chapter 13. sealing abandoned wells and boreholes

169 Pages·2007·7.52 MB·English

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technology and increased numbers of well-trained and experienced hydrogeologists, should
lead to the solution of many of our present groundwater contamination problems.

CHAPTER 13.

SEALING ABANDONED WELLS
AND BOREHOLES

Abandoned wells and completed boreholes need to be sealed carefully to prevent pollution of
the groundwater source, eliminate any physical hazard, conserve aquifer yield, maintain
confined head conditions, and prevent poor-quality water of one aquifer from entering
another.

The principal objective of sealing abandoned wells is to restore, as far as possible, the
original hydrogeologic conditions. Before being sealed, the well should be checked to insure
that there are no obstructions that may interfere with effective sealing operations. This
inspection is especially important in wells that could conduct undesirable water into aquifers
yielding potable water.

If the casing has not been grouted, it may be removed by hydraulic jacks or by bumping the
casing up using a hammer. A vibration hoarser can also be used to remove casing. If the
casing is in good condition, a trip-type casing spear operated by a fishing string can be used
to remove it. Casing cutters are used to separate the drive shoe from the lowest casing to
facilitate removal of the casing.

Removal of liner pipe from some wells may be necessary to assure an effective seal. Liners
or casings opposite water-bearing zones should be removed car perforated with a casing
ripper beforehand to assure proper sealing throughout these zones. The upper portion of the
casing should be removed to assure contact of the grout with the wall of the hole, to form a
watertight plug in the upper 15 to 20 ft. Exceptions to this procedure may be permitted where
the annular space around the casing was carefully cemented when the well was originally
drilled.

To seal an abandoned well properly, the groundwater conditions at the site must be
considered. When the groundwater occurs under unconfined conditions, the objective is to
prevent the percolation of surface water through the well bore or along the outside of the
casing to the water table. This is accomplished by grouting the entire well bore. When
confined conditions exist, the sealing operations must confine the water to the aquifer in
which it occurs. This prevents the loss of confining pressure that results from uncontrolled
flow from the aquifer.

In flowing wells, the water level must be lowered to control the flow before placing the seal.
Flow can be controlled by introducing high-specific-gravity fluids to stop the flow, extending
the pipe high enough above the land surface to stop the flow, or by pumping the problem
well or nearby wells to create a drawdown in the well to be sealed.

Depending on the flow conditions encountered, three types of seals can be used to properly
abandon a flowing well. Intermediate seals are placed between confined water-bearing
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formations having different static heads; thus, no water can pass from one aquifer to another.
Bridge seals are cement or weighted wood plugs placed beneath the major aquifers. The
lower part of the hole can be filled, if desired, with disinfected fill. Top seals are placed
above the aquifers and form the base for the neat cement grout. Concrete or cement grout
used to fill the well below the water level should be placed from the bottom up, by methods
that will avoid segregation or dilution of material.

13.1. ABANDONMENT of WELLS

All boreholes greater than 10 feet deep or which intersect a water table and all groundwater
monitoring wells should be properly abandoned. One should consider the following factors in
determining whether a borehole or monitoring well should be abandoned: purpose, location,
groundwater quality, age and condition of the well or borehole potential for groundwater
contamination and well or borehole construction.

13.2. GENERAL TIMELINES FOR ABANDONMENT

A borehole should be abandoned within 3 working days after its use has been discontinued.

Any permanent groundwater monitoring well no longer being used to gather information on
geologic or groundwater properties should be abandoned within 60 days after its use has been
discontinued.

Any groundwater monitoring well found to be acting as a conduit for groundwater
contamination shall be abandoned immediately.

Potable water wells should be abandoned according to local rules.

13.3. GENERAL ABANDONMENT PROCEDURES

Boreholes. Any borehole intersecting the Water table or greater than 10 feet deep, whose use
has been discontinued, can be abandoned according to the following as applicable.

Monitoring wells impermeable annular space seals. A permanent groundwater monitoring
well known to be constructed with an impermeable annular space seal can be abandoned
according to the sealing requirements after the protective cover pipe and ground surface seal
have been removed arid the well casing cut off at least 4 feet below the ground surface. The
well casing may be completely removed during abandonment by pulling the well casing,
overdrilling around the casing and then pulling the well casing out of the ground or by
drilling out the well casing completely. If the well casing is to be removed, the well shall be
sealed as the casing is removed.

Monitoring wells - permeable annular space seals, wells in waste areas and monitor wells on
known hazardous waste sites. A groundwater monitoring well not known to be constructed
with an impermeable annular space seal or located in an existing or planned future waste
disposal or treatment area shall be abandoned by removing the protective cover pipe and the
ground surface seal and then completely removing the well casing. The well casing can be
pulled out of the ground as the well is filled according to the sealing requirements.
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Sealing requirements. Boreholes and groundwater monitoring wells should be abandoned by
complete filling with neat cement grout, Bentonite cement grout, sand-cement grout, concrete
or bentonite-sand slurry, Bentonite chips or granular Bentonite. A tremie pipe should be used
to abandon groundwater wells and boreholes greater than 30 feet in depth or with standing
water. Groundwater monitoring wells and boreholes greater than 100 feet in depth should
also be sealed with a tremie pipe-pumped method. Bentonite may be used as a sealing
material without the use of a tremie pipe under the following conditions:

1. Granular bentonite may be used for boreholes and groundwater monitoring wells less than
25 feet deep and when there is no standing water above the filter pack seat

2. Bentonite pellets may be used for boreholes and groundwater monitoring wells less than
50 feet deep and the depth of standing water is less than 30 feet.

3. Bentonite chips may be used for boreholes and groundwater monitoring wells which are
greater than 4 inches in diameter and less than 250 feet deep and the depth of standing water
is less than 150 feet.

13.4. SEALANT SETTLEMENT

Any settling of the sealant material shall be topped off. Sealing material may be terminated 3
feet below the ground surface in agricultural areas to avoid interference with agricultural
activities. A native soil plug should be placed on top of the settled sealing material in such
cases.

13.5. ABANDONMENT DOCUMENTATION

All borehole and permanent groundwater monitoring well abandonments must be
documented. Special forms may be required by regulatory agencies. Decontamination
procedures used during abandonment should also be documented so that no cross
contamination is allowed to occur.

13.6. CONCLUSIONS

Although known cases of groundwater contamination affect only a small percentage of all
groundwater resources in the United States, most groundwater that is contaminated lies in
highly populated areas. Therefore, the impact of the contamination is greater because of the
number of people affected. Drilling contractors can take an active role in protecting their
customers from groundwater contamination by constructing safe wells and making sure that
old wells are abandoned properly.

CHAPTER 141.

1 The material in this chapter is based on and drawn from the AASHTO copyrighted
publication Manual on Subsurface Investigations - 1988, as described in the
'Acknowledgement' section of this book.
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SUBSURFACE EXPLORATION
TOOLS AND EQUIPMENT

The geotechnical parameters which affect design and construction of a facility must be
investigated and evaluated. A specific, well-integrated and flexible subsurface exploration
program, generally conducted in several phases, is necessary to develop the maximum
amount of geotechnical data for reasonable costs.

14.1. GENERAL PLANNING

The purpose of the "Office Reconnaissance" is to obtain and evaluate as much information as
possible about the project area in the early planning stages. This information is then utilized
in developing a subsurface exploration program which will determine the characteristics of
the materials and structures in the ground to the degree necessary for the location, design and
construction of the facility.

Although the level of importance of the various parameters will vary with the project type
and location, preliminary information should be obtained regarding the following subsurface
conditions during early stages of design:

1. Soil and Rock Stratigraphy
2. Hydrological Conditions
3. Soil Classification, Density and Consistency
4. Rock Quality and Discontinuities
5. "Mixed Face" Conditions
6. Obstructions (such as boulders)
7. Hazards (such as methane gas)

14.2. EXPLORATION PROGRAM

The project geologist, hydrogeologist or geotechnical engineer, when formulating the
subsurface exploration program, must carefully evaluate the variety of methods and
procedures which are available, in order to maximize the amount of information obtained and
minimize associated costs.

14.2.1. Exploration Plan

An initial exploration plan is prepared from the information evaluated during the preliminary
investigation phases; it includes location, spacing and depths of explorations, and sample
type and interval. There are many variables involved in the formulation of this plan. The
objective should be the development of the maximum amount of subsurface information
through the use of the minimum number of boreholes. The initial plan should be flexible.

The proposed boring locations should always be checked against actual field conditions,
prior to commencing the explorations, as modifications or adjustments may be required due
to access or other restraints.
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14.2.2. Types of Borings

The following terminology can be used for boring identification during the various phases of
the investigation program

Pilot Borings. Pilot borings are conducted during the preliminary or initial investigation
stages of the project. These borings are placed at scattered locations to obtain only sufficient
information to enable the project manager to:

· Establish the preliminary profile.

· Estimated the preliminary quantities of soil and rock items of construction involved in the
project.

Control Borings. Control borings are the designated first-phase design borings which are
conducted at selected and key locations.

Verification Borings. Verification borings are additional design banngs which are scheduled
following the analysis of the control borings.

14.2.3. Exploration Spacing

The locations of the explorations are subject to runny variables and depend on the uniformity
of the geological units and the type of facility proposed. If the subsurface conditions in the
project area are well known, and the stratification is simple, with relatively thick individual
strata of consistent physical properties, relatively widely-spaced explorations may be
sufficient. If, however, erratic and rapidly changing conditions exist, more closely spaced
explorations will be required.

Inclined borings may be used to advantage in exploring inclined strata and various
subsurface irregularities. Inclined boreholes furnish information in both a vertical and
horizontal direction.

Borings are generally staggered to give the broadest coverage.

The term "subsurface explorations" usually implies test borings; however, a variety of
exploration methods such as hand or machine excavated test pits and probings conducted at
selected locations may meet the project requirements and minimize investigation costs.

Critical-Area Explorations. In areas where the preliminary investigations indicate critical
geological conditions such as a highly irregular and shallow bedrock surface, swamp deposits
or underground caverns, it may be desirable to obtain explorations on a closely spaced grid
pattern in the area of concern.

14.2.4. Sampling Requirements

An objective of the subsurface exploration program is to obtain samples that are
representative, or nearly so, of the in sin' soil and rock conditions. The sampling
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requirements, including type and interval, are subject to the same geological variables and
project requirements which control the location and depth of the explorations.

Generally, representative in situ samples should be obtained at every change in soil strata and
at an interval not to exceed 5 ft. vertically. Ibis interval may be increased in thick uniform
deposits or be decreased in the more complex sediments.

It may be advantageous in areas of erratic conditions or immediately beneath foundation
bearing elevations to obtain representative samples continuously. Rock core, due to the
nature of the sampling devices, is usually obtained continuously.

The recovered sample is examined and logged by qualified field personnel and a
representative portion or portions are selected and preserved, usually in glass jars.
Occasionally, it may be preferable to preserve the entire sample that is recovered for more
detailed analysis and testing.

14.3. Right-of-Entry, Permits and Utilities

Prior to commencing the actual field exploration program, the owners of the property where
the work will be performed must be contacted and permission obtained to conduct the work.

This permission should cover rights of access and conduct of the work and any special
provisions required by the property owner, such as working hours and cleanup.

Certain public and private property, such as navigable waters, railroad property and public
streets, may require special permits for right-of-entry. These working permits may involve a
fee or special insurance and are usefully obtained by the drilling contractor.

The location of any underground or overhead utilities must be determined before
commencing the drilling operations.

Regardless of the procedures followed or who obtains the venous rights-of-entry and
clearances, these functions should be completed prior to moving any equipment into the field

14.4. Borehole Location Tolerance

The allowable tolerance for the field locations of the explorations in relation to the plan
locations is subject to major variations depending on the level of the exploration program,
degree of complexity of the subsurface conditions, and anticipated use of the information
obtained. The borehole location tolerances should be established by the project geologist or
geotechnical engineer for the specific site investigation and may vary from one foot to tens of
feet. Site access and underground utilities may dictate locations beyond the limits of any
established tolerances.

14.5. Drilling Equipment

A great variety of conventional and modified drilling rigs are available. Test boring
equipment is manufactured in a number of sizes and styles, ranging from small, handheld
portable drills and augers, to missives off-shore mineral exploration equipment. The selection
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of the drilling equipment is an important aspect of any subsurface exploration program. The
equipment must be capable of meeting all, or as many of the project requirements as
possible, have sufficient mobility and possess the ability to convert rapidly from one drilling
technique to another. Hydraulic-feed machines are usually preferable, especially when they
can maintain a constant advance pressure through varying formation densities, which
minimizes erosion and disturbance of the in sin' materials. The project requirements or site
conditions may be such that special drilling or sampling equipment is required or that
supplemental logistics are necessary in realizing the project goals.

14.6. Exploration Methods

The selection of the specific drilling equipment and methods to be used for a particular site
investigation are dependent upon a number of factors. These may include site accessibility,
equipment availability, and geologic conditions, in addition to economic and environmental
considerations.

New exploration technology which is still in various development or experimental stages
shows a potential for possible future utilization in minimizing subsurface unknowns. These
methods include long distance horizontal boreholes, acoustic borehole logging, and a variety
of recently developed geophysical methods.

14.7. Borehole Advancement

The more commonly used borehole advancement techniques may be classified into six
groups, depending on the method used is displacing or removing material during penetration
of the borehole. They are:

• Displacement Boring
• Wash Boring
• Percussion Drilling
• Rotary Drilling
• Auger Boring
• Continuous Sampling
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The quality of information obtained from the various methods vanes with the character of
the subsurface geologic conditions; therefore, careful consideration must be given when
selecting the desired method. It may be necessary to employ more than one method in
advancing a particular borehole.

Displacement Borings. This method is the most simple and economical test boring
procedure in non-caving ground. There is no attempt to stabilize the borehole and closed
samplers such as the split tube, cup, or piston sampler are forced in a closed position to
the required sampling depth. This method is generally employed in preliminary
reconnaissance work where only general subsurface information is required.
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Wash Borings. This method involves advancing steel casing, as required, and washing
out the material to the bottom of the casing or desired sampling depth below the casing,
with a variety of chopping bits. The drill rods and chopping bits are alternately raised and
dropped, with some hand rotation, to break up the material within the casing; the
loosened materials (cuttings) are then carried to the surface by the recirculating drilling
fluids.

The borehole may be stabilized with casing, water, or drilling mud. Open samplers such
as the split or solid tube types, are then driven into the "undisturbed" material at the
bottom of the borehole.
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Rotary Drilling. Rotary drilling is a very versatile and adaptable technique which may be
used with a range of equipment models and sampling devices. Rotary drilling consists of
advancing a cased or uncased borehole by rapid rotation and pressure on the drill bit
which cuts and grinds the sediments at the bottom of the borehole into small particles
called "cuttings". These cuttings are subsequently removed from the borehole by
pumping air, water or drilling mud from a surface reservoir through the drill rods to the
bottom of the borehole.

Auger Borings. The use of rotary auger drilling methods is a rapid and economical
method of conducting subsurface explorations. There are certain inherent limitations
when using this type of procedure, which should be carefully evaluated for the site
specific exploration programs.

Auger boring techniques may be divided into three categories depending on the type of
auger equipment used: