Vízügyi Közlemények, 1966 (48. évfolyam)

4. füzet - Rövidebb közlemények és beszámolók

(78) STUDIES ON SEALING LAYERS OBTAINED BY STABILIZING LOCALLY AVAILABLE SOILS By dr. I. Bogárdi and dr. L. Gáspár, Civ. Engrs. (For the Hungarian text see pp. 323) Efforts aimed at the reduction of seepage losses led to the investigation of types of soil sealing, which can be realized in a short time and which involve low construction and maintenance costs. In many instances lull impermeability is not required and the comparison of economics of the benefits obtainable by a certain degree of sealing with costruction costs may indicate an optimum solution, consisting of a seepage barrier, rather than an impervious lining. In Chapter 2 soil stabilization methods used in road building are considered from the point of view of their sealing properties. Experiences gained in road building practice are utilized, yet at the same time particular conditions of hydraulic construction are also allowed for. As indicated by laboratory evidence, silty clay soils which occur frequently in Hungary can be stabilized successfully with the help of cut-back bitumen, or cement and fly-ash from thermal power stations. Soil mechanical properties of basic materials have been tested at the laboratory with results summarized in Fig. 2. Soil mixtures of different composition were compared on the basis of their strength (Marshal stability in the case of bituminous stabilization, uni-axial compressive strength for others) and permeability (coefficient к in Marshal cylinders and tanks of 80 cm diameter) characteristics. Effects of exposure to atmospheric influences were studied by accelerated tests, using ultra-violet radiation and repeated cycles of freezing and thawing. Concerning stabilization with cut-back bitumen it was found that a k-value of 5 • 107 to 10

_

e cm/sec could be attained by adding 7% bitumen of quality HB-O and 4% cement of quality С 400 to silty sand, and this /с-value decreased with age (Fig. 10.) Good results were obtained also with silty-sandy gravel soil stabilized with less cement (2%) and bitumen of higher viscosity (quality HB-1) as, shown in Table III. The optimum molding water content for reducing seepage is with 12 to 15% higher than that determined by the compaction test, which yielded 9% (Figs. 9, 8). Both strength and elasticity of stabilized soil were appreciably higher than required by highway specifications. After exposure to ultraviolet radiation Marshal stability increased, elasticity was lower and permeability higher (Fig. 11). After the freezing test a loss in strength and an increase in permeability of about one order of magnitude was observed. Accelerated weathering test were found to impair properties of the stabilized soil within the permissible limits. Three types of soil stabilization with fly ash were investigated (Fig. 15). Basic fly-ash alone was found to be unsuited for soil stabilization (Tests C, and C

2), since the mix lost its strength under water and its permeability increased gradually. Favourable results were obtained by the combined use of fly ashes which showed an acidic reaction, or which contained medium amounts of lime, and cement (5 to 10% flyash plus 4% cement of quality С 500). Here coefficient к decreased gradually (А"иос1

ау

5 = = 107 cm/sec) accompanied by an increase of strength. As demonstrated by tests A, E, G

x, G

3 and G

3 basic fly ash can be used for subtituting hydraulic additives in bituminuous soil stabilization. In fact samples prepared instead of 4% cement of С 400 quality, with 6% fly ash and 7% of HB-O quality bitumen showed at 112 days age a . /c-value of 5 • 10~

7 cm/sec (test D). In the case of silty sand soils used, cement for soil stabilization was inferior for sealing purposes to bitumen or fly ash. The soil mixture selected on the basis of strength properties (Fig. 12.) and containing 9% cement of С 500 quality had /с-values from 9 • 10~

6 to 2 • 10~

5 cm/sec. Two methods are avaiable for realizing soil stabilization. The family of machines used on the mix in place method performs mixing without transporting soil. The family of machines based on the Howard-type mixer is described as one which found wide application in Hungary. Sloping surfaces could also be stabilized — perhaps after minor modifications with the mix in place method. For small and irregular cross sections frequently encountered in hydraulic engineering practice (irrigation