This is the second technical brief in the series on site stabilization and maintenance developed through cooperation among the Center for Archaeological Research at the University of Mississippi, the Tennessee Valley Authority, and the National Park Service. The series is based upon existing knowledge and stabilization project experiences to provide programmatic guidance appropriate for problem solving. As baseline information, the series demonstrates the highly variable conditions surrounding archeological site loss, discusses alternatives, and suggests how applicable stabilization techniques can be modified to meet needs.
Information exchange is an important objective of this series. The National Clearinghouse for Archaeological Site Stabilization is organized as a central location at which to seek information as well as to foster interactions among governmental agencies, professionals, and the private sector. It is one solution to the concern for improving technology transfer in historic preservation. The address and telephone number of the Clearinghouse are given at the end of this technical brief.
In an absolute sense, the preservation of archeological sites is an unattainable goal, since the aging process of all materials is ongoing. Techniques are available, however, to retard losses to site integrity that are the result of natural and/or cultural processes. To the extent that the processes that cause these losses can be slowed, resources can be stabilized and protected, and in that sense, preserved. Intentional site burial is offered as another of several alternatives that may prove to be appropriate means of achieving that goal.
Covering archeological sites is not a new phenomenon, since the natural burial of sites is a common occurrence. The Murray Springs site in southern Arizona (Haynes and Hemming 1968) and the Bacon Bend site in eastern Tennessee (Chapman 1978) were covered as a result of gradual colluvial and/or alluvial deposition, while the ancient Roman cities of Herculaneum and Pompeii were rapidly sealed by volcanic activity. Natural covering in these instances has worked in much the same manner as artificial covering: some kinds of artifacts and ecofacts are well preserved, while the loss of other kinds may be accelerated.
In reality, stratified archeological deposits may be viewed as microcosmic cases of site burial. Each succeeding occupation or each succeeding flooding episode buries the preceding deposits and to some extent, protects the earlier, lower occupational levels from changes that may be the result of physical and chemical processes on the land surface. Cultural and environmental changes proceed at sufficiently gradual rates so that in most cases succeeding depositions are chemically and biologically compatible with the lower levels and decay of the lower levels is not accelerated. Mechanical loss of the lower levels of sites occurs as new pits are dug, new posts are set, and as a result of bioturbation associated with the latest in the series of occupations. Like the burial of the Murray Springs and Bacon Bend sites, differential preservation of the various artifact classes is an accepted property of archeological sites.
The burial or intentional covering of archeological properties has been used as a means of protecting resources from natural or mechanical loss (U.S. Department of the Interior 1975; Jensen 1976; Chace 1981; Klinger 1982; Garfinkel and Lister 1983; Thorne, et al. 1987; Wilkie, Aide, and Knox 1986). Most of the completed intentional site burial projects that have been reported are in or adjacent to construction areas. An annotated bibliography describing some of those efforts is given at the end of this technical brief.
Design of an Effective Project
The objective of this technical brief is to provide guidance on design of an effective project for intentional site burial. It identifies the process by which an archeological program manager can: (1) evaluate the components of the site, (2) measure potential impacts, including decay processes, against the goals for protecting the site; (3) assess the benefits of intentional site burial; and (4) specify the methods and procedures to be used in the project, including cost considerations.
Documented cases of site burial can be referred for background technical and methodological support when a new project is being considered, but every site that is considered for artificial covering must be treated as a separate case. This is due in part to the extent of variability among individual sites as well as the degree to which the components of a single site will vary internally. Each site incorporated into a stabilization design must be judged on its particular internal and external components, even though several sites, in close proximity to each other, may be scheduled for treatment.
Evaluation of Site Components
Since the stabilization of an archeological site follows an orderly sequence of events (Thorne 1988a), a site's archeological components will have already been defined at the beginning of a preservation project. Testing for National Register eligibility will have demonstrated the range of artifacts that must be protected, including bone, shell, ceramic and lithic artifacts, wood and charcoal, and the variety of features that must be considered in the development of a preservation plan. In order to complete the evaluation of site components that is necessary for the development of a design for the burial of a particular site, additional information on components other than artifacts will be required. These data may go beyond those that are collected during the course of normal archeological investigations and may include: pH determinations taken from a number of loci within the site; data indicating ongoing and potential oxidation/reduction processes; and soil samples of the site's matrix as well as the underlying strata. Many approaches to the analysis of archeological soils have been taken. One example is from resistivity surveys (Carr 1982). The definition for site matrix follows Mathewson (1988) and Mathewson and Gonzales (1989). (See Figure 1 below.) The soil samples may have to be tested for compression strength and permeability.
The collection of these data will allow the development
of reasonable estimates of how a site's artifact and ecofact components
have reacted to their physical and chemical environments through time.
A model can then be derived to predict how the artifact component will
be affected by the placement of an artificial covering. These additional
data are necessary for the proper selection of the fill material that
will be used to cover the site, since chemical and organic compatibility
of both the site and fill are necessary. Post-burial monitoring assessments
must also rely on these data since they will form the baseline from
which all evaluations will be made.
Measurement of Impacts and Setting Goals for Protection
The design plan for intentional burial must be conceived in a manner that will insure that maximum protection is afforded the resource while minimizing any negative effects caused by such an overburden. In order to determine the best design, a multidisciplinary team of specialists is recommended. This team should include an archeologist, a geologist, and an engineer. Each will have specific responsibilities in developing the stabilization design plan.
Their efforts must be integrated and not performed as a series of independent steps. The archeologist must define the various classes of artifacts that are to be preserved and indicate what classes, if any, may ultimately be unprotected or lost (more about this later). The geologist must review the basic preservation (artifact class) requirements that have been set by the archeologist. The geologist should understand the mechanisms of artifact decay and on that basis be able to prescribe fill materials that will best fit the preservation needs of the resource. Finally, the engineer will be charged with designing the mechanics of the burial procedure. His or her level of understanding must extend from fill acquisition and placement to the hydraulic properties of the site and how hydraulic changes will affect the site's contents. He or she will also be responsible for designing the placement of the fill so the site components will not warp as a result of heavy equipment movements or the weight of the fill column over time.
During the process of arriving at the best plan for covering a site, engineering and geologic design criteria that are imposed on the project may seem unreasonable or unrealistic from the perspective of the archeologist. Such instances should be overcome through team negotiations. Successful design negotiations will be dependent on a shared understanding of the preservation needs of the resource by all team members. Frequently, however, design standards that must be met are set by the land managing agency or by one of the organizations that have established governing regulations for construction, e.g., the American Society for Testing Materials (ASTM). When external criteria of this kind must be taken into account, the archeologist must be prepared to yield to those constraints. The best solution to problems of this kind may be to seek innovative ways of meeting required standards while protecting the archeological resource.
Predictions about future reactions must be based on an understanding of the decay processes that operate on the various components of an archeological site. Mitigation of the effects of the decay processes and the external forces that impact the resource is the primary goal of the stabilization effort. A secondary goal of not accelerating ongoing decay or adding new destructive processes must be a consideration of the stabilization design.
Chemical processes related to oxidation and reduction combined with soil pH characteristics are most likely to be the primary causes for naturally stimulated site content loss. Cyclical wetting/drying and freezing/thawing can affect the decay process through both chemical and mechanical means. Culturally derived disturbance can act as a catalyst that will speed up chemical and mechanical loss. Biological degeneration of a resource is accelerated by macroorganism activities, e.g., burrowing animals. Any environmental alteration that increases the number of microorganisms will hasten the decay of some classes of artifacts and ecofacts.
Current preservation technology, combined with the nature of the decay processes that affect archeological materials, is such that not all artifact classes can be protected simultaneously. Mathewson (1988) has outlined some of the problem areas in site burial, and the model of site decay can be useful in both understanding and predicting how the preservation-natural decay process will proceed (Figure 1).
Even though our current technology limits how far we can go toward total stabilization, we can successfully design against further loss of some of a site's components. As can be seen from the Mathewson model, some design criteria will promote the preservation of one kind of artifact while accelerating the loss of some others. This suggests that when stabilization through burial is considered, the design team may need to allow the loss of certain classes of artifacts for the sake of preserving others. The natural decay processes that occur as a midden develops will almost always include an aspect of differential preservation of artifact classes. The artificial burial of a cultural deposit will embody those some aspects, but in a carefully conceived framework.
To illustrate the usefulness of the model, it is possible through inspection to predict that if a continuously dry environment is designed, preservation of some aspects of the site will be enhanced while other artifact classes will remain unaffected. This type of dry environment would be nearly identical to those found in the caves of semi-arid and arid areas. In most parts of the United States we are unlikely to construct a continuously dry environment. We can, however, engineer site covering plans in such a manner that the hydraulic character of the site is altered dramatically from its natural state and a site's matrix is dried considerably. Conversely, and as can be seen from the model, the rate of loss to more classes of artifacts increases with the level of moisture.
Again, drawing from the model, if an alkaline environment is artificially created, bone, shell, and the granular lithic assemblage would be better preserved while plant material, soil attributes, and metal artifacts would be lost at a faster rate. If by design the site stays continuously wet and an aerobic state exists, decay of all organic artifacts and remains will be hastened.
Benefits of Intentional Site Burial
The difficulties of covering a site are more apparent than real and can be overcome through a stabilization program that is designed with care. Advantages will accrue to the resource that have been previously unavailable. Protection from loss can be extended to both natural and cultural processes. (A convenient summary of these processes appears in Mathewson 1989.)
Protection from cultural
Protection from natural
An obvious advantage of site burial is that surface erosion of the archeological matrix is eliminated when a new land surface is produced. Similarly, future freezing and thawing can be eliminated by designing the fill depth to exceed the depth of the frost line.
Newly created land surface or strata can also provide relief from the absorbed effects of acid rain as well as serving to shed rainwater. Sites that are within reservoir or lake drawdown zones or along the splash zone of lake margins are not considered to be prime candidates for this treatment. Earth burial is only an appropriate means of stabilizing sites in wave impact environments if the design plan includes some form of hard covering that will protect the newly created surface, e.g., riprap, bulkheads, or filter fabric. (See Thorne 1988b for discussion of the use of filter fabrics as protective hard covering.)
Revegetation should be a part of the stabilization plan to insure land surface stability, and the newly created land surface can be used for a variety of purposes within specified limits. In specific instances, surface stability can be assured while cash crops are being cultivated on the newly placed fill. Care must be exercised in allowing agricultural production to continue after fill is in place (Figure 2), and there must be regular monitoring to insure that post-burial damage is minimized (Figure 3).
If properly designed, the site and the superimposed fill can be used as extra-load bearing strata for parking lot construction. The broadest possible post-burial uses of the new land surface should be anticipated during the preservation planning and design phase. The design criteria for those uses must be incorporated into the stabilization program.
Project Methods and Procedures
Several other concerns must be taken into account before proceeding with the actual burial of an archeological resource. First among these is the establishment of a reference or benchmark system so that the site and specific loci within it can be relocated in the future. This is particularly important if future scientific investigation is specified as a goal for the burial project. Permanent markers should be set and appropriate maps should be marked to indicate the location of the site. Pertinent benchmark data should be noted on the maps as well as contained in a written report. Clear and easily located on-the-ground marking is particularly important if the site is likely to be in a construction impact zone in the future. If post-burial use of the newly created stratigraphic layer is anticipated, benchmark placement should be done in such a manner as to accommodate the new use.
To insure that there is no inadvertent mixing between the archeological matrix and the covering material, the installation of some form of horizon marker may be desirable. A number of filter fabrics are available that can serve such a purpose (Thorne 1988b). Most of the filter fabrics are chemically inert and have a relatively long lifespan if exposure to the sun is minimized. Alternatively, culturally sterile sand, gravel, furnace slag (1/4"), or a clay-gravel lens may be placed between the fill and the site matrix to clearly mark the stratigraphic contact zone. Gravels and other naturally occurring materials may be used, but only if they have a zero distribution within the archeological matrix. Care must also be taken to insure that the horizon marker does not alter the chemical or hydrostatic character of the cultural deposit unless such a change has been intentionally incorporated into the burial design. The use of tightly compacted clays or clay-gravels has the potential of altering the permeability of the site as well as introducing pH variations.
Burying the site
Monitoring the site
Post-burial monitoring, as recommended here, will have several meanings. Any of these can be applied to any site, but each must be used on a site-by-site basis. At its lowest level, monitoring will be completed to do little more than regularly ascertain the condition of the surface of the site and to have those observations recorded. At the next level, site condition observations will be made, problems of stability noted, and some effort will then be made to rectify any problems. These two views of monitoring deal with the condition of the newly created surface and not with the buried archeological site. The most complex of the monitoring procedures will deal with determining the condition of the archeological matrix. Since the resource will no longer be readily accessible, the decision must be made during the design phase of the project as to which monitoring approach will be taken. If it is to be the latter course, evaluation procedures must be incorporated into the total burial plan.
Post-burial test areas should be determined, and these loci must be noted on appropriate maps and tied to the system of benchmarks that will be set to mark the location of the site. If subsurface evaluations are planned for a number of years, a schedule of specific test loci should be prepared. The location of these successive test units will be in part dependent on the types of evaluation data that are to be recovered. Generally, if test excavation and boring placements are to be used, they should be designed to avoid known or suspected archeological feature locations. Any form of blind subsurface testing such as boring always has the potential of damaging or destroying significant features unless locations are incorporated into the burial design.
Electronic monitoring of subsurface conditions also should be considered during the design phase of the project. Various metering gauges are available to record pressure and soil movement both vertically and horizontally. Magnetic displacement gauges that are designed to measure movement under road surfaces are appropriate for use in an archeological context and their use will not require any subfill disturbance.
If sufficient testing is completed prior to working out a burial design and if the burial design is properly conceived, no additional disturbance should accrue to the cultural deposit as a result of placement of the fill. Responsibility for the adequacy of the design will be in the hands of the geologist and the engineer and, given an appropriate design, site monitoring should be concerned with only the condition of the superimposed fill material.
To insure that the appropriate monitoring regimen is adequately carried out, a schedule should be prepared that details a timetable for the process and the extent to which testing will be taken. A statement specifying the intent of the monitoring as well as the schedule for inspection visits should be incorporated into an agreement that will be signed prior to completing the plans to bury the resource.
Material costs will include the benchmarks, the filtering agent and its installation, the fill material and its transportation and placement, and any remote sensing monitoring devices. If revegetation of the newly created surface is a part of the design plan, acquisition and installation of cover plants will form part of the total cost. If fertilizer must be used as a part of such a plan, care must be taken that it not affect the chemical composition of the archeological deposits.
Monitoring costs for the proposed period should be anticipated, and a mechanism must be devised to insure that future funding is available. Most previously completed projects recommend subsurface testing over a period of years that typically goes beyond the ability of any agency to guarantee funding. The direct consequence is that agencies will be reluctant at best to enter into such an agreement, since budgetary structuring is established on a year-by-year basis.
It may be that the most direct route to post-burial monitoring funds will be through a specific line item request in the operations and maintenance budget of the appropriate branch of an agency. In many instances, the construction unit of an agency will relinquish responsibility for a project once construction is completed, and maintenance for the post-burial period will become the responsibility of another branch. Communication of responsibility between branches is an absolute necessity if these responsibilities are to be adequately met.
To insure that adequate funds are available and that the monitoring process is regular and timely, funds should be placed in the line item budget on a yearly basis. Once the item becomes a specific part of a budget, continuation on a yearly basis will be less difficult and will insure also that the monitoring effort meets preservation requirements.
Request for Assistance
Information exchange about site stabilization is available
Dr. Robert M. Thorne
Archeological Site Stabilization Bibliography
The National Clearinghouse for Archaeological Site Stabilization maintains a bibliography that is intended to support the conceptualization, design, and development of site stabilization and preservation projects. The bibliography is divided into four sections: (1) Philosophy, (2) Technical Support, (3) Management Recommendations, and (4) Practical Applications. Annotations follow some of the entries and provide the user with a brief but sufficient sketch of the entry. As new source materials become available they are entered into the reference work.
Section 1 - Philosophy provides an overview for site preservation and stabilization and provides the user with sufficient background to philosophically justify archeological site stabilization projects. Section 2 - Technical Support draws together technical information that is generally unknown to archeologists. Reliance on and knowledge of these data are integral to the design of stabilization projects, particularly if cost effective and innovative stabilization measures are to be put into place. Section 3 - Management Recommendations contains a mix of projects to provide the user with an idea of how site stabilization has been approached. Section 4 - Practical Applications is devoted to the presentation of specific archeological site stabilization case histories. Data contained in these case histories will provide an insight into the planning and implementation of stabilization projects.
For a copy of the Bibliography, write to the address listed above.