Safety Assessment of the Archaeological Tunnels in Copán – Honduras
The main aim of this thesis, by the author Felipe Cardoso Vale Pires, is to establish a basis for further development of the safety assessment of the archaeological tunnels of Copan.
THE ANCIENT CITY OF COPAN
Copan was a major Maya settlement established at the floodplains of the Copan River, in the western highlands of present-day country of Honduras, about 400 km from its capital, Tegucigalpa.
Discovered by the Europeans colonizers in the sixteenth century, its ruins comprise several temples, palaces, ball courts, and associated monuments that have long been considered among the most beautiful architectural and sculptural monuments of the ancient Americas.
Central America was home to several cultures, most notably the Maya, before the Spanish Colonization. The Maya were a Mesoamerican indigenous people that inhabited areas of contemporary southern Mexico, Guatemala, Belize, Honduras and El Salvador. They created a sophisticated agricultural system, supplemented by forest, river, and seashore resources, to support a population that reached the millions at its peak. Archaeology has revealed many interconnected and dense cities, with an agricultural infrastructure well adapted to varied highland and lowland environments.
In Copan, rulers were enthusiastic patrons of architecture and each king changed the city landscape by adding monuments at the same area, one atop the other. Many structures were built or renovated along the centuries, culminating in monuments like the Hieroglyphic Stairway, the largest and longest hieroglyphic carved text in the Americas. This collection of buildings was not scattered in the territory, but built in a dense religious center, known as the Main Group.
Copan was a multi-ethnic center built at the margins of the Copan River, at the southeastern frontier of the Lowland Maya culture. Copan’s Classic era settlement was comprised by the actual ruins of the Main Group, the surrounding residential remains and its hinterlands. Its location and structures combine convenience and ideology. The bulk of the settlement occupied an area of approximately 25 km2 in a pocket of the Copan river valley.
The valley runs longitudinally from northwest to southeast along the Copan River.
The landscape is dominated by rugged topography. High mountains and their adjacent foothills delineate the river valley, constricting its width to a few hundred meters in most places, or widening it out to form pockets of alluvial soil.
The core of the site, Copan’s Main Group, is located in the middle of the largest pocket of the entire valley, about 12 km upstream from the border with Guatemala. The Copan pocket area is about 600 m above sea level, but adjacent mountains rise as high as 1,400 m creating a milder climate in the valley than in the surrounding regions.
The average monthly temperature is 18ºC. The region has marked seasons, with a dry summer (from November to April), and a wet winter (from May to October). The rainfall is more regular at the pocket than in locations nearby, with an average annual rain of about 1440 mm.
Brief Chronology of the Acropolis Construction
The structures on the surface of the Main Group are actually the last components of a series of additions carried by the 400-year dynasty that ruled over Copan. Nevertheless, the sequence of alterations introduced by each king seems to have followed a pattern, since its final spatial template is common to other Maya settlements. This template combines the following principles:
1) Emphatic reference to a north-south axis in the site organization;
2) Formal and functional complementarity, or dualism, between north and south;
3) The addition of elements on east and west to form a triangle with the north;
4) The presence in many cases of a ball court as transition between north and south;
5) The frequent use of causeways to emphasize connections among the cited elements, thereby underscoring the symbolic unity of the whole layout.
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Architectural and Construction Features
One building strategy differentiates the ancient Copañecos (i.e. Copan inhabitants) from other Maya communities: in Copan, both ordinary and monumental superstructures where built over durable stone platforms.
The term “structure” in Mesoamerican architecture has a special meaning. Structures (with capital “S”) are architectural entities added by all the modifications they suffered through time. They are composed of at least three main components:
⦁ Platforms – are leveling layers that provide an essentially horizontal surfasse;
⦁ Substructures – are an assemblage of elements that support a building;
⦁ Superstructures – are mainly the usable buildings supported by substructures and their associated elements: roof-combs (ornamental structures that topped Mesoamerican monuments), free-standing walls and altars.
Cultural Heritage
The combination of ideological, religious and political propaganda left an impressive built heritage at Copan site. In the second half of the 19th century, in order to protect the recently rediscovered monuments, the Honduran government delimited an area of about 45 ha around the Main Group and its surroundings, creating the archaeological park of Copan.
Later, UNESCO listed the Main Group as World Heritage Property in 1980, and was followed by the Republic of Honduras that listed it as a National Monument in 1982.
THE ARCHAEOLOGICAL TUNNELS OF COPAN
The common practice of constructing one building over another in Mesoamerican sites usually means that only the last version of the monumental architecture is fully available. However, in Copan, a unique situation uncovered a full sequence of monumental construction. The exposed section of the eastern edge of the Acropolis reveals a vast accumulation of stratified buildings, platforms and plazas.
This river cut, known as corte, is the result of hundreds of years of river course natural action. At some point of the Classic Period, the Maya artificially diverted the river channel. The artificial mitigation of the natural processes associated with an active river was stopped after the city decline, letting the river meanders again. Its widening floodplain became responsible for the undercutting and destruction of a portion of the Acropolis.
Added to centuries of erosion, an earthquake in the 1930’s threw the top of three East Court buildings into the river, including parts of Temple 22, partially destroying some of the structures recorded by the first archaeologists of Copan.
Known since the “discovery” of the site, the corte was misinterpreted by the first researchers as layers of foundations for the apparent structures of the Acropolis.
⦁ John Owens, a staff member of the Peabody Museum of Harvard University Expedition, seems to have been the first to understand the importance of the corte in the end of the 19th century.
⦁ In the 1930’s, Gustav Stromsvik started to excavate tunnels under Structure 10L-11, while Edwin Shook prepared the first scaled drawings of the corte.
⦁ Also in the 1930’s to prevent further catastrophic collapse, the Carnegie Institution of Washington carried out the diversion of the river.
After a 30-year span of small-scale archaeological projects, the modern period of research begun using tunnels starting from the corte, the architectural stratigraphy of the Main Group was explored. Buildings at the center were excavated and reconstructed, and monuments and inscriptions were recorded and studied.
Between the end of 1970’s and beginning of the 1990’s the corte suffered many stabilization campaigns. The interventions included the removal of the debris along the riverbank, construction of a new massive artificial tallus against its base, application of facing stones over the corte exposed elevation and construction of masonry walls mimicking the features that were buried inside the stabilization layer in order to preserve the visual impression of the site. The tunnels dug through it, in turn, were either stabilized with stone masonry lining or left unlined.
In the end of the 1980’s, the Copan Acropolis Archaeological Project was established, uniting past and new projects under the same direction. Between 1989 and 1996, the Early Copán Acropolis Program of the University Of Pennsylvania Museum opened long extensions of tunnels under the Acropolis.
These tunnels combined with the earliest ones, form the most extensive system of tunnels ever excavated at a Maya site, covering the eastern two-thirds of the Copan Acropolis and reaching approximately 3,650 meters.
In the late 1990’s it was created the Integral Project for Conservation of Copan Archaeological Park and under this program, the tunnels received the last interventions. Periodic collapses were followed by stabilization and backfilling actions.
By the year 2000, they preferred to build stabilization walls within tunnels that were experiencing collapse, instead of backfilling, as originally planned. These plans changed based on the recommendations of an Austrian structural engineer that emphasized that dirt infill would settle over time and permit the collapse of up to ten centimeters of tunnel.
Several small stabilization projects were executed after the end of the excavations, adding stretches of stone masonry walls to areas deemed as prone to collapse. However, as each management plan developed for the archaeological site mentioned, the safety of the tunnels and their monitoring system were not adequate yet.
Even though the tunnels created access to the treasures under the monuments, they also have some drawbacks such as:
⦁ During the archaeological excavations damage was caused since the tunnels received heavy traffic as workers dug in, carried material out and built stabilization walls.
⦁ The opening to tourism of many tunnels generated variation of the humidity, since the open tunnels allow for differential circulation of air. This was added by an increase in carbon dioxide and light inside the tunnels, thus fostering biological growth and material deterioration. The constant introduction of foreign particles to spaces that had been buried and isolated for up to 1500 years creates an environment that is unsafe for the plastered façades, one of the most important features discovered through the tunneling.
Tunnel Features
According to the Plan for the Long-term Conservation of the Copan Acropolis Tunnels, 3.65 km of tunnels have been excavated under Copan’s Acropolis.
To better understand the geometry of the tunnels and their spatial relations, a 3D survey was started in 2015. The ongoing project already surveyed half of the open tunnels to date, checking the existent maps and adding information to a previous database.
From the total extension of the presently open tunnels, around 52% have been surveyed. According to the information recorded in the existing master plans, around 18% of the total excavated tunnels were back-filled, 47% were stabilized (regardless of the type of stabilization) and 35% were left “as-excavated”.
Excavated Materials
The excavated material varies from very stable in lower levels to fairly unstable at upper levels:
⦁ The lowermost layers of the Acropolis consist of dark, clay-rich river mud (barro).
⦁ The larger part of the material excavated is a mixture of dark reddish-brown earth (tierra café oscuro) mixed with construction debris, river cobbles and broken lime plaster.
⦁ There are also small tunnel sections of red-colored (tierra roja) and yellow-colored (tierra amarilla) earthen fill.
⦁ At the Inferior Level most of the tunnels were left unlined. At the Superior and Third level, when collapses occurred, the tunnels were stabilized using masonry bonded with a mortar containing a low amount of Portland cement.
⦁ The most unstable material was used in later structures. These structures were buried with material composed by a yellow sandy earth (girún). The girún, considered as the less stable material, has little occurrence in the maps.
Tunnels Stabilization
In 1942, the stabilization was achieved by widening the tunnels and building masonry walls and arches along the sides and ceilings of the open spaces. The masonry additions support the loose earthen material fill to decrease the risk of collapse.
The stabilization methods from the 1990’s and beyond changed. The tunnel widening process was the same but the stabilization structures were built such that they avoided to directly touch the original walls with the installation of plastic tarps between the new masonry and the original when it was necessary that they were in contact.
In recent stabilization projects, new walls were built with an average width of 50 cm. The dimensions and regularity of the stones used in the linings also vary considerably between different projects and tunnels, both in source and shape, but not in type of material.
Structural Damage
The main concern related to the tunnels is their stability, hence, the possibility of collapse. The available data indicates that collapse is mainly associated to the type of material excavated and the occurrence of water intrusion.
METHODOLOGY
This study focused on a detailed study of the history of the site in order to allow an understanding of the architecture and construction methods of underground structures:
⦁ The shape of the tunnels under the Acropolis was studied using a 3D survey and typical section shapes were established for the study.
⦁ The material excavated by the tunnels is the filling of the structures, which was associated with the behavior of unsaturated soils for the evaluation of geotechnical stability and resistance to shear stresses.
⦁ The stability of unlined tunnels and masonry lined tunnels was analyzed using numerical methods with the PLAXIS 2D software and compared with results from analytical methods for circular section tunnels.
⦁ The tunnels were simulated isolated and together, and sensitivity analyses were performed in order to assess which parameters should be studied further.
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CONCLUSIONS
The main steps performed to assess tunnels stability were the study of the available information, literature review, modeling and analysis. The main conclusions are:
1 – Unlined Tunnels Stability
About 1/3 of the tunnels of Copan are kept unlined. These tunnels, typified as T1 (symmetrical) and T2 (asymmetrical by the presence of an ancient side wall) are expected to have been excavated in a geomaterial with a water retention curve in-between a coarse sand and a silt. The overall results indicate that, regardless of the depth or section type, unlined tunnels become unstable (i.e., the safety factor is close to 1.0) when the surrounding geomaterial reaches a saturation degree of about Sr = 80% ± 5%.
Since water infiltration may easily lead to such a degree of saturation, the conclusion is that unlined tunnels may suffer collapse mechanisms during the rainy season. Since the analyses reported in this thesis have been carried out on simplified 2D schemes, considering mechanical properties taken from the literature, the quantitative conclusions are unavoidably affected by a margin of uncertainty.
In order to make a conservative estimate of the maximum acceptable degree of saturation in the soil surrounding the unlined tunnels, a minimum acceptable safety factor of 1.5 was introduced. In such conditions, the limit value leading to tunnels collapse is Sr = 60% for tunnels T1 and Sr = 70% for tunnels T2.
2 – Lined Tunnels Stability
About 2/3 of Copan´s archaeological tunnels are lined with stone masonry in response to local failures or to avoid collapses.
Among all the models simulated, none reached safety factors below 6.0, and therefore none can be considered prone to collapse. This high safety level does not include properties reductions for the masonry and must be considered carefully.
3 – Sensitivity Analysis
The parametric analysis on the mechanical data of the lining indicates that variations in masonry compressive strength cause relevant changes in the safety, while masonry elastic modulus variations has weak effects on the results.
Therefore, and as the hydrostatic pressure has not been considered in the present study, it is possible to state that, according to the conditions established here, lined tunnels in the silty sand adopted as geomaterial do not fail when the soil becomes saturated and have adequate safety levels, as far as drainage is granted.
Collapse is governed mainly by the saturation level of the geomaterial, with failure at about 80% saturation.
The influence of adding new tunnels is much relevant: the original condition would allow a maximum saturation of slightly less than 70%, while the most conservative condition with a lateral tunnel addition would allow only a maximum saturation of 60%. The value for the ‘realistic’ section is 70%, which may be considered as reference in the current state of knowledge.
4 – Limit Safety Saturation Level
The assessments indicate that the unlined tunnels collapse when the surrounding geomaterial exceeds saturation levels of about 80%, while lined tunnels stay stable even if the geomaterial becomes fully saturated. However, considering all the uncertainties associated with the performed analyses, a limiting safety factor of 1.5 should be considered.
Therefore, the maximum allowed saturation level of Copan’s Acropolis underground at the current state of knowledge is between 60% and 70%. Conservatively, the lower value should be considered as the maximum degree of saturation to be accepted to have decent safety conditions in the tunnels.
Recommendations
⦁ 3D models would be of value to further understand the interactions between the different types of sections dug through the pyramidal structures of the Acropolis undergrounds.
⦁ A better 3D representation of the tunnels is highly recommend, including more information on the external masonry layer of the Late and Terminal Classic superficial temples.
⦁ Regarding the geomaterials, it is necessary to further investigate the different layers that comprise the Acropolis and their specific characteristics.
⦁ Regarding the several types of masonry used as lining, it is important to check the real geometry of the linings and of the excavated tunnel behind it.
⦁ Regarding material properties of the lining, the compressive strength and elastic modulus of the masonry impact the safety factors and should be assessed on site.
⦁ In the case of the geometrical features, non-destructive techniques such as boroscopic camera, impact-echo and/or georadar can be applied if the opening of an inspection “window” in the masonry is not feasible.
⦁ Regarding the material properties, investigations using techniques such as flat-jack (for compressive strength) and sonic testing (for the elastic modulus) are recommended.
