Methods of wetting loess soils in the borrow area for fill in impervious sections of a dam

Abstract

(282.255.1) and V. I. KorsuntsevWhen building rolled earth structures like the Nurek dam, which has a loess clay core with a volume 80 million m 3 and coarse gravel supporting shells with a volume 45 million m 3, the moisture content of the soil has a substantial influence on its compactibility.Adequate soil density with minimum compacting effort can be achieved only at moisture content approaching the optimum. Depending on the compacting machinery, loess soil has an optimum moisture content in the range of 13-18%. With moisture contents below 5-8% or over 22-24%, mechanical compacting is practically ineffective. The method used for wetting the soil is therefore important. Wetting of loess soil can be carried out either after placement or in situ at the borrow area. Transport of the soil in trucks prior to wetting results industin the atmosphere and on the roads, which is unacceptable from the safety standpoint, so that every effort has been made to investigate methods for wetting the soil directly In the borrow area. Due to the presence of macropores in loess soil, water penetrates readily and quickly into the soil in sire. The seepage coefficient in the vertical direction is A -102 to A 9 103 cm/sec, being nearly 100 times greater than in the disintegrated structure.The clays of the Langar borrow area are fairly homogeneous both in area and depth. Their average physical and mechanical properties are characterized by the data of Table 1. The investigations were aimed at determining the best means for wetting the soil in the borrow area, ensuring the best penetration of the moisture. Surface wetting and wetting by means ofboreholes and pits were investigated.After removing the vegetation the borrow area was divided into sections by dikes 30-40 cm high (Fig. 1). In the two-stage experiments preliminary wetting was carried out on small areas (sections) 2 x 5 m. The sections, separated from each other in order to avoid mutual interference, were wetted in sets of 3 to 4 with a restricted water supply to the area.The experiments continued for more than three years. Wetting by means of boreholes and pots was studied at the same time. In order to develop a technology of prewetting transporting and placing the wet soil,and establishmerit of an optimum depth of submersion of the sites and in order to determine the necessary wetting time, the investigations were continued in 1965 on a larger scale. The sections were made with dimensions 10 • 10 m and wetted in separate sectors (of 2 to 4 sections each) with sheets of water 10, 25, 40, and 55 cm deep, remaining under water for 2, 6, and 10 days. The advance of the wetted front is shown in Table 2. A moisture content over 20% was assumed to define the boundary of wetting.The depth of wetting and degree of wetting were obtained by means of drill holes. Sampling for moisture content was carried out every 0.5 m throughout the depth. The holes were made prior to wetting the sites, on the first or second days and after I5, 30, 60, 90, 120, and 270 days following wetting. It was ...