Landslide Investigation in Machu Picchu World Heritage, Cusco, Peru (C101-1)

Landslide Investigation in Machu Picchu World Heritage, Cusco, Peru (C101-1)

Landslide Investigation in Machu Picchu World Heritage,Cusco, Peru (C101-1)

Kyoji Sassa (ICL) ・ Hiroshi Fukuoka (Kyoto University, Japan)・ Raul Carreno (Grudec Ayar, Peru)

Abstract. The Japanese landslide expert team conducted landslideinvestigation in and around Machu Picchu Citadel since March 2000in cooperation with the Instituto Nacional de Cultura (INC) and theInstituto Nacional de Recursos Naturales (INRENA). The investigationresults and the cooperation scheme between ICL and the Governmentof Peru are introduced.

In the past, probably a series of retrogressive landslides scrapeda part of the mountain ridge of Machu Picchu slope along a shearband almost parallel to the present slope. The flat area was formedby landslides on the mountain ridge. Inca people were likely to haveconstructed a citadel on this flat part of mountain ridge. Landslidedebris provided them weathered debris and soils possible for farming.When undercutting by river erosion reached the level of anothershear band, another series of retrogressive landslides haveproceeded along the shear band near or a little bit higher than thepresent river bed. The process was faster in the landslide block (no. 1)including the Hyram Bingham road, and delayed in the landslideblock (no. 2) including the Inca’s citadel because the river erosionto the slope was stronger for block no. 1 due to sharp curvature ofriver route. The slope deformation affecting the citadel part is notreal landslide at present, but it is a precursor stage of landslides,namely it can become to be a real landslide as the result of retrogressivedevelopment of landslides from the Urubamba River andfrom the block no. 1 side.

The initial slope monitoring using extensometers by the Japaneseteam of the Disaster Prevention Research Institute of KyotoUniversity (DPRI/KU) started with cooperation from INC and INRENAfrom November 2000. One year monitoring in 2001 was presented.After the establishment of the International Consortium on Landslides(ICL) and the International Programme on Landslides (IPL) in2002, the initial cooperation agreement on Machu Picchu betweenthe Government of Peru and DPRI/KU has developed to the cooperationbetween the Government of Peru and the ICL. The InternationalProgramme on Landslides (IPL) C101-1 ‘Landslide investigationsin Machu Picchu’ consists of six groups including Japanese,Italian, Czech-Slovakian, Peruvian-Canadian groups in 2005. TheJapanese team installed new four sets of long-span extensometers,three sets of GPS receivers, a Total Station with three prism mirrortargets in 2004 and started monitoring of the displacement.

Keywords. Risk evaluation, prediction, monitoring, precursor stageof landslide

1. Background and History of Machu Picchu Project

Prediction or identification of precursor phenomena oflarge scale landslides is not an easy task. Large-scale landslidesdo not occur in the same place in a short returnperiod compatible to life period of human beings. It is akind of geological process and the return period of largescalelandslides is usually very long in the order of thousandyears or tens of thousand years or even longer.Before the Hyogoken-Nanbu earthquake took place,most of the Japanese people regarded that earthquakesoccurring in the order of thousand years was almost outof scope in present planning of disaster prevention measures.However, such earthquake caused great damagesto mega-city area of Kobe. So now it is clearly understoodthat even infrequent phenomena such as movement ofactive faults and earthquakes should be seriously consideredand people prepared for that. Frequency of activefaults and large-scale landslides are rather similar, thoughthe casual forces of phenomena are different; faults bycrustal horizontal stress, landslides by gravitational verticalstress.

Protection of mega-city from earthquakes is very important,and we, researchers involved in the field of disasterprevention should focus our study toward a morereliable prediction of site and time. Though the disastercaused by landslides is not so great in the number of deathcomparing to earthquakes, the Mayuyama landslide inUnzen, Japan, 1792 killed 15 000 people (Sassa 1999) andthe most recent Las Colinas landslide killed around600 people in El Salvador in 2001. So landslides cause nota little disaster.Entering twenty-first century, we are more and moreaware of the value of the environment, especially regardingthe invaluable cultural and natural heritage. Those are veryfragile treasure for humanity, which cannot be rebuilt oncethey were destroyed. People worked for economic developmentin the last century, and the industrial progress andeconomic development is still very important for the baseof society, but at the same time, we have noticed that weshould protect and leave our invaluable treasures of humanityto the next generation so long as possible.

2. Monitoring of Extensometers in Machu Picchu

To evaluate landslide risk, monitoring of the ground deformation, the geological drilling, monitoring of shear displacement and ground water level and/or pore water pressure inside drill holes are vitally important and necessary. Without such investigation, neither landslide risk,nor the safety of slope can be reliably evaluated in the convincing way. Without such reliable landslide risk evaluation, neither effective landslide remedial works can be planned, nor high costs of remedial works can be approved.
As the first step of quantitative investigation, two types of simple extensometers were installed in the Machu Picchu slope in 2000. One is a handmade manual reading extensometers, using a pulley and a super invar wire (a special kind of metal with least influence of temperature), movement of the distance between two points is mechanically enlarged by 5 times and indicated on the dial with a pointer.

Another type is theoretically the same, movement was also mechanically enlarged by 5 times, and it is recorded on a recording paper continually. The recording drum is rotated by landslide movement, while the recording pen shifts in a steady speed using a dry battery driven mechanical clock.

Extensometers have merits, which are not affected by moisture or atmospheric pressure in the air and cause less trouble and very reliable because of very simple mechanical recording system. They can be easily handled and understood by every body without special knowledge, which are different from sophisticated monitoring systems such as EDM (electronic distance meter) and GPS (global positioning system) and others.

Fig. 1Photo of the Machu Picchu Citadel from a chartered helicopter taken by K. Sassa 2000

Conclusions

The results of investigation by the Japanese team conductedsince March 2000 and the background of IPL C101-1 andslideInvestigation in Machu Picchu were introduced.Major findings of the investigation by the Japanese groupare:

1. Retrogressive landslides have probably proceeded inthe past and are still on-going in the Machu Picchuslope. The progress of landslide evolution stage is differentin block 1, 2, 3. Present stages of each block areinformative for the assessment for the future processof landslide activities.

2. A shear band, possibly a potential sliding surface ofblock 2 was found. It is probably gently dipping to thefront slope. Ground water is likely to flow downwardpossibly along the shear band. The ground water mustbe poor in the top of block 2 and rich in the lower slope.It may be the cause of possible head scarp which maybe splitting Plaza.

3. Ground water along the shear band in block 2 and alsoretrogressive landslides around block 2 (namely in theborder to block 1, in the toe cliff near the UrubambaRiver) will be major causal factors of block 2. Reductionof those causal factors must be effective landslideremedial measures.

4. We may conclude that there is a necessity of furtherinvestigation to evaluate landslide risk in MachuPicchu, and it will be possible to protect Machu Picchufrom landslide risk by reducing landslide causes basedon the reliable investigation results though it is not aneasy task in all of the scientific, technological, socialand financial aspects.

Acknowledgments

The Japanese investigation team was well received by therelated agencies in Peru and obtained significant supportand cooperation. We acknowledge the following personsand institutes for their cooperation to this investigation:Dr. Romulo Mucho, Vice Minister of Mine, Dr. VictorBenavides, and colleagues of INGEMMET (PeruvianInstituto Geologico Minero y Metalurgico) for their cooperationand coordination of many of Peruvian counterparts.

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