Athens Acropolis: Geotechnical Features of the Bedrock of Western Civilization

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The Acropolis of Athens is the most striking and complete ancient Greek monumental complex extant in our times. It is an architectural treasure that belongs not only to the Greek patrimony but also to the worldwide cultural heritage. It is situated on a 157 m high hill that rises in the basin of Athens. Its overall dimensions are approximately 170 × 350 m. 

In the 5th century BC, the Athenians, empowered from their victory over the Persians, built a new circuit wall and planned an ambitious building program comprising a large number of monuments that transformed the rocky hill into a unique complex. In the 8th century BC, the Acropolis gradually acquired a religious character with the establishment of the cult of Athena, the city’s patron goddess. These monuments describe classical Greek thoughts and art. These monuments include:

1. The Parthenon: a temple of Doric order of particularly large proportions that dominates the summit of the Acropolis hill. 

2. The Propylaea: The monumental entrance building to the sanctuary, famous for its impressive coffered ceilings.

3. The temple of Athena Nike, a small temple of the ionic order, notable for its elegance and grace. 

These buildings were built with dry masonry consisting of white marble and characterized by ingenious planning and flawless construction. The foundations of these buildings were made up of limestone.

The monuments have survived for almost fifteen centuries through wars, explosions, bombardments, fires, earthquakes, and thefts. Extensive restorations have been carried out throughout the 19th and at the beginning of the 20th century that gave the Acropolis monuments the appearance, they have today. This article describes the geotechnical features of the Acropolis of Athens. 

1. Geology of Acropolis Hill 

The Acropolis hill is a trapezoidal-shaped block of grey limestone resting on the marls and sandstones of the Athens schist rock. The following points describe the geology of Acropolis hill: 

1. Between the limestone mass and the underlying bedrock, a thin layer of conglomerate is interjected. Locally, on the upper surface of the bedrock, conglomerate horizons are met. 
2. The bedrock is covered at places with an eluvial soil cover of small thickness coming from the weathering of the schist bedrock.
3. The grey limestone is well exposed on the top of the hill. It has closely spaced joints and some of the older fissures have been filled with red marl and coarse calcite crystals. 
4. The top of the hill has been leveled with artificial fill up to 17 m thick, which is retained by the circuit wall. 
5. The artificial fill that forms the plateau where the monuments stand has a varying composition. This fact is mainly attributed to the different historical stages of backfill construction.
6. The rock mass quality shows significant local differentiations. The summit and the inner part of the hill consist mainly of compact to thick platy limestone and show a low degree of weathering with sparse closed discontinuities and favorable tectonic characteristics. 
7. On the other hand, the slopes clearly show a higher degree of weathering (leading to systematically jointed rock mass), especially, in the case of the east and north slopes. Intensely fractured rock mass, the unfavorable orientation of the discontinuities, and increased action of environmental agents had led to the development of local instabilities, rock falling, and the formation of caves and clefts in the Acropolis hill. 
8. The discontinuities of the permeable limestone formation allow for regular drainage of the hill. The water from the rainfalls seeps into the faults and fractures in the upper part of the Acropolis. Therefore, springs form at the base of the hill because the underlying schist bedrock is practically impermeable.

2. Materials Used in the Construction of the Acropolis Monuments

Limestone from several adjacent hills was used for the construction of the superstructure of many of its buildings. Dolomite and marly limestone were used for the foundation of its monuments and the circuit wall. Marble was used in the interior of all the great buildings of the Acropolis. Besides these materials, limestone and conglomerate were also extensively used in the construction of walls, beams, columns, and roofs.

3. Foundation of Acropolis Monuments

A plethora of buildings, small or large, once stood on the Acropolis hill throughout its long history. Out of those, today, only three stand intact: the Parthenon, the Temple of Athena Nike, and the Propylaia. However, the foundations of other ancient structures are still clearly visible on the Acropolis hill. 

Most of the above buildings were founded directly on natural rock. However, in some cases, mainly due to the irregular geometry of the rock at the location selected for the erection of each monumental structure, typical foundations from limestones had to be constructed. The foundations details of the remaining monuments on the Acropolis are discussed below. 

3.1 Foundation Details of the Parthenon 

The Parthenon was constructed in the middle of the Acropolis hill. It stands on the top of the rock. Only three-quarters of the monument possesses foundations. The following points describe the foundation details of the Parthenon: 

1. Limestone ashlars were used for its foundation, which was quarried especially for its construction. 
2. The highest depth of foundation is 20 m in the south-east corner.  
3. Foundation was made up of 22 courses. The bottom course consists of a single row of stretchers and the second course consists of a single row of headers. The third course has four rows of stretchers, and the fourth course has two rows of headers. This sequence was followed again from the fifth course up to the 19th course.
4. The three top courses follow the usual principle. The third course from the top only has headers, the second course from the top only has stretchers, and the first course also has only headers.
5. The interior of the foundation is not accessible. However, most of the researchers believe that it is probably a complete block of stone. 

From a geotechnical design point of view, this type of foundation based upon the natural limestone rock allows for high values of bearing capacity and settlement from the action of the structural loads.

The foundation design creates a firm base for carrying the vertical superstructure loads safely. Additionally, the dry masonry structure assures a relatively flexible foundation base capable of absorbing high energy arising from the propagation of seismic waves. 

3.2 Foundation Details of the Propylaea

The Propylaea consists of a main building and two wings on the north-west and south-west sides. The south-west wing is following the ridge of the rock on which it stands. The north-west wing, called the Pinakotheke, is situated lower than the main building and standing on a high podium. The Pinakotheke consists of a rectangular room and a porch with three Doric columns facing south. The following points describe the foundation details of the Propylaea: 

1. The foundation of the north-west building was built with large limestone blocks onto the leveled rock, consisting of isolated walls with backfilling. 
2. On the western side, the foundation is part of the Acropolis circuit wall. However, it is perceived as the podium of the Pinakotheke.  
3. The ancient backfilling of the podium was fairly dense and prevented the structure from major failures. However, the demolition of the medieval structures and the intensive archaeological excavations that replaced the fill with loose earth has contributed to the accumulation of rainwater inside the foundation. Therefore, the chances of an increase in horizontal earth pressure have increased. 
4. On the south side, the visible part of the building is a luxurious site and it represents the main portion of the Pinakotheke. The outer foundation blocks of this portion are made of marble and create a special form of a wall. This wall was formed in the pedestal shape on the front side and filled with large blocks on the backside. 

Today, the Pinakotheke suffers from structural deformations with 3 cm of general settlement and small horizontal deformation observed in the North Wall. Also, a general settlement of 3 to 7 cm, outward displacement, barreling, and inclination of the west wall has been observed. 

In 1955, to secure the western part of the north-west wing, a concrete floor was constructed. The excavation fill was removed and wells filled with reinforced concrete were created. Recently the floor was repaired with carbon fibers reinforcement.

3.3 Foundation Details of the Athena Nike Temple

The temple of Athena Nike lies at the south-west edge of the Acropolis. The core of the temple foundation is a strong tower of a wall. The tower wall was constructed with masonry of limestones and its internal part was filled with soil. The temple was demolished serval times and its members were used to create a defense wall around the Acropolis. It has been restored three times to date (1835–1844, 1935–1940, 2003–2010). During the recent restoration, flooring was installed beneath the temple in the form of a stainless steel grid over the basement space.

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