PSEUDOSINKHOLE OCCURRENCES IN BRASILIA, BRAZIL

Brasilia, the capital city of Brazil, is located in the central region of the country. Climate in the area is semitropical with an annual ra infall of 1500 mm. The geological environment in the area consists of low-grade metamorphic rocks. Slates of varying colors, metasiltstone, and quartzite beds are present. Over the Precambrian rocks is a lateri tic layer varying in thickness from centimeters up to 30 m. Latosol do minates the existing plateaus, while laterite crusts and immature soil s are dominant in the transition zones between plateaus and river vall eys. Erosional problems related to the lateritic terrains were known p rior to the settlement of the city in 1961. During 1986, erosion becam e a serious threat when several pseudosinkholes occurred in the urban area. Occurrence of pseudosinkholes resulted in condemnation of an are a of 300,000 m2 and the demolition of several buildings. Preliminary s tudies indicated at the time that underground erosion and pseudosinkho les were generated by shortening of the percolation path of groundwate r due to the progress, toward the residential area, of large gullies. This produced an increase in the hydraulic gradient, resulting in the removal of latosol particles. Occurrence of pseudosinkholes in other a reas of the city has led the Institute of Geosciences to investigate t he problem in more detail. Studies have been conducted looking for cor relation between pseudosinkhole occurrence and geologic, geomorphic, g eotechnical, and urban development features. Recent results show a muc h more complex process then previously thought. Field data suggests th at termite activity and recharge of the water table by inadequate disp osal of residential sewer systems are directly related to the problem.