FAILURES IN THE AMAZON RIVERBANKS, IQUITOS, PERU A.Carrillo-Gil University of Engineering & A.Carrillo Gil S.A.,Consulting Engineering,Lima,Peru L. Dominguez University of Engineering,Lima & The Maritime Authority of Peru ABSTRACT: The review of geological and getechnical conditions releted to the instability phenomena in the Amazon riverbanks near to Iquitos city in Peru have been presented. The Amazon river does not have a defined riverbed, therefore it flows forming meanders in the low-jungle of Peru, on a soft tropical soil formed by clays, silt and sands. The movement and withdrawal of the river, that has made an island disappear, has originated erosion in its left margin and sedimentation in its opposite margin, typical condition of the hydraulic behavior of this river each 100 years. This fact produces landslides that occur with great regularity along changing course of the river. The evaluation of stability of natural and cut slope in tropical soil involves in many cases a considerable degree of uncertainty. 1- INTRODUCTION The stability of the riverbanks in the Peruvian Amazon jungle present a great number of technical problems not existing in other places, since in very few regions of the world are present the atmospherical, environmental or hydrological conditions that prevail in this region, adding to these factors the lack of conventional construction materials. The erosion and sedimentation phenomena that alternatively occur in both margins of the rivers, and the continuous course changes between the subsequent years, present additional problems and large challenges to the application of the knowledge of the geotechnical engineering, so much in the low jungle that is between 80 and 400 meters above mean sea level, as in the high jungle between 400 and 1000 meters above mean sea level.. Fig.1 Overview of Reference 3 Landslide, 1994, Iquitos - Perú
Fig.2 - Schematic movements of the Amazon River
water when the level of the oceans having 100 meters above of the existing now (330,000 years ago) and began to fluctuate during several glacial and interglacial periods forming terraces throughout the water courses, dropping to 100 meters below of the level during the last Glacial Era (17,000 years ago) and remaining in these deep channels the large rivers, between them the Amazon river, raising afterwards to the current level (6,000 years ago) The accomplished studies establish that in the high jungle and in the limits of the low jungle are found so much igneous rocks as sedimentary, while in the low jungle prevail saprolitic soils originated by the sedimentary rocks of the terciary and quaternary and they are formed mainly by sandstones, shales and clays. The general description of the geomorphology of the Amazon region indicates that the low jungle is substantially flat and as said remain, its height varies between 80 to 400 meters above mean sea level. Due to this small difference of elevation the rivers flow slowly, getting in the dry station the apperance of lakes. This region of the Amazone plain, can be indicated as advanced erosion type. The Amazon plain is characterized by his great humidity and soil covered by a dense tropical vegetation. Fig.3 - Cross sectional illustration of the bed movement Two factors that influence of preponderant way on the bank failures of the Amazon river are undoubtedly the behavior of the tropical saprolitic soil and the special characteristics of the river within the Peruvian Amazon region; this is why we emphasize of general way these two aspects trying to explain the phenomena occurred since several years ago in this region. (Fig.1) 2- GEOTECHNICAL SETTING The general geology considers that a large part of the Amazon region has stayed covered during the periods interglacial periods of the quaternary by an interior sea of shallow 3- MORPHOLOGICAL PROCESS OF THE RIVER The Amazon river does not have a defined riverbed, it reflects forming meanders on a soft alluvial soil of thickness variable. The meanders are displaced downstream originating erosion and sedimentation, near the zone of Iquitos city, the river is split into two arms by an Island. If it is compared the information of the movement of the Amazon river for 1948 with that of 1994 we can observe that the erosion of the Island is of almost 2,500 meters (Fig. 2). The erosion of the head of the Island originated by the flow of the river, has permitted the advance of the waters meander up of this but not to it used to, causing its deformation upstream as well as downstream. The deformation of the upstream meander has modified the riverbed of the eastern arm stressing its curvature and narrowing the
Island, getting to break it in February 1994. As consequence of the break of the Island, the eastern riverbed is becoming the main bed of the river, carrying out more than 90% of the liquid and solid transportation of the flow of the river; this flow prevents the free delivery of the western arm flow, originating loss of speed of the flow and the sedimentation in all the eastern bed (Fig. 3). The revenue of the water flow by the western arm will be every time smaller due to the sedimentation being formed in the left margin.in a near future, in low waters, there will not be revenue of water to this arm. 4- ORIGINS OF MEANDERS MOVEMENT To offer some explanation to the movement of the meanders of the Amazon river, we present below the factors that they can originate them: Soil with low gradient and smoothly sloped toward to the East, in the order of 1: 20,000, that offers greater or smaller resistance to the water flow. The changes of water level between flood and ebb times, that reach fluctuations from 10 to 12 meters. The tectonic movements in the Amazon zone that are small, however the surface of the land bark suffers level changes, originating possible displacement in the bed of the rivers, this is as if a great flat platform and slightly inclined is moved slowly through a geological time of 100 to 200 years, modifying the riverbeds of then returning to their original state causing again disorders and instability in their banks. Finally, the periodical change of the magnetic axis of the earth that originates a difference of attraction located in the course of the river, producing greater levels in the banks and as consequence a change in the form of the meanders. 5- EVOLUTION OF INSTABILITY OF THE RIVERBANKS According to what is shown previously, the Amazon river has impacted strongly on the riverbank of reference 1 between the years 1948 and 1972 and on the riverbank of reference 3, between the years 1993 and 1994, being produced phenomena of instability similar in the zone of Iquitos city. Phenomena that go advancing downstream initially as Fig.4 - Observed safety factors, References 3 landslides
erosion to end afterwards as sedimentation and therefore stabilization of the slide critical area. During more than 40 years they have been producing landslides that have damaged considerably different types of engineering works placed in the banks of the Amazon river, having been evaluated their stability throughout different methods and calculation procedures, finding evidently, safety factors very low (Fs = 0.963) when the river impacts directly on the critical border, and increasing gradually according the river is going far creating sedimentation and throwing safety factors that go growing (Fs = 1.341, 1.684, 1.848, etc) until showing reasonably high stability, that assure stable engineering works during a time that varies from 50 to 100 years in some instances, returning after this period to present critical instability states. (Fig. 4) 6- FINAL COMMENTS The results shown in this paper provide a global vision of the stability problems of soils of the Peruvian wet tropic, generated by the changing morphology of the rivers that originate important risk situations in some cases, and increasingly growing stability in others that permits to establish the development of new behavior standards for the riverbanks of the Peruvian Amazon that in the future can be predictable with certain approximation considering their evolution in the geological time of hundreds of years, since now in certain areas it has already passed the danger, and maybe within 100 or more years, the problem return to be present and the safety factors of the banks decrease gradually until to become unstable and to produce large landslides as they occurred in sites and dates of study. Due to the fact that is not yet possible to arrive to a real determination of the engineering properties of saprolitic soils of the Peruvian Amazon that allow to formulate a rational forecast about the behavior of these geotechnical materials, it is convenient to project engineering structures for a given useful life, according to the probable safety factors of the banks of the river, being estimated the probable and necessary investments, in some instances effective and in others unnecessary due to the fact that the traditional criteria does not insert satisfactorily in many of the cases of instability studied, considering finally that the Peruvian Amazon is located in a region of a very singular world in light of their geotechnical occurrences and of climate that create very difficult wet tropical soils to predict and handle in the construction of the earth works. 7- REFERENCES Carrillo-Gil, A.,(1978), Characteristic of Tropical Soils in Peru, Latin American Magazine of Geotecnique, Vol. IV, Nº4 pp. 207-216, Caracas - Venezuela (in Spanish). Carrillo-Gil, A.,(1983), Stability Problems in Iquitos, Peru, Proc. VII PanAmerican Conference on Soil Mechanics and Foundation Engineering, Vancouver -Canada. Carrillo-Gil, A.,Carrillo Delgado E.,Cardenas, J.,Flores, J., 1993, Stabilization alternative in the Wet Tropical Region for Pavements, Proc. of the VII National Conference on Soils Mechanics, Lima - Peru (in Spanish). Carrillo-Gil,A.,Carrillo, E.,Cardenas, J.,Robalino M.,(1994), Characterization of Tropical Soils of Peru, X National Congress of Civil Engineering, Lima, Peru (in Spanish). Carrillo-Gil, A.,Carrillo, E.,Cardenas, J.,(1995), Properties of the peruvian tropical soils, Paper submited to X Panamerican Conference on Soil Mech. and Foundation Eng., Guadalajara, Mexico. Dominguez, L., (1994), Morphological changes on the Amazon River, Conference in Ricardo Palma University, Lima, Perú. (in Spanish).