Slope stabilization in residual soils of peru

Transcription

1 SlopeStability Engineering, Yagi, Yamagami & Jiang 1999 Balkema, Rótterdam, ISBN Slope stabilization in residual soils of peru A.CARRILLO-GIL University of Engineering, Lima, Peru A. CARRILLO-ACEVEDO A. Carrillo Gil S.A. Consulting Engineers, Lima, Peru. ABSTRACT: The main objective of this paper is to present a real case occurred in residual soils from the Peruvian amazon plane, in order to show the positive effects of the stabilization with drainage and the pore pressures dissipation that previously had originated large landslides in the season of the annual water level decrease of the Amazon River. This happens in a very short time and decreases something more than 12 meters in a fast way. This effect decreases the shearing strength of the saprolitic soil underlying, producing instability in its banks and important damages in the works of civil engineering over the surface. The results of the practiced instrumentation allowed a better planing and distribution of the drains in the affected area as well as an interpretation of the registered movement with biaxial inclinometers and the water pressures with pneumatic piezometers. All of them were associated with the extensive rains of the area, the movement of the riverbed and the rapid drawdown of the water, minimizing the risk and creating better possibilities for future investments. 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 amazon 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. To offer some explanation to the movement of the meanders of the Amazon river, it is present below the factors that can originate them: Soil with very low gradient and smootly 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 are small, however the surface of the land bark suffers level changes, originating possible displacement in the bed of the rivers. According to what is shown previously, the Amazon river has impacted strongly on the riverbank between the

2 years 1948 and 1972 and on the others (330,000 years ago) it also to fluctuate Figure 1.- View of slope tipical failure in tropicals soil of Peru, Iquitos, 1994 riverbank, between the years 1993 and 1994, being produced phenomena of instability. Phenomena go advancing downstream initially as 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 considerably damaged different types of engineering works placed in the banks of the Amazon river in the region of Peru, when the river impacts directly on the critical border, and increasing gradually according to the river is going far (Figure 1). 2-GEOLOGYCAL AND GEOTECHNICAL SETTING The general geology considers that a large part of the Amazon region has stayed covered during the interglacial periods of the quaternary by an interior sea of shallow water when the level of the oceans had 100 meters above of the existing now during several glacial and interglacial periods forming terraces throughout the water courses, dropping to 100 meters below of the original 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

3 ANÁLISIS Y DISEÑO EN LOS SUELOS PERUANOS Figure 2.- Measured Horizontal Displacements Amazone plain, can be indicated as advanced erosion type. The Amazon plain is characterize by its great humidity and soil covered by a dense tropical vegetation. 3- SLOPE FAILURE MECHANISM The statistical analysis of the movement of the Amazon River performed between the years 1991 and 1996 clearly established that the landslides have occurred during the stage of water level decrease in the river. This is different in instead of what occur in other parts of the world where the rains that are presented during the decrease of the level of the water of the those which river originate the landslides. We consider that as a phenomenon of rapid drawdown that affects the bank, because of the water level decreases to an average of 12 meters in to very short time. This rapid drawdown is interpreted as a process that increases the undrained deformation of the saturated zone in the affected banks. In other words, the

4 Figure 3.- Cross Section with Piezometric reaction of the stability of the banks to the rapid movement when the water level decreases is similar to the response occurred in an open cut in which is produced a forced alleviation, due to material that previously was offered as lateral support and that was suddenly removed. In this case, as a consequence of the imbalance produced by the rapid drawdown of the river, there is water that remains within the porous structure of the soil, since its level does not decrease to the same speed that the water level. This phenomenon causes an increase in the weight of the bank body, as in the pore pressure with the soil. This effect reduce the shearing strength of the soil, which, together with the effects of the river, causes the ladslides (if it has not been possible to evacuate the water tricked within the soil of the bank). 4- LANDSLIDES CONTROL MEASURES The system of installed deep drainage is efficient and it has generated an adequate drainage during the critical stage of

5 ANÁLISIS Y DISEÑO EN LOS SUELOS PERUANOS drawdown of the Amazon river in 1996, 1997 and In the better behavior area we put 31 horizontal drains of 30.meters of length, spaced each 3 meters with a slope of 3 o and diameter of 4". In the adjacent section we installed wells with radial drains that arrived to lengths understood between 15 to 25 meters The measures analyzed indicate a small displacement in direction to the Amazon river in the stage of water level decrease, and backward displacement when the water level rose. The comparison of the results obtained demonstrate that the movements registered before have reduced considerably, probably due to the effective operation of the deep drainage system, and the additional effects produced by the sedimentation that originated due to movement of the riverbed of the Amazon river. The results of the final piezometrics measures indicate that, as a rule, the dissipation of the pore pressures in almost all cases has been effected in correspondence with the decrease and increase of the water level. So, we found a good behavior in the drainage system installed in the critical zones. The piezometers that were installed in the zone of the last landslide (1994) from the beginning of their readings showed irregularities with respect to the dissipation of the accrued pore pressures after of the decrease of the river. It must be noted that in the location zone of these instruments was not practiced nor any deep drainage system or treatment for maintenance. 5- CONCLUSIONS The deep drainage system, instaalled in the studied area (by means of wells with radial drains as well as by horizontal drains) has contributed effectively in the stabilization of these banks, and the analysis of all the measures taken during the several months of work with the instruments, prove that there is a substantial improvement in the stability conditions of the platforms included in the study, conditions that can improve in the future due to more sedimentation that presumably could be produced in the place by effect of the change of the Amazons riverbed. The results shown in this paper provide a global vision of the stability problems of soils in 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 aproximation 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, considering finally that the Peruvian Amzon 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. ACKNOWLEDGMENT The permission of The Maritime Authority of Peru (ENAPU-PERU) to publish this paper is gratefully acknowledged REFERENCES A. CARRILLO-GIL, S.A., Consulting Enginers, 1998, Stability Control of the Riverbanks in Iquitos, Peru, Thenical Report to ENAPU S.A., Lima, Peru.

6 CARRILLO-GIL, A.,1978, Characteristic of Tropical Soils in Peru, Latin American Magazine of Geotecnique, Vol. IV, Nº4 pp , Caracas - Venezuela (in Spanish). CARRILLO-GIL, A.,1983, Stability Problems in Iquitos, Peru, Proc. VII Pan- American Conference on Soil Mechanics and Foundation Engineering, Vancouver - Canada. 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, Proc. X Pan- American Conference on Soil Mech. and Foundation Eng., Guadalajara, Mexico (in Spanish). CARRILLO-GIL, A.,DOMINGUEZ, E.,1996, Failures in Amazon riverbanks, Iquitos, Peru. Seventh International Symposium on Landslides, Trondheim, Norway. CARRILLO-GIL, A.,1997, Peculiarities of tropical saprolitic soils of Peru, XIV International Conference on Soil Mechanics and Foundation Engineering, Hamburg, Germany. CARRILLO-GIL, A., 1998, Analysis and Design in the Tropical Soils of Peru,. VIII GEO LIMA 98, Lima, Perú.