Tunnel engineering in the Iron Age: geoarchaeology of the Siloam Tunnel, Jerusalem

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Abstract

The Siloam Tunnel (ST) is the best-identified biblical structure that can be entered today. We use geological, structural, and chemical features of ST and its internal deposits to show that it is an authentic engineering project, without any pre-existing natural conduit that could have guided its excavators. Radiometrically and historically dated to ∼700 BCE, ST pinpoints the technological advance in leveling techniques that was essential for the construction of such a long tunnel without intermediate shafts. A combination of geological and archaeological evidence demonstrates that the circuitous route of ST and the final meeting of the two excavating teams are associated with continuous modifications of the plan to allow acoustic communication between hewers and the surface teams. Hydraulic plaster was applied throughout the tunnel in order to seal voids of dissolution and tectonic origin. Organic material accidentally entrapped in the plaster was carbon 14 dated, and speleothems were dated by U–Th, both corroborating the historic and epigraphic evidence ascribing the engineering advance in tunneling techniques to the Judahite King Hezekiah.

Introduction

Among ancient techniques, constructing a tunnel between two distant points involves mastering several sophisticated fields, including engineering, architecture, geodesy, hydraulics, and geology. Understanding the methods of construction of ancient tunnels may shed light on early stages of these sciences, since they must have been developed through practical needs. As no ancient text describes the technical principles of tunneling prior to the Roman Period, such a study becomes naturally field-oriented. ST in Ancient Jerusalem (Fig. 1) is unique in the availability of a combination of records associated with its construction: biblical narratives which describe the purpose of the project, backed by Assyrian documentation of the campaign of Sennacherib against Judah; the Siloam Inscription, discovered in the tunnel, depicting the dramatic encounter of two excavating teams; and finally physical and geo-archaeological field evidence [19], [24], [37], [42], [51].

ST represents a major advance in tunneling techniques, being a long (533 m) tunnel without man-made intermediate shafts present. Earlier Assyrian tunnels were constructed in short segments between successive intermediate shafts, spaced a few tens of meters apart [9], [43], [46]. Reducing long tunnels into short segments allowed easy underground connection and ventilation, thus avoiding the need for sophisticated tunneling techniques. In spite of several attempts to understand the tunneling techniques implemented in ST, several major enigmas regarding the tunnel have, until the present, not been resolved: (1) why did the two tunneling teams follow such a twisting circuitous route (533 m) rather than the more obvious straight line course (320 m)? (2) How did two teams following such a complex route manage to meet one another? (3) What is the meaning of ‘zdh’ in the Siloam Inscription and what is its significance for understanding the meeting method?

One of the most interesting, and widely accepted solutions offered in the past is termed here ‘the karst hypothesis’ (KH). It was originally proposed by Sully, an English architect who never visited ST, who wrote: “Since those who have inspected the tunnel mention a cleft in the rock at the point where the excavators met, it seems to me that this cleft is the explanation of the course taken by the excavators in forming this tunnel. Probably the cleft extended from the Virgin's Fountain (Gihon Spring) to the Pool of Siloam, and a small quantity of water would at times trickle through” [53]. This rationale appealed to many others [1], [2], [3], [4], [26], [27], [30], [31] because it seemed to solve the above-mentioned enigmas with one simple theory. In a well-known publication in the journal ‘Science’ Gill [27] expanded KH and concluded that ST (as well as other waterworks beneath ancient Jerusalem) ‘was fashioned essentially by skillful enlargement of natural (karstic) dissolution channels’. If true, KH implies that ST is not a special technical achievement, but rather an elegant adaptation of a natural feature.

Subsequent observations have questioned KH [22], [44], [51], but did not present a comprehensive study, as pointed out in a recent review: “A comprehensive explanation of the way the tunnel was planned and cut is still a desideratum” [44]. In the present study we focus our research where scientific techniques permit a detailed analysis of the technical background of ST. In a preliminary study [23] we used radiometric dating of natural (flowstone) and man-made (plaster) materials in ST to constrain the time of its construction, and redefine its relation with the biblical narrative and the Siloam Inscription. Here we use geological techniques to study the plasters and sediments deposited within the tunnel, in addition to its structural geology, morphology, and hewing anomalies, in order to clarify its natural vs. man-made features. Finally, we compare the collected data with evidence from natural karst features from Jerusalem and vicinity in order to test KH. An integration of the new data enables us to elucidate the nature and significance of the technological advance manifested by ST.

ST was excavated within limestones of the Bi'na Formation of Turonian age [26]. In Jerusalem, the formation is generally thickly bedded, gently dipping toward the SE and frequently karstified, as exemplified in the immediate area of the Gihon Spring itself. The spring used to ebb and flow in the past – a property characteristic of karst springs. ST was hewn in the upper part of the Mizzi Ahmar unit, which is massive to well bedded, stromatolithic and frequently pinkish in color.

We have performed a comprehensive study of karst caves in and around Jerusalem, comprising over 1000 caves in the Bi'na formation [14], [18], [20], [21]. The caves can be classified into three groups: (1) vadose shafts; (2) maze caves; and (3) chamber caves. The vadose shafts formed in the unsaturated zone; they resemble vertical cylinders in shape, tapering upward into a fissure. While active, such shafts act as vertical conduits carrying vadose water down, towards the regional watertable. In the studied region, individual shafts are commonly some tens of meters deep.

Maze and chamber caves in the area formed below the regional watertable, under confined and unconfined conditions, respectively, and have smooth curvilinear walls typical of slow-moving water [21], [36], [52]. A few of these caves reach a total length of several hundred meters up to several kilometers. The maze caves are networks of interconnected narrow passages, most of which developed preferentially along the intersection of a bedding plane and a vertical fracture. These structural discontinuities are consistently observed along the walls and ceilings of cave passages. Chamber caves consist of a void whose width dimension is close to its length. They also form along structural discontinuities, which are observed along their walls. Structural discontinuities are essential for the incipient stage of cave formation in compact limestones, as the water can initially flow only along such hydraulically conductive fissures [33], [39]. Such structural discontinuities appear along all karst passages in the studied region.

Section snippets

Methods

With the above evidence in mind, a systematic study of large and small morphological features including joints, fractures and voids within and near ST was carried out. All features were carefully surveyed in relation to the three-dimensional morphology of ST and the surface topography (Fig. 1, Fig. 2, Fig. 3), using a Brunton compass and Disto electronic distance measuring device.

We drilled the sedimentary and man-made materials within ST, and studied the core samples utilizing standard

Geological structure

We found several mostly small, isolated karst voids, mainly at the southern segments of ST, but no evidence of a continuous karst conduit anywhere along the tunnel. A vadose shaft forms the vertical segment of the Warren Shaft system close to the start of ST (Fig. 1) and a similar vertical void, with smooth walls, projects almost above the Gihon Spring. Four small shafts or kamenitzas [17] are exposed on the rock surface almost directly above ST (Fig. 1, Fig. 2, Fig. 3), serving as local drain

The central portion of ST

The Siloam Inscription and field evidence in the central portion of ST contain the most important indications pertaining to the meeting of the two teams and the way it was achieved; consequently we start the detailed discussion in this segment. The central portion of ST is defined here as the segment between 30 m upstream (northward) and downstream (southward) of the meeting point. The hewing marks, deviations and false starts clearly indicate the meeting point of the two teams [55], leaving no

ST in the context of tunneling and measurement techniques in the ancient world

The origin of tunneling techniques is linked to ancient mining. This knowledge had been applied for transporting water underground and bringing it to the surface using long tunnels at Urartu and Mesopotamia [24], [34]. The ancient Qanat technique, still widely used in Iran, involves excavating a slightly-inclined tunnel branching out from the bottom of a shaft and meeting at about half way with excavators from neighboring shafts. The method was feasible because the small distance between

Conclusion

During most of the 20th century many puzzling aspects of ST have been attributed to natural geological (karst) processes. We have demonstrated that ST must have been engineered and hewn by man without a pre-existing natural conduit, using two lines of evidence: (a) the natural features of ST – the lack of any continuous fissure or remnant of a karst passage, and the natural sediments covering the ancient plaster, and (b) the artificial features – the plaster covering bare bedrock, and the false

Acknowledgements

The Israel water commission and the National Center for Cooperation Between Science and Archaeology provided financial assistance. The team of the Israel Cave Research Unit at the Hebrew University assisted in the field work. Shlomo Ashkenazi and Yehuda Peled of the Geological Survey of Israel carried out the drilling program. Radiocarbon dating was carried out at the Research Laboratory for Archaeology and History of Art, Oxford University, and 230Th/234U dating at the Open University, Milton

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