Summary

Following up a suggestion that packed sand could be used as an anchorage material for anchoring cables in boreholes, tests were carried out on direct models to study the loads that different depths of granular media would sustain, whilst other conditions such as grain size, moisture content and model dimensions were changed. Theoretical work was carried out to try and evaluate the possible load bearing capacity.

Resume

Suite à une suggestion sur la possibilite d'employer du sable compacte pour ancrer des cables dans les trous de sondage, on a fait des essais sur modèles directs afin d'etudier les charges que differentes profondeurs de milieux granulaires subiraient, lorsqu'on faisait varier les autres conditions, telles que la granulometrie la teneur en eau et les dimensions du modèle. On a aussi cherche à determiner theoriquement la valeur possible de la capacite portante.

Zusammenfassung

Nach einem Vorschlag, dass gedichteter Sand als Verankerungsmaterial fuer Kabelnverankerung in Bohrlocher angewandt könnte, wurden Versuche an direkte Modelle durchgefuehrt, urn die Belastungen zu untersuchen, welche die verschiedene Tiefe von körnigen Mitteln unterstuetzen wuerden, wahrend andere Bedingungen, wie Kornabmessungen, Feuchtigkeitsinhalt und Modellabmessungen geandert werden. Es wurde theoretische Arbeit durchgefuehrt, um die mögliche Tragfahigkeit zu ermitteln.

Introduction

Techniques of anchoring cables to a rock foundation have been perfected during the last decade, due mainly to the growing awareness of the importance of utilising large volumes of rock for the foundations of major structures. The technique is, basically, to pass the cable into a deep rock borehole and then to pump in cement grout. Recent work on some aspects of rock foundations has emphasised the need for a method of anchoring a cable temporarily. An important application is found in the latest approach to «in situ» testing of rock strata, where the reaction load is taken by anchored cables instead of by the roof of a tunnel. This report describes work carried out on one inexpensive and simple method of temporary anchorage. It explores the possibility of using a granular media within the borehole to support the load. This follows from a verbal suggestion made by F. F. Ferguson late of the Cementation Co. Ltd. This may prove a feasible approach if the load bearing characteristics were found to be suitable since the anchorage medium could easily be removed with a high pressure jet of water. Although the work described is biased towards its use for «in situ» rock testing, many other commercial applications can be envisaged. Pull out tests on direct models were conducted to study the effects of the important variables. In considering the theoretical aspect, all simple analyses gave results much lower than those experienced in practice. The main theoretical work was based on an elastic analysis of the media under an assumed shear pattern. To the authors knowledge no previous work has been conducted on either the experimental or theoretical aspects of this topic.

Experimental work

The experimental work performed consisted of tensile tests on direct models of an anchorage in a borehole. The variables considered quantitatively were:

  • The depth of granular media.

  • The model dimensions.

  • The grain size and material of the media.

During the tests some qualitative observations were made to study the effect of:

  • Vibration, both before and during loading.

  • Repeated loading.

  • Load holding.

  • Extreme moisture content variations.

Two models were constructed, the details of which are shown in Figs. 1a and lb. A series of stainless steel locating points were glued onto the outer surface of the tube of the smaller model to enable longitudinal strain measurements to be taken. All tests were carried out on an Universal testing machine of 100 ton capacity. In the first series of tests performed on the small apparatus, the material used was that portion retained between No. 52 and No. 25 B. S. sieves during the sieving of a beach sand. The depth of sand above the surface of the piston was varied from 3½ in. to 14 in. The procedure during a test was to place the piston end of the anchor rod inside the tube and then to pour the required depth of sand in whilst maintaining the rod and tube vertical. In general, precautions were taken to reduce external vibration during filling and loading. Tension was then applied axially between the rod and the tube, the load being applied at a fairly constant rate. During four of the tests, deflection readings were taken longitudinally, at ½ ton intervals for loads,above 1.5 tons. Tests were continued until a clear peak could be seen on the load extension graph (see Fig. 3). To avoid damaging the apparatus all tests were discontinued at a load of 5 tons. A total of 28 tests were conducted on this model, but only twelve of these gave a failure result.

This content is only available via PDF.
You can access this article if you purchase or spend a download.