2315
Design and installation of buried large diameter HDPE pipelines in a coastal area
Project et installation de tuyaux enterrés de grand diamètre en zone côtière
Bellezza I., Mazzier F., Pasqualini E.
Dep. SIMAU – Università Politecnica delle Marche, Ancona, Italy
D’Alberto D., Caccavo C., Serrani C.
SPS - Società Progettazione Servizi s.r.l., Ancona, Italy
ABSTRACT: The present paper deals with the main geotechnical aspects of the design and installation of two adjacent HDPE large
diameter pipelines along the Adriatic Sea (Italy) coastline. The pipelines -270 m in length and 2 m in diameter - are conceived as
buried collectors of polluted runoff water, to convey to sanitation prior to discharge into the sea. Considering that pipes are below the
water table uplift analysis is detailed, showing three possible approaches in static conditions, whereas in seismic conditions a method
is proposed that include the build-up of pore-water pressures during earthquake. As far as prediction of vertical deflection is
concerned, the backfill loosening due to sheet piles extraction has been modelled by assuming no compaction (dumped backfill).
Despite this assumption, theoretical short term deflection represents a lower bound of measured deflections.
RÉSUMÉ : Cet article décrit les principales problématiques géotechniques du projet et de l’installation de deux tuyaux enterrés
adjacents de grand diamètre sur le littoral adriatique italien. Les tuyaux – longs de 270 m - ont la fonction de collecteurs enterrés pour
l’eau de ruissellement polluée, pour permettre sa dépuration avant du déchargement dans la mer. En considérant que les tuyaux se
trouvent au-dessous du niveau de la nappe d’eau on a analysé le problème du possible soulèvement en conditions statiques utilisant
trois différentes méthodes. En conditions sismiques on a proposé une méthode qui considère le développement de pressions
interstitielles positives excessives durant le tremblement de terre. L’ovalisation du tuyau a été calculée par une méthode de littérature
en considérant un remblai sans compactage pour tenir compte de l’extraction des palplanches utilisées pendant l’excavation de la
tranchée. Les valeurs calculées de l’ovalisation initiale représente un limite inferieur de l’ovalisation mesurée.
Keywords : uplift, pipe of large diameter, deflection
1 INTRODUCTION
Urban and infrastructural development often involves
vulnerable areas such as coastlines. To prevent pollution of the
sea from runoff water of a nearby urban area and crowded roads
a system of buried collectors are to be built along a stretch of
the Italian shoreline of the Adriatic Sea. In such a way, the
collected runoff water will be conveyed to sanitation before
discharging into the sea. A preliminary hydraulic study allowed
to define different drainage basins, and for each basin an
adequate collector is required. This paper deals with the design
and execution of the first part of the system, concerning a
collected water volume of about 1300 m
3
, for which the
designers opted to realise the collectors by two adjacent buried
pipelines of 2000 mm in internal diameter and 270 m long.
ADRIATIC SEA
COLLECTOR
RAILWAY
PIEZOMETER
Figure 1. Plan view of the working area
The design and execution of geotechnical works in coastal
area must in general face regulatory requirements,
environmental and aesthetic concerns, public attention.
As far as engineering problems are concerned, the designer
must take into account the objective difficulties connected with
the critical location (e.g., underwater excavation, tidal and storm
waves, risk of uplift). For the specific case, additional
constraints are represented by the closeness of the working area
to an important railway (Figure 1), and consequent limited
accessibility for materials and machinery. Moreover, local
authorities required to minimize the working area, avoiding the
occupancy of the beach for the overall length of the collectors
through the entire duration of the works. To comply with this
requirement, a staged execution was envisaged.
2 OUTLINE OF THE DESIGN
2.1 Pipe material
Corrugated HDPE pipes were selected (Table 1). HDPE offer in
general significant advantages in terms of costs, corrosion
resistance, ease of handling and jointing over more traditional
materials such spheroidal cast iron. The selected HDPE pipes
are manufactured in modules 6.0 m long (less than other
materials, e.g., cast iron) which allowed solving the problem
linked to area accessibility. In particular with the use of 6 m
long modules it is possible to reach the beach area passing
through a narrow railway underpass. For longer pipe modules, a
more expensive marine transportation would be requested.
