Lezkairu Utilities Tunnel

Case Study

Lezkairu Utilities TunnelFrancisco de Asís Ramírez Chasco, Ph.D.1; Andrés Seco Meneses, Ph.D.2; and Eduardo Prieto Cobo3

Abstract: Underground utility tunnels present some unique constructional and operational considerations that require specific approaches toachieve optimal system performance and maintenance. From the construction point of view, perhaps the main issue is the avoidance of theaccumulation of water in the surroundings of the structure. The design and construction of an adequate drainage system is important inavoiding added pressure on side walls and subslab pressure on the bottom concrete slab, as well as avoiding contact of water with thestructure that could result in infiltrations that could impair the functionality of the construction and lead to ensuing and major repair costs.From the point of view of management and operation, some aspects of the overall design, such as coordinating different economic agents andensuring the safety and integrity of the facility (from accidents as well as sabotage), require considerations that are not present in other typicalcivil engineering projects. This article relates these considerations to a specific case of an underground utility tunnel, or gallery, of approx-imately 7,885 m that was recently built for a new urban expansion area in the city of Pamplona, Spain. DOI: 10.1061/(ASCE)SC.1943-5576.0000071. © 2011 American Society of Civil Engineers.

CE Database subject headings: Utilities; Tunnels; Drainage; Geosynthetics; Spain.

Author keywords: Utilities tunnel; Gallery system; Drainage; Geotextiles.

Introduction

The General Urban Development Plan (P.G.O.U.) of Pamplona,drafted in 1990, foresaw the expansion of the city at its southernend, in an area known as Soto de Lezkairu. The development layoutof this new urban area, under a plan to build up to a total of 5,000housing units for approximately 16,000 people, has the shape of anL, consisting of three groups of city blocks arranged following anorthogonal frame (Fig. 1).

The adaptation of an 860; 000 m2 rural setting to the needs of anarea zoned for development required the construction of a completenetwork of infrastructures and utilities; and having a “virgin” space,in which it was possible to carry out the project design with com-plete freedom, was an advantage from a technical point of view.On the other hand, it is well known that, in rural settings, theinstallation, maintenance, and expansion of utility trenches entailsconsiderable financial cost, not to mention the inconveniencethat construction in public areas presents to the citizenry. Taking allof these variables into consideration, the construction of anunderground system of galleries was decided on. The total invest-ment expected for the development was over 50 million Euro(US$70.68 million), with an estimated construction period of30 months. These figures, which refer to the total urbanizationof the area, include not only the building of the service tunnel,but also include the construction of the road network and pedestrianzones, public parks, lighting, street furniture, and preparation of

parking areas. With respect to the service tunnel (civil worksand internal installations), the estimated investment in the projectamounts to 24.2 million Euro (US$34.2 million), with a partialcompletion period of 14 months, which as of now, with generalwork still in progress, may be considered fulfilled.

General Description of the Network of Tunnels

The underground siting of the network of tunnels within theorthogonal layout of the road system of the new development willsimplify the future work of expanding and/or repairing any of theservices placed in the gallery and will minimize inconveniences tothe population of the area. The design of the network that will reacha length of 7,785 m is hierarchical; the “main galleries,” with alength of 5,328 m, will house the general distribution systems ofchannels and services, whereas the “secondary galleries” willextend the utilities to the edge of the plots.

Structurally, the main gallery is a box of reinforced concrete,with a rectangular cross section and dimensions of 3:10 ×4:60 m (2:50 × 4:00 m useful space), built in situ, with its interiordistributed in two spaces, one on top of the other, separated by awalkable steel grate, enabling each one to hold different types ofpipes and services. As indicated in Fig. 2, the upper subspace, ar-ranged in trays, carries the conduits for the street lighting system,electrical power, telecommunications, traffic light network, and theservices of the gallery itself (emergency detectors, lighting, venti-lation, and detection of toxic gases); the water-supply pipes(100 mm ϕ pipe) also run through this upper space, under the trays.The lower sublevel is devoted to the wastewater collection system(315 mm ϕ PVC pipe) and the solid waste pneumatic collectionsystem (500 mm ϕ steel pipe). The floor of this second sublevelhas a 2% cross-sectional slope on both sides, converging on a gra-ting that allows the evacuation of possible water leaks that mighttake place during the operation of the system and that is incorpo-rated into the general drainage network of the entire gallery.

The secondary gallery tunnels connect at 90° at the junctionswith the main gallery tunnels and are made up of 2-m-long prefab-ricated modules of reinforced concrete that constitute one single

1Strategic Project Coordinator, Pamplona, Spain.2Assistant Professor, Public Univ. of Navarre, Campus Arrosadia,

31006 Pamplona, Spain (corresponding author). E-mail: [email protected]

3Professor, Agronomy Engineer, Public Univ. of Navarre, CampusArrosadia, 31006 Pamplona, Spain.

Note. This manuscript was submitted on February 16, 2010; approvedon May 7, 2010; published online on May 12, 2010. Discussion periodopen until October 1, 2011; separate discussions must be submitted for in-dividual papers. This paper is part of the Practice Periodical on StructuralDesign and Construction, Vol. 16, No. 2, May 1, 2011. ©ASCE, ISSN1084-0680/2011/2-73–81/$25.00.

PRACTICE PERIODICAL ON STRUCTURAL DESIGN AND CONSTRUCTION © ASCE / MAY 2011 / 73

Pract. Period. Struct. Des. Constr. 2011.16:73-81.

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http://dx.doi.org/10.1061/(ASCE)SC.1943-5576.0000071




interior space of 1:50 × 3:00 m. The distribution systems and serv-ices that constitute the utilities of each plot run inside this maingallery. In ascending order, these consist of a pneumatic solid-waste collection system, encased in a 500 mm ϕ steel pipe; twotrays that support the fiberglass telephone network and cableTV network, respectively; a waste-water collection system runningthrough a 200 mm ϕ PVC pipe; 100 mm ϕ cast-iron pipe for watersupply; and, finally, a tray that holds all the electric power-tracksystems (Fig. 3).

Construction Process

In a linear construction project, such as the one described in thisarticle, the timing of tasks inevitably forces the execution of variousactivities to overlap. However, the following paragraphs describeeach phase of the execution of the project in sequential order,including the most characteristic information about each of them.

Excavation

Since the land surface was available, an opencast excavation wasdone, adapting the side slopes to the natural slope of the terrain. Inareas with a solid substrate, in which it was geotechnically admis-sible to adopt a strict slope, the area was overexcavated to haveenough space to place the retaining walls of draining material oncethe gallery was built. The excavation was performed at an averagedepth of approximately 6.50 m, which involved a total excavatedvolume of approximately 345; 000 m3. Bulldozers and backhoe ex-cavators were primarily used to accomplish this task, resulting in anaverage excavation of 1;500 m3=day. Fig. 4 shows an approximateview of the stratification of the different layers of material found inthe operational site.

Installation of Drainage Pipes

One of the main problems that need solving in this type of construc-tion work is the presence of water, both in pressure exerted on the

side walls and the subslab pressure on the bottom concrete slab, aswell as in the infiltrations that can occur throughout the construc-tion process, and these can have important effects on the systemswithin. For that reason, in this stage of the construction, an effortwas made to be particularly careful in designing an undergrounddrainage system that would ensure the fast removal of water fromaround the galleries.

In the main galleries (Fig. 2), three 400 mm ϕ PVC drainagepipes were installed on top of the excavated and leveled terrain.Two of them were set out laterally to collect the water from thegradual upward and backward slope of the side walls and the waterthat flows through the drainage pipes of the secondary galleries thatconnect to this one. The third drainage pipe runs under the framethat constitutes the gallery and has a dual purpose: it collects thewater that could accumulate over the gallery bottom concrete slab ifthere was a rise of the water table, and it can collect possible leak-ages from the water supply and sewage networks that run inside(Fig. 5). For this latter purpose, 0:40 × 0:40 m cast-iron gratingswere installed at a distance of 50 m in the bottom concrete slabof the gallery.

Once the pipes were installed, the entire bottom of the trenchwas leveled with filtering material and covered with a geotextilefabric that extends to the crowning of the slopes to avoid contami-nation of the filtering material that could result from the washingout of fine elements from the soil (Fig. 6). Given the time of theyear when this was carried out (autumn–winter), the execution wasrather problematic at times, resulting in a low performance ofapproximately 100 m=week. This lower performance was causedby operations of laying drainage pipes, spreading the geotextilelayer and covering it all with filtering material.

With regard to the secondary galleries, the cross section (Fig. 3)shows the drainage design: after excavating the natural terrain, a20-cm-deep layer of 60–80 mm ϕ granular material was spread;160 mm ϕ PVC drainage pipe was laid on top of it, placing it underthe bottom concrete slab of the gallery; subsequently, a second20-cm-thick layer of fine granular material (6–12 mm ϕ) wasspread to obtain a perfect leveling; and on top of it was laid a thin10-cm-thick concrete slab reinforced with rebar that ultimatelyserved as a support to the prefabricated concrete elements thateventually constituted the secondary gallery.

Once the prefabricated modules that constituted the frame of thesecondary gallery were in position, the lateral drainage pipes (PVC160 mm ϕ) were installed and the trench was refilled with selectedmaterial. These lateral drainage pipes were then connected to thecorresponding lateral drainage pipe of the main gallery.

Structural Calculation of the Tunnel

The structural dimensions of the main tunnel were established onthe following assumptions:1. The proposed drainage system is sufficient to evacuate the

water circulating through the ground, and therefore no hydrau-lic pressures are expected on the structure.

2. As a consequence of the preceding assumption, the side wallsof the structure will only be subject to the pressure of the soil(consisting of granular quarry material) for which the follow-ing parameters have been considered:a. Density of the soil ρ ¼ 2 t=m3

b. Angle of repose ϕ ¼ 30°c. Cohesion c ¼ 0d. Soil-wall friction ¼ 0

3. The following forces on the upper slab of the structure havebeen taken into account:a. Weight of soil bearing on the tunnel: soil thicknesses

of between emin ¼ 1:5 m and emax ¼ 2:5 m have been

Fig. 1. Location of Lezkairu in relation to the urban continuum ofPamplona

74 / PRACTICE PERIODICAL ON STRUCTURAL DESIGN AND CONSTRUCTION © ASCE / MAY 2011


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considered. Accepting the least-favorable situation, anexcess load of 5 t=m has been assumed.

b. Excess loads for practical use (roads open to vehicular traf-fic) have been estimated at 1:4 T=m2.

4. As total effective load within the tunnel, an excess load of0:8 T=m2 has been assumed.At the depth of the foundation slab, it was assumed that the ter-

rain has a load-bearing capacity of 5:00 kg=cm2, with a modulus ofsubgrade reaction on 30 × 30 plate, K ¼ 10:00 kg=cm3. The esti-mated loads for the interior paving of the gallery are those of thegrating itself, 40 kg=m2 and the carrying machinery (for reference,a Honda HP500 all-terrain power carrier that measures 2:14 ×0:65 × 1:10 m and weighs 197 Kg was used) plus the load beingtransported, for a total load of 800 Kg=m2. Fig. 7 shows a cross-sectional diagram depicting the pressures of soil, excess loads andterrain loads on the main tunnel.

Execution of the Galleries

As previously mentioned, the main galleries were constructed withcast-in-place concrete methods They are actually a frame of rein-forced concrete that does not require further explanation from acivil engineering perspective.

To comply with the Spanish Seismic Design Code, NCSE-94,the execution of the utilities tunnels as being of “special impor-tance” was considered, insofar as that is how this type of construc-tion is classified in this Code. Basic seismic acceleration forPamplona is 0:392 m=seg2 (BA=g ¼ 0:04), and the contributioncoefficient is K ¼ 1. The construction engineering design teamassumed a granular cohesive soil of medium compaction with acoefficient of 1.4 in an open structure with low ductility.

Fig. 8 shows the positioning of the reinforcement in the bottomconcrete slab and side walls, and Fig. 9 consists of a photo of theforming of the top slab.

The forming of the concrete of the side walls and top slabwas performed using moving “carretones” or carriages (Figs. 10and 11). The carriages used have a length of 12 m, which allowedthe execution of complete stretches between concrete expansionjoints. By using these devices, an output per carriage of up to12 m=day was obtained, a critical performance given the lengthof the gallery being built. At specific times in the execution, therewere up to four carriages in use simultaneously. Secondary galleries(with lengths that varied between 5 m and 20 m) were built using2-m-long reinforced concrete prefabricated modules. The joints,the most delicate element in the construction, were made from

Fig. 2. Main gallery with the various services that run through it



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neoprene with a subsequent layer of water-repellent mortar(Figs. 10–12).

The perpendicular layout of the galleries could make the move-ment of relatively long elements more difficult. To avoid this prob-lem, extra width was allowed at every tunnel junction, providing ausable space of 6:50 m × 6:50 m and free height of 4.90 m. Thevolume is designed with 30 cm thick walls, 30 cm thick top con-crete slab, and a 50 cm thick base or foundation slab (Fig. 13).

Gallery Safety: Equipment

From the point of view of the safety of the operation and mainte-nance performance, three basic aspects were considered for thisproject: accessibility for materials and small machinery, aerationand fire-fighting systems, and security against trespassers. As it hap-pens, the optimization of some of these aspects almost necessarilyimplies penalizing others (e.g., optimizing accessibility reducesthe amount of control; minimizing internal routing through thegalleries means having to make available a larger number of accessnodes with a consequent impact on the development at the surface).Achieving a functional optimum is always difficult, and, in someinstances, attention must be given to priorities that are not purelytechnical, such as those that involve security.

Access of Materials and Small Machinery

Because of security considerations regarding the facility itself, aswell as minimizing the effects of any elements emerging at thesurface of the area being developed, only one access was made

Fig. 3. Cross section of the secondary utility gallery that connects from the main gallery to the individual plots

Fig. 4. Cross section of strata of operational site



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available, large enough to allow the safety and ease of entry forboth personnel and materials (Fig. 14).

Because it was anticipated that during the duration of the con-struction, it would be necessary to move materials of considerable

size and weight (pipes, pumps, parts for the distribution andcollection systems), this single-access node was equipped with a2:00 × 8:00 m freight elevator to provide easy and safe accessfor workers and allow access to small loading machinery that will

Fig. 5. Drain pipes in the main galleriesFig. 7. Calculated loading of structure

Fig. 6. Spreading of the anticontamination geotextile



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make it easier to transport materials throughout the entire course ofthe main galleries. The main drawbacks to this decision are the longdistances that will occasionally be necessary to access the entiregallery system. The major advantages lie in the ease of controllingonly one access point and the minimization of its effects at thesurface. This is not the only exit provided for the transit of person-nel outside the gallery, because as already previously mentioned,“outlets” have been provided at all the junctions or intersections

between the various branches comprising the gallery, and at eitherend. Because the junctions are 150 m apart, in case of emergency,internal distances in the gallery are in no case greater than 75 m.

Fire Protection Sand Ventilation

In Spain, the basic design requirements related to fire protection ofbuildings and facilities are covered in form NBE-CPI-96. However,this regulation does not consider the measures that must be adopted

Fig. 9. Forming of the top slab

Fig. 8. Reinforcement of the bottom plate and gables in the main galleries



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in projects, such as accessible utility tunnels. This means that thesolutions adopted, both for fire detection and fire extinction, reflectsubjective criteria that, although perhaps taking caution to the

extreme, entail adopting excessive measures that would naturallymake their implementation more expensive. In any case, thesolution adopted in this case differentiates between the main andthe secondary galleries, primarily on the basis of two characteristicsthat set them apart: first, their length (secondary galleries havelengths that vary between 4 and 20 m) and being ending branches(not connected to others); second, the lesser importance of thedistribution and collection systems in secondary galleries.

Fig. 11. Detail of joints

Fig. 10. Joint details

Fig. 12. Detail of joints



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Fig. 14. Access node for the entry of materials

Fig. 13. Gallery junction node



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The main galleries, with a total length close to 5,300 m, havebeen divided into sections of approximately 400 m, creating airtightcompartments separated by fire-resistant doors (RF-90 min) andveneer firewalls of perforated brick, coated with 2 cm of cementmortar on both sides. Wall penetrations have their annular spacesfilled with intumescent mastic. Each one of these compartments isventilated through suction grilles located up on the sidewalk in130 × 110 × 100 cm concrete manholes with 70 × 70 cm2 top-nodular cast-iron grilles and droplet separators to prevent waterfrom entering the gallery. The grilles connect to the gallery throughmanholes with 500 mm ϕ PVC pipes that are held to the wall sheet-ing with clamps and protected with mass concrete backfill HM 20.

Forced extraction is carried out by units within isolated boxes,capable of working in fire risk areas at 400°C for 2 h. They arelocated in the gallery junction nodes, and evacuate stale air througha circular duct, concrete pipe ASTM 1,000 mm ϕ, with a horizontalcourse until it reaches the closest median strip and emerges to thesurface, first through a rising manhole and vertical duct made out ofconcrete built in situ, 160–120 cm ϕ (interior), and, subsequently,through vents or “metallic mailboxes,” of 3-mm-thick stainlesssteel, with an equilateral triangular cross section of 1.3 m on theside, and 275þ 76 cm high, from the level of the lawn or pave-ment. These metallic elements incorporate a trap door for accessand step-irons for emergency egress.

Moreover, every 15 m throughout the gallery system, there arespot optical smoke detectors and 95 manual alarm switches foractivation in case of fire. Both the detectors and the switches con-nected to the alarm switchboards would activate interior sirens,acoustically signaling the fire. There are also powder fire extin-guishers every 50 m throughout the main galleries and in eachsecondary gallery and a CO2 fire extinguisher next to each ofthe main control panels of the gallery.

Emergency evacuation from the interior is provided by ventila-tion manhole shafts with step-irons and a double trapdoor that can-not be reached from the exterior. The uptake shafts will be locatedat the junction nodes of the branches of the main gallery. At the endof each secondary gallery, there is an air intake in a reinforced con-crete hatch with a top grille and an antidrip grille. The junction ofthe secondary galleries with the main gallery has also been com-partmentalized, usng the same features used in sectioning the maingallery. The goal is to isolate any fire outbreak that might occur in asecondary gallery, keeping it isolated from the rest of the system.

Safety in Regard to Trespassers

Access to the utilities within the tunnel is required by personnel ofall the individual utility companies on a regular basis. Although thispaper cannot, for obvious reasons, go into great detail about thecontrol elements used, it can be noted that, beside having a safetydoor, sensing devices have been installed in the access node and instrategic locations throughout the gallery that are deactivated whenauthorized workers insert an electronic identity card and enter analphanumeric code that can be modified periodically. Protectionagainst acts of sabotage is supplemented with a closed-circuit

television (CCTV) surveillance system connected to the center ofoperations.

Management of the Facility

One of the more complex aspects of this type of construction isdesigning and regulating a management system that, without losingnecessary control, is flexible and effective enough to allow any typeof action in the various systems and services placed in the system ofgalleries. Problems arise as a consequence of the multiple admin-istrative and economic agents that converge, first in the construc-tion and subsequently, in the operation of the facility. First, both theconstruction itself and its resultant costs are the responsibility of thedeveloper who owns the land and will control its development andthe sale of housing units upon it. However, second, ownership ofurban facilities and equipment (parks, gardens, roads, streets, andutility tunnels) and their maintenance and regulation fall uponthe municipal administration. Third, ownership of each utilityservice network (water supply, electric power, telephone, and cablenetworks) falls upon the different companies that supply thoseservices. Last, ensuring the safety of the facility as a whole, asa matter of public order, falls upon the corresponding law-enforcement agencies and authorities.

Achieving a balance between private interests and public author-ity regulations can be difficult, so cooperation between all partiesconcerned is imperative. In the present case, the municipaladministration has tried to make three key aspects compatible:the general maintenance of the gallery system from the “construc-tion” perspective, free access to the facility by utilities companies,and global control of the facility from a safety point of view.In essence:1. The municipal administration “receives” the entire develop-

ment (including the system of utility tunnels) and assumesthe costs derived from its maintenance and upkeep, takingdirect care of the global management and maintenance ofthe structural elements of the utility tunnel and the generalelements of security, access, and functionality;

2. The utility companies that own and/or manage the servicesassume the costs of maintenance and upkeep and the expansionor update of the appropriate pipes or raceway and undertake tomake the payment of a fee to the municipal administration forthe general maintenance of the gallery system and its protec-tion and control facilities; and

3. The local police, either directly or through agreements withother law-enforcement forces, assume responsibility for thesafety of the system, controlling access to the facilities andmanaging, by a specific program, the response to alarms.At the time of the writing of this article, the facility is not yet

in use, given that the development of the area has not been com-pleted. It will be necessary to wait some time before the systemperformance can be assessed and the adjustments or modificationsdeemed necessary to improve its functionality can be carriedout.



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Documents

Lezkairu Utilities Tunnel