Subsurface sensors
Subsurface sensors are contained in layers below the top structural layers. These sensors provide researchers with information about soil temperature, structure and water content.
There are two categories of subsurface pavement sensors.
- Traffic load response: Sensors that measure the response resulting from dynamic forces applied by traffic.
- Environmental: Sensors that measure the response due to slower changing forces like climate, material expansion/contraction, shrinkage, creep.
Specification and performance history for an individual sensor type is available upon request.
Dynamic Pore Water Pressure Gauge (DW/XD)
Soil Moisture (NP)
Water Table Level (OS)
Static Pressure Gauge (PL/PT & XL/XT)
Frost Depth Resistivity Probe (RP)
Positive Pore Water Pressure (SW/XS)
Drainage Volume {Tipping Bucket} (TB)
Thermocouple (TC)
Soil Moisture (TD)
Soil Moisture / Frost Depth (WM)
Dynamic Pore Water Pressure Gauge - DW/XD
Function: Measures rapidly changing positive pore water pressure in over-saturated thawing base and soil layers subjected to dynamic loading conditions. A “XD” sensor, mounted to the DW sensor, is a thermistor necessary for temperature correction of the measured pressure.
Description: The Dynamic Pore Water Pressure Gauge is a pore water pressure-sensing device consisting of a semi-conductor strain gauge piezometer.
Manufacturer: Geokon 3410S Dynamic Pore Water Pressure Gauge.
Location and quantity: DW/XD sensors are embedded into the base and subgrade layers near the wheelpaths of both asphalt and PCC test cells. There were originally 52 DW (and 52 XD) sensors located in 4 PCC, 6 hot-mix asphalt, and 1 aggregate surface MnROAD test cells.
Sensor location tables: DW/XT (Excel spreadsheet)
Current Status: Most DW/XD sensors have not been tested for functionality recently, therefore their status is unknown.
MnROAD Data Base Tables: Limited data has been collected from this type of sensor. Data is not readily accessible at this time, but future data will be input into the MnROAD database table: MNR.DW_VALUES, MNR.XD_VALUES
Function: Measures vertical pressure in base and subgrade layers subjected to dynamic loads.
Description: The Dynamic Pressure Gauge (PK) is a small diameter, soil stress cell consisting of a liquid-filled hollow steel cell approximately 50.8mm (2") in diameter and 12.7mm (.5") thick, with an electrical pressure transducer housed within the cell. The transducer responds to changes in pressure applied to the surface of the sensor by the material in which the sensor is embedded.
Manufacturer: Kulite model 0234 Soil Pressure Cell.
Location and quantity: PK sensors are embedded into the base and subgrade layers near the wheelpaths of both asphalt and PCC test cells.
There were originally 66 PG sensors located in 4 PCC, 11 hot-mix asphalt, and 4 aggregate surface MnROAD test cells.
Sensor location tables: PK (Excel spreadsheet)
Current Status: These sensors have performed well. Most PK sensors have not been tested for functionality recently, therefore their status is unknown.
MnROAD Data Base Tables: Data has been collected during various load response testing sessions. Data can be found in MnROAD database table: MNR.PK_VALUES.
Function: Measures vertical pressure in base and subgrade layers subjected to dynamic loads.
Description: The Dynamic Pressure Gauge (PG) is a pressure-sensing device consisting of two 6 inch (152 mm) diameter steel plates (welded together around their rims) connected to an electrical pressure transducer by a steel tube. The space between the plates and within the tube is filled with a liquid. The transducer responds to changes in pressure applied to the plates by the material in which the sensor is embedded.
Photos: 1 | Installation
Manufacturer: Geokon 3500 Dynamic Soil Pressure Cells with Ashkroft K1 Transducers.
Location and quantity: PG sensors are embedded into the base and subgrade layers near the wheelpaths of both asphalt and PCC test cells. There were originally 106 PG sensors located in 8 PCC and 7 hot-mix asphalt MnROAD test cells. As test cells were reconstructed, functioning PG sensors were removing and reinstalled into newer test cells.
Sensor location tables: PG (Excel spreadsheet)
Current Status: These sensors have performed well. Most PG sensors have not been tested for functionality recently, therefore their status is unknown
MnROAD Data Base Tables: Data has been collected during various load response testing sessions. Data can be found in MnROAD database table: MNR.PG_VALUES.
Neuton Probe (Neutron Probe) - NP
Function: The “NP” designation represents a neutron probe access tube. A device was lowered into the vertical tube in order to measure the moisture content within the surrounding soil at various depths. Data was collected manually at various times
Description: A Neutron Probe (NP) access tube consists of a 2-inch I.D. Schedule 40 PVC pipe 8 feet long. Soil moisture measurements were obtained by lowering a low-level radioactive measurement probe into the pipe.
Manufacturer: A NP access tube is simply a PVC pipe with a rubber stopper installed into the top opening to keep water and debris out of the pipe The neutron probe device manufacturer was Troxler.
Location and quantity: NP access tubes were originally placed 2 to 6 feet from the roadway centerline. Some NP access tubes were placed in the shoulder/ditch area. There was at least one NP access tube installed in each of the 40 MnROAD test cells. Some test cells had 2 or 5 installations of NP access tubes throughout a cell.
Sensor location tables: NP (Excel spreadsheet)
Current Status: Manual readings of using a neutron probe ceased shortly after MnROAD started operation. It was discovered that the PVC access tubes scattered the signal from the probe, and therefore the technique was too difficult to continue. Licensing of the neutron probe also became a logistical problem. Little data (if any) is available from this technique.
MnROAD Data Base Tables: Due to the trouble with the technique, a database table does not exist.
Water Table Level (Open Standpipe) - OS
Function: The “OS” designation represents an open standpipe access tube. A device is lowered into the vertical tube in order to measure the depth to the water table. Data for the original installations was collected manually at various times. More recent installations have pressure sensors installed at the bottom of the tube to detect standing water pressure (head)
Description: The Open Standpipe consists of a 15-foot deep PVC pipe, with holes to allow the water table level to stabilize within it. Measurements can be obtained by lowering a water-sensing probe into the pipe. More recent installations have included automated pressure transducers inserted into the bottom of the tube.
Manufacturer: Original OS was a simple PVC wrapped with fabric.
Location and quantity: OS sensors were originally placed 25 feet from the roadway centerline (just beyond the shoulder). There was originally one OS sensor installed in each of the 40 MnROAD test cells. Newer test cells 31 and 52 have added an OS sensor underneath the centerline of the roadway.
Sensor location tables: OS (Excel spreadsheet)
Current Status: Manual readings of the OS sensors ceased after March 2002. Sensors in newer cells have been automated and are continuing to function.
MnROAD Data Base Tables: For recent (automated) sensor installations, data has been collected automatically either at 15 minute or 1 hour intervals. Data can be found in MnROAD database tables: MNR.OS_TRANS_RAW_VALUES, MNR.OS_COAX_RAW_VALUES.
Older data, collected manually can be found in MnROAD database table: MNR.OS_MANUAL_VALUES.
Static Pressure Gauge - PL/PT and XL/XT
Function: These sensors measure horizontal or vertical pressure in base and subgrade layers caused by environmental loads (moisture, temperature, settlement). A “XL” or “XT” sensor, mounted to the PL or PT sensor, respectively, is a thermistor necessary for temperature correction of the measured pressure. Data is automatically collected once every 15 minutes, due to the slow rate of change of the measured pressure and temperature within the soil.
Description: The Static Lateral Pressure Gauge (PL) and Static Soil Pressure Gauge (PT) is a pressure-sensing device consisting of two 6 inch (152 mm) diameter steel plates (welded together around their rims) connected by a steel tube to a vibrating wire sensor. The space between the plates and within the tube is filled with a liquid. The vibrating wire sensor responds to changes in pressure applied to the plates by the material in which the sensor is embedded.
Photos: 1
Manufacturer: Geokon 4800E Earth Pressure Cell
Location and quantity: PL/PT sensors are embedded into the base and subgrade layers near the centerline of the roadway of both asphalt and PCC test cells. There were originally 13 PL (&13 XL) and 53 PT (&53 XT) sensors located in 4 PCC, 6 hot-mix asphalt, and 1 aggregate surface MnROAD test cells. As test cells were reconstructed, functioning PL/PT sensors were removing and reinstalled into newer test cells
Sensor location tables: PL | PT (Excel spreadsheet)
Current Status: These sensors have performed well. Most PL or PT sensors have not been tested for functionality recently, therefore their status is unknown.
MnROAD Data Base Tables: Data has been collected both manually and automatically during the life of each sensor. Data can be found in MnROAD database tables: MNR.PL_VALUES, MNR.XL_VALUES, MNR.PT_VALUES, MNR. XT_VALUES.
Function: Measures depths of freezing and thawing fronts in the granular base and subgrade. Data for the original installations was collected manually at various times. Some installations became automated.
Description: The Resistivity Probe (RP) sensor is a vertical series of electrical resistivity gauges used to indicate when a substantial portion of the pore water in a soil has frozen.
Manufacturer: RP sensors were fabricated by Shannon and Wilson, the instrumentation consultant involved with the original construction of MnROAD.
Location and quantity: RP sensors were originally placed in several locations within a test cell to characterize frost profiles. There was originally at least one RP sensor installed in each of the 40 MnROAD PCC test cells. Considering each probe had 41 sensors, the measurement points at MnROAD totaled 4223.
Sensor location tables: RP (Excel spreadsheet)
Current Status: The use of RP sensors was discontinued in spring 1999. Data collection and processing was found to be very labor intensive.
MnROAD Data Base Tables: Data from original RP sensors can be found in the MnROAD database table: MNR.RP_VALUES.
Positive Pore Water Pressure - SW/XS
Function: Measures slowly changing positive pore water pressure in oversaturated soils. A “XS” sensor, mounted to the SW sensor, is a thermistor necessary for temperature correction of the measured pressure
Description: The Dynamic Pore Water Pressure Gauge (DW) is a pore water pressure-sensing device consisting of a pressure sensitive diaphragm with a vibrating wire attached to it.
Photos: 1
Manufacturer: Geokon 4500SL Vibrating Wire Piezometer
Location and quantity: SW/XS sensors are embedded into the base and subgrade layers near the centerline of both asphalt and PCC test cells. There were originally 49 DW (and 49 XD) sensors located in 4 PCC, 6 hot-mix asphalt, and 1 aggregate surface MnROAD test cells.
Sensor location tables: SW/XS (Excel spreadsheet)
Current Status: M ost SW/XS sensors have not been tested for functionality recently, therefore their status is unknown.
MnROAD Data Base Tables: Data has been collected automatically every 15 minutes over the life of each sensor. Data can be found in MnROAD database tables: MNR.SW_VALUES, MNR.XS_VALUES
Drainage Volume (Tipping Bucket) - TB
Function: Measures the amount of water draining from the outlet of an edge drain.
Description: The Tipping Bucket (TB) device consists of a divided “bucket” and an electronic counting switch. Water from a pavement edge drain falls into one side of the divided “bucket” until it is filled, at which point the bucket tips or “rocks”, such that the other side of the bucket then begins to collect the drain water. An electronic switch counts the number of tips of the bucket. Knowing the quantity of water in the bucket and the time between tips, edge drain flow rates can be determined.
Manufacturer: The original tipping buckets (TB) were designed by Mn/DOT and constructed by a consultant hired to manufacture and install the MnROAD sensors.
Location and quantity: TB sensors are installed at the end of the outlet of edge drains in several MnROAD test cells. There were originally 14 TB sensors located in 5 PCC and 2 hot-mix asphalt MnROAD test cells. Newer test cells have also been equipped with TB sensors
Sensor location tables: TB (Excel spreadsheet)
Current Status: Automated readings from the original TB sensors ceased after June 2003. Sensors in newer cells continue to function.
MnROAD Data Base Tables: Data has been collected automatically every 15 minutes over the life of each sensor. Data can be found in MnROAD database tables: MNR.TB_VALUES, MNR.TB_CALIBRATIONS.
Function: A “TC” sensor measures the temperature of a material in which it is embedded. Data is automatically collected once every 15 minutes.
Description: The Thermocouple (TC) sensor is a pair of dissimilar metal alloy wires (copper and Constantine) connected together near the point of measurement. Thermocouples generate an open-circuit voltage, called the Seebeck voltage, which is proportional to the temperature difference between the hot (point of interest) end and a reference junction
Photos: 1 .
Manufacturer: Omega Type-T thermocouple wire.
Location and quantity: TC sensors within pavements are typically mounted on a vertical “tree.” This mounting method allows vertical temperature profiles to be determined, which are very important to the understanding of pavement response. Since TC sensors were mounted on “trees”, which often span both the surface and subsurface layers, it is difficult to characterize their quantities (originally at least 1000).
Sensor location tables: TC (Excel spreadsheet)
Current Status: These sensors perform very well, with over 95% of the original TC sensors still functioning after 12 years of service.
MnROAD Data Base Tables: Data was been collected automatically every 15 minutes over the life of each sensor. Data can be found in MnROAD database table: MNR.TC_VALUES (current year), MNR.TC_VALUES_#### (where #### indicates one year of values).
Time Domain Reflectometer - TD
Function: Measures the moisture content of base and subgrade materials. Data was collected manually from 1993-1998, but is now being collected automatically.
Description: The Time Domain Reflectometer (TD) [or TDR] sensor measures changes in the dielectric constant of the soil due to changes in the soil moisture content. The device is excited by a high frequency electromagnetic pulse, which is sent down 2 or 3 parallel wave-guides or prongs. The velocity of the reflected wave is measured and correlated to soil moisture content.
Photos: 1
Manufacturer: Original TD sensors were designed by Mn/DOT researchers and fabricated by a consultant hired during the original construction of MnROAD. More recent installations have used Campbell Scientific model CS610 and CS615 TDR sensors.
Location and quantity: TD sensors were originally placed in each of the test cells in the outer wheelpath and/or at the centerline of the roadway. Some cells had replicated installations that included additional TD sensors in the shoulder area. Original TD sensors were installed in a vertical stack of 7 sensors to capture the moisture content at various layers below the pavement surface.
Sensor location tables: TD (Excel spreadsheet)
Current Status: Data from the TD sensors was manually collected from 1993-1998. The operational status of the original TD sensors is unknown at this time. Since 1999, many new TD sensors have been installed and connected to an automated data collection system.
MnROAD Data Base Tables: Data was been collected automatically once every hour for newer TD sensors. Data can be found in MnROAD database tables: MNR.TD_VALUES, MNR.TD_ERRORS, MNR.TD_RECALC, MNR.TDR_LOG, MNR.TD_AUTOMATED.
Soil Moisture/Frost Depth - WM
Function: Originally, “WM” sensors were used to measure the moisture content of base and subgrade materials. More recently, WM sensors have used to indicate frost depth within the pavement structure.
Description: The Watermark block (WM) sensor measures changes in electrical resistance due to changes in the soil moisture content.
Photos: 1
Manufacturer: WM sensors used at MnROAD are manufactured by the Irrometer Company.
Location and quantity: WM sensors were originally placed next to the TD sensors in each of the test cell. This included replicated installations in several test cells. Similar to TD sensors, WM sensors were installed in a vertical stack of 7 sensors to capture the moisture content at various layers below the pavement surface.
Sensor location tables: WM (Excel spreadsheet)
Current Status: These sensors perform very well, with most of the original WM sensors still functioning after 12 years of service. These sensors continued to be installed to indicate frost depths.
MnROAD Data Base Tables: Data was been collected automatically every 15 minutes over the life of each sensor. Data can be found in MnROAD database table: MNR.WM_VALUES