Machu Picchu: Construction of the Lost City of Incas

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Machu Picchu is the royal site of the Inca ruler Pachacuti and an incredible monument known for its estate and sanctity. The engineering techniques used in the construction of Machu Picchu, in conjunction with the natural environment, are remarkable in the history of civil engineering. The site was recently declared as one of the seven wonders of the world.

The construction of the monument started in 1450 A.D. and is a great example of the ancient engineering skills of the Inca people. Following the collapse of the Inca empire, the monument was abandoned after a century, during the Spanish conquest. In 1562 A.D., the monument was burnt down and what we see today are the remnants left of a great marvel of civil engineering. 

Although there are no well-written documents about the monument, the remnants show that the Inca people had developed advanced technologies related to hydrology, urban planning, hydraulics, foundation, and robust concretion methods.

The technical planning of the Machu Picchu monuments was the key to the site's permanency and functionality. The people of Inca used the construction techniques in such a way that the retreat was not reduced to ruins even after many years of abandonment. Despite the precarious locations of the monuments over the undulated terrain, with the use of basic hydrological principles, the Inca people turned the site into a royal estate.

One cannot fully realize the triumph that Machu Picchu symbolizes without first knowing the site constraints that the ancient civil engineers tackled. The engineering needs of planning and construction should be perceived in context with the geology and climate of the site. When these elements are observed as a whole, Machu Picchu becomes the Lost City of the Inca in all its glory.

1. Geology of the Machu Picchu Site 

Machu Picchu is located at 1400 km from the south of the equator on the east side of the Peruvian Andes. The monumental site is found near the mainstream of the Amazon River. The following points describe the major geological features of the Machu Picchu site:

1. Machu Picchu is located at an altitude of 500 m above the bottom of a valley. The valley was formed as a result of tectonic movements and due to downcutting by the Urubamba River. 
2. The Urubamba River flows along three sides of the Machu Picchu ridge top.
3. Machu Picchu is also surrounded by the ice-capped peaks of two mountains, Mt. Veronica to the east and Mt. Salcantay to the south.
4. A total of 40 km² area is occupied by the monuments of Machu Picchu between the high plateau and the Peruvian Andes.
5. The site of Machu Picchu is 250 million years old and consists of white to grey colored granite, characterized by the abundance of quartz, feldspar, and mica.
6. The main reason behind the durability of the monuments of Machu Picchu is the mineralogical constituents of the granite. The people of Inca masterfully utilized the rectangular joint pattern of the granite in building stones to construct the monuments that ensured the retreat's longevity.
7. The most important geological features of the Machu Picchu site are the abundant rock fractures and numerous faults.
8. Two primary faults are named for the two famous peaks, Machu Picchu fault and Huayna Picchu fault. These are the reverse faults formed due to sliding of the wedge-shaped structural block against the peaks on either side.
9. The structural blocks are acknowledged as Graben. The Inca people built their retreat on the Graben.
10. The location of the Machu Picchu fault influences the water springs that emerge nearby the site of the monument.      
11. Augmented permeability along the gradient segments of the fault system permits the precipitation to infiltrate and then arise at the spring site. Thus, it provided a regular water supply to the Incas.

2. Site Selection

Tourists describe Machu Picchu as a breathtaking site due to its extraordinary design and orientation. The three main factors believed to be behind the selection of the site by the Inca planners and engineers are: the roaring Urubamba River surrounding the site on three sides; the presence of Graben structure between two local faults generating two sharp peaks of the Machu Picchu, and Huayna Picchu Mountains. A rounded profile like a half-orange is visible from the Machu Picchu site, which represents the peak of Putucusi mountain across the Urubamba River.

Even today, all three peaks are believed to be holy mountains by locals. Further away, the triangle-shaped Yanantin mountain and the glacier-capped flanks of Veronica mountain are located at an elevation of 6,257 m.

Inca engineers had to find a solution for water supply before selecting the site for their royal estate, which was eventually devised by Pachacuti, a famous civil engineer of the Inca empire.

After the approval for the Machu Picchu site by the Inca's emperor, the engineers had to further examine the ridge top characteristics for building and construction purpose. The main characteristic features of the site that persuaded the Inca's emperor are as follows:

1. There was a reliable spring for all water supply solutions
2. The dense rainforests were the solutions for the irrigation uncertainty
3. There was an abundance of good and fractured rock for building purposes
4. The site provided great view of the mountains and the river

3. Engineering Planning

After the approval for the thickly forested site of Machu Picchu, the civil engineers separated and laid the agriculture area apart from the urban area. To protect the royal estate, the engineers constructed an outer wall by incorporating the natural topography. Also, an inner wall was constructed between the agricultural and urban areas. The inner wall had only one gate to serve the Inca trail from the capital city.          

The orientation of the main gate was decided in such a way that when the visitors passed through the gate, they would be mesmerized by a framed view of the holy Huayna Picchu Mountain.

The engineers used the practices of providing the big building with a large plaza in between the upper and lower sectors in the urban area. This required intensive site work, massive walls, and substantial filling to create flat building sites and a central plaza.

The royal residence and the major temples were located in the upper urban sector. The urban area was located at such a place that the water of the perennial spring on the north slope of the Machu Picchu Mountain could be carried by gravity to the center of the upper urban sector.

The canal was designed to discharge the water directly into the fountains, and the royal residence was constructed adjacent to the fountains. A series of 16 fountains paralleling a long stairway was continuing down the steep slope by serving both the upper and lower urban sectors with a domestic water supply.

4. Building Foundations and Stone Walls

Proven technologies and an utmost care were adopted by the Inca people for constructing the buildings, and that is one of the reasons Machu Picchu endured through several centuries. Much of the work done by Inca's engineers is not visible until excavated. The unseen Machu Picchu exists in its subsurface drainage system and the widespread foundations that lie beneath the buildings and terrace walls.

Archaeologists who excavated the Machu Picchu site concluded that 60% of its building efforts went into site preparation, foundations, and drainage.

After inspecting the steep colluvial slopes of the site, the civil engineers realized that the Incas had excellent knowledge of slope stability technology. Although the slope stability analysis of a colluvial slope is very complex and perplexing, the accomplishment of slope stability at Machu Picchu exhibits that the Inca engineers used the solutions common to today's geotechnical engineering.

4.1 Foundations

Construction of wall foundation started with the placement of small rocks at the bottom of the excavated portion to create a firm platform for the foundation. The rocks became larger as the foundation rose nearer the ground line. The thickness of the wall kept varying at different locations, but most of the structures had a 0.8 m thick wall. Some of Machu Picchu's monuments required a sturdier thickness, such as retaining wall at the east of the royal residence, which denoted the boundary of the west side of the urban sector and the beginning of the central plaza.

At some locations, the engineers of Inca provided a living granite rock or huge in situ rocks for their foundation base, which was first shaped to provide a ledge or platform for founding. Examples include the Sun temple and the Condor temple.

Before laying the foundation, the site of Machu Picchu was very irregular and contained large and small rocks, giant boulders, and pieces of granite covered with vegetation. The engineers levelled the site according to the footprint of a particular monument. Then they constructed a retaining wall to the south-east to denote the boundaries and defined the platform for the patio and of the enclosure.

In this space, the engineers used the combination of small and large transportable rocks for backfilling the area. After that, the construction of the foundation started by placing the base rock over flat rocks and stones. Later on, to obtain a uniform level in the interior of the patio, the engineers again did the backfilling with gravel. 

Over this layer, the engineers placed pulverized and fragmented granite of a natural origin, a product that is resulted from the working and polishing of the stones.

Further, engineers used the yellowish earth for floor finishing because of its materialist properties such as high impermeability and degree of compatibility. It was also placed in the interior of the enclosure with a defined gradient toward the outside. They also put it in the patio by providing a slope towards the drainage channels. Thus, in this manner, they kept rainwater out of the interior of the enclosures and prevented ponding in the patio.

The engineers built hundreds of ordinary terrace walls at Machu Picchu with different types of subsoil conditions ranging from rocky soil to decomposed granite.

4.2 Stone Walls

In wall construction, the building stones were alternately placed in the wall lengthwise and then crosswise for stability. A header stone was placed at intervals to tie the front and back wall faces together. The stone was about 0.8 m wide and extended from one face of the wall to the other.

Many of the stones had top or bottom indentations that helped fit them together in a nesting manner. Corner stones had such indentations to provide additional stability. The walls were laid with a batter of 4%.

A mortar consisting of clay and earth mixed with small stones was used in many of the walls. Once workers placed a course of stones over a layer of fresh mortar, they placed additional mortar in the center of the wall.

At Machu Picchu, at least 18 different types of stone walls are still visible. Patterns on these stone walls are the examples of the finest carving done by the Inca people. There are even examples of temporary stone walls used as an inclined plane for moving large blocks upward.

5. Hydraulic Works at Machu Picchu

The Sun temple at Machu Picchu was the urban focal point of the entire site. A solar observatory and religious center were also located near to the Sun temple with a series of 16 fountains. The functional and aesthetic look and the construction layout of the fountains make them a noteworthy example of planning and construction triumph of civil engineering.

All the 16 fountains were constructed in a series except the third fountain, which was constructed to allow the bypass flow so that the extra water can be diverted. Water for the fountains was taken from the natural spring, which was located on the north side of the Machu Picchu Mountain. For this purpose, a canal was constructed using the principle of gravity flow. The canal delivered the water to the first fountain, which was located at the top in the series of fountains. The use of hydrology and hydraulic engineering in ancient time highlights the awareness about engineering principles among the Incas.

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