Surface and Subsurface Hydrology and Drainage Design
Expert Subsurface Solutions
We have extensive experience with a wide range of traditional and innovative subsurface exploration techniques and applications, and we contract only qualified and knowledgeable exploration contractors to support our subsurface exploration programs. The subsurface exploration methods we employ are specifically tailored to match the environment, site access, available equipment, and design concepts. Many of the projects that we have completed have involved unconventional techniques for obtaining the subsurface information needed to properly design the project. Key to our success in obtaining quality geotechnical data has been our ability to communicate and work with the exploration contractor and our extensive knowledge of the capabilities of various types of exploration equipment.
Soil and Bedrock Methods
Our qualified exploration subcontractors offer a variety of subsurface exploration methods, equipment and technologies. We select the most appropriate exploration method(s) to provide the most relevant subsurface information for a given project.
Hollow Stem and Solid Flight Auger Drilling
Hollow-stem auger drilling is a relatively fast, easy, and cost-effective geotechnical exploration technology which employs hollow drilling augers (with center drill rods) to advance a soil boring in relatively unconsolidated/non-lithified soil and/or decomposed rock down to depths of up to effective of approximately 150-300 feet (depending upon the subsurface conditions encountered). The hollow augers support the borehole wall from collapse while serving as a conduit to allow for the advancement and retrieval of various geotechnical soil and rock sampling tooling and/or subsurface instrumentation. Hollow-stem auger drilling is a proven and established geotechnical exploration method, and has proven to be the gold-standard for conventional geotechnical exploration programs.
Hollow-stem auger drilling does have limitations though, and proper considerations should be made prior to the commencement of a hollow-stem auger drilling program in order to evaluate its suitability, benefits, and risks for a given project.
Air and Mud Rotary Drilling
Rotary drilling is a technique which employs a rotating drill bit that grinds/pulverizes the soil/rock as the drill bit advances and then uses a medium (such as compressed air, water, or drilling mud) to transport the drill cuttings up to the ground surface. Rotary drilling is most commonly associated with water and oil well drilling, but also plays an important role in geotechnical exploration. Rotary drilling often incorporates the usage of driven steel casing to help reinforce the drilled borehole and prevent borehole collapse, while facilitating cuttings transport out of the borehole. Rotary drilling creates a borehole (either cased or open-hole) which serves as a conduit to allow for the advancement and retrieval of various geotechnical soil and rock sampling tooling and/or subsurface instrumentation.
Rotary drilling is especially useful for drilling in very coarse and/or dense soils, as well as bedrock, as it is relatively unaffected by the presence of hard materials (such as rock). Mud rotary drilling is also very useful for collecting undisturbed samples of sand soils which are located below the groundwater table, as the drilling mud serves to equalize the hydrostatic pressures between the open borehole and the surrounding formation, which reduces the potential for sand heave and increases the potential for the collection of an undisturbed sample (important for soil liquefaction analysis). However, rotary drilling requires an additional level of effort and support equipment and therefore can be relatively expensive to conduct and/or difficult to mobilize to remote areas.
Geoprobe Direct Push Coring
Sonic Drilling Technology
Sonic drilling is a subsurface exploration technique that strongly reduces friction on the drill string and drill bit due to liquefaction, inertial effects, and a temporary reduction of the porosity of the soil generated by a high-frequency vibration applied directly to the drill stem at the drill head. In addition to vibration, sonic drilling uses both the rotation and downforce of the drill rig and drill casing to advance the borehole. Sonic drill stems incorporate both an inner core barrel and an outer sonic drill casing to penetrate the substrate, stabilize the borehole, and collect continuous, relatively undisturbed soil samples.
Sonic drilling techniques can recover a fairly large sample specimen (depending upon the casing diameter employed), successfully sample large-diameter granular soils (gravel and cobbles), and are somewhat effective at recovering in-tact samples of frozen soils. Sonic drilling is also fairly efficient, results in limited ground disturbance, and is devoid of drill cuttings. Sonic drill rigs can also be equipped with standard geotechnical sampling equipment (SPT/MPT, Shelby tubes, etc.). However, sonic drilling is a fairly new, equipment-intensive technology, and therefore, is generally fairly cost-prohibitive for moderate to small-scale geotechnical exploration projects.
Diamond Rock Core Drilling
Diamond rock core drilling is typically employed in geotechnical exploration applications when samples of the bedrock need to be evaluated for engineering properties. Diamond rock core drilling can be very time consuming and expensive, and is usually reserved for projects which aim to utilize the bedrock as a source of construction materials (i.e. quarry, mine, etc.) or for projects which will bear directly on, or into, the bedrock body itself.
Test Pit Excavation
Oftentimes, factors such as project budget, site access, subsurface composition, and/or drill rig availability, make geotechnical soil boring/coring methods impractical, inappropriate, and/or cost-prohibitive. Therefore, we often employ mechanical excavating equipment to conduct subsurface explorations in the form of open test pit excavations. Excavation equipment is fairly commonplace (even in more remote areas), relatively inexpensive to operate, and tracked excavators (with skilled operators) can negotiate almost any type of terrain. Therefore, mechanical excavators play an important role in geotechnical exploration.
Test pit excavations can reveal a lot of visual information about the shallow subsurface that soil boring/coring cannot, including (but not limited to):Vertical and horizontal material contacts;
Actual particles size of large-diameter soil particles (i.e., gravel/cobble/boulders);
Nature and extent of any buried debris;
Differentiate between bedrock and large boulders in the shallow subsurface;
The collection of large (bulk) samples.
However, test pit excavations have several drawbacks, including (but not limited to):
Excessive ground disturbance;
Limited exploration depth (depending upon equipment size and excavation footprint);
Lack of ability to conduct in-situ geotechnical testing/sampling such as SPT/MPT, Shelby tubes, etc.
Geophysical Subsurface Exploration and Testing
We are all well-versed in several geophysical methods, equipment, and data reduction techniques useful in geotechnical exploration and evaluation. Geophysical exploration is an non-invasive tool which uses the measurement of material properties to help assess potential subsurface conditions. The field of geophysical exploration has grown substantially over the past 30 years, and has earned a good reputation as a relatively cost-effective, non-invasive tool to help supplement traditional subsurface exploration techniques and/or provide stand-alone subsurface information. Despite their obvious benefits, geophysical exploration techniques should not be viewed as an equal alternative to standard subsurface exploration and careful planning should be used at the beginning of a project to determine what the most appropriate exploration technique/method may be given the assumed subsurface conditions. Geophysical exploration will not always be appropriate for a given project.
Ground Penetrating Radar
Ground Penetrating Radar (GPR) is a non-invasive, cost-effective technology which uses pulses of electromagnetic energy to image features in the shallow subsurface. NGE-TFT provides GPR services to projects across the state where conventional subsurface exploration methods (drilling, excavation, etc.) and/or inspection methods (rebar, slab thickness, etc.) may be prohibitive or undesirable due to equipment mobilization, access, buried utility conflicts, archeological/historical restrictions, or various other factors which exclude physical disturbance of the area being imaged. GPR can also be used in conjunction with standard exploration and inspection techniques to provide an additional level of subsurface information to a project at relatively little cost or effort. NGE-TFT has successfully employed GPR technology to image buried pipelines and other buried utilities as well as to image shallow bedrock and to quantify organic overburden thicknesses.
GPR technology works best in dry, electrically non-conductive materials such as sand and gravel, as well as construction materials such as concrete and asphalt. The effective resolution of GPR varies based on the electrical properties of the materials being imaged and of the antenna frequency employed, but generally it can range from a few inches to upwards of 20-30 feet. Nevertheless, GPR technology does have limitations, and some surface and subsurface conditions are prohibitive to effective GPR data acquisition. Generally, a brief site assessment should be conducted prior to the commissioning of a GPR survey to evaluate the site conditions and determine if GPR technology may be a suitable exploration/inspection method for a given project. New and/or unproven applications for GPR are constantly being discovered and tested, so contact NGE-TFT and let us work together to determine if GPR is a suitable technology for your next project!
Electrical Resistivity
Electrical Resistivity (ER) applies direct current or low-frequency alternating current into the subsurface to determine the potential difference between two points revealing the resistivity and conductivity of the geological subsurface units. This exploration technique is a quick and inexpensive way to image the subsurface while leaving the ground surface unscathed. A number of electrical resistivity arrays can be utilized depending on the scope and goals of the project and can be used independently or in conjunction with other exploration techniques including borehole drilling. Uses of ER exploration include: geologic mapping of shallow bedrock and permafrost, volume estimates of shallow subsurface leaks and liquid bodies, locating mine shafts and voids, mapping the extent of aquifers, aquifer contamination evaluations, and archaeological studies. NGE-TFT has successfully used ER to gather resistivity data to be used for the design in grounding systems for electrical substations and cell towers. We also employ the soil box resistivity test as a laboratory analysis of the resistivity of soil samples from the field. This method is mainly used to determine the potential for corrosion of pipes and other metals buried in the tested soils. Electrical Resistivity is limited to summer use and should be employed in soils that are unsaturated with rain water. Generally, a brief site assessment should be conducted prior to the commissioning of an ER survey to evaluate the site conditions and determine if ER technology may be a suitable exploration/inspection method for a given project. New and/or unproven applications for Electrical Resistivity are constantly being discovered and tested, so contact NGE-TFT and let us work together to determine if ER is a suitable technology for your next project!
Downhole Seismic
We use Seismic downhole testing to gain information regarding the seismic wave velocities of the materials being tested. Once obtained, we can relate the compression (P) and shear (S) wave velocities to important geotechnical material properties and can be used (for example) in our foundation design or seismic analyses.
SPT Hammer Efficiency Analysis- SPT hammer analysis is a geophysical method of measuring the amount of energy that is transferred from a SPT drop hammer to the drill stem (and subsequently to the SPT sampler). SPT hammer analysis can be critical to the thorough evaluation of potentially liquefiable sand deposits.
The SPT analyzing equipment measures the hammer energy that is transferred to a standard drill rod section that has been instrumented with accelerometers and strain gauges and is linked directly into the advancing drill stem. Each time the SPT hammer strikes the drill stem, a force and acceleration are measured by the SPT analyzing equipment and are recorded for later data processing. The data output is later used to adjust the measured N-value to the normalized N60 for standard 60% energy transfer into the rods. Compensating for widely variable efficiencies from different SPT rigs and hammer types improves the reliability of soil strength estimates used in geotechnical designs.
Instrumentation and Monitoring
Subsurface instrumentation and monitoring are important facets of geotechnical exploration as it allows us to establish baseline measurements of subsurface conditions (e.g., groundwater table elevation, ground temperatures, etc.) and monitor how they change with time and as the result of various environmental and man-made factors. We have extensive experience with various subsurface instrumentation equipment and procedures and are well-versed at developing monitoring programs geared to extract the most useful information from a given site for the least amount of disturbance, effort, and cost.
Well Casing Installation
Groundwater Level Monitoring - Groundwater level monitoring wells are a very common geotechnical exploration and monitoring tool which allow for the periodic and/or continual measurement of the groundwater table elevation at a given location using a water level meter and/or piezometer. The well casing is most often constructed of small-diameter PVC pipe, which is subsequently slotted/perforated and/or open at its bottom end to allow groundwater to flow freely through the well casing. The depth and length of slotting/perforations is predetermined by the engineer/geologist on-site and usually corresponds to a depth interval where the groundwater table occurs. The well casing is inserted into an open borehole, corehole, or excavation and then the annulus/excavation is backfilled with various materials, including (but not limited to): drill cuttings/spoils, sand, gravel, bentonite, grout, etc. The well casing typically extends above-grade (also known as a stick-up or wellhead) and can (if need be) be protected by a security vault constructed of steel, wood, or some other protective material. Sometimes, the well casing is cut-off below grade to protect the monitoring well from surface impacts (i.e. vehicles, equipment, animals, vandals, etc.). In this scenario, a subterranean vault (such as a manhole) is often employed to protect the well casing and allow for casing access.
Thermistor Casing - Thermistor casing issued to provide a vertical conduit into the subsurface to allow for the insertion of ground temperature monitoring equipment (i.e., thermistor strings), which are used to record the temperature of the surrounding soil/bedrock. Thermistor casing typically consists of small-diameter PVC pipe, which is sealed at each pipe joint and at its ends in an effort to prevent any groundwater from infiltrating into the casing. Groundwater which seeps into a thermistor casing is subject to freezing; which can plug the inside of the thermistor casing and obstruct instrumentation and/or can freeze instrumentation into the casing, preventing its removal. In areas where casing leakage is expected, seamless tubing is sometimes placed inside of the casing to add an additional level of groundwater protection and help ensure proper instrumentation. The thermistor casing is inserted into an open borehole, corehole, or excavation and then the annulus/excavation is backfilled with various materials, including (but not limited to): drill cuttings/spoils, sand slurry, grout, etc.
Inclinometer Casing - Thermistor casing are specialized plastic pipes that are used to take inclinometer measurements. The casings, which come in different diameters, typically have two sets of precision machined grooves which serve as guides for various instruments (e.g.,inclinometers, downhole seismic geophones, etc.) to follow during measurements and which keep the instruments oriented in a specific direction. Inclinometer casings are typically installed for slope stability monitoring in natural slopes and/or constructed fill/cut slopes. The casings are grouted into place using a grout mixture designed for the specific subsurface conditions at a given project. We have successfully installed inclinometer casing for a number of projects with varying subsurface and surface conditions including within the intertidal zone.
Groundwater Level Monitoring
Pore Water Pressure Monitoring
Ground Temperature Monitoring
Ground Settlement Monitoring
Inclinometer Monitoring
Soil and Rock Sampling
The collection of representative subsurface samples is key to the development of relevant and accurate geotechnical evaluations and assessments. There are a wide variety of sampling methods available, and each is suited to a particular exploration method(s) and information goals. Oftentimes, multiple sampling methods will be utilized for a given project depending upon the nature of the subsurface materials and the scope of the proposed design. We have extensive experience with several common geotechnical sampling methods, and we have summarized the general purpose and procedure for each method and some their individual pros and cons.
Standard and Modified Penetration Tests
Pros
Can collect a relatively undisturbed sample;
Can provide resistance information be help evaluate the relative consistency of a given soil;
Relatively common and inexpensive to collect;
Re-usable equipment.
Cons
Can result in the collection of size-biased samples of coarse-grained soils;
Cannot sample effectively competent bedrock or very coarse-grained soils;
Difficult to sample permafrost soils;
Can be difficult to obtain sufficient sample of coarse-grained soil to comply with ASTM test standards.
Thin Walled Shelby Tube Sample
Pros
Can obtain an undisturbed sample of fine-grained (silt/clay) soils;
Relatively common and inexpensive to collect.
Cons
Cannot advance sampler in medium to coarse grained soils or bedrock;
Does not provide quantifiable soil resistance information;
Must be transported and stored to help reduce vibration and other inertial effects; that can alter the condition of the sample prior to testing.
One test per sample tube.
Direct Push Macrocore
Pros
Relatively fast and inexpensive to collect;
Can collect a continuous sample;
No drill cuttings generated (minimizes potential for contaminated soil cuttings).
Cons
Can result in the collection of size-biased samples of coarse-grained soils;
Cannot sample effectively competent bedrock or very coarse-grained soils;
Difficult to sample permafrost soils;
No drill cuttings generated (for visual observation)
Sonic Core Sampling
Pros
Collects a relatively large sample volume
Relatively fast sample collection
Can effectively sample coarse-grained soils
Generates limited drill cuttings
Cons
Collects a disturbed sample
Can be difficult to effectively sample permafrost or competent bedrock
Comparatively expensive to other sampling methods
Limited availability in Alaska
