Machu Picchu: Prehistoric Public Works

Kenneth R. Wright, P.E.
Consulting Engineer
Wright Water Engineers, Inc.
Denver, Colorado
Presenter, 2003 APWA Congress

Machu Picchu, the royal estate of the Inca Ruler Pachacuti, is a breathtaking monument to the ancient engineering skills of the Inca people. Machu Picchu was established in AD 1450. It was finally abandoned in AD 1572. However, it likely ceased normal operation by AD 1540 due to the collapse of the Inca Empire.

Although the Inca did not have a written language, the well-preserved remains of Machu Picchu show that they had an advanced understanding of such principles as urban planning, hydrology, hydraulics, drainage and durable construction methods. By studying the Inca's engineering techniques, in conjunction with the natural environment, we are able to supplement existing archeological theory on the Inca's practices, religion and the significance of Machu Picchu.

The technical planning of Machu Picchu is surely the key to the site's longevity and functionality. The Inca's careful use of hydraulic, drainage and construction techniques ensured that the retreat was not reduced to rubble during its many years of abandonment. These techniques, combined with a strong knowledge of hydrology, were what made it a grand and operational retreat high in the most rugged of terrain.

Site Selection
The Machu Picchu site is extraordinary; modern visitors say it is breathtaking. The Inca planners and engineers would have judged the site to be well suited for a royal estate. First, it is surrounded on three sides by the steep, roaring Urubamba River. Second, the site is on a graben between two regional faults that created the two sharp peaks of Huayna Picchu and Machu Picchu Mountains. Across the river is the steep Putucusi peak that has a rounded profile like a half orange when viewed from Machu Picchu. Even today, all three peaks are considered to be holy mountains by the local Quechua Indians. Further away is the triangular-shaped mountain called Yanantin and the glacier-capped flanks of Mt. Veronica. Some 20 kilometers south is Mt. Salcantay at an elevation of 6,257 meters (20,530 feet). An arrow stone on the Intiwatana summit points toward Mt. Salcantay.

But without the perennial spring on the north slope of Machu Picchu Mountain, there would have been no Machu Picchu to admire in the early morning. Prior to the Emperor's selection of the site for his royal estate, the civil engineers would have had to determine that the water supply would be suitable. Pachacuti was one to be reckoned with, and the public works engineers had to be sure of their water supply evaluations.

A detailed public works engineering description of the Machu Picchu miracle is presented in Machu Picchu: A Civil Engineering Marvel.

City Planning
The thickly forested site of Machu Picchu was cleared and then the civil engineers saw that an agricultural area could be laid out separate and apart from an urban area to the north. To help with security, an outer wall would be constructed incorporating the natural topography, and an inner wall between the agricultural and urban areas would be penetrated with only one gate to serve the Inca Trail from the capital city of Cusco. Nearer the city, on the Inca Trail, a three-sided guardhouse would control access, and there would also be a narrow branch trail. Security for this branch trail would be supplemented by the use of a drawbridge-like structure on the steep cliff of the Machu Picchu fault.

The one gate through the inner wall would be the Sacred (Main) Gate oriented so that as one passed through the gate, a framed view of the holy Huayna Picchu Mountain would surely impress visitors. Just inside the gate, which could be closed, there would be storehouses and llama pens to facilitate the delivery and unloading of goods.

Consistent with Inca city planning practices, there would be upper and lower sectors in the urban area, with a large plaza in between. This would require intensive site work, massive walls and substantial filling to create flat building sites and the central plaza. The plaza may have served as a quarry for the one-million-plus granite stones used at Machu Picchu.

Engineering Infrastructure
The grand plan for Machu Picchu also included longevity, for without a timeless life of Machu Picchu, the Emperor would not be well served. The civil engineers of Machu Picchu knew from the technology gleaned from preceding empires and other conquered peoples stretching from north to south that a sound and well-conceived infrastructure was needed.

The infrastructure would need to rely on an incorporated knowledge of hydrology, hydraulics, agriculture, urban drainage, sanitation, soils and foundation technology, structural engineering and a "tool box" of construction methods, along with good survey control of elevations, distances and alignment.

Water Supply
The siting of the ancient Inca royal estate of Machu Picchu on a high mountain ridge between two prominent peaks would appear to be an unlikely location for ancient people to have found a pure and reliable groundwater source. The royal estate would not have been built at this location if it had not been for the availability of the perennial spring that the Inca engineers found and developed on the steep north slope of Machu Picchu Mountain. Nearly 2,000 millimeters (79 inches) of annual rainfall, a modestly-sized tributary drainage basin, igneous bedrock and extensive faulting associated with the site, collectively provided Pachacuti and his engineers with a reliable domestic source of water. This water supply made it feasible for the Inca ruler to build his sanctuary at this spectacular location. Machu Picchu supported a resident population of about 300 people, with a maximum of 1,000 when the royal entourage would visit, for nearly a century, from AD 1450 to about AD 1540, with final abandonment by remaining Indians in AD 1572 (Rowe 1990).

Water Delivery
Machu Picchu had as its urban focal point the Temple of the Sun, a solar observatory and a religious center, along with a series of 16 fountains. The aesthetic and functional layout and construction of the fountains make them a notable example of pre-Colombian public works engineering and planning. The fountains are in series except for the Sacred Fountain (No. 3), which now has an optional flow bypass to allow water delivery directly from Fountain No. 2 to Fountain No. 4. The bypass is likely a modern addition.

While the water supply for the fountains was derived from a natural spring, an effective spring collection works on the north side of the adjacent Machu Picchu Mountain enhanced the yield of this spring. A canal to carry domestic water by gravity from the Machu Picchu spring to the city center at Fountain No. 1, the uppermost fountain, is built on a series of stone-walled terraces. The locations of the water supply canal and the fountains are prescribed by the constraints of gravity flow of water from the primary spring.

To even the most casual visitor, the operating fountains at Machu Picchu provide a special treat. The sight and sound of jetting water is an attraction, just like in modern cities, especially on a hot summer day. The 16 fountains of Machu Picchu were laid out and designed to provide domestic water for the population, to enhance the urban environment and as a manifestation of the power of the Inca ruler Pachacuti.

Fountain No. 1 was constructed adjacent to the doorway of the residence of the Inca ruler, providing him with the first opportunity to utilize the imported water supply. Fountain No. 3 is known as the Sacred Fountain because it is adjacent to a stone of adoration (huaca) and enigmatic window of the Temple of the Sun.

The Sacred Fountain has finely finished carved stone blocks and four niches for ceremonial objects. As part of the Sacred Fountain, a specially carved rock adjoins the fountain on the east overlooking the Eastern Urban Sector, the Urubamba River below and the high mountains and sharp peaks in the distance. The carved rock near the jetting water would have provided an impressive backdrop for special ceremonies conducted by the Inca priests.

The water flows in series, from Fountain No. 4 to Fountain No. 16, until it is discharged to an underground stone conduit. From the stone conduit, water flows to the Main Drain of Machu Picchu via a steeply sloping channel next to a long staircase.

The vertical drop between Fountain No. 1 and Fountain No. 16 is approximately 26 meters. Except for Fountain No. 16, each of the fountains can be reached via common stairways and walkways. Fountain No. 16 is a private fountain accessible only from the Temple of the Condor.

The spring collection works, the Main Canal, and the 16 fountains of Machu Picchu represent the work of a civilization with sophisticated water-handling capabilities. Machu Picchu is a pinnacle of the architectural and engineering works of the Inca civilization, which adopted public works technology from preceding civilizations and then carried the technology to new heights.

Urban Drainage
The drainage infrastructure constructed by the Inca at ancient Machu Picchu represents a significant public works achievement. The difficult site constraints associated with the nearly 2,000 millimeters per year of rainfall, steep slopes, landslides and remoteness all posed drainage challenges which were met successfully by the Inca. The technical analysis of the Inca drainage works demonstrates that the drainage criteria used were reasonable and the implementation uncanny. For instance, we found that drainage outlets were properly sized for the rainfall, runoff factors, and areas. While in the field examining the drainage system, it would seem that the Inca would have had the equivalent of an urban drainage manual similar to the ASCE Manual of Practice No. 77, except that the Inca had no written language.

The agricultural terraces of Machu Picchu provided rich planting soils from the floodplain of the Urubamba River on otherwise impossibly steep slopes. Not only did the terraces provide flat ground surface for maize production, they protected against erosion and landslides common in the area and helped the Inca demonstrate their dominion over the land. The Inca civil engineers built the terrace walls so well that even after nearly four centuries the walls were still intact when Hiram Bingham cleared Machu Picchu in 1912. The terraces were constructed not only for agricultural purposes, but to create an amenity as they fit the hillsides and ridges like a well-tailored glove on a rough hand. Our study of prehistoric vegetation at Machu Picchu showed that the hundreds of terraces were mostly used for growing a high-status crop of maize.

Road Building
When the young Yale history professor Hiram Bingham came upon Machu Picchu in 1911, he was impressed with the quality of engineering that had gone into the site some four centuries before by early Americans. Bingham went back to Machu Picchu in 1912 as director of a Yale/National Geographic Society expedition to clear and document the ruins. Bingham's description of his explorations filled the entire April 1913 National Geographic magazine and brought Machu Picchu to the world's attention; Bingham opined that "The Inca were good engineers." He could also have stated that the Inca civil engineers were good road builders because by the time Conquistador Pizarro arrived, the Inca were operating and maintaining some 14,000 miles of road system that stretched from Ecuador to Chile and from the Pacific Ocean to high Andean settlements after crossing mountain passes with 18,000-foot elevations. Bingham described the main Inca Trail leading into Machu Picchu as a masterpiece of prehistoric Inca road building.

One example of a civil engineering road-building feat that escaped Bingham's sharp eye, however, was right there at Machu Picchu, but it lay undetected in the forest cover of the lower East Flank for another 87 years before being uncovered in 1999. The Machu Picchu Inca Trail extension proved to be a fine example of route selection, slope stabilization, environmental design and construction; all accomplished without a written language, iron, steel or the use of the wheel. While Machu Picchu was described in the Wright/Valencia book, the book dealt with the new Inca Trail only in passing, because the data on the trail was still being processed at the time of its writing.

The Inca technique for constructing wall foundations started with the careful placement of smaller rocks in the excavation bottom to create a firm bedding. The rocks became larger as the foundation rose nearer the ground line. One finds a typical wall thickness of 0.8 meter (2.6 feet) at the ground line though the Inca civil engineers were not bound by such a rule. Some of Machu Picchu's structures demanded a more sturdy thickness, such as the retaining wall east of the Royal Residence, which marked the edge of the Western Urban Sector and the beginning of the central plaza. Here, huge stones were shaped, moved into position and carefully fitted for stability.

In some instances, the Inca engineers selected a "living" granite rock or huge in-situ rocks for their foundation base, which would first be shaped to provide a ledge or platform for founding. Examples include the Temple of the Sun and the Temple of the Condor.

The Inca had learned about long-lived infrastructure and building for the centuries from their predecessors near Cusco at Pikillacta and Chokepukio. They also learned much about building technology from the Tiwanaku Empire centered near Lake Titicaca, and they learned to avoid building in the floodplain, perhaps based upon the experiences of the Moche Kingdom on the north coast. From all of these earlier people, the Inca ascertained the need to include solid foundations, good drainage, good agricultural soil and a reliable water supply. As a result, their buildings have withstood the ravages of time and tropical rainforest tree roots.

Technology Transfer
The Inca Empire, although short lived, was the culmination of thousands of years of Andean civilization. From their predecessors they had inherited a body of statecraft and much of the physical infrastructure for the empire. They had also inherited and participated in a great Andean culture based on religious ideas that were thousands of years old. That they were the inheritors of this great tradition does not in any way diminish their own achievement, however. It was the peculiar Inca genius for organization that allowed them to make profitable use of their cultural inheritance. They, alone, of the late Andean societies were able to weave together the disparate elements of the many Andean cultures through military prowess, ideology, and extraordinary statecraft by drawing on thousands of years of cultural inheritance. In terms of geographical extension, military power and political organization, the Inca created the greatest of the pre-Colombian empires.

Native Americans built Machu Picchu before the arrival of the Spanish conquistadors. It was abandoned after the Inca Empire collapsed, and it endured under a thick rainforest until the 20th Century. Scientists, engineers and laymen alike continue to marvel at the wonders of Machu Picchu and its magical ambiance. The mystique of Machu Picchu is in its details: the hydrology of the water supply, the hydraulics of the canal and fountains, and the blending of man's work with the challenging natural topography and environment with which the public works engineers were faced.

One cannot fully appreciate the accomplishment that Machu Picchu represents without first considering the site constraints faced by its ancient civil engineers. The Inca engineering feats of planning and construction must be viewed in context with its setting, geology and climate. It is when these details are viewed as a whole that Machu Picchu becomes the Lost City of the Inca in all its glory: the destination of travelers from throughout the world.

Kenneth R. Wright is a consulting engineer with Wright Water Engineers, Inc., who studies water use and handling by ancient civilizations. This year he is working in Olympia, Greece; Mesa Verde National Park, Cortez, Colorado; Arles, France; and Machu Picchu, Peru. At the 2003 APWA Congress in San Diego, he will give a presentation entitled "Machu Picchu: Prehistoric City Planning & Civil Engineering" at the Public Works Historical Society Luncheon on Monday, August 25.