INTRODUCTION
Maputo is the capital city of Mozambique and the most important city of the country. It was built on elevated Tertiary and Quaternary deposits and is bordered by an extensive slope which separates the upper part of the town (up to 60 m height) from the downtown area (almost at sea level). Most of these deposits look stable for engineering purposes, however time after time problems such as tilting of buildings, instability of roads due to moving ground, soil erosion and slope instability are reported. Slope failures caused severe damage to roads, private homes, drainage and sewage systems, water supply sources, collapse of the waste collecting and disposal systems, deposition of red sediments on basic infrastructures (football ground, schools), and in the beach area, displacement of hundreds of families mainly affecting the impoverished. Since these failures presented a hazard to the public, considerable attention was given to stabilisation and corrective measures. The use of these measures should be preceded by a prior identification of failure mechanisms in order to recommend the adequate preventative techniques, but are often taken without real assessment of the degree of slope stability.
Slope stability
The field of slope stability encompasses the analysis of static and dynamic stability of slopes of earth and rock-fill dams, slopes of other types of embankments, excavated slopes, and natural slopes in soil and soft rock. Slope stability investigation, analysis, and design mitigation is typically completed by geologists, engineering geologists, or geotechnical engineers. Geologists and engineering geologists can also use their knowledge of earth process and their ability to interpret surficial geomorphology to determine relative slope stability based simply on site observations.
Water can increase or decrease the stability of a slope depending on the amount present. Small amounts of water can strengthen soils because the surface tension of water gives the soil a lot of cohesion. This allows the soil to resist erosion better than if it were dry. If too much water is present the water may act as a lubricant, accelerating the erosion process and resulting in different types of mass wasting (i.e. mudflows, landslides, etc). A good example of this is to think of a sand castle. Water must be mixed with sand in order for the castle to keep its shape. If too much water is added the sand washes away, if not enough water is added the sand falls and can not keep its shape.
GEOLOGICAL SETTING
The built up area of Maputo lies on top of a sedimentary sequence of varying thickness and mineralogical composition, mostly of marine, fluvial and aeolian. These sequences are grouped into Tertiary formations (Inharrime, Santiago and Ponta Vermelha) and Quaternary formations (Matola, Machava, Malhazine, Congolote, Costa do Sol, Xefina, Inland dunes, Alluvial deposits and Tidal and Shoreline deposits).
The geological structure is relatively simple, being dominated by two normal faults of NNE/SSW trending, the Polana and Infulene faults. The Infulene Fault led to the formation of the low-lying valley of the Infulene River while the Polana Fault caused the formation of the escarpment analyzed in this study. This fault runs parallel to the coast being the boundary between Ponta Vermelha Formation to the West and the Xefina Formation to the coast. To the South of the city the coastal slope curves inland along the Maputo Estuary leaving a 100 to 1000 m wide strip of Alluvial Deposits between its foot and the estuary. The height of this natural slope formed by these tectonic events varies from 10 to 50 m. The characteristic angles of the problematic areas are typically 20 degrees observed at the long abandoned estuarine slope at Nações Unidas Avenue and also of the North of the city and about 40 degrees observed at the coastal slope facing and close to the sea. Locally a slope angle of 60 degrees is observed.
SLOPE STABILITY ANALYSIS
Historical notes and causes
Slope instability problems started to be a cause of concern in the last decade when the first cases were registered.
The first cases coincided with the increasing population density and development of informal and formal construction projects in the immediate coastal zone. Much of the development was above the neutral line of slides. In the late 1990s gullies were opened in the slopes in Polana-Caniço residential area near the entrance of Main Campus of the Eduardo Mondlane University and small landslides were registered in the Friedrich Engels Avenue. In February 2000 an extremely heavy, unusual and short-lived rainstorm hit Maputo with 400 mm recorded in 4 days (30% of the mean annual precipitation) and the problems of slope stability were exacerbated. Water flow into the top of the slopes saturated the soil and reduced its strength, triggering slope instability as well as causing deep gully failures. In addition, several isolated cases of damage to infrastructures occurred throughout the city:
1.) Located along Vladimir Lenine Av., close to the Supreme Court building, it seems that unstable ground has led to folding of the asphalt and systematic problems of road bumps;
2.) Vladimir Lenine Av., in the same area as above, the construction of a multi-story building called “Millennium Hotel” was abandoned, apparently due to unstable ground problems;
3.) Patrice Lumumba Av., behind Girassol Hotel, grows concern over continuous soil erosion problem;
4.) Julius Nyerere Av., five buildings are subject to subsidence and slope instability, causing panic among residents;
5.) Friedrich Engels Av., a retaining wall and road integrity are subject to continuing erosion and slope instability;
6.) Marginal Av.(Costa do Sol area), the construction of Quatro Estações Hotel was abandoned long ago, apparently due to subsidence and slope instability (Along the coastal line);
7.) Marginal Av.(Costa do Sol area), retention wall and coastal erosion protection trees have been destroyed by sea waves and the road is in danger (along the coastal line).
CONCLUSIONS
The stability of slopes in Maputo City has been analysed and soil engineering properties tested. From these studies it can be concluded that the soils of the study area are generally medium dense to loose up to very loose silty very fine to medium sands. These soils are normally consolidated and their structure becomes unstable in the presence of water.
Most of the slopes studied are unstable. A sensitivity analysis was carried out to generate alternative measures that support decision making by technicians aiming to solve slope stability and soil instability problems in Maputo. It showed that morphology is one of the main factors controlling mass movement, and it played an important role in slope instability. The topographic difference between the upper and down town and the respective slope angle have contributed to landslide and slope instability hazard in Maputo City.
To qualify to log this EarthCache you have to answer the following questions and sent it to me (do not place it in your log):
1.) What do you understand about mass wasting?
2.) What is the angle degree of the slope at this site?
3.) Do you see any corrective measures taken to make these slopes more stable?
4.) Describe any damages to buildings or structures in this area which are affected by slope instability.
5.) Describe the slopes at the other sites as mentioned above if you’ve visited them.
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Resource and acknowledgement: Dept of Geology, Eduardo Mondlane University.
