TransportationTunnel Boring

Construction of the longest road tunnel in Mexico

FIG.1
Plant location of Acapulco’s Alternate Roadway Project in Acapulco City, Mexico.

FIG.1-Plant location of Acapulco’s Alternate Roadway Project in Acapulco City, Mexico.

Construction of the longest road tunnel in Mexico is part of Acapulco’s alternate roadway to the scenic roadway project. This article provides a brief summary regarding details of the construction since the project began. Work was done by skilled engineers and Mexican workers that includes outstanding execution of the work in an urban and tourist area. This project will have a strong impact on the steady development of the port of Acapulco.

Project location

Acapulco’s Alternate Roadway Project is currently under construction in the city of Acapulco, located in the Guerrero state of Mexico. It is an 8-km (5-mile) long roadway that starts at the Icacos neighborhood, heading toward Acapulco’s airport, crossing a drill-and-blast tunnel that is 3,160 m (10,350 ft) long under Veladero Park, a mountainous area. The tunnel overburden reaches a depth of 380 m (1,250 ft) approximately at the central point of the horizontal alignment, and continues with a 4-km (2.5-mile) long elevated road to connect to the existing Viaducto Diamante toll road (Fig. 1).

Tunnel construction has already been finished using two portals for drilling and blasting operations; Brisamar Portal on the west side and Cayaco Portal on the east side (Fig. 2).

Geometric data

Only two main excavation cross-sections were considered in the executive design. The first one corresponds to the running tunnel for a three-lane vehicle section, with crosssection areas between 120 and 130 m2 (1,290 and 1,400 sq ft) taking into consideration different geological and geotechnical conditions to be encountered (Fig. 3).

The second one corresponds to eight emergency bays, each one 50 m (164 ft) long, laid down within the tunnel outline, located at 400 m (1,312 ft) from each one with excavation cross-sections between 156 to 169 m2 (1,680 to 1,820 sq ft), in accordance to geological and geotechnical conditions to be encountered (Fig. 4).

The tunnel’s horizontal alignment is completely straight, having a small curve within the vertical alignment on Cayaco portal, because there were some differences in levels when tunnel excavation started.

Tunnel slopes were designed in such a way that rainwater does not run down into the tunnel at any time.

Construction process of the running tunnel and bays, primary lining of the tunnel and equipment used

Drill-and-blast construction was planned to be used for the entire tunnel excavation, adjusting the excavation sequence as a function of the geological-geotechnical conditions, resulting from the exploration carried out during the design stage. Only seven direct borings for sample recovery and six transient electromagnetic borings at distances between 370 and 530 m (1,213 and 1,740 ft) were executed. Taking into consideration that most of the direct exploratory borings were near the portals, 70 percent of the tunnel distance was not explored properly, and rock formation parameters were inferred in large measure. Because of these reasons, the executive design presented several constructive sequences to cover different mechanical conditions of the rock formations, that were only indicative of measures to be applied by the contractor.

FIG.2
Schematic plan location showing Brisamar and Cayaco portals, and emergency bays along tunnel alignment.

FIG.2-Schematic plan location showing Brisamar and Cayaco portals, and emergency bays along tunnel  alignment.

Because of the uncertainty due to the lack of information about geological and geotechnical conditions of the site’s rock formations, tunnel excavation was planned to be done in three stages; the top-heading section first, keeping a 4-m (13-ft) bench for the lower section, that would be alternatively excavated after having advanced 500 m (1,640 ft) in the upper middle section. Drilling patterns and explosives were consequently adjusted accordingly to found rock-formation conditions.

Tunnel excavation at bays was done in two stages for the top-heading section, due to the larger dimensions. Bench excavation was also done in two stages, allowing access to the face at all times.

Primary support was in direct relationship to rock quality. A combination of steel fiber shotcrete in thickness from 5 to 20 cm (2 to 7.8 in.), IPR profile steel arches for poor to very poor rock conditions, in accordance to criteria established by Bieniawski for rock mass rating (RMP) determination was used. For regular rock conditions, it was recommended a 5-cm (2-in.) thick layer of steel-fiber reinforced shotcrete, plus 6-m (20-ft) long friction rock bolts at the top covering a 160-degree area was used.

Micropiles umbrellas and rock bolts.

For very bad to bad rock conditions where underground water was leaking or flowing throughout joints and cracks, a systematic array of micropile umbrellas were installed first to prevent cave-in formations after each blasting, making it easier to advance safely under the micropile umbrella protection. 37 micropiles were originally considered for installation along the running tunnel arch, distributed at a 40-cm (15-in.) distance from each other. Micropiles were made of special steel pipe (10 cm or 4 in. inner diameter) and were installed simultaneously as boring drilling was being done (Fig. 5).

FIG.3
Cross section of the running tunnel.

FIG.3-Cross section of the running tunnel.

FIG.4
Cross section at the bay tunnels.

FIG.4-Cross section at the bay tunnels.

Drilling length for each micropile umbrellas was 12 m (40 ft), allowing 3 m (10 ft) length overlap between each umbrella, giving 9 m (30 ft) of reinforcing length to each one. The number of micropiles installed was directly related to cross-section stability for rock conditions and water inflow, and thus this number was decreased or increased to cover tunnel arches and walls of the top heading as required. Steel arches and lagging installations were mandatory under these circumstances, as the micropile umbrellas needed a complementary support as tunnel excavation progressed.

FIG.5
Micropile umbrella array for running tunnel section.

FIG.5-Micropile umbrella array for running tunnel section.

Machinery used for excavation and primary support.

Excavation of the running tunnel was done efficiently with great-performance machines, which reduced work cycle times and allowed for excavation production rates for the top heading section of more than 9 m (30 ft) long per 24-hour work day, when geological conditions were fair. This equipment is as follows:

  • Three-boom, electro–hydraulic jumbo for drilling.
  • Electro-hydraulic, self-propelled mobile concrete sprayer.
  • Telehandlers 5 t (5.5 st) capacity and reach till 8 m (26 ft) height.
  • Wheel loaders, 16.73 t (18.4 st), 170 hp, 3.10 m3 (33.3 sq ft) bucket.
  • Hydraulic excavator, 30.5 t (33.6 st) with hydraulic hammer.
  • Backhoe loaders of 6.79 t (7.5 st), 74 hp, 0.73 m3 (7.8 sq ft) bucket.
  • Extraction of excavated rock was carried out with many dump trucks, 14 to 16 m3 (150 to 172 sq ft) capacity, owned by members of the local union.

Geological and geotechnical conditions encountered during construction.

The previous geological and geotechnical works consisting of direct and indirect exploration carried out for the executive project, were not enough for a proper evaluation of geologicalgeotechnical conditions on rock formations where the tunnel alignment was located.

Good practice in engineering recommends having sample recovery borings along the tunnel alignment at distances between 150 m and 300 m (492 and 984 ft) for a tunnel in rock with fair conditions, which means that in this case it was at least necessary to have 16 borings for direct exploration, complemented by geophysical borings for exploration to 300 m (984 ft) depth. Instead only seven direct sample recovery borings were made.

This article presents the longitudinal tunnel profiles that show the differences between the real rock conditions encountered in comparison with the rock conditions predicted by the executive project (Figs. 6a and 6b). Figure 6b is the result of a very close follow up to geological-geotechnical reports that were taken every day as tunnel excavation progressed by both portals. The most remarkable difference is a very bad to bad rock condition encountered in the first 465 m (1,525 ft) of the tunnel starting from Brisamar Portal, when the executive project indicated a regular-quality rock formation.

In this stretch, some cave-ins happened when there was an omission to install micropile umbrellas at places where rock fractures presented an unfavorable angle, allowing rock wedges to destabilize the tunnel section.

FIG.6A
Geotechnical conditions in accordance to the executive project Acapulco’s Alternate Roadway Project.

FIG.6A-Geotechnical conditions in accordance to the executive project Acapulco’s Alternate Roadway Project.

FIG.6B
Actual geotechnical conditions encountered.

FIG.6B-Actual geotechnical conditions encountered.

FIG.7
Monthly tunnel excavation progress at the top heading of section by Brisamar portal (L) and monthly tunnel excavation progress at the top heading section by Cayaco portal.

FIG.7-Monthly tunnel excavation progress at the top heading of section by Brisamar portal (L) and monthly tunnel excavation progress at the top heading section by Cayaco portal.

For the remaining length of the stretch, the real conditions of the rock formations in comparison to the ones indicated by the executive project, were also quite different, as can be seen in the comparison chart.

It is suitable to mention that, since the beginning of the work, several approaches were made to complement the geological-geotechnical exploration that was missing. However, this attempt was not successful.

Progress accomplished during tunnel construction

Figure 7 shows the monthly progress for the tunnel excavation at the top heading section by both portals, underlying a good production achievement for several months, to get 230 m (754 ft) maximum for a single heading, once most of the problems that arose during the start of the work were solved.

Descriptions of details found during tunnel excavation for each portal are given in the following section.

Tunnel excavation by Cayaco portal.

Tunnel excavation by Cayaco Portal started on April 17, 2014, through a medium hardness metamorphic rock. Two micropile umbrellas, 12-m (40-ft) long were installed as indicated by the executive project, with steel arches separated 1 m (3 ft) from each other. On June 23, 2014, after 114.45 m (375 ft) of tunnel excavation, the first cave-in event occurred suddenly after a blasting execution, due to the presence of unfavorably wedging formations in the arch of the top heading section. The blasted section length was 3.4 m (11.2 ft). Seepage water ran throughout fractures of the wedging rock. The time taken to surpass this event was seven days, after placing steel fiber shotcrete to stabilize sliding wedges, the installation of 13 steel arches and scaffolding formwork covering the entire area, and the pumping of hydraulic concrete to fill most of the volume left by the cave in event, which was close to 450 m3 (588 cu yd) After 120 m (390 ft) of tunnel excavation, there was another cavein of minor proportions, due to similar rock wedging problems and water leakage. Steel fiber shotcrete and seven steel arches were installed, and formwork scaffolding and concrete were pumped for support of the area affected. As tunnel excavation progressed and regular to bad rock conditions were encountered, it was necessary to keep installing steel arches and steel fiber shotcrete, facing one more event of minor problems by fractured rock and water leakage. However, it is very important to say that production daily rates increased significantly as tunnel-worker skills and coordination improved.

Unfortunately, on Nov. 19, 2014 tunnel works were suspended at Cayaco Portal, because of social problems between the state government and former land owners, after a tunnel length of 746.3 m (2,450 ft) had been reached. Under these circumstances there was only one way to make the tunnel connection, which was through the Brisamar Portal.

Tunnel excavation by Brisamar Portal.

Tunnel excavation by Brisamar Portal began on May 17, 2014 through very weathered granite formations and water leakage. These rock formations were classified as very bad to bad quality, in accordance to the geologicalgeotechnical conditions found. The construction method for the first 465 m (1,525 ft) of tunnel required systematic installation of micropile umbrellas, steel arches separated between 1 and 1.5 m (3 and 5 ft) and a shotcrete layer 20 cm (7.8 in.) thick for primary support. Drill-and-blast operations were only partially used for excavation of the cross-section as mechanical excavation employing excavator and hydraulic hammer was necessary for tunnel stability reasons at the top heading.

FIG.8
Tunnel breakthrough on Oct. 27, 2015 by Brisamar Portal. It concludes tunnel excavation at the top heading section.

FIG.8-Tunnel breakthrough on Oct. 27, 2015 by Brisamar Portal. It concludes tunnel excavation at the top heading section.

Production rates achieved for tunnel excavation after the first 465 m (1,525 ft) improved as the rock conditions upgraded to fair condition, although several areas with bad rock were found, where some small caveins occurred in association with the presence of water leakage. The environmental conditions inside the tunnel began to be a problem, as ventilation calculations for the additional length of tunnel by Brisamar Portal had not been adjusted. The problem was solved by replacing the whole ventilation system with more powerful and bettersuited fans, and ducting to the actual conditions of the work.

It is important to mention that some important water inflows were found at this heading and the overall flow was close to 30 L/s. Fortunately, rock conditions had improved and there was not a potential stability problem, as drilling and hoses were used to conduct all water inflow.

Tunnel excavation by Brisamar Portal was successfully finished on Oct. 27, 2015, making the breakthrough with the tunnel stretch that had been excavated by Cayaco Portal (Fig. 8).

Special considerations for work at the Brisamar Portal

Taking into consideration that the Brisamar Portal is located at one side of Joyas de Brisamar, a private high-class residential development within an urban area, it was of high priority and importance to minimize the construction impact on buildings and especially the life quality of residents. It was mandatory to take special measures to reduce all noise of drilling and blasting, as well as to buffer the effects of vibrations caused by blasting. The task was not easy, because residents reacted quickly to oppose the execution of the work, to the extent that legal action was taken and work was suspended on a judge’s order. The relationship with representatives of the residents of Joyas de Brisamar was often strained and with many complaints. After several meetings, the following measures were taken:

  1. Blasting was not allowed during night shifts.
  2. Electronic detonators were used to reduce noise blasting and detonating cord was not used.
  3. The use of low explosives for tunnel excavation for the first 500 m (1,640 ft) was restricted below the zone of influence of the houses and buildings. Only high explosives were used.
  4. Neighbors were kept informed about blasting times through written notes delivered to the Joyas de Brisamar administration and use of a siren system to alert prior to the execution of any blasting.

Thanks to these measures, the buildings located within the radius of influence of blasting suffered only minor cracks and/or damage. Vibration and noise were satisfactorily controlled within allowable limits.

Construction procedure for tunnel lining

Geometric sections of the final lining for the running tunnel section and bay section are shown in Figs. 1 and 2, appreciating the thicknesses of hydraulic concrete and reinforcing steel, as indicated by the executive project. These thicknesses correspond to theoretical sections of excavation, because actual thicknesses are based on geological rock conditions and measures taken to avoid over excavation and cave-ins during the stage of excavation.

Pouring the final lining of hydraulic concrete was originally planned using two monolithic and collapsible steel forms, 15 m (50 ft) long. Each piece was supplemented with two steel form sections for the bays (Fig. 9), designed exclusively for the ceiling and walls; one for each portal, as well as several modular sections of metal formwork for curve, walkway and a starting short wall section where the tunnel formwork overlaps.

FIG.9
Final lining construction activities at a bay area. The steel form has been assembled.

FIG.9-Final lining construction activities at a bay area. The steel form has been assembled.

With activities having been suspended by Cayaco Portal, the program related to the tunnel lining was fitted to the actual conditions of the work schedule, so both monolithic steel forms were armed and introduced by Brisamar Portal. Activities for pouring concrete in curves and walkways began on March 9, 2015 by Brisamar Portal, and activities for pouring concrete at the tunnel upper section and walls continued until June 19, 2015. Due to logistical issues related to the activities for reinforcing steel-bars installation, a geotextile liner combined with a water proofing geomembrane was fixed to the shotcrete primary lining.

It is important to emphasize the simplicity in the design of the steel formwork for concrete pouring in the bay areas, manufactured by a Mexican company recognized for its technology and ingenious design, making an easy fitting of both steel forms (running tunnel and bay).

Conclusions

Tunnel construction within an urban area has faced several special situations, due to the systematic use of explosives. However, the excavation was successfully completed and there is no doubt that it will bring great benefits to the people of Acapulco. National and international visitors will enjoy the benefits too.

The use of the electronic initiators for blasting operations was a successful measure for a substantial reduction of noise and vibrations.

The geological monitoring carried out in each blast was very useful for determining corrective actions required to be applied at subsequent blasts.

The work highlights the importance of adequate planning of prior geological-geotechnical studies, as well as of its magnitude and scope, in such a way that the executive project gets all the elements for the proper design of the primary tunnel support, and the best tools possible to avoid uncertainties during tunneling construction.

Micropile umbrellas were shown to be a good tool of support for safer and faster tunnel excavation when facing bad to very bad rock conditions.

All facts mentioned have led to an excellent step further in training young Mexican engineers and skilled workers, to continue building the great tunneling projects that Mexico needs, with Mexican contractors specialized in tunneling and underground works.

The average advance rates in linear meters of excavated tunnel, made in combination with good coordination, fair to good rock conditions, highproduction equipment and specialized machinery are very significant, close to 230 m (754 ft) per month for a single tunnel heading,

Excellent ventilation and preservation of good environmental conditions inside the tunnel must be the starting point for the selection of the equipment that will be used in these tasks, thus ensuring the safety, health and efficiency of the engineers, technical workers and technical staff that work in underground projects.

References

“Executive Project for Acapulco’s Alternate Roadway Project to the Scenic Roadway,” (2011). General Direction of Highways of The Secretary of Communications and Transportation. Commission of highway Infrastructure and airports of Guerrero State. Technical Manual for Design and Construction of Road Tunnels—Civil Elements, (December 2009). US Department of Transportation. Federal Highway Administration.

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