Al Ain Trunk Sewerage, UAE

ESC carried out the design calculations on behalf of the Main Contractor and has installed the sheet piles and two levels of struts which was necessary according to the design. All designs were based on the British Standards.

Introduction

Al Ain is the fourth largest city in the UAE. The ever increasing population of this oasis city has increased the demand for additional infrastructure facilities. Amongst them is the need for sustainable sewerage network for the city. The Zakher Waste Water Treatment plant receives an average daily flow of 108,964cu.m (according to the 2007 estimates). The present sewerage infrastructure is believed to be severely overloaded. Abu Dhabi Sewerage Service Company (ADSSC) has planned to rehabilitate and refurbish the existing sewerage infrastructure and construct the new network in order to meet the current and future demands.

Gulf Contractors Company (GCC) is appointed as the Main Contractor for one of the packages to construct a new sewer pipe along the Zakher - Rewah road (close to Abu Dhabi – Al Ain truck road) and Hyder Consulting as the Consultant. The project involved laying of the sewer pipe, construction of manholes etc along the sewer line alignment. ESC is entrusted with the work of installing a temporary retention system for the proposed NDM pits. The pits required an excavation up to 15.24m from the existing ground level.

Site Stratigraphy

In general, the overburden soils at site consisted of a 3m thick dense slightly silty sand followed by very dense silty cemented sand (SPT N value > 50) to about 12.5m followed by Moderately weathered Sandstone (bed rock). However thin layers of very dense silty sand were observed between the rock layers, which made the conventional sheet pile installation methods unsuitable. Water table was observed to be at 6~7m depth.

Sheet Pile Retention System

The original design was to install concrete caissons with sequential excavation until final formation level is reached. Due to the high water table and very hard ground conditions (rock) it was advised to go with embedded sheet pile wall which is faster and technically sound solution. Working platform for the installation works was lowered with the help of sloped excavations to limit the length of the sheet pile to 12m. 2 levels of strutting were installed for additional lateral support (refer figure 1).

Modeling was performed using an effective stress analysis based on BS8002, using the analysis software ReWaRD. Struts were designed in accordance with BS5950.

Cross section illustrating the sheet pile retention system with 2 levels of strutting.

Construction

Installation was done in three phases. First phase consisted of installation of the temporary retention wall to secure the power cable. This was followed by sloped excavation and creating a platform for the crawler crane. Third phase consisted of installation of the sheet piles for the cofferdam. Pre-drilling was carried out before installation of sheet piles owing to very dense and rocky ground conditions (refer figure 2). Installation of the sheet piles was done, by vibrating the sheet piles to required level with a vibro hammer mounted on a 50T crawler crane. Guide beam is used to keep the sheet piles in alignment (refer figure 3).

The above retention system is adopted at three pits. One pit was 10m x 10m whereas the other two pits were of 8m x 8m. A total of 310 sheet piles (NSP3W and FSPIV), 12m length, were installed for cable protection and for the main pits. Corner piles are fabricated according to the site requirements and were installed to reduce the water seepage into the pits to facilitate the dry working platform at the formation level.

To add to the lateral stability of the retention system, two level strutting was required (refer figure 4). After completion of the sheet pile installation and necessary excavation is done to the first level of strut, strutting was done with 305UB97 beams. This was followed by further excavation to the second level strut. 533UB101 beams were used for the second level strutting. A total 26 Ton of beams were installed for the strutting works.