There are no standard quality control or assurance measures on ground freezing method. However, there is a publication by (FWHA 2009) that describes ground freezing methodology and basic guidelines. The FWHA publication gives the quality guideline for brine only.
Some of the quality assurance requirements for the Ground Freezing method include preparing the site with enough surface water drainage, pipes insulated with polyurethane foam, backup power generation, and a refrigeration facility. Depending on the intricacy of the project, there may be numerous factors.
Construction quality is achieved by meeting established requirements, as detailed in project plans and specifications, including applicable codes and standards. Quality Control (QC) and Quality Assurance (QA) are terms applied to the procedures, measurements, and observations used to ensure that construction satisfies the requirements in the project plans and specifications. QC and QA are often misunderstood and used interchangeably.
Quality Control refers to procedures, measurements, and observations used by the contractor to monitor and control the construction quality such that all applicable requirements are satisfied. Quality Assurance refers to measurements and observations by the owner or the owner's engineer to provide assurance to the owner that the facility has been constructed in accordance with the plans and specifications.
Table 1 shows the components of QC/QA monitoring programs for Ground Freezing. The entries in the table are a list of typical items, not a list of all methods that could be used for QC/QA. Some QC procedures and measurement items may also serve as QA procedures and measurement items.
Table 1. Typical components of QC/QA monitoring programs.
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Topics |
Items |
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Existing QC/QA Procedures & Measurement Items |
QC |
Material Related |
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QC | Process Control |
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QA |
Material Related |
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| QA |
Process Control |
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Material Parameters |
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| Performance Criteria | System Behavior |
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Emerging QC/QA Procedures & Measurement Items |
Material Related |
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| Measurement Items |
Process Control |
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I. Drilling and Freeze Pipe Installation
- Location and Orientation of Freeze Pipes
Locate all existing utilities prior to Freeze Pipe installation. Conflicting utilities shall be relocated, or pipe locations adjusted accordingly to prevent damage due to Freeze Pipe installation.
- Logging of Drill Cuttings
Maintain boring logs for each Freeze Pipe that shall include soil stratification, depth and thickness of obstructions encountered, time required to drill, advancement rates and any record of drilling fluid loss.
- Deviation Measurement and Plotting
Perform verticality survey of each Freeze Pipe following installation by using a calibrated gyroscopic device or other instrumentation to establish deviations from planned location. Alignment plots of vertical refrigeration pipes can be completed with the data obtained by downhole inclinometer instruments.
- Observation of Welding or Other type of connection
All subsurface connections on a refrigeration pipe should be welded. Threaded couplings are not acceptable.
- Pressuring Testing of Freeze Pipe
After completion of all welding, prior to the installation of a joint of refrigeration pipe in the ground, the pipe should be pressure-tested to at least 125% of its working pressure to ensure no coolant leaks in the ground.
II. Installation of Coolant Distribution Manifold
a. Confirmation of Pipe Size and Flow Rate Computations
Use the correct size of the freezing pipe (ranges from 5 to 15 cm diameter) for proper distribution flow rate of the coolant.
b. Pressure Testing of System
Pressure test involves the coolant distribution system connections for leak detection, and all leaks discovered during this testing shall be repaired before the Coolant is introduced. Coolant distribution system should hold the pressure for a minimum of 15 minutes.
c. Installation of Insulation
Polyurethane foam (instrumented with thermocouples) or any other material meeting the technical specifications is needed to prevent frost penetration or thawing surrounding any affected utility. The thickness, continuity and the integrity are to be checked for the best performance.
d. Pressure Gauge Installation
The pressure gauge shall be used to monitor the pressure in the coolant distribution manifold. The pressure gauge shall be checked for its location, orientation, calibration, and tolerances.
Before installing, the flow meter and temperature monitoring systems should be inspected to ensure that are free from damage and their compatibility with the coolant being used. The location of installation shall be verified with technical specifications. Checking the orientation ensures they operate correctly and should be calibrated frequently by a qualified technician.
f. Flow Meter Installation
The flow meter should be in a section of the pipe where the flow is fully developed. Once installed, it should be tested to ensure it is operating within the specified tolerances.
g. Supply and Return Temperature Monitoring Installation
The temperature sensors should be integrated with the control system of the ground freezing system to provide real-time monitoring and control of the coolant supply temperature. Also for Individual Pipe(s), Return Temperature Monitoring system should be installed.
III. Freeze Plant
- Calibration of Control Panel
- Heat Exchanger Inspection
- Cooling Water Supply
- Ammonia Detector Test and Calibration
- Ammonia Dump System Test
Freeze Plant is used to chill the coolant consisting of either a single Chiller Unit or several combined Chiller Units. It must always operate appropriately in accordance with labor and safety requirements and as required to meet the Ground Freezing performance objectives.
The Freeze Plant is an important component of the ground freezing system and should be defect free. It should have stand-by equipment and replacement components readily available to prevent failure and the ground temperature monitoring functions work as required. A stand-by chiller may be used to assist initial freezing but shall be kept on site during the maintenance phase of the ground freeze operation.
Provide an emergency generator capable of providing 100% standby electrical generating capacity with automatic switching from line to generator.
Heat Exchanger is used transfer heat between coolant fluid and the ground being frozen. Its inspection must be done in a timely manner. Developing Standard Operating Procedures, Checklists, Documentation and Reporting can help in quality control. The inspections must be done by certified personnel with necessary skills and knowledge.
The ammonia dump system should be equipped with ammonia sensors that can detect any leaks or releases of ammonia and shut off the Freeze Plant. The sensors should be checked regularly and calibrated.
The control panel of the Freezing Plant system should be calibrated as per the specific equipment and components used in the panel as well as manufacturer recommendation and industry standard.
IV. Groundwater Monitoring
a. Verification of Piezometer Installation and Construction
Piezometers are installed to monitor and evaluate the ground water gradients. They must be checked for defects before installation.
b. Evaluation of Groundwater Gradients
To evaluate and monitor ground water gradients, as per technical specifications provided by the owner, a minimum number of groundwater monitoring points shall be be (3) standpipe piezometers and one (1) center observation well.
As per industry standards, a minimum the groundwater shall be monitored in the (3) standpipe piezometers and one (1) center observation well-constructed as part of the supplemental investigation.
c. Verification of Transducer Depths
The Transducers should be installed at a depth where they can provide accurate readings without being affected by external factors like soil movement. The technical specifications provide the depths at which they are to be provided.
V. Ground Temperature Monitoring
a. Calibration of Sensors
The accuracy of the sensors is critical to ensure that the freezing process is proceeding as intended. To calibrate the sensor, submerge it in a known fluid such as water and record multiple readings to ensure consistency. Comparing the readings from the sensors to the known temperature of the calibration bath, adjustments may need to be done if there’s a significant difference. Document the calibration process including the date, time and results.
b. Verification of Depth Placement
Use a calibrated measuring tool to measure the depth of sensors, take multiple measurements at different points along the sensors to ensure accuracy. Compare the measured depth to the installation plan to verify the placement.
VI. Instrumentation
Instrumentation plans must include a description of all equipment and monitoring processes. Instrumentation plans should include the following criteria.
- Connection and Testing of Coolant Manifold Pressure Sensors
- Connection and Testing of Coolant Flow Meters
- Connection and Testing of Supply/Return Temperatures
- Testing of Freeze Plant Control Panel.
- Connection, Testing and Verification of Groundwater Monitoring Wells
- Connection and Testing of Ground Temperatures
- Method of drilling and supporting the temperature monitoring, piezometer, and observation well boreholes, including type of temporary casing and drilling fluid.
- Methods used to monitor temperature of Freeze Zone, coolant and refrigeration plant, system pressure and minimum performance criteria to be achieved prior to the start of excavation.
- Methods of confirming frozen structure closure and adequate strength, and thickness of Freeze Zone (initial and for extended limit). As a minimum, this shall include center- hole piezometers in each of the permeable layers.
- Method of installation of instrumentation capable of monitoring ground deformations due to freeze development and thaw, and that are fully functional within and/or adjacent to the frozen ground.
- Method of installation of instrumentation capable of monitoring ground deformations of the excavation face due to creep.
VII. Formation Freezing
The data collected by the sensors should be recorded without omitting any to plot and evaluate the performance of the formation freezing.
a. Acquisition, Plotting and Evaluation of Ground Temperatures
To determine and evaluate the temperatures of the frozen ground zone and verify the dimensions of the frozen zone, a minimum number of Temperature Monitoring Bore wells are to be installed around the site and locate some closer to existing utilities to monitor the ground temperatures away from the freezing front.
b. Acquisition, Plotting and Evaluation of Coolant Distribution Sensors
To monitor loss of circulation and/or loss of coolant, the coolant distribution must be installed with sensors. These sensors actively collect and evaluate the data to ensure proper distribution of the coolant.
c. Acquisition, Plotting and Evaluation of Groundwater Data
The piezometers shall collect the groundwater data and evaluate the groundwater data. The calibration must be done before installation.
VIII. Excavation
a. Confirmation of Internal Groundwater Levels
Monitoring the bore wells or monitoring wells located within the freeze zone. The data collected by the groundwater monitoring system should be verified by collecting the data by another method.
b. Verification of External Groundwater Level Isolation
A site investigation should be conducted to determine the groundwater conditions. Installation of boreholes and monitoring wells helps to measure groundwater levels. Geophysical methods or pumping test or tracer test can be performed and evaluating the levels of monitoring wells can verify the isolation from external groundwater.
c. Installation of Convergence Sensors
Convergence sensors like Convergence Gage Arrays are to be installed on the shaft sidewalls to measure creep. These sensors must be calibrated on a regular basis. They must be free of defects before installation.
d. Inspection of Insulation Application
Plans for insulating frozen ground during excavation and subsequent construction are to be mentioned in the instrumentation plan. A quality inspection to check the integrity of the insulation must be done to expect its best performance.
FHWA (2009). US Department of Transportation Federal Highway Administration, “Technical Manual for Design and Construction of Road Tunnels-Civil Elements”, Report No. FHWA-NHI-10034.
Shuster J. A. (1981). Engineering quality assurance for construction ground freezing. Engineering Geology. 18(1-4): 333-50.
Sopko, J. A. (2023). Ground Freezing for Deep Shaft Excavation Shaft 17B-1 New York City Water Tunnel No. 3 New York, New York.