ABSTRACT

Data are presented from two sites in southeast England, where problems were encountered in tunnelling through ground containing flint. In response to the difficulties, substantial laboratory testing programmes were initiated to determine the strength of the flints. It is shown that the strength varies very considerably, even within a single flint particle and that the practice of estimating unconfined compressive strength by a single multiplier applied to the point load strength index (Is(50)) may give misleading results. The preparation of UCS specimens for testing was found to be very difficult and it is recommended that tunnel machine capacity is assessed relative to Is(50).

RESUME

Des donnees sont presentees sur deux sites dans le sud-ouest de l'Angleterre,où le perçage d'un tunnel dans un sol contenant du silex a pose des problemes. En consequences de ces difficultes, des programmes substatiels de recherche en laboratoire ont ete inities afin de determiner la resistance des silex. Il est demontre que la resistance varie considerablement, même au sein d'une particule de silex, et que la pratique de estimation de la resistance en compression non confinee, (UCS) par le biais d'un simple coefficient applique au 'point load strength index (Is50)' pourrait donner des resultants trompeurs. La preparation de specimens UCS destines aux tests s'est averee tres difficile et il est recommande que la capacite de la machine de perçage de tunnels soit calculee relativement au Is50.

ZUSAMMENFASSUNG

Daten von zwei Standorten in Suedosten von England, wo Probleme beim Tunnelbau durch feuersteinhaltigen Grund antraffen, sind angegeben. Wegen diesen Schwierigkeiten wurden Laboratoriumsforschungsprogramme eingeleitet, um die Festigkeit von Feuersteinen zu bestimmen. Es wurde gefunden, dass die Festigkeit sehr unterschiedlich war, selbst bei einzelnen Feuersteinstuecken, und dass die Praxis, die unbegrenzte Kompressionsfestigkeit (uKf) auf Grund eines einzigen Multiplikationsfaktors vom Punktladefestigkeitsindex (Is50) abzuschatzen, zu irrefuehrenden Resultaten fuehrt. Die Vorbereitung der uKf Pruefstuecke war sehr schwierig und es wird vorgeschlagen, dass die Kapazitat der Tunnelbaumaschine in Abhang der Is50 bestimmt wird.

Introduction

Flints occur widely in the chalk areas of the United Kingdom. They are present both within the chalk, generally concentrated in bands, and above the chalk as the residue of weathering or processes of re-deposition.

Tunnelling projects in these areas often encounter difficulties in removing the flints. The particles are commonly of cobble or boulder size and must be broken into smaller fragments to be removed. Hence the strength of the flint particles is of considerable interest to the tunnelling contractor. Data are presented from two projects in the southeast of England, one at Brighton where the tunnel ran through Upper Chalk and the other at Cray, where the tunnel ran through the deposits immediately overlying the Upper Chalk. For the Brighton project only six point load test results on flint samples were available prior to tendering; for the Cray project, there was no information. At both sites, problems were encountered during construction. Strength testing was undertaken as part of the investigations into the difficulties and included unconfined compression testing on cylindrical and cuboidal specimens, point load strength index tests.

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