a survey of ancient geotechnical engineering techniques in

SAHC2014 – 9th International Conference on
Structural Analysis of Historical Constructions
F. Peña & M. Chávez (eds.)
Mexico City, Mexico, 14–17 October 2014
A SURVEY OF ANCIENT GEOTECHNICAL ENGINEERING
TECHNIQUES IN SUBFOUNDATION PREPARATION
B. Carpani1
1
ENEA
Brasimone Research Centre, Camugnano (Bologna), Italy
e-mail: bruno.carpani@enea.it
Keywords: ancient foundation techniques, Greeks temples, antiseismic construction techniques
Abstract. The paper presents an overview of a particular type of construction techniques relating with foundation engineering in antiquity: the subfoundation preparation practices employed in many ancient monuments for dealing with hydrogeological and seismic hazards.
The purpose is to illustrate and discuss a number of technological achievements in the ancient
world and provide a significant insight into the geotechnical skills of ancient architects and
builders. The case studies here presented aim to give evidence that, under certain cultural
and environmental conditions, ancient societies acquired an awareness of the risks linked to
the earth phenomena that led to the development of geotechnical engineering techniques, sophisticated even by modern standards. Several examples show a clear understanding of some
basic principles of foundation engineering, such as the need to keep the soil moisture content
under control and to assure an uniformly distribution of the building loads. This survey is focused on Greek and Near East cultures ranging from Bronze Age to classical times and extending over an area from southern Italy to ancient Mesopotamia. The analysis includes some
of the most imposing monuments of antiquity, such as the fortification walls of Troy and the
colossal archaic temple of Artemis at Ephesus – known in antiquity as one of the Seven Wonders of the World - besides examples of remarkable foundation systems from lesser known
archaeological sites like the Greek colonies on the north coast of the Black Sea.
B. Carpani
1
INTRODUCTION
The laying of stable foundations represented one of the most serious problems for ancient
builders. Since the early stages of urban civilization in Neolithic times, ancient cultures have
struggled to develop building technologies to give strength and solidity to their constructions.
In line with the evolution of increasingly complex urban systems, religious and government
buildings gained more and more in importance and size. Not infrequently, these imposing
structures were sited in highly seismic areas and founded on soils of poor load-bearing capacity. Despite this, as shown by the stratigraphy of many archaeological deposits, damage and
failures due to natural hazards like earthquakes and subsidence did not induce the communities to abandon those sites, which were places often charged with symbolic and sacred value.
So, ancient builders had to face difficult building problems and in their effort to cope with it,
developed an awareness of the risks associated with bad soil conditions and seismic phenomena that led to advanced geotechnical techniques. This view is supported by the most of the
examples chosen, which show ingenious technical solutions to improve the bearing capacity
of the underlying soil by both providing uniform loads distribution and maintaining appropriate moisture content at the construction base.
Focusing on sub-foundation preparation techniques, this paper presents an overview of related practices that provides clear evidence that the interaction between soil and foundation
was carefully considered, and that past builders and architects achieved a good understanding
of geotechnical problems, displaying a remarkable capacity for innovation.
Considering that many of the foundation features here illustrated recall modern antiseismic technologies, in particular base isolation, such distinctive techniques shall be discussed from a seismic perspective, trying to evaluate their effectiveness in improving earthquake performance.
Regarding the historical-geographical coordinates, the collection of case studies here presented, which makes no pretend to being exhaustive, range from the ancient Greek world to
the Near East, covering a period of time spanning from the middle Bronze Age to the end of
classical times.
2
BRONZE AGE IN ANATOLIA
The Bronze Age architecture in Anatolia is characterized by the extensive use of timber
frames to strengthening building structures. Since the early Bronze Age (BA), stone or mudbrick walls were reinforced by inserting at regular intervals rows of runner-beams held in position by cross-ties. During Middle BA, the system evolved through vertical posts extending
from the foundations to the roof and thus creating a three-dimensional timber framework in
the structure of the building (figure 1). It is generally accepted that a not secondary purpose of
this constructional practice, which continued to be used until the present day, were to give additional elasticity to the structure so as to improve the seismic resistance of the building [1].
This view is supported by experience of past earthquakes, which shows a good structural performance of this type of building.
2.1
The palaces of Açemhöyük and Beycesultan
The excavations of the settlement mounds of Acemhöyük and Beycesultan (figure 2) uncovered occupation levels stretching back to the 4th-5th millennium BC, revealing the extensive remains of high developed cities. Both the cities reached a peak of prosperity during the
first half of the 2nd millennium, a period dominated by massive palatial buildings.
Besides extensive use of wood, the foundation system shows special provision for improving stability. In several parts of the Beycesultan palace, a distinctive feature is the bedding of
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Ancient Geotechnical Techniques in Subfoundation Preparation
timber logs laid transversely to the direction of the wall, upon which the first stones are laid
(figure 3) [1]. At first, it was thought that such substructure was unique in its kind, but this
view was soon contradicted by the excavations of the palace of Acemhöyük, where a very
similar foundations system was found. However, compared to Beycesultan, the Acemhöyük
subfoundations show an interesting difference. While in the former case the logs are laid directly on the ground, in the latter they rest on a layer of large, projecting limestone base-slabs
often four meters wide (figure 4) [2]. The real purpose of this arrangement deserves further
investigation, but the first impression brings to mind antiseismic systems based on rolling devices. On the contrary, there is no doubt about the exact date of construction of the
Acemhöyük structure. Thanks to the well-preserved timbers (bark has been preserved), dendrochronological analysis has shown that the trunks were all cut the same year, 1774 BC [3].
Figure 1. Reconstructions of wall structures from Beycesultan (Lloyd 1965)
Figure 2. Map of ancient Anatolia
Figure 3. Palace of Beycesultan: timber logs under wall foundations (Lloyd 1965)
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B. Carpani
Figure 4. Acemhöyük Palace: timber logs laid on stone slabs; they were all cut in 1774 BC. (Özgüç 1966)
2.2
The great walls of Troy
The fortification walls that can be still admired, belong to the level VI of the settlement
(1700-1300 BC). They were built of great stones hewn square, tightly fitted together through
all the thickness (4.5 m at least). An interesting feature of this structure is that its foundations
do not reach the bedrock. According to excavations report, the ancient builders deliberately
left a layer of hard-packed earth (ranging from 20 to 120 cm) between the bedrock and the
foundation base (figure 5). This subfoundation preparation has been interpreted by the archaeologists as an antiseismic device, “a cuschion of earth” acting like a simple “shock absorber” [4]. For a critical discussion of the subject see [5, 6].
Figure 5. A view of Troy wall (left, Dörpfeld 1902) and a section showing on the right the layer of earth between foundation and bedrock (Blegen 1953)
3
3.1
SUMER
The Temple Oval at Khafājah
Khafājah is an archaeological site in the lower Mesopotamia, a few kilometers east to Bagdad, where the remains of the Sumerian city of Tutub were brought to light in the early 1930s.
The most impressive discover was a great religious structure of elliptical shape (figure 6)
named Temple Oval and dated to the Early Dynastic II period (2750-2600 BC). The excavations have revealed that the Temple foundations rest on a gigantic subfoundation that consists
of a bed of sand at least 8 m thick (figure 7) [7]. Deep soundings showed that a natural deposit
is out of question, since at no point outside the Oval was sand found, and that the whole space
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Ancient Geotechnical Techniques in Subfoundation Preparation
occupied by the sand bed was excavated removing existing buildings and graves. Considering
that the Oval covers an area of about 8,000 m2, some 64,000 m3 of soil had to be excavated
and carried away to prepare the huge cavity, which was then filled with the same amount of
sand. Furthermore, it has been ascertained that the sand was quite pure, with no traces of organic materials, and therefore it must have been brought from outside the city, very probably
from the bank of the Diyala river (about 11 km away) and presumably filtered to removed
impurities. Once the filling was completed, the surface was carefully levelled and upon the
sand, walls foundations in sun-dried bricks were built to a height of 1.20-1.40 m. Over the
sand, the space between the footings was then filled with clay, which was packed down and
tamped to form a thick and hard mass in which foundations were embedded. To complete the
artificial platform, on the top of the clay a layer of bricks was finally built.
Figure 6. Temple Oval: reconstruction (left) and view of foundation ring (Delougaz 1940)
What was the purpose of such an enormous amount of labor is a matter of debate among
specialists. The excavators rejected the suggestion that the layer of sand would have drained a
marshy ground, in favour of an explanation that takes religious meaning, in particular ritual
purification, into account. On the other hand, it is indisputable that a sand foundation, when
effectively retained, has many advantages like assuring both even load distribution and good
volumetric stability in case of changing in moisture content. Moreover, under certain conditions, a sand layer could mitigate the effect of seismic shocks. But whatever the reason it may
has been, the archaeological evidence shows that the building process of the complex was carried out according to a detailed plan and using advanced methods, including a well-developed
knowledge of vaulting techniques [7]. However, as we see in the next paragraph, a misunderstanding about the actual role played by sand bedding may pose serious threat to the monument stability.
Figure 7. Temple Oval: a section showing the enormous subfoundation (left), and a detail of sand bedding b
under foundations a (Delougaz 1940).
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4
EGYPT
The laying of sand bedding under foundations masonry was a fairly common constructive
method in ancient Egypt, especially for buildings sited inside the alluvial flood plain of the
Nile Valley. An interesting example of such subfoundation technique at its well-developed
stage is represented by a colossal sacred building, dated to the middle of 6th c. BC, discovered
in the ancient delta city of Mendes. The excavations show that the shrine was built on a depression 6 m deep where the foundations, consisting of a massive stone platform, rest on a
layer of sand 2,30 m. thick, held in place by mud brick retaining-wall. Such a preparation, defined sand-box foundation [8], acts as a floating foundation and essentially meets two basic
needs. First, in the absence of bedrock, it provides an equal distribution of the bearing pressure on the alluvial soil, and second, it avoids settlement, being the sand a good draining material that is substantially immune from the destabilizing action of the yearly inundation of the
Nile. From this perspective, the use of pure and clean sand, instead of sandy gravel or silty
sand, acquires a geotechnical meaning, avoiding fines content transport over the fill due to
water flow, which could lead to differential settlement. It should be noted that the above considerations are valid only if the layer of sand lies in a effectively confined space and, in fact,
when during the Mendes excavations breaches in the perimeter walls were made, the resulting
alteration of sand constraint caused a small but significant settlement of about 1 cm [8]. Far
more serious consequences occurred for the Great Hypostyle Hall at Karnak when, in 1899, a
trench was dug to drain water from the foundations [9]: the sand on which the plinth foundations rested, lost its bearing capacity causing the collapse of eleven columns (figure 8). More
recently, in 1989, a similar disregard for the stabilizing function of confined sand, involved
another famous monument, the Amenhotep Temple at Luxor. Following the fortuitous discovery of a statues cache under the stylobate, a trench was excavated cutting the sand resting
on the alluvial soil. Shortly after, the perimeter columns began to leaning dangerously and to
escape from a disastrous collapse, they had to be dismantled and re-erected on concrete foundations [8].
Figure 8. Great Hypostyle Hall at Karnak: collapsed columns (Legrain 1900)
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Ancient Geotechnical Techniques in Subfoundation Preparation
5
5.1
GREEK FOUNDATION ENGINEERING
The Temples of Paestum and Metapontum
“Why in the Greek colonies of Italy, from Metapontum to Paestum, did the sixth century
BC builders dig a trench in the rock on which their temples were erected, and filled it with
sand, used this strange pillow like a device for supporting their foundations?”.
With these words prof. Antonino Giuffrè wondered about what purposes such foundation
practice would serve for. Giuffrè, until his death in 1997 the leading Italian expert in seismic
analysis of heritage buildings, went on observing that the sand bedding recalls modern antiseismic systems based on the decoupling of the foundations from the ground [10].
A well investigated example is the Athenaion of Paestum, a marvelous Doric temple built
at the end of the 6th c. BC. To lay the foundations under the colonnades and the cella walls,
deep digging were driven down to reach the travertine bedrock, in which trenches were cut
and filled with a sand bedding 0,50 m thick. Upon the sand, large travertine slabs of 1,85-2,35
m width were laid down to forming continuous footing foundations, which were then
strengthened by means of transversal binding walls (figure 9). A modern geotechnical analysis clearly stated that the foundation system was very well conceived, showing a high safety
factor toward undrained ground rupture, even in case of seismic load [11].
As above mentioned, this foundation technique was not only systematically used in Paestum (e.g., the archaic Temple of Hera or Basilica, 550-540 BC), but was also developed over
a wider area, including Metapontum, where the most ancient applications were found (e.g.,
the so-called Temple AI, 570-560 BC). The fact that both cities were Achaeans colonies, suggests this foundation practice were possibly imported from the homeland.
Figure 9. Athenaion of Paestum: section showing the foundations layout with sand bedding (Pescatore 1991)
5.2
The Temple of Artemis at Ephesus
“The most wonderful monument of Grecian magnificence, and one that merits our genuine
admiration, is the Temple of Diana at Ephesus, which took one hundred and twenty years in
building, a work in which all Asia joined. A marshy soil was selected for its site, in order that
it might not suffer from earthquakes, or the chasms which they produce. On the other hand, to
avoid that the foundations of so huge mass rest upon a loose and shifting bed, layers of trodden charcoal were placed beneath, with fleeces and wool upon the top of them.”
This passage from the Naturalis Historia by Pliny the Elder (36.95), describing one of the
Seven Wonders of the World, is of special importance to all which are involved in seismic
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B. Carpani
researches, because it is the only ancient source where it is reported explicitly that precautions
against earthquakes were taken in antiquity. In particular, some see in that layers of charcoal
and wool an early intuition of seismic base isolation system.
Actually, the scarce and fragmentary archaeological evidence (figure 10) does not enable
us to ascertain the truthfulness of each aspect reported by Pliny, but nevertheless it shows indeed very ingenious constructive methods, perfectly adequate to the adverse soil conditions.
The author has been carrying out an in-depth study on the subject [6], what follows is therefore only a brief account of some salient facts.
During the course of the 6th c. BC, the Greek colonies on Asia Minor’s western coast developed social, economic and political conditions that enabled the major religious centers to
embark upon the erection of colossal temples on a scale never before attempted.
The building of the Artemision began around 550-540 BC over a sacred area at the mouth
of the ancient Kraistos River. The sediments transported and the frequent floods resulted in a
marshy alluvial soil that posed seemingly insurmountable technical problems to the ancient
architects [12]. Until then, the main methods of laying foundations were based on spread or
continuous footings beneath the bearing elements (colonnades and walls), but, for the first
time in Greek architecture, an huge stone platform 112 m long, 57 m wide and around 1,15 m
thick was built [13], a veritable mat foundation indeed. According to a reconstructive hypothesis of the temple superstructure, each column transfers to the foundations a load of at least
100 ton [14]; that means a pressure on soil not greater than 1 kg/cm2, a value that should not
exceed the allowable bearing capacity of the soil.
But what about the charcoal and fleeces bedding mentioned by Pliny? The excavations
have revealed that the foundation platform actually rest on a bedding of white clay 0.10-0.20
m thick [15]. The clay was carefully spread and levelled all over the laying base to provide an
even surface to the foundation slabs, and in some cases it was mixed with burnt material.
However, also layers of ash and charcoal have been detected in lower level of the archaeological stratification (figure 11). It is worth noticing that both clay and charcoal are effective materials for waterproof layers against rising dump, thanks to their characteristic of
impermeability and adsorbency respectively. At the time, this property of charcoal was well
known, as emerges from the passage of Diogene Laertius (Lives II.103) where it is reported
that the advice of put charcoal under the Artemision foundations came from Theodoros of
Samos, the chief architect of the other great temple on the Ionian coast, the Heraion of Samos,
the construction of which had begun a few years before the Artemision (560-550 BC).
Considering that the Ephesian had to struggle against geotechnical problems over the centuries, it may be ask why they did not move the temple to a better site. Apart the issue regarding ritual landscapes and sacred geographies [6], the Pliny’s answer is that they thought that
alluvial soils were immune from earthquakes. This belief might has two main explanations:
empirical observation of the earthquakes effects (in rare cases buildings on alluvial deposit
showed minor damage than those built on rock, e.g., 1986 Kalamata earthquake) and/or theoretical elaboration about what causes earthquakes. Many Greek philosophers observed earthquakes effects and tried to give a rational explanation to the phenomenon. One of the main
causes was related to the existence of great caves beneath the ground; such caves, when hit
with force by “winds” (the Aristotelian pneuma) or other subterranean forces, induce earth
tremors and eventually collapse causing major earthquakes. In this view, it is reasonable to
infer that sedimentary soils, being free from cavities, are aseismic (for these reasons Egypt
was believed immune to earthquakes).
Finally, some words about the subfoundation bedding as an early effort at seismic base isolation. As the ancient sources clearly show, layers under foundations did not deal with seismic
precautions, but with the need to create a waterproof course. Actually, the cited antiseimic
8
Ancient Geotechnical Techniques in Subfoundation Preparation
device seems rather pertains to what today is called microzonation (defined on the USGS
website as “the identification of separate individual areas having different potentials for hazardous earthquake effects”). Nevertheless, the structural conception of the temple fits well
several present-day seismic engineering criteria, where the clay bed is a far better solution
than the layer of fleeces of wool. It should be considered, however, that it is no easy to assess
all the purposes behind a specific building technique out of the synthetic view characterizing
the ancient technical knowledge. Interpreted thus, the foundation system of the Artemision,
with its massive platform resting on a sliding surface, fits well as precursor of seismic base
isolation, but it corresponds even more with Frank Lloyd Wright definition of his Imperial
Hotel at Tokyo as a building designed to float on the mud "like a battleship floats on water" in
case of earthquake.
Figure 10. Artemision: early excavations with the remains of the foundation platform (Hogarth 1908)
Figure 11. Artemision: section showing charcoal layer under cella foundations (Bammer 1984)
5.3
The Temple of Hera at Samos
The temple of Hera on the Ionian Greek colony of Samos was one of the most important
sanctuaries of the Greek world. It shared with his rival in Ephesus enormous dimensions and
geotechnical problems due to marshy soil. The construction of a first great dipteral temple began around 575 BC under the direction of the Samian architects Rhoikos and Theodoros and
was probably finished in 550 BC. As previously mentioned, Theodoros had recommended to
place a layer of charcoal under the foundations of the Artemision of Ephesus. He was renowned as a genius of its time and went down in history as a foundations expert, so it is ironic
that the temple named after him ended in a total failure shortly after its completion because of
the inadequacy of its foundations [16]. In fact, considering the weight of columns 12 m high
plus entablature, the pressure on the soil would be around 2 kg/cm2, a value that exceeds
twice the allowable bearing capacity of alluvial soil [17]. Around 540-530 BC the temple was
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B. Carpani
dismantled and replaced about 40 m further west. This second Dipteros, of even larger dimensions, was never finished, but the foundation remains witness the impressive effort made to
improve the stability of the structure. The massive strip footings consist of eleven courses of
limestone slabs for a total height of 2.50 m and 4 m in width at the bottom; this foundation
rests on a 20 cm layer of gravel that cover a trench 1 m in depth filled with pure sea sand (figures 16-17) [17].
Figure 16. Cross-section through the foundation of the second temple (Kienast 1991)
Figure 17. Comparison between the foundations of the first (left) and second temple (right) (Kienast 1991)
It is not clear whether the foundations failure was due to a gradual subsidence process or to
a sudden catastrophe and, in fact, there is not a general consensus amongst the archaeologists
about a possibly seismic cause. It has been argued that, since no traces of fire damage nor any
debris or rubble that might be interpreted as sign of a sudden collapse were found, an earthquake could not have been the cause of the disaster because it would have been destroyed the
superstructure, but hardly the substructure [16]. Actually, it seems that, in presence of alluvial
deposit of saturated sands, earthquake-induced soil liquefaction may be a more than plausible
explanation.
5.4
The Temple of Apollo Epicurius at Bassae
This famous temple was built around the middle 5th c. BC in a remote area of the Arcadia
mountains (present-day northeastern Messenia) at 1.130 m above sea level. Also thanks its
lonely position, the temple structure has for the most part survived. Considering the high
seismicity of the site, the temple structural integrity witnesses the ability of its builders to de10
Ancient Geotechnical Techniques in Subfoundation Preparation
sign and construct a de facto earthquake-proof structure. Moreover, the temple is located on a
sloping ground of bad geotechnical quality, a much weakened folded rock with a low bearing
capacity. The foundations were built with a mixed system where a sort of mat foundation consisting in thick layers of limestone slabs and boulders, and gravelly soil contained by retaining
walls, isolates the platform from bedrock. Within this mass the spread foundations of the columns were placed (figure 18). Furthermore, a layer of clay on the bedrock was found under
the east platform, while in others places, the bedrock fractures were filled with the same clay
material. According to the archaeological report [18], much of temple’s successful performance is ascribable to the quality of these foundations, which, besides assuring optimal pressure distribution and good draining, also incorporate a seismic base isolation design.
Professor J. M. Kelly, who was asked to review the foundation deposit, is of the opinion that
such a hypothesis is not implausible [18].
Figure 18. Bassae: North-South Section of the temple foundations (Cooper IV, 1992)
5.5
The Temple of Athena at Ilion (Troy settlement)
Another temple founded on an artificial layer of sand is the temple of Athena in the Hellenistic city of Ilion, on the top of Troy mound settlement. The building work began at the middle 3rd c. BC. The difficulty of reaching a solid base in the deep artificial deposit, brought the
ancient builder to develop massive foundations consisting of strip footings 5,40 m high resting on a 3,70 m high sand bed (figure 19) [19].
Figure 19. Ilion, Temple of Athena: section of the foundation and view of remains (Dörpfeld 1902)
5.6
Greek colonies on the Black Sea
An amazing example of technical skill and capacity for developing technical innovation
comes from the Greek colonies on the north shore of the Black Sea. In particular, the excavations at the site of ancient Olbia in present-day Ukraine, have brought to light the remains of a
very impressive and original foundations practice [20 - 21]. Olbia was founded by settlers
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B. Carpani
from the Ionian city of Miletus in the late 7th c. BC, over an area with peculiar geologic features. In fact, stone deposits are quite absent on the Olbia territory, while loess deposits are
very common. Loess is known for developing into very fertile soils (a factor that probably
influenced site’s choice) but also to be a very problematic foundation soil. The loess cohesion
is mainly due to clay coating over particles of silt or similar size grains. When it gets saturated,
the binding strength that holds together the soil particles is lost, leading to soil collapse. Thus
to retain stability loess must be in unsaturated condition. To cope with the collapsible nature
of this typology of soil, very ingenious mitigation measures were taken to ensuring the stabilization of moisture content and improving the structural strength of the foundations. By the
early of 4th c. BC, it was developed a new method for building foundations that consists of
alternate layers of soaked and tamped loess, and tamped ash and charcoal (figure 20). Once
soaked and tamped, loess turned into a solid block while ash and coal worked as waterproof
courses; moreover, the continuous foundations were also provided with a drainage system
[22]. All the temples, the agora buildings, the defensive walls and so on, were built upon these
foundations, which proved to be extraordinarily solid. During the excavations of the colossal
foundations of the city walls (5 m thick) in the early 1900’, the attempt to cut through it by
means of spades failed due to its strength, very similar to that of stone [23].
Figure 20. Olbia foundations: temple of Zeus on upper left, Great Stoa on lower left (Wasowicz 1975) and a
public building (Belin de Ballu 1972)
6
CONCLUSION
There is clear evidence that ancient cultures developed an awareness of geotechnical problems and related risks. This view is supported by the findings from many archaeological sites,
which show that the need to build monumental buildings on unfavourable foundation soils led
– not without failures – to the development of ingenious practices, displaying both good understanding of geotechnics basics and capacity of innovation.
The analysis also reveals that most of the techniques and arrangements here described are
consistent with present-day technical knowledge. Soil improvement techniques, for example,
12
Ancient Geotechnical Techniques in Subfoundation Preparation
were widely used to control moisture content at the construction base and improve the bearing
capacity of soils and foundation materials. The lying of artificial layers under foundations like
sand or clay bedding reminds modern anti-seismic techniques, in particular base isolation.
Regarding their effectiveness to improve building performance towards earthquakes, it should
be noted that modern researches on low-technology base-isolation techniques based on sliding
substructure, such as friction base-isolation or soil liquefaction base-isolation [24-25-26], confirm the soundness of most of the ancient techniques above described, as the use of clay and
confined pure sand layers.
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