A Highway Drainage Marvel
A pre-Columbian engineering legacy
By
Kenneth R. Wright,
T. Andrew Earles,
David Foss,
Alexander Merle-Smith
The noble and ancient profession of highway drainage engineering has long been exemplified in contemporary literature by Roman public works. Even in the remote, rugged lands of Lebanon or Syria, one might stumble upon the remains of an attractive arched stone culvert that carried a Roman road over a watercourse.
In the Western Hemisphere, long before Columbus sailed for America, the prehistoric Inca highway builders of the Andes set an even higher standard for modern drainage engineers to emulate. Near one of the world's archaeological jewels, Machu Picchu, we found an ancient highway culvert that turned out to be a civil engineering marvel.
 |
| From inside the drainage culvert, under the Inca Trail, the outlet frames a view of the holy mountain of Putucusi. |
After crossing the culvert dozens of times during our 11 years of engineering research at the Lost City of the Inca, we one day noticed an unusual structure on the legendary Inca Trail in 2002. Ken and Ruth Wright, while stopping for a few minutes to polish their field notes, looked down and realized that the structure beneath them was something special. Later that day, Dr. Andrew Earles crawled into the structure opening and cleared out years of accumulated brush and debris; sure enough, it was a carefully built conduit under the fabled Inca Trail.
When we returned in 2004 for more nearby archaeological excavations along the Inca Trail, we decided to examine the enigmatic structure in more detail. What we found surprised even Ken Wright, who has studied drainage practices for over 40 years. Here was a granite stone highway culvert built to perfection. It had endured 500 years of service and climatic extremes without failing, and yet it went unnoticed by scientists even after nearly a century of onsite archaeological studies starting in 1912, when Yale professor Hiram Bingham rediscovered Machu Picchu.
Our 2004 field research at Machu Picchu was aimed at supplementing the American Society of Civil Engineers Press book Machu Picchu: A Civil Engineering Marvel. Our new studies were focused on an Inca security station, religious shrine, and trailhead halfway between Machu Picchu and the Gate of the Sun that is high up on the mountain ridge. During a lull in the archaeological excavations, research leader Ken Wright chose engineer David Foss to help document the new culvert. Together with research associate Alexander Merle-Smith of Virginia, they measured and sketched the culvert, analyzed the upstream drainage, inspected the nearby Inca Trail, and concluded that, indeed, the Inca civil engineers were competent specialists in highway drainage, even though they had neither a written language, nor the wheel, nor even iron and steel.
Near the culvert, high-status terraces lie on both sides of the Inca Trail. They were used for growing maize, likely for producing the religious beer still known as Chicha. We know what the Incas grew on the terraces because of our past palynology studies. The soil samples consistently showed maize pollen in the Machu Picchu terrace soil samples.
The ancient highway culvert is inlet-controlled with a length of 2.1 meters (6.9 feet). Like many modern culverts, provisions were made by the Incas for its use as a pedestrian underpass, complete with a stairway on its bottom. Engineering parameters associated with the culvert are given in Table 1.
Using previously determined Inca urban drainage criteria used at Machu Picchu, for the drainage basin area upstream of the culvert of 2 hectares (5 acres), one would expect the inlet to have been sized at 1.3 square meters (14 square feet). However, the drainage basin does not contain impervious urban areas, but forest and agricultural terraces. The basin rational formula coefficient of runoff is judged to be low. The Incas' use of an inlet cross-sectional area of 0.5 square meter is judged to be reasonable. This tells us the Inca Highway builders did not just use runoff estimates from nearby Machu Picchu, but selected criteria suitable for a rural basin. The granite steps in the upstream and downstream channels, coupled with shaped stone stairs in the culvert, tell us that the culvert also served as a passage for farm workers so that these people could go from field to field without interfering with traffic above.
 |
| An ancient drainage culvert carries runoff from left to right, under the highway leading into Machu Picchu from high in the Andes. |
A unique drop inlet exists only 15 meters (50 feet) upstream from the culvert entrance. At times when the peak drainage flow exceeded the drop inlet's capacity, excess flow would cascade over the stone retaining wall, in a controlled manner, and into the channel below.
From inside the culvert, looking out to the view beyond, the engineering research team members could hardly believe their eyes. Perfectly framed by the culvert exit was the holy mountain of Putucusi, a mountain that, even today, is worshipped by the local Quechua Indians, descendants of the Incas. We know from past studies that the Inca engineers were good at environmental design, which included framing spectacular views with windows and doors, but from inside a culvert? This framing may have set a new environmental design standard, even for the Incas.
 |
| The outlet of an upstream drop inlet was well formed by Inca workers over 500 years ago. It has endured unscathed over the centuries. |
Nearby, we measured the batter of the main Inca Trail supporting walls. Here, we encountered another surprise. Inca walls, almost universally, are battered at about 10% for strength and aesthetics. But these Inca Trail walls were nearly vertical, and yet they had endured for nearly 500 years. It was evident that the construction work was performed with a high standard of care.
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The many agricultural terrace walls nearby are all battered, as are most Inca walls all over Peru. The Incas had strict building codes, but apparently the Inca building code did not govern the elite highway builders; otherwise these walls would also have been battered. The Incas must have had special design standard exceptions for the highway builders whose crews built and maintained over 20,000 kilometers (14,000 miles) of highways up and down the Andean spine of South America. Our resident Peruvian archaeologist, Dr. Alfredo Valencia, opined that maybe the walls had lost their batter due to 500 years of earth pressure and creep. While this opinion at first seemed to have merit, it was discounted after we noted that the vertical walls were consistent; soil creep would not be so uniform over a long stretch of trail where supporting walls reached 4.4 meters (14 feet) in height.
Several days after exploring the wonders of the Inca Trail culvert and after thinking about the hundreds of workers laboring to construct the Inca Trail leading into Machu Picchu some 500 years ago, we decided to further reflect on the spectacular drainage structure and the view from inside the culvert. We did this by climbing to the summit of the holy mountain of Huayna Picchu and viewing the far-off culvert as it would have been seen from the summit by an Inca nobleman or priest while asking the Earth Mother to make the Inca Trail endure forever as a transportation artery into Machu Picchu.
March-April 2006
A Highway Drainage Marvel
A pre-Columbian engineering legacy
By
Kenneth R. Wright,
T. Andrew Earles,
David Foss,
Alexander Merle-Smith
The noble and ancient profession of highway drainage engineering has long been exemplified in contemporary literature by Roman public works. Even in the remote, rugged lands of Lebanon or Syria, one might stumble upon the remains of an attractive arched stone culvert that carried a Roman road over a watercourse. In the Western Hemisphere, long before Columbus sailed for America, the prehistoric Inca highway builders of the Andes set an even higher standard for modern drainage engineers to emulate. Near one of the world's archaeological jewels, Machu Picchu, we found an ancient highway culvert that turned out to be a civil engineering marvel.
|
| From inside the drainage culvert, under the Inca Trail, the outlet frames a view of the holy mountain of Putucusi. |
After crossing the culvert dozens of times during our 11 years of engineering research at the Lost City of the Inca, we one day noticed an unusual structure on the legendary Inca Trail in 2002. Ken and Ruth Wright, while stopping for a few minutes to polish their field notes, looked down and realized that the structure beneath them was something special. Later that day, Dr. Andrew Earles crawled into the structure opening and cleared out years of accumulated brush and debris; sure enough, it was a carefully built conduit under the fabled Inca Trail.
When we returned in 2004 for more nearby archaeological excavations along the Inca Trail, we decided to examine the enigmatic structure in more detail. What we found surprised even Ken Wright, who has studied drainage practices for over 40 years. Here was a granite stone highway culvert built to perfection. It had endured 500 years of service and climatic extremes without failing, and yet it went unnoticed by scientists even after nearly a century of onsite archaeological studies starting in 1912, when Yale professor Hiram Bingham rediscovered Machu Picchu.
Our 2004 field research at Machu Picchu was aimed at supplementing the American Society of Civil Engineers Press book Machu Picchu: A Civil Engineering Marvel. Our new studies were focused on an Inca security station, religious shrine, and trailhead halfway between Machu Picchu and the Gate of the Sun that is high up on the mountain ridge. During a lull in the archaeological excavations, research leader Ken Wright chose engineer David Foss to help document the new culvert. Together with research associate Alexander Merle-Smith of Virginia, they measured and sketched the culvert, analyzed the upstream drainage, inspected the nearby Inca Trail, and concluded that, indeed, the Inca civil engineers were competent specialists in highway drainage, even though they had neither a written language, nor the wheel, nor even iron and steel.
Near the culvert, high-status terraces lie on both sides of the Inca Trail. They were used for growing maize, likely for producing the religious beer still known as Chicha. We know what the Incas grew on the terraces because of our past palynology studies. The soil samples consistently showed maize pollen in the Machu Picchu terrace soil samples.
The ancient highway culvert is inlet-controlled with a length of 2.1 meters (6.9 feet). Like many modern culverts, provisions were made by the Incas for its use as a pedestrian underpass, complete with a stairway on its bottom. Engineering parameters associated with the culvert are given in Table 1.
Using previously determined Inca urban drainage criteria used at Machu Picchu, for the drainage basin area upstream of the culvert of 2 hectares (5 acres), one would expect the inlet to have been sized at 1.3 square meters (14 square feet). However, the drainage basin does not contain impervious urban areas, but forest and agricultural terraces. The basin rational formula coefficient of runoff is judged to be low. The Incas' use of an inlet cross-sectional area of 0.5 square meter is judged to be reasonable. This tells us the Inca Highway builders did not just use runoff estimates from nearby Machu Picchu, but selected criteria suitable for a rural basin. The granite steps in the upstream and downstream channels, coupled with shaped stone stairs in the culvert, tell us that the culvert also served as a passage for farm workers so that these people could go from field to field without interfering with traffic above.
|
| An ancient drainage culvert carries runoff from left to right, under the highway leading into Machu Picchu from high in the Andes. |
A unique drop inlet exists only 15 meters (50 feet) upstream from the culvert entrance. At times when the peak drainage flow exceeded the drop inlet's capacity, excess flow would cascade over the stone retaining wall, in a controlled manner, and into the channel below.
From inside the culvert, looking out to the view beyond, the engineering research team members could hardly believe their eyes. Perfectly framed by the culvert exit was the holy mountain of Putucusi, a mountain that, even today, is worshipped by the local Quechua Indians, descendants of the Incas. We know from past studies that the Inca engineers were good at environmental design, which included framing spectacular views with windows and doors, but from inside a culvert? This framing may have set a new environmental design standard, even for the Incas.
|
| The outlet of an upstream drop inlet was well formed by Inca workers over 500 years ago. It has endured unscathed over the centuries. |
Nearby, we measured the batter of the main Inca Trail supporting walls. Here, we encountered another surprise. Inca walls, almost universally, are battered at about 10% for strength and aesthetics. But these Inca Trail walls were nearly vertical, and yet they had endured for nearly 500 years. It was evident that the construction work was performed with a high standard of care.
The many agricultural terrace walls nearby are all battered, as are most Inca walls all over Peru. The Incas had strict building codes, but apparently the Inca building code did not govern the elite highway builders; otherwise these walls would also have been battered. The Incas must have had special design standard exceptions for the highway builders whose crews built and maintained over 20,000 kilometers (14,000 miles) of highways up and down the Andean spine of South America. Our resident Peruvian archaeologist, Dr. Alfredo Valencia, opined that maybe the walls had lost their batter due to 500 years of earth pressure and creep. While this opinion at first seemed to have merit, it was discounted after we noted that the vertical walls were consistent; soil creep would not be so uniform over a long stretch of trail where supporting walls reached 4.4 meters (14 feet) in height.
Several days after exploring the wonders of the Inca Trail culvert and after thinking about the hundreds of workers laboring to construct the Inca Trail leading into Machu Picchu some 500 years ago, we decided to further reflect on the spectacular drainage structure and the view from inside the culvert. We did this by climbing to the summit of the holy mountain of Huayna Picchu and viewing the far-off culvert as it would have been seen from the summit by an Inca nobleman or priest while asking the Earth Mother to make the Inca Trail endure forever as a transportation artery into Machu Picchu.