Page 45 - Failure Analysis Case Studies II

Failure Analysis Case Studies I1
D.R.H. Jones (Editor)
8 2001 Elsevier Science Ltd. All rights reserved 31

FAILURE OF A FLEXIBLE PIPE WITH A CONCRETE
LINER

MARK TALESNICK* and RAFAEL BAKER
Department of Civil Engineering, Technion, Israel Institute of Technology, Haifa 32000, Israel
(Received I5 Sepfember 1997)
Abstract-This study documents the functional failure of a concrctc lined steel sewage pipe. Symptoms of the
pipe failure are presented. Failure of the pipe system can be attributed to incompatibility between the
mechanical behavior of the pipe and the methodology employed in its design. The underlying cause of the
failure may be traced to a lack of sufficient backfill stiffness. In situ testing was used to evaluate the stiffness
of the side backfill. The existing pipe-trench system condition was analysed numerically and a criterion
developed for the consideration of the structural integrity of the pipeline. 0 1998 Elscvicr Science Ltd. All
rights reserved.
Keywords: Corrosion protection, fitness for purpose, pipeline failures.

1. INTRODUCTION

The present paper documents a failure of a large diameter concrete lined steel sewage pipe, buried
in a clay soil profile. The project consisted of a 3.5 km long gravity pipe in central Israel which
failed before being placed in service. The present contribution documents the failure of this pipe-
trench system. Field and laboratory testing provided significant insight into the probable cause(s)
of failure. The case study accentuates some basic design principles, as well as the use of simple field
tests as an effective diagnostic tool to evaluate site conditions.

2. DESIGN, CONSTRUCTION AND SITE CONDITIONS
The sewage pipeline was designed and constructed in central Israel during 1992-1994. The design
called for a steel pipe with an inner diameter of 120 cm and a wall thickness of 0.64 cm. The inner
surface of the pipe was lined with an aluminum based cement of between 1.8 and 2.2 cm thickness.
The primary purpose of the inner liner was to provide protection of the steel pipe from the affects
of the corrosive sewage flowing inside. The outer surface of the pipe was covered by a 2.5 cm thick
concrete layer.
The design of the pipe-trench system was based on a flexible pipe criterion. This implies that
the pipe maintains structural and functional integrity by mobilizing lateral resistance from the
surrounding soil. The pipe was designed to withstand static soil loads alone.
A design section of the piptrench system is shown schematically in Fig. 1. The pipe invert was
founded at a depth of between 4.5 and 5.5 m below the ground surface, depending on the natural
topography. The natural soil consists of a highly plastic clay (CH, liquid limit: o, = 62%, plasticity
index: Ip = 36%). A perched water table (depths of as little as 2-3 m) exists in part of the project
area. The design specified the excavation of a 2.5 m wide trench (twice the pipe diameter), placement
of a 20 cm thick layer of poorly graded gravel (GP) with a particle size between 16 and 20 mm. The
pipe was placed directly on the gravel layer. Following placement of the pipe section the design
specified that (a) dune sand (SP) with calcareous concretions (Dso = 0.17 mm and D,,, = 0.12 mm)
be placed around the pipe to a height of 30 cm above the pipe crown elevation; (b) above that

Author to whom correspondence should be addressed.
Reprinted from Engineering Failure Analysis 5 (3), 247-259 (1998)