Elsevier

Powder Technology

Volume 101, Issue 1, 4 January 1999, Pages 17-30
Powder Technology

Load diversion by embedding in crushed salt: Experimental results, Part I

https://doi.org/10.1016/S0032-5910(98)00119-3Get rights and content

Abstract

The semihydrostatic model permits the analytical calculation of the pressure distribution in a vertical cylindrical borehole filled with waste packages and crushed salt. The pressure components depend on the axial and radial coordinates. Two test stands of CFRP with different wall roughnesses were erected for the experimental verification of the model. The sensors manufactured for measuring the radial and axial pressures are characterized by high sensitivity, great thermal resistance and adequate stability. The measuring techniques used are capable of measuring the pressures with high axial position resolution. The investigations in the small test stand LEISA I with a smooth wall, described in this paper, show a clear dependence of the measured vertical pressures on the fine grain fraction of the bed. The vertical pressures rise with increasing fine grain fraction. A correlation between the pressures and the densities of the respective grain fraction cannot be observed. The lateral pressure coefficients are determined from the radial and vertical pressures measured. They can be used together with the measurements of the wall friction coefficients in a shear apparatus to calculate the pressure distributions. There is good agreement between measured and calculated pressures. This agreement will further improve when a bottom plate with differently arranged vertical pressure sensors is completed and a lambda meter is built for exactly measuring the lateral pressure coefficient.

Section snippets

Problems and objectives

Current plans of the Federal Office for Radiation Protection (BfS Bundesamt für Strahlenschutz) provide for the emplacement of all types of solid and solidified high- and intermediate-level waste, especially waste involving considerable heat generation, in a mined geological repository in underground rock salt formations. Emplacement will either be in vertical boreholes or alternatively in horizontal drifts.

For emplacement according to the borehole technique, the packages are lowered from the

Theoretical models

There is a multitude of theoretical approaches and models to calculate the vertical and radial pressures exerted by granular materials inside cylindrical walls. The best known model is that of Janssen [3]. This theory assumes that the vertical pressures on the cross-sectional area are homogeneously distributed. Full friction acts on the side surfaces between the granular material and the wall. The ratio of horizontal to vertical pressure is assumed to be equal to an empirical constant (lateral

Test stands

In calculating the pressure distribution according to the different models it is generally assumed that the cylindrical walls are rigid and immobile. This condition is fulfilled in the case of emplacing waste packages in vertical boreholes drilled into underground rock salt formations. It must therefore also be fulfilled for the experiments in the laboratory. For this purpose, two test stands of CFRP (carbon-fibre-reinforced plastic) with diameters of DB=0.25 m (LEISA I, Fig. 2) and DB=0.6 m

Filling the test stand

For the measurements at the LEISA I test stand described in this paper, crushed salt with a grain size of 0–10 mm is used varying the fine grain fraction XF. The fine grain fraction defined here as the fraction of the bed in the range of 0–2 mm varies between 0 and 1.0 in step widths of 0.2.

Test material is produced by sieving the fine grain fraction of 0–2 mm out of the total 0–10 mm fraction and adding it proportionally to the 2–10 mm fraction, depending on the requirement. The test material

Measured pressure distributions

For the fine grain fractions XF=0, 0.2, 0.4, 0.6, 0.8 and 1.0 the vertical and radial pressure components are measured in relation to depth for uniform filling. In the case of known geometrical dimensions of the test stand it is possible to determine at the end of each experiment the current filling height or depth below the crushed salt surface during the filling process from the weight of the material in the feeder, which is always recorded at the same time as the pressure values. On account

Calculated pressure distributions

In order to calculate the pressures, all relevant parameters are substituted in the equations. In the literature, the wall friction coefficient between granular material and silo wall is commonly used for calculating vertical pressures in silos. Good agreement is observed, in part, between measured and calculated values. The wall friction coefficients are seldom less than 0.4 in such experiments. Values in the range of 0.12, as for the combination of smooth CFRP wall/crushed salt, are not

Radial distribution of the pressures

In Janssen's model, the vertical pressure is independent of r and constant over the cross-section. According to the semihydrostatic model, the pressure components change both with the axial and with the radial coordinate. The vertical pressure should increase from the central axis of the borehole towards the wall. The value pv=0 at r=0 does not occur due to the finite geometrical extension of the sensors.

In the literature, different results are reported for the pressure distributions on the

Conclusions

The semihydrostatic model as a generalization of Janssen's model permits the analytical calculation of all pressure components in vertical, cylindrical boreholes filled with crushed salt and waste packages. The pressures are dependent both on the axial and the radial coordinate. Differences from Janssen's theory are mainly found in the zone of low depths. The pressures according to Janssen are always lower than the pressures calculated according to the semihydrostatic model. The generalized

List of symbols

gacceleration due to gravity
klateral pressure coefficient
kcorrcorrected lateral pressure coefficient
kuncorruncorrected lateral pressure coefficient
prradial pressure component
ptvertical shear stress
pvvertical pressure component
pvmaxmaximum vertical pressure
pφazimuthal pressure component
rradial borehole coordinate
RBborehole radius
XFfine grain fraction
Xwmoisture content
zaxial borehole coordinate
δangle of wall friction
μiinternal friction coefficient
μWwall friction coefficient
ρSdensity of the

References (24)

  • D.M. Walker

    Chem. Eng. Sci.

    (1966)
  • J.K. Walters

    Chem. Eng. Sci.

    (1973)
  • S.C. Cowin

    Powder Technol.

    (1980)
  • S.C. Cowin

    Powder Technol.

    (1985)
  • V. Sundaram et al.

    Powder Technol.

    (1985)
  • M. Shahinpoor

    Appl. Math. Modelling

    (1981)
  • W. Feuser

    Powder Technol.

    (1997)
  • J.C. Williams et al.

    Powder Technol.

    (1987)
  • E. Barnert, H. Brücher, K. Kroth, Einlagerungs- und Bohrlochverschlußtechnik—Abschlußbericht für die Projektphase...
  • W. Feuser, Messung mechanischer Eigenschaften von Salzgrus und Überlegungen zur Semihydrostatischen Einlagerung von...
  • H.A. Janssen

    Z. Ver. Dtsch. Ing.

    (1895)
  • S.C. Negi et al.

    Can. Agric. Eng.

    (1977)

    • Cited by (0)

    View full text
    Copyright © 1999 Elsevier Science S.A. All rights reserved.