Published: July 15, 2022
501 Types of Instrumentation
501.1 Monitoring Wells
Monitoring wells are a type of instrument used to measure groundwater levels. They can also be used for collecting groundwater samples and testing aquifer parameters. Installation involves drilling a test boring, inserting riser pipe with a screened interval into the boring, backfilling with granular material (filter pack) around the screened interval, sealing the remaining annulus above the sensing zone, and providing a protector cover. The screened length may vary, but commonly are sized for the anticipated range of the seasonal fluctuation of the groundwater and may extend for the majority of the boring depth. Geotechnical Engineering Circular No. 5 and ASTM D5092 provide guidance for construction of monitoring wells.
501.2 Open Standpipe Piezometers
Open standpipe piezometers are a type of monitoring well with a small diameter (two inches or less) and a screened interval. Their purpose is to measure groundwater levels and piezometric pressures within isolated water-bearing formations in soil or rock. Open standpipe piezometers are designed and installed in similar fashion to monitoring wells.
Inclinometers are used to monitor lateral movement of a soil or rock mass through the use of an inclinometer probe or tilt indicator probe inserted into an inclinometer casing. The inclinometer probe is a wheeled instrument, which uses force-balanced accelerometers to measure its inclination from vertical. The inclinometer casing is grooved along its length on the inside so as to accept the wheels of the inclinometer probe and maintain its horizontal orientation. Inclinometers can be used for both quantitative and qualitative measurements of slope movements.
501.4 TDR Cable
TDR (Time Domain Reflectometry) is a method used to measure subsurface deformation utilizing a vertically or horizontally placed coaxial cable to monitor the location of a shearing plane in a soil or rock mass. An electronic signal sent down the length of the cable is reflected back from the other end. Any points of deformation (kinks, bends, breaks, or elongations) in the cable will cause increased resistance in the reflected signal, allowing the locations of these deformations to be determined along the length of the cable. Prior to installation, the TDR cable is crimped at regularly spaced intervals to provide calibration reference points. The location of the shearing plane or the location of the vertical subsurface movement in a moving soil or rock mass can therefore be determined by referencing the crimped intervals. If TDR cable installation is proposed on a project, contact the District Geotechnical Engineer for guidance.
501.5 Other Instrumentation
Other types of geotechnical instrumentation may be installed if project-specific circumstances warrant their use. Other instrumentation may include, but are not limited to, pneumatic piezometers, vibrating wire piezometers, extensometers, and strain gages. In addition, instrumentation used for pressuremeter testing, sonic testing, and seismic testing may be installed. The use of other geotechnical instrumentation will require prior approval of the District Geotechnical Engineer.
502 Monitoring Wells/Piezometers
Construct the monitoring well or piezometer with a minimum 1-inch nominal inside diameter to allow for monitoring groundwater levels. If groundwater sampling, pumping, and other testing of the formation under study is desired then a minimum 2-inch nominal inside diameter should be utilized. The following materials are required to install a monitoring well or piezometer.
502.1.1 Riser Pipe
Furnish flush-threaded, schedule 40 PVC solid pipe. Typical sections are 2.5-foot, 5-foot, and 10-foot long with watertight connections. Other materials, such as stainless steel, ABS, and fluoropolymer material, may be used for riser pipe if approved by the District Geotechnical Engineer.
502.1.2 Well Screen
Furnish flush-threaded, schedule 40 PVC pipe, machine slotted. Wire wound stainless steel, pre-packed screens and porous tips may also be used for well screens, if approved by the District Geotechnical Engineer.
The screen should be otherwise identical material to the riser pipe, with the same connections. Typically, well screens are commercially available in 2.5-foot, 5-foot, and 10- foot long sections. For groundwater monitoring, only a generic slot size of 0.010-inch openings is recommended. For any installation where sampling or testing from the well then select the slot size of the screen based upon the grain size distribution of the aquifer material, according to ASTM D5092. If a naturally developed filter pack is utilized, the slot size of the screen should retain at least 70% of the formation material. If an artificial filter pack is used, the slot size of the screen should retain at least 90% of the filter pack.
502.1.3 Bottom Cap
Furnish a flush-threaded or slip bottom cap made of PVC or other approved material for the bottom end of the well screen. Do not attach bottom caps in monitoring wells with PVC solvent glue or any other material that could alter the groundwater chemistry.
If a natural filter pack, or aquifer material is such that the collection of fines with the installation is anticipated, include a sediment sump as part of the bottom cap. A sediment sump should be a small section of riser pipe placed below the screen to allow for accumulation of fine sediment without clogging of the screen.
502.1.4 Top Cap
Furnish a flush-threaded cap, slip-type removable cap or expandable plug made of PVC or other approved material, designed to fit snugly on the top end of the riser pipe. Vent the cap.
502.1.5 Filter Pack
Furnish either a naturally developed or artificial filter pack. The filter pack should be installed beneath the bottom cap and extend above the top of the screen a sufficient length to ensure that the water levels and quality is not being impacted by the rest of the well installation. The filter pack should be sized to minimize the migration of the natural soil particles from the ground formation into the well screen.
- Naturally Developed Filter Pack. With a naturally developed filter pack, the granular material of the formation is allowed to collapse around the screen and riser. A naturally developed filter pack may be considered for formations that contain predominantly coarse grained, free-draining materials. This type of filter pack is appropriate when the grain size distribution of samples from the studied formation indicate that the effective grain size (D10) is greater than 0.01 inch and the uniformity coefficient (D40/D90) is greater than 3. If the natural formation does not meet these criteria or if the grain size distribution of the natural formation is not known, an artificial filter pack should be used.
- Artificial Filter Pack. An artificial filter pack consists of a granular material deliberately placed within the annulus between the screen and the outside of the boring. An artificial filter pack, sometimes referred to as sand pack, gravel pack, or well pack, is composed of hard durable granular material that has been washed, screened, and dried and is chemically inert, such as silica sand.
For groundwater monitoring only clayey materials use a generic artificial pack consisting of Global® #5 quartz sand of equivalent. Ensure that the artificial filter pack material meets the following gradation.
Table 500-1. Filter Pack Gradation
|U.S. Standard Sieve Sizes||Opening Size in Inches||Total Percent Passing|
For any installation where sampling or testing from the well then furnish a filter pack meeting the requirements of ASTM D5092.
Pre-packed screens may also be used in place of artificial filter packs.
502.1.6 Bentonite Seal
Furnish bentonite (hydrous aluminum silicate, sodium montmorillonite or calcium montmorillonite) powder, granulation, chips or pellets (coated or uncoated) to create a low permeability seal at the top of the filter pack. Make sure that the filter pack is sufficiently placed to allow for minor migration of the seal into the granular material without impacting the screen.
A bentonite grout, as described in Section 502.1.7.a, may also be used to form the bentonite seal. However, an additional 6 to 12-inch layer of fine sand will also need to be placed in the upper portion of the filter pack to minimize infiltration of grout into the sensing zone.
Furnish one of the following types of grout to backfill the boring from the bentonite seal to the ground surface.
- Bentonite Grout. Furnish a mixture of powdered or granular bentonite with water. Typical bentonite grout mix consists of 1 to 1.25 pounds of bentonite per gallon of water. Use of granular bentonite with a hydration inhibiting catalyst is acceptable.
When the backfill zone is above the water level in the boring, dry granular bentonite or bentonite chips may be used to backfill the boring annulus.
- Bentonite/Cement Grout. Furnish a mixture of powdered bentonite and Type I Portland Cement. Typical bentonite/cement grout mix ratio consists of 6 to 7 gallons of water and 3 to 9 pounds of powdered bentonite per one 94-pound bag of Portland Cement.
- Neat Cement Grout. Furnish a mixture of Type I Portland Cement and water. Quick setting types of cement should not be used in a neat cement grout. Typical neat cement grout mix consists of 6 to 7 gallons of water per one 94-pound bag of cement.
502.1.8 Protector Cover
Furnish a protector cover according to Section 504.
502.2 Boring Construction and Preparation.
502.2.1 Boring Construction
Advance borings using hollow stem augers or other approved techniques. Construct the boring in a manner to minimize the development of a mud “skin” or smearing on the walls of the boring. Keep the boring clean and open for the full depth prior to the installation of the monitoring well. Install the monitoring well after the boring is completed to the desired depth. If the boring is not being drilled for a geohazard, continuous sampling may be necessary to determine the limits of the saturated zone(s).
502.2.2 Diameter of Annulus
Construct the boring so as to maintain a minimum of a continuous 1-inch annulus between the inside of the augers (or the wall of the boring if solid stem augers are used) and the riser pipe and screen. If the diameter of the boring is inadequate to maintain a 1-inch annulus, then ream the boring to the required diameter.
502.2.3 Determining Well Interval
After completion of the sampling, determine the saturated zone in which the instrument will be installed in. Determine the limits of the filter pack, or sensing zone, so that it maximizes the majority of the saturated zone, or zone of interest. The filter pack should not extend beyond the saturated zone and should allow for the bentonite seal. Based on the length of the filter pack, size the well screen length so that it is completely encompassed within the filter pack.
502.2.4 Depth of Boring
Advance the boring to 6 to 12 inches below the planned depth of the filter pack. If the boring has been drilled deeper than the planned depth, backfill the additional boring depth with bentonite grout or bentonite chips or pellets. If bentonite chips or pellets are used, hydrate in lifts while backfilling. If grout is used, place grout to the depth of the base of the filter pack and allow time for the grout to harden before installing the filter pack. Install grout according to Section 502.3.5.
502.3 Standard Installation
The following installation procedure is applicable to a single monitoring well installed in soil. For monitoring wells installed in bedrock and special installations, refer to Sections 502.5 and 502.6. Figure 500-1 shows a typical monitoring well installation.
Figure 500-1. Typical Monitoring Well Installation
502.3.1 Initial Filter Pack Placement
Install the filter pack from the bottom of the boring, grout or bentonite to the depth of the bottom cap.
502.3.2 Instrument Construction
Assemble the full length of the instrument (bottom cap, well screen and riser pipe) either at the ground surface prior to placement in the boring or as the pipe configuration is being lowered into the boring inside the hollow stem augers. Place the instrument such that the installation is centered with the borehole. The well screen should be placed to ensure proper placement of the filter pack and the riser pipe should be placed to ensure a proper seal around it. Place centralizers so that they do not impede or obstruct the placement of filter pack, bentonite seal, or grout. Place the top cap on the top end of the riser pipe prior to placement of any additional material.
502.3.3 Filter Pack
After the instrument has been placed, fill the annulus with the filter pack up to the top of the intake or sensing zone. Typically, the top of the well screen will be at least two feet lower than the top of the filter pack. Do not allow bridging (obstruction preventing the full or proper placement) of the filter pack to occur. Remove the hollow stem augers gradually as the filter pack is placed, taking care to not allow the top of the filter pack material to drop below the bottom of the hollow stem augers.
502.3.4 Bentonite Seal
Place a minimum two-foot thick bentonite seal directly into the annulus. Ensure that bridging of the material does not occur, allowing for complete filling of the annulus with impermeable material. Remove the hollow stem augers gradually as the bentonite seal is placed, ensuring that the top of the bentonite seal does not drop below the bottom of the hollow stem augers.
If bentonite grout is used to form the seal, place a 6 to 12-inch layer of fine sand (sugar sand) in the upper portion to the filter pack to minimize the infiltration of grout into the sensing zone.
After the bentonite seal is in place, fill the remainder of the boring annulus with grout using a tremie pipe for placement from the bottom to top. Continue grout placement until the consistency of the return grout material at the ground surface is similar to the original grout. The hollow stem augers will remain in place until the grout reaches the ground surface at which time gradually begin removing the hollow stem augers. Add additional grout as needed to ensure that the top of the grout material does not drop below the bottom of the hollow stem augers.
Confirm the top of grout surface 24 hours after placement to ensure the grout has not shrunk or dropped. If the top of grout surface has dropped to a level more than 36 inches below the ground surface, then place additional grout.
502.3.6 Protector Cover
Install a protector cover at the ground surface according to Section 504.
502.3.7 Installation Report
Record field measurements on an installation report representing the actual construction of the monitoring well. Report measurements to the nearest 0.1 feet, referenced to the ground surface. At a minimum, include the following information on the installation report:
A typical monitoring well installation report is presented in Appendix B.\
Upon completion of installation, develop the instrument. At a minimum, for those instruments that are to be utilized only for groundwater level observations, first surge and then bail or pump until the purged water is clear. Any instrument that is to be utilized for in-situ testing of the hydrogeologic conditions of the monitored interval which will result in a high inflow of water will be further developed. The most common methods of monitoring well development include mechanical surging and bailing or pumping, over-pumping, air-lift surging, and jetting. Begin all methods of development slowly and gently and increase in energy as the monitoring well is developed until sufficient energy is applied to disturb the filter pack, thereby freeing the fines and allowing them to be drawn into the monitoring well.
Monitor the development process quantitatively through the use of direct reading instruments. These include single or multi-parameter instruments that measure the water quality parameters of turbidity, pH, conductivity, and temperature. A well will be considered developed when the water has cleared and the water quality parameters have stabilized.
502.5 Bedrock Installation
A monitoring well or piezometer may be installed to monitor groundwater levels in bedrock. This application may include, but is not limited to, slope stability explorations, mine or karst investigations, and new construction evaluations. The monitoring zone may span the entire bedrock interval or may isolate a specific zone within the bedrock. Figure 500-2 shows a monitoring well installation in bedrock with a void.
Figure 500-2. Monitoring Well Installed in Bedrock with Void
Generally, a filter pack is not utilized in a bedrock installation due to the narrow annulus between the screen and the sides of the boring. If a filter pack is required, the boring may be reamed with a tricone roller bit to increase the diameter of the boring and facilitate the installation of a filter pack. A pre-packed screen may also be used in a bedrock monitoring well installation.
Attach a shale trap or basket to the riser pipe just above the well screen to allow for placement of the bentonite seal.
When voids are the studied formation, such as in an underground mine or a karst investigation, place the screened section within the void. To permit grouting of the annulus, attach the shale trap or basket to the riser pipe in the zone of competent rock just above the void.
Once the shale trap or basket is in position, place the bentonite seal in the annulus. A two-foot bentonite seal is generally sufficient. However, if the screen is placed within a void, use a minimum three-foot seal above the shale trap to ensure a good seal. If the bedrock above the well screen is highly fractured, increase the length of the bentonite seal so that two feet of the seal is above the highly fractured zone. Backfill the remainder of the annulus with grout and finish the installation according to the standard monitoring well installation procedure.
502.6 Special Installations
Monitoring wells/piezometers may be installed for special conditions or purposes, which include, but are not limited to permeability/permissivity studies and multiple installations.
In this application, also known as time-lag or slug test, a monitoring well is used to study the permeability of the water bearing zone. Here the inflow of water from the surrounding soil mass is critical. The filter pack must be at least as permeable as the formation being studied, but not so open-graded that it allows infiltration of the soil particles and possible fouling of the filter pack and clogging of the well screen. Proper installation and development of the monitoring well is essential in this type of installation.
502.6.2 Multiple Installations
Monitoring wells piezometers are most commonly installed with a single riser pipe/well screen assembly in a borehole. Multiple installations can include a cluster of closely spaced installations or nested installations (multiple riser pipe/well screen assemblies installed in the same borehole) or a multiple-port well.
Use the following materials to install an inclinometer.
503.1.1 Inclinometer Casing
Furnish ABS pipe in 5 or 10-foot long sections, 2.75-inch outside diameter with snap lock “quick connections.” Other size casings and different materials and connections may be used if approved by the District Geotechnical Engineer. The inside of the pipe has machined grooves at four equidistantly spaced locations along the longitudinal axis. All joints will have watertight connections to prevent grout from entering the casing during installation.
503.1.2 Bottom Cap
Furnish a permanently fixed ABS cap with a watertight connection designed to seal off the bottom of the inclinometer casing.
503.1.3 Top Cap
Furnish a removable ABS cap designed to fit snugly onto the top of the inclinometer casing.
503.1.4 Bentonite/Cement Grout
Furnish a mixture of powdered bentonite and Type I Portland Cement to backfill the boring. Match the strength of the grout mix to the strength of the formation into which the inclinometer is being installed, according to Table 500-2.
If directed by the District Geotechnical Engineer, make one set of three 2-inch by 2-inch grout cubes in the field for compressive strength testing. Ship these to the location specified by the District Geotechnical Engineer.
Table 500-2. Grout Mix Design for Inclinometers
|Application||Grout for Hard Soils and Rock
(SPT > 4 blows per foot)
|Grout for Soft Soils
(SPT ≤ 4 blows per foot)
|Material||Weight||Ratio By Weight||Weight||Ratio By Weight|
|Water||30 gallons||2.5||76 gallons||6.6|
|Portland Cement (Type I)||94 lbs
|Bentonite||± 25 lbs.||0.3||± 39 lbs.||0.4|
|Notes:||~100 psi 28-day compressive strength. Similar strength to hard clay.||~4 psi 28-day compressive strength. Similar strength to very soft clay.|
- Grout Mix Design Source is Durham Geo Slope Indicator
- Mix the water and cement together first to maintain the water-cement ratio prior to the addition of the bentonite. If the bentonite is mixed first it is difficult to maintain the water-cement ratio.
Adjust the bentonite mix ratio until grout has a heavy cream consistency. Thin grouts will result in separation of the solids and water. Thick grouts will be difficult to pump
503.1.5 Protector Cover
Furnish a protector cover according to Section 504.
503.2 Boring Construction and Preparation
Advance the test boring to the termination depth or top of competent bedrock using hollow stem augers or other approved method. Take a minimum of 10 feet of rock core if the failure surface is suspected to be within the overburden soils. If the failure surface is suspected to be within the bedrock, core bedrock to a minimum depth of 20 feet below the anticipated location of the failure surface. If bedrock is impractically deep, the test boring may be terminated in soil as long as the bottom is at least 30 feet below the anticipated failure surface.
Upon completion of the drilling activities, and prior to placement of the inclinometer casing, inspect the inside of the boring for obstructions and depth. If the inspection indicates that the boring has caved in or soil and/or rock cuttings have accumulated in the boring, either ream the boring with a tricone bit, flush, or re-drill to remove the obstructions.
Once the boring has been completed to the specified depth and is clear of obstructions and debris, install the inclinometer casing as outlined below. Figure 500-3 shows a typical inclinometer installation.
503.3.1 Initial Grout Placement
Prior to installation of the casing, place bentonite/cement grout through a tremie pipe placed the bottom of the boring, until approximately the lower half of the boring is filled.
Figure 500-3. Typical Inclinometer Installation
503.3.2 Inclinometer Casing
Assemble the full length of the bottom cap and the inclinometer casing at the ground surface or as it is lowered into the boring. Insert the inclinometer casing into the boring, through the hollow stem augers, and rest the bottom cap directly on the bottom of the boring displacing the initial grout placement. If the inclinometer casing cannot be inserted the entire depth of the boring due to an obstruction, remove the casing, ream out the boring, replace the grout and re-install the casing. Do not force the inclinometer casing into the boring, especially by utilizing down pressure from the drill rig. To counteract the buoyant forces from the water and grout within the boring, fill the casing with clean, potable water or place a weighted rod inside the entire length of the casing. Place the top cap on the top end of the casing prior to the placement of any additional material.
Place one of the two sets of opposing grooves within the casing in-line with the assumed direction of movement, as shown in Figure 500-4. Mark the grooves of the casing with the A-A’ axis (direction of movement) and the B-B’ axis (perpendicular to the direction of movement). If the orientation of the grooves is not perfectly aligned upon completion of the placement of the casing, do not attempt to rotate the casing to re-align. This may result in spiraling of the casing.
Figure 500-4. Typical Inclinometer Casing Grooves and Axes
503.3.3 Bentonite/Cement Grout
Fill the remaining annulus between the outside of the inclinometer casing, placing grout through a tremie pipe inserted approximately two feet into the existing grout. As the grout is added to the boring, any water located within the boring will be displaced to the ground surface. Continue grout placement until the consistency of the return grout material is similar to the original grout. Begin removing the hollow stem augers gradually after the top of the grout reaches the ground surface and the return material has the proper consistency, ensuring that the top of the grout material does not drop below the bottom of the hollow stem augers. Place additional grout as needed until all the augers are removed.
Confirm the top of grout surface 24 hours after placement to ensure the grout has not shrunk or dropped. If the top of grout surface has dropped to a level more than 36 inches below the ground surface, then place additional grout.
503.3.4 Protector Cover
Install a protector cover according to Section 504.
503.3.5 Installation Report
Record field measurements on an installation report representing the actual construction of the inclinometer. Report measurements to the nearest 0.1 feet, referenced to the ground surface. At a minimum, include the following information on the installation report:
A typical inclinometer installation report is presented in Appendix B.
504 Protector Covers
A protector cover is placed at the top of any boring containing geotechnical instrumentation to protect it from contamination, debris, and vandalism. This cover may either be above-ground or flush-mounted. An above-ground protector is typically preferred, however, a flush-mounted protector must be used wherever an above-ground protector would interfere with motor vehicle traffic, such as inside guardrail, anywhere in the roadway or shoulders, or within a paved or grass median, which is not protected from traffic by a guardrail or concrete crash barrier.
504.1.1 Above-Ground Protector Cover
Furnish an aluminum or steel casing, typically five feet long and a minimum four inches square or in diameter, with a lockable aluminum cover at the top.
504.1.2 Flush-Mounted Protector Cover
Furnish a flush-mounted protector cover consisting of a steel casing, usually 9 to 12 inches long, with a cast-iron lid at the top. The cast iron lid is fixed down with threaded stainless steel bolts with lubricant and contains a gasket to minimize water collection within the cover. The top of the lid is typically marked as either “Monitoring Well” or “Test Well.”
504.1.3 Portland Cement Concrete
Furnish Type I Portland cement concrete.
Furnish a weather resistant keyed brass padlock to secure the above-ground protector cover. Key the lock to a specific ODOT key code, which is available upon request.
504.2.1 Above-Ground Protector Cover
After completely backfilling the annulus between the instrumentation and the boring with grout according to the previous sections, excavate an approximate 2-foot wide by 36 to 42- inch deep conical/cylindrical hole at the ground surface around the top of the instrumentation. Cut the inner casing or riser off below the anticipated top of the protector casing to a sufficient depth so that readings can be performed.
Mix the Type I Portland cement concrete with water according to manufacturer’s directions. Place the protector cover in the ground around the top of the instrumentation and the fill the excavation with Type I Portland cement concrete around the bottom of the protector cover. Set the protector cover so that the top of the protector casing is approximately two to three feet (24 to 36 inches) above the ground surface and it extends at least 9 inches below the ground surface.
If the installation is for an inclinometer casing, center the installed inclinometer casing within the protector cover so that the clamp and collar on the pulley system, if used when taking readings, can be attached to the head of the installation without interference from the protector cover. Fill the annulus with sand to within six inches of the top of the protector casing to hold the free end of the inclinometer casing firmly in place. Ensure that the top cap is in place on the inclinometer casing before pouring the sand. Drill a weep hole through the protector cover just above the top of the concrete to allow an outlet point for any water accumulation within the sand fill.
Install the brass padlock to secure the cover. Provide a copy of the padlock key to the District Geotechnical Engineer if requested. A typical above-ground protector cover installation is shown in Figure 500-6.
Figure 500-6. Above-Ground Protector Cover
504.2.2 Flush-Mounted Protector
Prepare the excavation and the Type I Portland cement concrete according to Section 504.2.1. Cut the inner casing or riser pipe off below the level of the ground surface to a sufficient depth so that readings can be performed when the lid is removed.
If the installation is for an inclinometer casing, center the installed inclinometer casing within the protector cover, so that the clamp and collar on the pulley system, if used when taking readings, can be attached to the head of the installation without interference from the protector cover.
Place the protector cover in the ground around the top of the instrumentation and the fill the excavation with Type I Portland cement concrete around the protector cover. Set the protector cover so the lid is flush with the existing ground surface or pavement.
A typical flush-mounted protector cover installation is shown in Figure 500-7.
Figure 500-7. Flush-Mounted Protector Cover
505 Method of Payment
Instrumentation is an engineering service to be performed and paid according to the engineering agreement and the Specifications for Consulting Services. The method of compensation for the work involved in Instrumentation will be on a unit cost basis. The tasks and corresponding units for this work are listed in the Proposal/Invoice form presented in Appendix E. The work includes technical supervision of the agreement; preparation, duplication, and delivery of records; furnishing all labor, tools machinery, materials, supplies, equipment, and utilities necessary and incidental to completing the work strictly according to these specifications.