System for Construction of a Railway Underpass

Case Study

System for Construction of a Railway UnderpassFrancisco de Asís Ramírez Chasco, Ph.D.1; Andrés Seco Meneses, Ph.D.2; and Eduardo Prieto Cobo3

Abstract: The increasing shortage of available spaces in urban areas resulting from the construction and consolidation of certain facilitiesrelated to basic services (such as transport) mean that the provision of infrastructure is sometimes excessively complex in relation to itsintended use. This article describes, in detail, the building of an underpass under railway tracks using a system of pushing a structure con-sisting of a ready-built reinforced concrete box using jacks. Implementing the system proved particularly delicate, given the constraint ofmaintaining normal rail traffic throughout the construction of the underpass. The primary responsibility was to design and effectively carryout a process that would adequately guarantee the safeguarding of the stability and maintenance of the track bed while the box was beingpushed underneath, at what was an undeniably shallow depth (approximately 1 m). The developed system, although laborious, proved to behighly efficient, given that at no time during the course of the work did there arise any circumstances that might jeopardize either the safety ofrail traffic or the physical integrity of the workers on site. DOI: 10.1061/(ASCE)SC.1943-5576.0000073. © 2011 American Society of CivilEngineers.

CE Database subject headings: Railroad tracks; Pedestrians; Urban areas; Construction; Overpasses and underpasses.

Author keywords: Railway track; Pedestrian; Urban area.

Introduction

The railway tracks on the approach to the station in the city ofPamplona, Spain—a city a population exceeding 16,000—separatetwo major areas of the city (see Fig. 1). The absence of a directpedestrian connection between the two called for the constructionof a crossing infrastructure to address this need, with the conven-ience and safety standards required in an urban setting. The urbanlimits of the two neighborhoods were no more than 90 m apart; itwas therefore a matter of designing the most direct possible con-nection, which would necessarily include a structure to cross thetracks at a different level.

The great profusion of overhead catenary wires in the crossingarea ruled out the option of building an overpass, as the clearanceneeded for the crossing structure would require a disproportionateconstructional effort if reasonable gradients were to be maintainedon the access ramps. The solution therefore indicated was an under-pass beneath the railway tracks, a simple solution from a structuralpoint of view but highly sensitive and complicated to construct,given that the work should not at any time interrupt the normaltraffic of trains to and from the station.

This article will center on the construction-related solutionadopted for the building of the underpass, although some brief de-tails will also be given of some aspects of the project, including costand construction time.

Structural Type and Choice of Construction System

Both the type of structure and the construction system to be usedwere studied at the same time, given that the former was fundamen-tally conditioned by the choice of the latter. The maintenance ofrailway traffic ruled out any “cut and cover” type of construction,so it was a matter of choosing between two a priori feasible sol-utions: “mine” type construction, with subsequent sectional erec-tion of load-bearing structures; or a process of “jacking” of apreviously fully completed structure.

Fig. 1. Track underpass location

1Strategic Project Coordinator, Pamplona Municipality.2Assistant Professor, Public Univ. of Navarre (corresponding author).

E-mail: [email protected], Public Univ. of Navarre.Note. This manuscript was submitted on March 29, 2010; approved on

June 22, 2010; published online on June 25, 2010. Discussion period openuntil October 1, 2011; separate discussions must be submitted for indivi-dual papers. This paper is part of the Practice Periodical on StructuralDesign and Construction, Vol. 16, No. 2, May 1, 2011. ©ASCE, ISSN1084-0680/2011/2-82–88/$25.00.

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

Pract. Period. Struct. Des. Constr. 2011.16:82-88.

Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TY

OF

SYD

NE

Y o

n 08

/29/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

http://dx.doi.org/10.1061/(ASCE)SC.1943-5576.0000073




“Mine” tunneling was rejected for two reasons, the first beingthe length of time of execution. In this sense, it was estimated thatthe duration of the work would be notably increased, because thesoil in which they had to work was infill, which would require con-siderable fragmentation in the preliminary stages. Related to this,mine tunneling was also rejected because the mining process en-tailed significant risks for the stability of the excavation itself andtherefore for the stability of the railway track. This finally led to theselection of the procedure of “jacking” as being suitable for thecircumstances.

Once the construction procedure had been chosen, the next de-cision concerned the type of structure that would best meet the needfor strength, not only in its final position (loading of earth andtrains), but also during the construction phase of jacking. Finally,a boxlike structure was designed, as a square or rectangular shapewas ideal both from the point of view of ease of construction andfrom the standpoint of presenting sufficient inertia to withstand thestresses resulting from the jacking process. As a bonus, the rectan-gular box, once placed in its final location, required no major op-erations to form the underpass itself.

The dimensions of the box were planned on the premise that theunderpass should not be narrow or oppressive; it was therefore nec-essary to have generous dimensions to minimize the undesirable“tunnel effect,” while exercising necessary caution concerningcosts, in line with the expected pedestrian traffic. Finally, a rectan-gle of 3:30 m × 4:00 m was chosen.

Internally, the project was finished with processes and equip-ment aimed to improve the convenience and security of theunderpass:• Antigraffiti treatment on the walls,• Lighting, and• CCTV cameras linked directly to the Municipal Police.

In view of the dimensions of the cross-section of the underpassand its short length and exclusively pedestrian use, it was not nec-essary to install a permanent ventilation system, since no pollutantgases were expected within the underpass. In any case, there was amore than adequate flow of fresh air through the two tunnelmouths.

Construction Procedure

The most important aspects of the construction process were pro-grammed according to the plan of operations detailed in the sub-sequent sections.

Excavation and Preparation of the Jacking Shaft

The dimensions of the excavation, and hence its volume, were in-fluenced by two conditions. The first concerned the length of thetrench excavation, which was determined on the basis of two re-strictive criteria: (1) maintaining a protective thickness of soil under

the tracks of at least 60–70 cm that, together with the height of thebox (3.30 m), meant a excavation depth of 4.00 m; (2) achieving agradient on the access ramp of no more than 6%, in accordancewith Spanish legislation on access for the disabled. The secondcondition was the space needed for the construction of the thrustwall, the installation of the thrust jacks, and construction of thesliding floor for the box, as well as the fabrication of the box itself.

All of these conditions dictated the dimensions of the excava-tion, which finally reached a size of 73 m long, 5.00 m wide, and4.00 m deep. Excavation and leveling were carried out with atracked backhoe that, with an operating weight of 26,900 kg, de-velops a power output of 132 kW, with a bucket of 1:39 m3. Thejacking shaft construction took four days.

Construction of the Sliding Bed and the Thrust Wall

The second stage consisted of the construction of the sliding bed forthe box and the thrust wall necessary to carry out the subsequentpush for the jacking. Fig. 2 shows both elements.

These auxiliary constructions were made in reinforced concrete;the sliding bed, 3:50 m × 0:40 m and 72 m long, was covered witha layer of polyethylene film to improve the sliding properties of thebox during the jacking. The concrete slab also had some short sidewalls to act as guides for the box (Figs. 3 and 4).

Construction of the Box and Preparation of theRailway Tracks

With the bed finished and its surface bared, a film of polyethylenewas placed over it to act as a break and a barrier between the slidingbed and the box itself, which was built directly on top of it. At the

Fig. 2. Sliding bed and thrust wall

Fig. 3. Thrust wall and support and sliding slab for the box

PRACTICE PERIODICAL ON STRUCTURAL DESIGN AND CONSTRUCTION © ASCE / MAY 2011 / 83


Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TY

OF

SYD

NE

Y o

n 08

/29/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

sides, sheets of 40-cm-thick expanded polystyrene were placed be-tween the guide walls and the box (Fig. 5). The structure of the box(4.10-m high by 3.80-m wide and 45.50-m long) was assembled intwo stages: first, the floor and side walls; and second, the top of thebox. The reinforcement used consisted of B-500 S corrugated steelbars with a characteristic yield strength of f MK ¼ 500 N=mm2; theconcrete to be used was specified as having a characteristic yieldstrength of f CK ¼ 25 N=mm2.

The cutting edge of the box was formed in a wedge shape asshown in Fig. 6, to facilitate its penetration into the soil into whichit was to be pushed.

While the box was being constructed, work was begun on thepreconstruction preparation of the sections of track beneath whichthe underpass was to go. This work, essential to improve the sta-bility of the rail lines, consisted of the following actions:

• Removal of concrete sleepers and insertion of wooden sleepersto consolidate the rails of each of the tracks, so that when theyare secured together, the track bed is transversely stabilized(Figs. 7 and 8);

• Installation of strong longitudinal bracing bundles to avoid set-tling and longitudinal deformation of the rails (Fig. 9).

Fig. 4. Thrust wall and support and sliding slab for the box

Fig. 5. Details of the jacked box and the sliding bed

Fig. 6. Profile with geometrical characteristics of the box

Fig. 7. Removal of concrete sleepers and insertion of wooden sleepers

Fig. 8. Removal of concrete sleepers and insertion of wooden sleepers



Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TY

OF

SYD

NE

Y o

n 08

/29/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

In this way, a firm supporting structure was achieved for therails, “sewn together” both longitudinally and transversely, ensur-ing a stable and solid temporary support. Figs. 10 and 11 show thedifferent elements employed in the bracing of the railway track bed,as well as their installation and fixing.

Repeated Jacking-Cutting Procedure

Given the nature of the material to be bored, which primarily con-sists of random fill with low strength and stability, a moderate rateof advance of 2 m=day was programmed. For the pushing process,seven hydraulic jacks were used with a capacity of 150–200 t perjack that, allowing for a safety factor of 1.5, were sufficient to movethe structure, which weighed approximately 600 t (Fig. 12). Thiswork lasted 31 days, since, on occasions, weather conditions madeit impossible to achieve the expected performance, and some dayswere lost completely.

After each operation of advancing the box forward and penetrat-ing the ground, it was necessary to remove the accumulated exca-vated material from inside. This work was done by mechanicalmeans, with a small backhoe loader (model Hitachi Zaxis 16) that,with an overhead clearance of 2.64 m, an overall width of 1.05 mand a total power of 12 hp, was ideal for this task. Another detail tobe planned was the system for rationalizing the subsequent stagesof pushing the structure. Given the short effective stroke of thejacks, it was necessary to establish a safe and efficient procedureto avoid the need to build a succession of thrust walls as the boxadvanced, with consequent increased expense and delays. It wasdecided to use a support system for the jacks based on very strongtransverse prismatic steel sections alternating between tubes thatwere also steel (Figs. 13 and 14). The system proved to be highlyefficient and more than adequately achieved its function of

withstanding the force of the jacks during the jacking process,while at the same time proving to be a quick and easy way signifi-cantly to shorten the execution time of the work.

The most crucial and delicate stage of the work was the passageof the box under the tracks. At the moment when the box began topass beneath the tracks, the whole system that initially rested on theballast would now rest on the IPE 600 steel forepoling beams,which were initially placed on the top slab of the box and separatedfrom it by bearings so they could slide on it. The procedure requiredthat, as the box went under the track, the maneuvering beams wouldpenetrate beneath the tracks, acting as a support for the platformthat had previously been braced both longitudinally and trans-versely (Figs. 15 and 16)

The method of working was as follows: as the box was on thepoint of passing under the tracks, the maneuver beams were pushed

Fig. 9. Longitudinal bracing of the sleeper/rail assembly

Fig. 10. Bracing of track bed and track, and positioning of maneuver beam

Fig. 11. Bracing of track bed and track, and positioning of maneuverbeam

Fig. 12. Hydraulic jacks against the reaction thrust wall



Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TY

OF

SYD

NE

Y o

n 08

/29/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

under the tracks in such a way that they bore simultaneously on thebox and on the earth. Subsequently, the pushing of the box began sothat the steel maneuver beams, now anchored to avoid moving,could move. Thus, at any given moment, the maneuver beams,the supporting structure they carried, and the tracks were all fixed.Fig. 17 illustrates the process.

Given the limited depth of earth between the upper slab of thejacked box and the position of the railway tracks, as well as the factthat this area was composed of loose gravel (ballast), in the processof jacking there was a partial localized collapse of the ballast wherethe rails rested on the area closest to the point of entry of the box,leaving the rails temporarily in the air, with no other support thanthat provided by the bracing achieved by means of previously in-serted longitudinal and transverse stiffening (Fig. 15). In this sit-uation, it was very simple to slide the IPE beams supported onrollers into position under the rails and to proceed to their manualfixing and consolidation.

Final Operations on the Track-Bed

Once the jacking of the box was complete and the box was in itsfinal position, the following operations were carried out:1. Withdrawal of supports, wooden sleepers and support beams;

Fig. 13. Precast concrete modules and steel pipes as thrust transfer forjacks

Fig. 14. Precast concrete modules and steel pipes as thrust transfer forjacks

Fig. 15. Maneuver beams passing under the live rails

Fig. 16. Maneuver beams passing under the live rails

Fig. 17. Construction jacking phases



Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TY

OF

SYD

NE

Y o

n 08

/29/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

2. Filling of approximately 40 cm of concrete above the box, toact in addition as a load-spreading slab once the operation wascomplete (Fig. 18);

3. Spreading of 30 cm of ballast, tamped and packed on top of theconcrete slab (Fig. 19);

4. Reinstatement of concrete sleepers, of the same type as the restof the track; and

5. Reinstatement of installations affected by the work (e.g., powercables, communications).

Dismantling of Auxiliary Structures and Items

Once the track was back in position, the sliding bed and thrust wallbuilt to act as a buffer for the jacking of the box were demolished,as were the vertical wedges and front support beam, prior to instal-lation of the wing walls of the box.

Security Measures during Execution of the Work

The safety of workers on construction sites in Spain is regulated byRoyal Decree 1627/1997, which establishes the obligation to drawup a Health and Safety Assessment for each individual project; thisHealth and Safety Assessment, which is included as part of thedocumentation of the Production Plan for the project, must identify

the risks entailed by each of the activities to be undertaken, theaction taken to eliminate them and, if this is not possible, protectivemeasures to be taken.

In view of the preceeding, and to not prolong the explanationunduly, this section shall make no comments on measures thatmight be considered general and habitual on any construction site,and shall focus briefly on the measures taken in two particular ac-tivities that may be considered more specific to this particular work.

Work to Be Done in the Vicinity of the Railway TracksAs already mentioned, the work had to be done without disruptingnormal rail traffic, and it therefore represented a potential dangerfor the operators who had to carry out their tasks in this situation.The first thing to be done was to obtain accurate details of all thetimes of trains that had to travel through the work zone, and to es-tablish an immediate communication channel with railway officialsto find out, in real time, any change in timetables or the presence ofextra trains. On the basis of this information, the organization of thework proceeded in such a way that operations to be performed onthe tracks were scheduled for times when it was certain that therewould be no rail traffic.

Work to Be Done inside the Box during JackingThese were the only operations that were not going to be carried out“in the open,” and the very nature of the work done (removal, load-ing and transportation of the earth entering the box during the jack-ing) involved the production of dust and a probable decrease inoxygen concentration, which might cause breathing and vision dif-ficulties for the operatives. The problem was very limited, given theshort length of the box and that there was always some sort of holeconnecting with the exterior, resulting from the partial collapse ofthe layer of ballast between the entry point and the position of therails. This situation resulted in the presence of some degree of airflow between the two mouths of the box, which undoubtedlygreatly mitigated the problem.

Nevertheless, a ventilation system was installed. Its dimensionswere as follows:• Throughput per horsepower of diesel motors q ¼ 2:5 m3=hp

per minute;• Throughput for personnel at workface q ¼ 3 m3=person per

minute;• Maximum length of blower pipe to the ventilator 70 m.

Fresh Air Requirements: The point of maximum power use isproduced during mechanical excavation and simultaneous clear-ance in a forward direction:

1Machine ¼ 12 hp × 2:5 m3=hp=min ¼ 30 m3=min Personnel

¼ 2 workers × 3 m3=worker

¼ 6 m3=min TOTAL 36 m3=min

In other words, the maximum requirements of fresh air at theworkface will be 0:6 m3=s.

Assuming a loss of 5% of volume per 100 meters of pipe, thevolume to be supplied by the ventilator is estimated to be

QV ¼ Qf =

�1� 5

100·Lpipe100

�¼ 0:6=

�1� 5

100·70100

�

¼ 0:622 m3=s

Calculation of Diameter of Pipes For the range of volume to besupplied, the diameter of the pipe is determined by the followingformula:

Dpipe ¼ 0:29 ·ffiffiffiffiffiffiffiQV

p¼ 0:29 ·

ffiffiffiffiffiffiffiffiffiffiffi0:622

p¼ 0:229 m ≅ 0:25 m

Fig. 18. Concrete slab and track ballast tamping

Fig. 19. Concrete slab and track ballast tamping



Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TY

OF

SYD

NE

Y o

n 08

/29/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

Pipe of 250-mm diameter will therefore be used.Velocity of Air in Pipe The velocity of the air circulation in the

pipe is determined by the following formula:

Vpipe ¼QV

Spipe¼ 0:622

π ð0:25=2Þ2 ¼ 12:67 m=s

Calculation of Resistance The dynamic pressure attributable toturbulence of the airflow in the pipe is calculated using the formula

Pdynamic ¼1:2 · V2

pipe

2 · 9:81¼ 1:2 · 12:672

2 · 9:81¼ 9:82 mwc ð1 MPaÞ

On the other hand, resistance because of friction against thewalls of the pipe is obtained by the formula

Pfriction ¼ 0:25 · 0:12 · 70 ¼ 2:1 mwc ð0:21 MPaÞThe total resistance to be overcome by the ventilator would be

about 12 mwc. Therefore, the ventilator to be installed should fulfillthe following conditions:• Air Discharge 0:622 m3=s• Pressure 12 mwc (1.22 MPa)

Interior Installations of the Underpass

These address the comfort and safety of the user. Three installationswere made. The first was lighting the underpass, achieved byinstalling 10 centrally placed units of watertight lights with36 W fluorescent tubes. The second was installing surveillancecameras connected directly to the offices of the Municipal Police.The third, and complementary to the previous two, was installinglockable gates at both ends of the underpass, allowing temporaryclosure in any circumstances that the authorities consider impairingthe safety of users.

Draining of the Work

The drainage system had to account for both the collection andchanneling of surface water on the access ramps and subterraneangroundwater runoff currently on the side walls of the box. It wasalso necessary to account for the fact that the collection level of thiswater was lower (∼4 m) than the general systems of rainwater col-lection of the adjacent housing development.

To intercept the flow of subterranean water over the lateral wallsof the box, two drainage pipes of ϕ 315-mm diameter were in-stalled; the surface drainage of the passageway was by means ofthree transversal grilles connected to a PVC pipe of ϕ 400-mmdiameter (Fig. 20). All the drainage pipes in turn were connectedto two precast concrete shafts 1,000 mm in diameter, a 50-cm shaftreinforcing ring of precast concrete, and a precast concrete cone of75 cm in height. The capacity of the collection of water drainingfrom the box and rainwater runoff is about 2 m3. In both wells wereplaced two submersible pumps, one in each. From the pumping

shaft to the rainwater recovery shafts of the Local Authority,two polyurethane pipes of DN 63 mm were installed (one inuse and the other in reserve).

The drainage pipe was connected to the pumping shafts from theinside of the in situ concrete wing walls and from the floor of finegravel on which the box slid during the jacking process.

Cost and Time Limit for Completion

The total construction cost amounted to € 681,072 in overallbudget, broken down into the following categories as shown inTable 1.

The project was completed within four months.

Fig. 20. Cross-section of the underpass with drainage pipes position

Table 1. Total Construction Costs

SUMMARY Euros

Jacking shaft 68,732.15

Fabrication work 134,532.80

Work on tracks 184,304.24

Jacking of box 142,774.60

Demolition of concrete structures 12,099.52

Landscaping, access, and finishing of underpass 133,590.78

Health and safety 5,038.00

Total overall budget € 681,072.00



Dow

nloa

ded

from

asc

elib

rary

.org

by

UN

IVE

RSI

TY

OF

SYD

NE

Y o

n 08

/29/

14. C

opyr

ight

ASC

E. F

or p

erso

nal u

se o

nly;

all

righ

ts r

eser

ved.

Documents

System for Construction of a Railway Underpass