risk language.  

Assem Elsayed is the Vice President and Practice Area Leader of GeoStructural Engineering at Geocomp. Assem has extensive experience with waterfront and marine structures, design of monopiles for wind farms, and support of deep excavation.