1、Standard Practice for Decommissioning Geotechnical Exploratory Boreholes AASHTO Designation: R 22-97 (2015) American Association of State Highway and Transportation Officials 444 North Capitol Street N.W., Suite 249 Washington, D.C. 20001 TS-5b R 22-1 AASHTO Standard Practice for Decommissioning Geo
2、technical Exploratory Boreholes AASHTO Designation: R 22-97 (2015) INTRODUCTION The preservation of groundwater resources is a very important issue in the design, construction, and maintenance of the highway infrastructure system. Most projects require some type of on-site subsurface geotechnical in
3、vestigation to properly design the facilities. These investigations typically involve progressing smaller-diameter (200 mm) exploratory boreholes up to depths of approximately 60 m. In some instances, cone penetrometers or instrumentation devices such as piezometers, monitoring wells, and vane shear
4、 tests are used to obtain subsurface information. By progressing exploratory boreholes that may penetrate the water table, a conduit for surface and subsurface aquifer cross-contamination may be introduced. In situations in which the investigation may lead to the contamination of groundwater resourc
5、es, it would be prudent to properly abandon the borehole. Construction of a borehole seal during decommissioning can add significantly to the time, material, and labor costs of geotechnical exploratory boreholes. Some states have regulations that mandate that all geotechnical exploratory boreholes m
6、ust be sealed. When that is the case, this standard practice is of limited use, and the reader should consult references pertaining to borehole sealing. If borehole sealing mandates are not in effect, this standard practice may provide some guidelines regarding which boreholes should be sealed, as w
7、ell as the procedures involved in this determination. 1. SCOPE 1.1. Types of BoreholesThis standard practice addresses the permanent decommissioning (closure) of the following types of geotechnical exploratory boreholes. It is specifically intended to address the closure in situations in which hazar
8、dous material or solid wastes are determined not to be present.1Included in the list of geotechnical exploratory boreholes are: 1.1.1. Boreholes progressed with temporary casing for obtaining soil, rock, and groundwater information; 1.1.2. Uncased boreholes progressed for obtaining soil, rock, and g
9、roundwater information using drilling mud or open-hole techniques; 1.1.3. Hollow-stem flight auger boreholes; 1.1.4. Observation wells for monitoring groundwater conditions; 1.1.5. Instrumentation boreholes (i.e., piezometers, Borros points, etc.); 1.1.6. Cone penetrometer boreholes; and 2016 by the
10、 American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law.TS-5b R 22-2 AASHTO 1.1.7. Soil characterization tests (vane shear, dilatometer, etc.). 1.2. Intent of PracticeThis practice describes the overall process of decommi
11、ssioning the holes and, as such, includes a systematic approach to completing the process. The typical steps entail an evaluation of the specific site characteristics and potential problems, a review of available decommissioning options including the null option if applicable, selection of an approp
12、riate sealant, and a method of sealing. Specific requirements for the quality control and performance of the sealed borehole are addressed. Record-keeping requirements are noted. 1.3. SafetyThis standard practice does not propose to address all safety concerns associated with its usage. It is the du
13、ty and responsibility of the user of this practice to establish appropriate safety and health practices and determine the applicability of other state regulatory limitations prior to use. 2. REFERENCED DOCUMENTS 2.1. AASHTO Standard: R 13, Conducting Geotechnical Subsurface Investigations 2.2. ASTM
14、Standard: D5299, Standard Guide for Decommissioning of Groundwater Wells, Vadose Zone Monitoring Devices, Boreholes, and Other Devices for Environmental Activities 2.3. National Cooperative Highway Research Program Reports: NCHRP Report 378, Recommended Guidelines for Sealing Geotechnical Explorator
15、y Holes NCHRP Project 21-4, Sealing Geotechnical Exploratory Holes to Protect the Subsurface Environment 3. SUMMARY OF GUIDE 3.1. Currently, there are numerous state agencies and private contractors who install geotechnical exploratory boreholes for the state transportation programs. Within these ag
16、encies, various state regulations and decommissioning procedures are in place. No national or international standards exist for abandoning geotechnical boreholes. This standard practice attempts to provide members of the transportation industry with guidelines and references to follow in evaluating
17、the need for sealing, different sealant materials, methods for sealing, and the associated record keeping for the process. 4. SIGNIFICANCE AND USE 4.1. This standard practice is intended to provide technical information and is not intended to supplant any federal, state, or municipal regulations tha
18、t may be in effect in a particular area. For a summary of practices of state transportation departments, see National Cooperative Highway Research Program Project 21-4, Sealing Geotechnical Exploratory Holes to Protect the Subsurface Environment, August 1995. 5. MATERIALS 5.1. See NCHRP Report 378,
19、Recommended Guidelines for Sealing Geotechnical Exploratory Holes, Chapter Six. 2016 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law.TS-5b R 22-3 AASHTO 6. PROCEDURE 6.1. Reaching the decision regarding the
20、appropriate type of decommissioning procedure for geotechnical exploratory boreholes is a complex task. Among other factors, the person making the decision must understand the hydrogeology of the area, be aware of any known aquifers in the vicinity, know where various groundwater levels exist, and k
21、now the depth and diameter of the borehole. From a practical point of view, the decision regarding the correct type of decommissioning must be studied prior to performing the actual field work. 6.2. The decommissioning process can be broken down into two major processes: backfilling or sealing. Thes
22、e two processes are defined as follows: 6.2.1. BackfillingThe practice of placing native soil cuttings or other materials in the hole as a part of the restoration process. This is largely an uncontrolled process and may only inadvertently produce a seal. 6.2.2. SealingThe knowledgeable and conscient
23、ious practice of constructing a permanent hydraulic barrier in the hole. This is a carefully controlled practice of hole decommissioning.26.3. The information for making the decision regarding when a geotechnical exploratory borehole should be sealed must be gathered from a broad spectrum of availab
24、le resources. The first step to follow in reaching a decision is to perform a review of the existing site data. Data can be collected from a number of sources, including but not limited to: a) Geologic reports, b) Topographic maps, c) Previous borings in the area, d) Water well records, e) Aquifer m
25、aps, f) Hazardous site maps, g) Survey of industry (pollutant potential), h) Survey of existing or proposed landfills, and i) State environmental protection agencies. Making a final decision may require input from various people skilled in different disciplines, such as hydrogeology, geology, drilli
26、ng, geotechnical engineering, and highway design. 6.4. The goal in decommissioning any hole is to leave, as far as possible, the hydrologic conditions in an unaltered state. This is particularly important where a public water supply is obtained from groundwater in an area. To determine whether this
27、is the case, maps of aquifers used for public water supplies can be obtained from state and local public health agencies and state environmental protection agencies. The need for aquifer protection is as great for private water companies as it is for water supply systems under municipal control. 6.5
28、. A review of geologic reports, water well records, and previous borings can help define where confined aquifers or multiple aquifers are present. Confined aquifers, which are sometimes called artesian aquifers, require special consideration for borehole decommissioning. The confined aquifer is over
29、lain by an impervious layer such as clay or glacial till. When a borehole penetrates the confining layer and enters the underlying aquifer, it is introducing a vertical pathway for aqueous constituents to enter the aquifer. Moreover, the material comprising the confining layer is likely to remain op
30、en after the removal of drill rods or casing, so that the avenue for possible contaminants will not be self-sealing. Multiple aquifers are similar to confined aquifers, in that several waterbearing strata of aquifer material are separated by confining impermeable strata. The 2016 by the American Ass
31、ociation of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law.TS-5b R 22-4 AASHTO determination as to whether the medium to be drilled is a confined or multiple aquifer should be made before drilling, or in the field by qualified personnel.
32、 6.6. After information is compiled from the aforementioned sources, it is necessary to make the decision as to whether backfilling or sealing is the most appropriate action. Some boreholes will encounter conditions that mandate sealing in order to safeguard the groundwater resources of the area. 6.
33、6.1. Any boreholes that encounter methane gas should be sealed as quickly as possible. Methane is a naturally occurring explosive gas that may be tapped at relatively shallow depths and can present a public safety hazard if it is allowed to vent indefinitely from a borehole. 6.6.2. As mentioned earl
34、ier, confined aquifers and multiple aquifers are unique geologic conditions, and they require sealing. It is especially important to seal a flowing artesian condition because the groundwater will carry fine material with it as it flows out of the ground, and this may cause future subsidence. Confine
35、d aquifers that do not flow to the surface should be sealed as well because of the possibility of cross-contamination of separate and discrete hydrogeologic units. 6.6.3. Boreholes drilled in areas where groundwater is the source of documented large public water supplies should be sealed. 6.6.4. In
36、a heavily populated urban or suburban area, the possibility of future contamination of groundwater is increased because of the density of potential spill sites such as gas stations, landfills, dry cleaning stores, and chemical factories. Boreholes that encounter groundwater in these areas should be
37、sealed. 6.6.5. Boreholes that are drilled in environmentally sensitive areas should be sealed. An example of this would be a piezometer installed in a wetland as part of an environmental impact study. 6.7. Some boreholes do not require sealing, but should be backfilled after use. If backfilling the
38、borehole with native cuttings is justified, care should be taken to compact the upper portion of the native material in the hole to minimize any future subsidence. With any borehole, an open hole should at least be backfilled and not left open because of the possibility of a hazard to pedestrians an
39、d others. 6.7.1. Boreholes that do not encounter groundwater require backfilling, but not sealing, because they do not disturb the hydrologic regime. 6.7.2. If a borehole is located entirely in granular material, it does not require sealing even if groundwater is encountered. After removal of the dr
40、illing equipment, granular material will collapse on itself, and the previously existing hydrogeologic conditions will be restored. 6.7.3. If a borehole is located entirely within material of low permeability, backfilling is adequate for decommissioning. An example of this would be a borehole drille
41、d through clay or dense glacial till over shale rock. None of these materials would be considered an aquifer. 6.8. The procedures for properly sealing a borehole are presented in Chapter 7 through Chapter 10 of NCHRP Report 378, Recommended Guidelines for Sealing Geotechnical Exploratory Holes. Note
42、 1The hole reentry method for sealing penetration holes (Chapter 10) can be used only if the hole remains open on its own after removal of tools. 6.9. It is important that the quality of workmanship and the skills of the drilling personnel who perform the sealing operation are guaranteed. Individual
43、 states should determine procedures to qualify personnel for this activity, such as a certification process. 2016 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law.TS-5b R 22-5 AASHTO 7. RECORD KEEPING 7.1. Pr
44、oper record-keeping practice is addressed in Chapter 11 of NCHRP Report 378. 8. INVENTORY OF SEALED BOREHOLES 8.1. In addition to maintaining the sealing records identified in Section 7 of this practice, the borehole should be located using a suitable coordinate system wherever possible. With the ad
45、vances in Global Positioning technologies and the incorporation of GIS application, complete records and locations of sealed boreholes will provide valuable information to the various state agencies charged with protecting valuable groundwater resources. 9. REFERENCES 9.1. AASHTO. Manual on Subsurfa
46、ce Investigations. American Association of State Highway and Transportation Officials, Washington, DC, 1987. 9.2. Clark, Lewis. The Field Guide to Water Wells and Boreholes. John Wiley, New York, NY, 1988. 9.3. Driscoll, Fletcher G. Groundwater and Wells. Johnson Filtration Systems, Inc., St. Paul,
47、MN, 1986. 9.4. Todd, D. K. Groundwater Hydrology. John Wiley, New York, NY, 1980. 10. ADDITIONAL APPLICATIONS 10.1. The focus of this standard practice is limited to geotechnical exploratory holes. This practice also contains information that may pertain to other activities performed by the transpor
48、tation industry that result in a vertical conduit being introduced into the ground in an area where protection of groundwater is a consideration. APPENDIX (Nonmandatory Information) X1. TERMINOLOGY X1.1. abandonmentsee decommissioning. X1.2. aquifera geologic formation, group of formations, or part
49、of a formation capable of yielding a significant amount of groundwater to wells or springs.3X1.3. artesiangroundwater that is under sufficient hydrostatic head to rise above the aquifer containing it. A flowing artesian condition is when water flows up to the surface through a drillhole. X1.4. artesian wella well tapping an aquifer bounded above and below by impermeable beds or beds of distinctly lower permeability than the aquifer itself. The water will rise in the well above the 2016 by the American Association of State Highway and Transportation Officials. All ri
