A challenge present in every project is managing risk and reducing or eliminating unknown elements prior to construction. Significant risk can be present when subsurface conditions are not documented or investigated prior to the start of work. Dealing with these unearthed items can be very costly to demolish, repair or reroute and can create a need for urgent redesign amidst the chaos of construction. Early use of ground penetrating radar (GPR) on a project can greatly reduce these risks.
Common Construction Issues
Unknown buried obstructions and utilities, resulting in costly construction delays, are commonly encountered for several reasons. Many older facilities lack complete and accurate records to pinpoint the locations of buried infrastructure. Additionally, abandoned items underground such as foundations, storage tanks, vaults and utilities were typically left in place due to the inherit cost of full removal. Any new project proposed in these locations will have to absorb costs and schedule impacts to address these items. Depending on what is found, the issue could be quite expensive to correct, especially if it ultimately involves environmental remediation.
Notification to the state sponsored utility locating service (e.g., 811 or Miss Dig) is required prior to commencing with any excavation. However, this service only applies to public utilities. Plant utilities will not be covered by this service and are the responsibility of the owner to determine impact and perform any necessary modifications. This is where a private service utility locating provider can fill in the gaps and reduce risks for new projects.
Proposed underground utilities also may require a connection to an existing buried utility in which the actual position is unknown. Identifying an accurate tie-point for these utilities can be critical, especially when proper slope must be maintained, such as for a gravity-drained sewer.
Voids and Sinkholes
Voids and sinkholes may exist below the surface with no indication above ground. These are typically formed when a buried utility or structure below is washed out or when improper backfill and subbase below grade settles, leaving a vacant pocket in the soil. Sometimes when this occurs in a paved area, the pavement above can bridge the void leaving a cavity below that is not visible above grade. These voids can be detrimental to large equipment or heavy traffic as well as for crane outriggers that may punch through the surface when an unknown void is below, thus creating a serious safety concern to personnel as well as a definite impact to plant operations.
Embedded Items in Concrete
Determining the condition of existing concrete slabs and walls is important when evaluating new equipment loads or deciding if a portion of the concrete can be cut out to route new utilities or structures. Common items embedded within concrete that can cause complications if encountered include:
- Electric and communication wires/conduits
- Drains and process utilities
- Reinforcing bars
- Building post-tension hairpins
The Solution – GPR and Utility Locating
If information on subsurface conditions can be obtained early in a project, the engineer can account for these items in the design and avoid costly impacts during construction. Projects where this information will be crucial include any projects involving:
- New equipment or building foundations
- New or rework to buried utilities
- Site work and grading
- Large crane setups
- Coring through walls and slabs
Matrix Technologies uses cutting-edge technology, ground penetrating radar (GPR) and radio utility locators to provide insight into subsurface conditions not visible above ground without requiring expensive excavations.
Ground Penetrating Radar (GPR)
Ground penetrating radar works by sending radar pulses from a transmitter into a material and the receiver records the time and strength of any reflected signal. Information is gathered as the reflected radar signal changes when it passes through a material with different properties, creating a parabola to identify an anomaly or obstruction.
Varying antenna frequencies transmit differently through each medium and are used for varying subsurface investigation purposes. Typically, the lower the frequency, the deeper the penetration. Therefore, Matrix Technologies, Inc. uses both a 400 MHz antenna and a 1600 MHz antenna in order to gain an accurate picture underground at various depths. The 400 MHz antenna can scan depths between 2 and 15 feet, depending on site and soil conditions. The 400MHz antenna is used for locating utilities, underground voids or other abnormalities. This antenna is operated using a three-wheeled cart allowing for large areas to be scanned in a short period of time.
The 1600 MHz antenna provides a detailed scan up to 18 inches deep. The 1600 MHz antenna is used for determining concrete thickness, rebar location and voids under pavement. This antenna can be operated using the large cart or a smaller handheld unit, allowing it to be used in smaller spaces or along vertical walls and ceilings.
This equipment is non-intrusive and does not emit radiation so plant operations occurring in proximity to the scanner are uninterrupted.
Although GPR is very useful for providing subsurface imaging without the need for excavation, it does have limitations that need to be considered. Site conditions such as soil conductivity and material properties can play a factor in the data the GPR is able to collect. Soil conductivity is the measure of how well electromagnetic signals pass through a medium. The lower conductivity, the further the signal can travel.
Low Conductivity (Deep GPR Penetration):
- Dry soil
High Conductivity (Shallow GPR Penetration):
- Saturated soil
The material composition and diameter of the target also factor into the GPR’s ability to “see” it. Metallic items reflect the radar signal much better than plastic objects. Also, a good rule of thumb for target size is that GPR can typically see an object 1 inch in diameter per foot of depth scanning (e.g., 4-inch diameter pipe, buried 4 feet).
Matrix Technologies also uses the RD 8100 radio detection utility locator. Radio detection works by sending radar pulses through the ground to detect electric current and metallic pipe. It uses five different antenna frequencies to optimize the level of precision for each utility/material to be located. To further enhance this equipment, Matrix Technologies uses an iLOC induction tracer clamp, which connects around pipe risers where they come above grade and electromagnetically traces underground utilities for a distance of up to 1,400 feet.
Ultimately, GPR and utility locating is a safe, valuable and non-invasive technique to gather subsurface information. No matter the scale of the project, having this service completed early is a cost-effective way to help eliminate expensive redesign/rework and adverse schedule implications due to inaccurate information in the design phase. Additionally, knowing what is hidden below grade greatly reduces safety concerns associated with excavation during construction and potential impacts to plant operations.
The Matrix Difference
Matrix Technologies is different from many other utility locating companies. In addition to using the latest technology, each of our GPR technicians are engineers with experience in site layout, underground utility and structural design. This knowledge helps technicians have a full picture and better understanding of site conditions, as well as be able to provide engineering solutions when items are unearthed in the field.
Matrix Technologies provides onsite field markings as well as a formal report documenting the results of the subsurface GPR scan. Additionally, Matrix Technologies can provide topographic surveying and 3D laser scanning to accurately document the field information. Matrix Technologies also can create and manage CAD and 3D models for site layouts and utility plans.
While subsurface investigation using GPR and utility locators is a critical step in the design process, it is typically supplemented with other field services and engineering. Matrix Technologies provides a wide range of field services and multi-discipline consulting engineering, thus creating a one-stop shop that eliminates the time, effort and potential coordination concerns associated with hiring separate contractors or engineers to perform this work.
Matrix Technologies is one of the largest independent process design services, industrial automation engineering, and manufacturing operations management companies in North America. To learn more about our manufacturing operations management capabilities and manufacturing process control solutions, including field services including GPR and utility locating or other engineering capabilities, contact Joe Leech, PE, Engineer 4 in the Mechanical & Facilities Design Department.
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