the asphalt lining of a large water reservoir near bengazi - schmid

8/12/2019 The Asphalt Lining of a Large Water Reservoir Near Bengazi - Schmid

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The Asphalt Lining of a Large Water Reservoir near Benghazi

Die Asphaltdichtung eines groen Speicherbeckens bei Benghazi

Reinhard Schmid

Abstract

The Great Man-Made River Project utilizes Libyas vast fossil groundwater reserves in the

desert in order to supply fresh water to the populated and fertile Mediterranean coast. Pipelines

are transferring the water to artificial reservoirs in the target areas. Near Benghazi, the largest

reservoir with a capacity of twenty-four million cubic meters has recently been completed and

sealed by an asphalt lining of more than one million square meters. Special equipment

designed for hydraulic asphalt has been utilised to place the lining on the steep slopes and on

the floor.

Zusammenfassung

Das Great Man-Made River Projekt nutzt die fossilen Wasservorrte in der libyschen Wste zur

Versorgung der dicht besiedelten Kste im Norden des Landes. ber Betonpipelines wird das

Wasser bis zu 1600 km in die in den Zielgebieten errichteten Reservoirs transportiert, von wo es

zur Trinkwasserversorgung der Stdte oder zur Bewsserung landwirtschaftlicher Anbaugebiete

verteilt wird. Das grte Speicherbecken, das Grand Omar Mukhtar Reservoir bei Benghazi,

erhielt eine Asphaltdichtung, bestehend aus je einer Binder- und Dichtungslage. Hierzu wurdespeziell fr den Bschungseinbau entwickeltes Gert eingesetzt, das von fahrbaren Winden

gehalten wird. Die Abdichtungsarbeiten wurden im Mrz 2007 abgeschlossen.

1 Introduction

The Great Man-Made River Project of Libya is one of the most ambitious engineering water

projects worldwide: it started in 1983 and encompasses the large scale abstraction of fossil

groundwater reserves in the desert and their long distance transfer via concrete pipelines to the

coastal plains with their fertile soils, where the large majority of the Libyan population is located.

Major phases of this project have been completed until now and water is permanentlydistributed to big Cities like Tripoli and agricultural lands in the coastal region (Figure 1).

Engineering landmarks are the five major well fields with more than 1000 wells up to 800 m

deep, the water conveying system of four meter diameter pre-stressed concrete pipes and

numerous pump stations, as well as big storage reservoirs in the target areas. The largest of

these reservoirs is the Grand Omar Mukhtar Reservoir, which was completed in March 2007.


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THE GREAT MAN-MADE RIVER PROJECT

Mediterranean Sea

Benghazi

Sirt

Tripoli

6004503001500

Km

Tazerbo

Ajdabiya

Sarir

Wellfield

Tazerbo

Wellfield

Sarir PCCPPlant

NE Jebel Hasouna

Wellfield

E Jebel Hasouna

Wellfield

Tobruk

Kufra

Jaghbo ubGhadamesBrega PCCP Plant

Egypt

Egypt

Algeria

Algeria

TunisiaTunisia

Sudan

Sudan

Benghazi Plains

Region

Al-Ghar dabi ya Plains

Region

Jefara Plains

RegionGrand Omar

Mukhtar

Reservoir

Figure 1: Libya water grid with Grand Omar Mukhtar Reservoir near the coast

2 The Grand Omar Mukhtar Reservoir

The coastal city of Benghazi is already receiving more than 200 000 m3of water per day from

well fields located 1900 km south. With the new Grand Omar Mukhtar Reservoir under

operation, additional water will be provided to irrigate the large fertile plains in the Benghaziregion.

There are many good reasons to choose asphaltic concrete as sealing material for a fill dam or

water reservoir. High reliability, low maintenance costs and economic construction methods

contribute to its long and very successful track record. In case of Grand Omar Mukhtar

Reservoir, the asphalt lining covers an area of 800000 m2on the floor and 270000 m2 on the

slope and it will retain an operational volume of 24 mill m3of valuable irrigation water. After

commissioning, this reservoir will provide water for 18.000 hectares of agricultural land. Further

extensions are envisaged in the near future.

3 General Design Features

The reservoir is surrounded by a 3600 m long ring embankment consisting of a crushed

limestone fill and a vertical chimney drain of fine sand. The upstream slope is protected by

coarse riprap whereas the downstream slope is overlaid by a drainage zone of crushed

limestone material with specified drainage capacities and sufficient stability to allow safe

operation of the asphalt paving equipment. The drainage zone was also designed to withstand

rain water during construction time, a property which was tested successfully during occasional

but very heavy rainfalls in winter. The drainage zone was placed in two layers of 15 cm eachwith the slope paving equipment, thus providing a very even and accurate formation for the

binder asphalt layer.


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The reservoir floor was milled five meter deep into the limestone underground by conventional

road mining machines. On the accurately excavated bedrock formation a regulating layer of 20

cm was sufficient to provide an even and stable base for the asphalt lining. The excavated

material was used as fill material for the embankment thus minimising haulage expenditures for

material from outside quarries.

Filling of the reservoir is accomplished through a concrete intake structure where seven steel

pipes allow for well regulated filling according to operational requirements. The outlet structure

is situated on the opposite side of the reservoir to ensure a certain water circulation of the

stored water. The draw-off is controlled by valves outside the reservoir.

4 Design of the Asphalt Lining

The lining consists of a binder layer of 8 cm thickness and a sealing layer of 6 cm thickness.

Crucial for the good performance of the asphalt lining under water load are the connections to

the concrete structures within the reservoir, in case of Grand Omar Mukhtar Reservoir, the inletand outlet structure on floor level. In both cases, special designs have been applied, taking into

account the flexibility of the asphalt and the rigidity of the concrete. Possible differential

deformations between asphalt lining and concrete structure require a special watertight joint

construction design. A copper sheet which is clamped to the concrete and installed in loop

profile will overbridge a possible opening of the joint between asphalt sealing and concrete wall

and thus guarantee a watertight sealing system even in this critical area and under

unfavourable load conditions.

All construction material such as joint filler, copper sheet and clamping plate not only fulfilled the

specified high quality standard but they have been successfully applied on numerous similar

projects.

At the dam crest the lining is connected to the wave wall. The joint is above the reservoir water

level and therefore not exposed to a permanent static water pressure. Expensive joint

construction is not required. The joint is filled with highly elastic joint filler which has good bond

to both the concrete wave wall and the asphalt lining.

5 Asphaltic Concrete Mix Design

As a mixture of crushed coarse aggregates, sand, filler and bitumen, with a suitable mixcomposition and if professionally placed, asphaltic concrete can be manufactured in a way to be

technically impermeable. The mix design had also to consider flexibility and stability

characteristics under extreme temperature variations of more than 70K. This was established

on the construction site laboratory with support of Strabags laboratory in Cologne, following the

MS-2 Guidelines of the American Asphalt Institute which define in detail all required steps for

the design evaluation. After verification of the specified quality on field tests outside and within

the permanent works, full scale asphalt placing started in November 2005.

6 Production of the Asphaltic Concrete

In order to produce high quality hydraulic asphalt, a conventional asphalt mixing plant which

was already on the construction site had to be technically modified and adapted to the specified


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American Standard. This included mainly an electronically controlled mixing process with

automatic recording of main mixing parameters such as weights, temperatures at various

process steps and mixing times.

The fully computerised plant was equipped with a precise weighting system that ensured a

highly accurate and consistent mix composition, as for example a variation in the bitumencontent of less than 0,1 %. The production capacity of the mixing plant was at 140 tons per hour

for binder asphalt and 120 tons per hour for dense asphalt, which allowed for a continuous

asphalt placing process of one placing unit.

For the entire asphalt lining covering an area of more than one mill m2, in total 370.000 tons of

asphalt were produced between November 2005 and January 2007. All aggregates were taken

from a limestone quarry situated 25 km from the construction site. Within the scope of the

quality control, all components of the asphalt mix were tested regularly in the asphalt site labo-

ratory.

7 Placing of the asphalt lining

On the embankment slope, the drainage zone and the asphalt lining were placed with special

paving equipment. All machines working on the slope were held and moved by winch wagons

which operated on the embankment crest (Figure 2). The paving machine equipped with a high

compaction screed, worked in a vertical direction placing asphalt from bottom to top in a width of

5,5 m each lane. In order to allow for a continuous paving process, a supply cart transported the

asphalt from the crest to the paving machine. Final compaction of the asphalt was achieved by

vibratory rollers which ran on separate winch wagons. On the floor a conventional road finisher

with a high compaction screed was used (Figure 3).

Figure 2: Asphalt placing on the reservoir slope


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Figure 3: Reservoir lining under construction

In this way the binder and dense asphalt layer were placed in two consecutive stages. After

verifying the specified quality of the sealing by various tests, a hot mastic coating was spread on

top of the lining at a thickness of 1,5 to 2 mm. The coating consists of approx. 30 % bitumen

and 70 % of filler. Due to the high sun intensity in Libya, a stabilising agent was added to the

bitumen. This final coating will protect the sealing against the ageing effects of the sun radiation,

thus conserving the high quality characteristics of the sealing for decades.

8 Sequence of Construct ion

In order to shorten the overall construction period, placing of the drainage zone on the slope

started when approximately 30 % of the embankment was still uncompleted and before the

asphalt mixing plant was operational. After execution of successful field tests, asphalt placing

started in one shift, and in spring 2006 a night shift was added for asphalt placing on thereservoir floor. Mainly for safety reasons, slope placing was restricted to the day shift.

Whereas interruption of asphalt works by rain were limited to a few events during the winter

season, dust and sand storms from the Sahara desert disturbed the placing works more

frequently and more severely.

In order to minimise interference between earth works and asphalt works and to avoid

interruptions, careful work planning with permanent adjustment to the actual work progress was

crucial for success. As a result of good co-operation the performance of the placing works met

the schedule and only four months after hand over of the last dam section, which included

almost 30 % of the entire slope section, the asphalt works were completed.


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9 Monitor ing System

In order to collect and monitor any seepage through the lining a ring drainage has been installed

along the dam toe which is connected to the drainage layer of the slope. Seepage water from

the reservoir slopes can be localised by fibre optical cable which has been installed at the toe of

the drainage zone. In addition, the water tables below and around the reservoir are monitoredaccurately by piezometer boreholes which have been drilled from the dam crest and the

surrounding area to a depth below the ground water table.

10 Impounding

Impounding of the reservoir will start in July 2007 after commissioning the pumping and

conveying system that will feed the reservoir with water. According to the designers programme

the filling procedure will be carried out in defined steps and will take approximately six months in

total.

Acknowledgement

The author wishes to thank all Parties involved in the construction works namely the Owner of

the reservoir, the Great Man-Made River Utilization Authority and their Consultant Brown and

Root North Africa as well as the local Main Contractor General Company for Building and

Construction and their Consultant LTCC from Benghazi.

Author s Address and Affiliation

Schmid, Reinhard, Dr.-Ing.

Strabag International GmbH

50679 Kln

Germany

[email protected]

the asphalt lining of a large water reservoir near bengazi - schmid

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