Problemática da caracterização de solos aluviais para fins de drenagem subterrânea.

Abstract

The objective of this work was to study the problems of interpretation and representativeness of alluvial soil hydraulic conductivity field results, to define the best method for its characterization, and to verify the most appropriate procedure for the analysis of these results, for underground drainage purposes. The work was conducted in the Irrigation Project of São Gonçalo, PB, where a Pilot Field of Underground Drainage Testing, of 1.3 ha, was established. It was also studied the horizontal and vertical spatial variability of the physical-hydraulic properties such as texture, apparent or global density and basic infiltration, and the characteristics of the water flow to the drains through discharge tests. For the study of hydraulic conductivity K, parameter of importance in the definition of drainage standards for the design of underground systems, four field methods (well, piezometer, basic infiltration and drainage discharge) were used and an indirect method based on granulometry . In general, a large horizontal spatial variation of the physical properties was observed, mainly due to the presence of mixtures of soil layers with textures muddy sand and sandy clay, with others more densified by high sodium concentration. It has been found that bulk density and basic infiltration data tend to have a normal distribution. The apparent density variation was 1.45 to 1.77 g cm -3 and the basic infiltration was 0.065 to 1.28 m day -1. In the case of hydraulic conductivity, there was a low correlation between values ​​obtained by the different methods in the same place. Analyzing the data by calculating the probability of occurrence it was defined that the values ​​of K follow the trend of a logarithmic distribution. Using this methodology it was verified that the representative values ​​(50% probability) of the different methods were of the same magnitude. It was also verified that, because the methods provide values ​​of K representing different layers of the profile, thus giving results of different magnitudes, their necessary representativity would be obtained through the horizontal spatial variability of the soil. From the traditional methods to determine K, well and piezometer, it was verified that they can be used in alluvial soils subject to some restrictions, such as adequate density and distribution of determinations, and dependence on the presence or absence of a lot of sub - superficial stratification. In the case of tests for discharge of underground drains, the drainage system was recovered and cleaned of the area of ​​1.3 ha containing manhole drains 60 cm long and 10 cm in diameter, with a total of 5 lines of drains of 70m in length, installed at a depth of 1.4m and 20m spacing. In this area, a symmetrical network of 25 observation wells and exit structures were installed in the drain lines to facilitate flow measurement. This area was recharged for 15 days, and during the discharge period, simultaneous measurements were made every two hours of the depth of the sheet in all wells and of the flow in the five drain lines. For the analysis of the data, Hooghoudt's formula for permanent flow, and Glover-Dumm's for non-permanent flow, the latter being the main objective of the work. It was found that the expression for non-permanent flow did not adequately characterize the effective hydraulic conductivity of the flow area, the drainage intensity factor, and the porosity of the profile that contributes to the flow of the drains. The use of the equation for permanent flow gave K values ​​closer to the representative values ​​obtained by the other field methods. However, due to the great textural stratification of the profiles, a greater number of tests are required under variable recharge conditions, in order to define the best procedure for analyzing the results