Page 48 - Failure Analysis Case Studies II

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5. EXPERIMENTAL PROGRAM

The experimental program consisted of two components. The first was laboratory testing of pipe
sections in order to determine their stiffness (stiffness factor = EO, vertical deflection or strain,
which induces cracking in the inner pipe liner and collapse loads. The second was a field investigation
which included opening of test pits at several sections along the pipeline. Excavation of the test pits
allowed for visual description of the soil-trench cross section, and performance of dynamic cone
penetration (DCP) tests within the sand backfill alongside the pipe. The field investigation was
limited to a 330 m pipeline segment.

5.1. Results of tests on pipe sections
Ring compression (bending) tests were carried out on three sections of pipe. Each section was
placed in a hydraulic press and loaded across its vertical diameter by a line load along the full
segment length. Throughout loading of each test section, vertical and horizontal deflections were
monitored. Visual physical damage to the inner pipe lining (cracking) was also recorded. Figure
3(a) presents the experimental load deformation curve of one of the pipe sections together with
observations with respect to crack development throughout the test. Figure 3(b) shows that the
results for the three sections are fairly similar.
Based on the data presented in Fig. 3 it is possible to obtain the following information:
The collapse load of the pipe section is between 50 and 55 kN/m. Collapse occurred at vertical
deflections of 63-87 mm which correspond to diametrical strains of 5-7%. It is noted that these
values characterize the unsupported behavior of pipe sections.
The maximum moment acting in the pipe section at the collapse load may be determined by
eqn (l), after Timoshenko and Gere [I]. For the pipes tested the maximum moments at collapse
varied between 5.M.O kN m/m,

where P is the collapse load per unit length as noted above, and R is the pipe radius.
The stiffness factor of the pipe (EI) can be determined based on the linear section of the force
deflection curve using eqn (2) [I].

where Ay is the vertical pipe deflection under load per unit length P.
The calculated stiffness of the three pipe sections was found to be approximately 13.5 kN m. It
is noted that the EI is an inherent property of the pipe section which is independent of
lateral support conditions. This experimentally determined pipe stiffness is representative of the
composite pipe cross section, which includes both concrete layers and the steel core.
Severe cracking of the inner liner wall (defined as a crack opening of 0.3 mm [2]) occurred at a
vertical diametric strain of approximately 1.2%. The working assumption used throughout the
investigation has been that cracking occurs at the same strain value irrespective of the support
conditions. Obviously the load required to impose this strain level is dependent upon lateral
support conditions.

5.2. Resuits offield investigation
Dynamic cone penetration testing was performed at several stations along the investigated portion
of the pipeline. Technical details of the testing procedure and interpretation of results may be found
in [3]. The testing was performed following excavation of the fill material down to the pipe crown.
Two or three DCP soundings were performed within each excavation to a depth of approximately
1.61.8 m. The end point of the sounding was located at a depth of approximately 0.5 m below the
pipe invert. The plots shown in Fig. 4 are typical results found at six stations. It is noted that, in