Hidrológiai Közlöny 1971 (51. évfolyam)

1. szám - Dr. Öllős Géza: A kutak vízhozamát befolyásoló tényezők

18 Hidrológiai Közlöny 1971. 1. sz. Special Conference Number, Szebellédy, L. Prior to investigating the causes and development of these processes, it should be remembered that the aim of the well — as a structure withdrawing groundwater — is to ensure sand-free water production at favourable hydraulic conditions. This requirement implies that the soil particles around the well have a fixed position, their geometrical configuration, the coefficient k of the soil-field are constant; at the same time the geometry of the withdrawal system of the well is alsó regarded as constant. Actually these conditions cannot be realized, at least not completely; owing to mechanical, chemical or biological causes a surplus of material may form in the vicinity of the well, in the natural soil around the gravel packing and even in the filter system. The clogging effect of this excess material usually increases in time. 2.1. The silting up of wells Research in recent decades indicated that the Sichardt- approach applied to wells, results mostly in a much too conservative and thus uneconomical design in most soil types. Development in the design philosophy—as indicated for instance by the investigations of Á. Kézdi and Gs. Szolnoky—can be expected from the analyzis, comparison of the granulometric curves for the soil and gravel pack. Investigations by Gs. Szolnoky imply, that movement of the soil particles around the well, the silting up of the well, is related not so much to the velocity and direction of seepage, but, first of all, to the diameter (D) of gravel particles in the pack and that of soil particles (d), and can be expressed by the ratio The safe diameter ratios N for soils, and the gravel filters found suitable, at 15, 50 and 85 percent passing, are as follows: N „=15 N di D 50 50 ~ ^

8

5 = d = 8, = 6, = 4, 85 T Fig.2. z.f(r) r

0 n Sig.3. Fig. 2 — 3. Interpretation of symbols and boundary conditions of the well—due to the seepage field with a permeability of k

v — is: H

2-Z* Q=nkIn — — íl In — ro V kJ r

0 The three ratio limits N need not be satisfied simultaneously since the gravel pack will support the surrounding soil even if the ratio N meets the above requirements at a single percentage value only. In case of the example indicated in Fig. 1, the gravel pack supports safely both soils a and b. In all instances when the permeability coefficient of the soil in the vicinity of the well varies in time, the yield becomes difficult to calculate. For this reason the suggestion put forward by P. Mauersberger for calculating the yield is of great interest: for the fully penetrating well in Fig. 2, when in the cylindrical space with a radius r

t around the well, the condition of k

k =f= k is satisfied, the yield In a uniform layer with permeability k the yield of the well be Q

ho

m. Thus Q 1 Qh, om -j . I k (I1) ln

y

i ro ln/i r

n As proved alsó by P. Mauersberger, this relation is valid alsó for a partially penetrating well, (see Fig. 3), if r

1—r

0 is negligibly small compared to the well diameter. 2.2. Corrosion Experience has shown the deterioration of wells to be possible even while groundwater of potable quality is pumped. Highly detrimental effects (e.g. "ti Fig. 1. Interpretation of the ratio numbers