San Esteban II

8/15/2019 San Esteban II

1/11

Experiences in the construction of the San Esteban II Hydro-Power Plant

Authors Augusto Rebollo Patmore. MSc Civil Engineer, Exec. MBA. International Director,Obras Subterráneas S.A.

Eduardo Lostalé Alonso. MSc Civil Engineer. Technical Director, Obras Subterráneas S.A.

Summary

The original San Esteban Hydro-Power Plant is located in the province of Orense, Galicia, northernSpain. Owned by energy corporation IBERDROLA GENERACION S.A.U. it was inaugurated in1950, and has four generating groups totaling 264 MW of power.

The newly built plant extension, called C.H San Esteban II, of 175 MW generating power capacity, isintended to increase the overall power generation capacity up to 439 MW. The plant is entirely builtunderground within the rock formations of the left abutment of the San Esteban dam.

We are presenting two distinct and quite unique aspects of the construction of the undergroundhydraulic system of the San Esteban II Project, that help reduce time and cost savings beneting theowner and the contractor.

1. Excavation of an inclined section of the headrace tunnel

The hydraulic system of the power plant is composed of the following elements (from the upstreamend to the downstream end): Intake Structure, Intake Tunnel, Headrace Tunnel, Discharge Tunnel,Discharge Structure and Discharge Channel.

The inclined section of the Headrace Tunnel was a design alternative to the more common solutionof a vertical shaft followed by a horizontal section to achieve the hydraulic jump required in the

headrace tunnel. The solution adopted was a 45º degree inclined shaft, which was more complexfrom a constructability point of view than the more common solution described, but presentedhydraulic advantages, and reduced the overall length of the hydraulic system.

Hydraulic advantages included reduced friction and energy dissipation at the shaft bottom, avoidanceof a heavier steel lining at the elbow, and greater hydraulic speed.

The inclined shaft´s diameter is 8.5 m. and is approximately 100 m. long. It was carried out usingcustomized Raise-Boring technology, with a 2.4 m. pilot hole carried out in advance of the full sectiondrill and blast excavation enlargement. The outputs achieved ranged between 5 and 6 m/day in thepilot hole and between 1 and 2 m/day in the drill and blast full section enlargement. Difculties of aninclined bore included heavy customized mechanical and electrical installations to effectively guide,hold and pull the raise bore machinery in the shaft.

2. Solution adopted for the Discharge Structure construction

An alternative construction methodology proposed by the contractor allowed the client to minimizethe interference of the construction of the Discharge Structure with the standard operational regimeof the upstream (San Esteban) and downstream (San Pedro) reservoirs. This in turn contributed toreducing the environmental impact to a very sensitive area, where agro-tourism plays a vital role.Due to both positive effects, the economic savings were signicant.

The original design of the construction phasing of the Discharge Structure considered that theriver levels had to be lowered twice, in two separate long periods of, given that the duration of theDischarge Structure construction was greater than the maximum allowed duration of “dry river bed

period” (4 months).


2/11

With the solution adopted by the contractor, the excavation for the Discharge Structure base wasdone with a large 6.3 m long x 13 m wide 20 m deep shaft built by drilling and blasting, in 7.5 months.This allowed the use the original mountain formation surrounding the shaft not to be excavated, andto be kept in place as a natural protection dam to hold water away from the base of the Structure.With the protection of this natural cofferdam, the structure was built avoiding any affection to theriver stream (pollution by construction sediments and debris), and allowing the remaining excavation

surrounding the Structure and the Discharge Channel to be built in a single 1.5 month period.


3/11

Experiences in the construction of the San Esteban II Hydro-Power Plant

Authors Augusto Rebollo Patmore. MSc Civil Engineer, Exec. MBA. International Director,Obras Subterráneas S.A.

Eduardo Lostalé Alonso. MSc Civil Engineer. Technical Director, Obras Subterráneas S.A.

Introduction

This presentation summarises two peculiar experiences in the construction of the hidropower projectof S. Esteban II, the excavation of Pressure inclined section, of the Head Tunnell and the constructionof the Discharge structure.

The inclined section of the Headrace Tunnel was a design alternative to the more common solutionof a vertical shaft followed by a horizontal section to achieve the hydraulic jump required in theheadrace tunnel. The solution adopted was a 45º degree inclined shaft, which was more complexfrom a constructability point of view than the more common solution described, but presented

hydraulic advantages, and reduced the overall length of the hydraulic system.

Hydraulic advantages included reduced friction and energy dissipation at the shaft bottom, avoidanceof a heavier steel lining at the elbow, and greater hydraulic speed.

The second experience an alternative construction methodology proposed by the contractor allowedthe client to minimize the interference of the construction of the Discharge Structure with the standardoperational regime of the upstream (San Esteban) and downstream (San Pedro) reservoirs. This inturn contributed to reducing the environmental impact to a very sensitive area, where agro-tourismplays a vital role. Due to both positive effects, the economic savings were signicant.

Work’s description

San Esteban hydropower project is located in Orense province, Galicia, NW Spain, and was broughtinto operation in mid fties of the past century, is the last but one downstream hydro system ownedby IBERDROLA GENERACION S. A. in the Sil River. The original San Esteban hydropower plantconsist in 4 groups with a total power of 264 MW.

The following picture (Photo 1) shows a general view of the existing reservoir.

Photo 1. View of S. Esteban’s dam hydro power plant.


4/11

The new project of S. Esteban II tries to increase the power trough the construction of a newunderground plant of 175 MW, located into the rockmass that supports the left abutment of theactual dam.

The excavations of this brand new hydropower project has been developed in two kinds of lithotypeswell-distinguished: granite and a dike of diabasic nature.

Overall, the quality of the granite rockmass has strong changes. The RMR index ranges between 40to 70 and the UCS tests of intact rock shows unsettled results between 35 and 105 MPa.

In the granitic massif also four main dikes and are major diabasic dike, with its RMR ranging between30 and 40, was encountered.

In general, drill and blast has been used, with different types of support: shotcrete, rockbolting andoccasionally steel ribs.

The use of explosives close to the existing dam and Power House, demands having a very strictcontrol of vibrations produced by blasts, and demands adapting shooting schemes to the rockcharactericstics with the purpose of minimize the effects in the described facilities.

Following it is shown (Figures 1 and 2) the general layout of the Project as well as a tridimensionalview of it. In this last gure all the auxiliary tunnels needed as entry/access can be easily appreciated.It must be emphasize that these auxiliary excavations totalize more than 1550 m of total length.

Figure 1. General longitudinal Cross Section of the hydropower project.


5/11

Figure 2. General 3D layout of the project.

The hydraulic pressure system, from upstream to downstream, is made of the following parts: Intake

structure, Intake Tunnel, Headrace tunnel, Pressure ramp, power house and transformer caverns,Tail Race Tunnel and Discharge structure and channel. This system is shown at the following gure(Figure 3).

Figure 3. Detail of the hydraulic pressure system.


6/11

1. First experience

It is called inclined high pressure section of the headrace tunnel, the conduction between the shaftand the turbine, and is divides by: upper horizontal stretch, sloping stretch and lower horizontalstretch.

The construction of the inclined shaft stretch with an inclination of 45º was really particular becauseof the method used. The section is circular with a diameter of 8.50 m and its length is 100 m.Previously for its construction a small tunnel with a diameter of 2.4 m was excavated using Raise-Boring method.

Following its construction method is described.

Firstly the pilot borehole was drilled and following it was proceeded to enlarge it upwards.

Secondly, after raise boring the benching was done downwards using the equipments describedfurther on, up to its full section.

The following gure (Figure 4) illustrated this construction procedure.

Figure 4. Construction method used at the inclined section.


7/11

The advanced ratio for the raise boring was 5.5m/day and 1.6m/day for benching using drill and blast.The execution’s cycle of benching with 2.8 m span, was: Drilling: 20h / Blasting: 1.5 h / Ventilation:0.5 h / Scaling and de-mucking: 4h / Installations: 3 h / Support application: 5 h / Others: 1 h. TotalCycle: 35 h.

Using 20 h effective hours, an average ratio of 1.6 m/day is obtained. The work was established with

3 shifts of 8 h each, and 5.5 working days per week. The following personnel was employed: 1 shiftresponsible / 2 Drilling technicians / 2 Auxiliary personnel / 1 Electro mechanical technician.

It was necessary to design complex facilities for the staff and equipments in the highest securityrequirements. Figure 5 shows theses facilities.

Figure 5. Detail of the facilities used to construct the inclined section.

Apart from these facilities, the following equipments shown at Photos 2 a 3, can be remarked:

Photo 2. Raise Borer Robbins Photo 3. Máquina Brokk 400


8/11

2. Second experience

The Tailrace tunnel is the existing stretch between turbine and the discharge structure and channel,at river restitution. It ends in a Discharge Portal system and in a water discharge channel. Photo 4shows the Discharge portal after its nalization.

Photo 4. Finished discharge Portal system. (downstream).

In order to affect the explotation of the two existing dams as little as possible and to restrict anynegative effects, the construction’s method was modied.

The original design of the construction phasing of the Discharge Structure considered that theriver levels had to be lowered twice, in two separate long periods of, given that the duration of theDischarge Structure construction was greater than the maximum allowed duration of “dry river bedperiod” (4 months). Figure 6 shows this procedure.


9/11

In the foressen solution in the rst campaign, from June to October it would be carried out the portal excavation

and the structure, that afterwards would be submerged until the next campaign. The rest of the structure would

be built over the water’s level.

In the second campaign the structure would be nished and the Tailrace tunnel and canal would be built.

Figure 6. Original design for the Construction of the Discharge Structure.

Instead of this solution, OSSA proposed an alternative method in which rstly a deep shaft wasexcavated and from these the stretch of tunnel down and up stream, keeping a rock pillar to avoidwater entrance from the existing dawn, was excavated. Afterwars, and using only one dam down, therest of the tunnel was constructed.

With the solution adopted by the contractor, the excavation for the Discharge Structure base wasdone with a large 6.3 m long x 13 m wide 20 m deep shaft built by drilling and blasting, in 7.5 months.This allowed the use the original mountain formation surrounding the shaft not to be excavated, andto be kept in place as a natural protection dam to hold water away from the base of the Structure.With the protection of this natural cofferdam, the structure was built avoiding any affection to the

river stream (pollution by construction sediments and debris), and allowing the remaining excavationsurrounding the Structure and the Discharge Channel to be built in a single 1.5 month period.


10/11

Following some illustrative schemes of this alternative methods used are shows (Figure 7).

Figure 7. Alternative construction method proposed by OSSA.

Photo 5 shows a general view of the discharge structure after its construction.

Photo 5. General View of the river at its ecological “dry river period” and of the nished structure.


11/11

The shaft’s excavation of dimensions 6.3m x 13.00m and a height of 20m was made by drill and blastmethod, using presplitting close to the river bed in order to affect as little as possible the rockmassand to avoid possible entries of water. For this purpose an Ingersoll Rand ECM 6660 equipment wasused, put in place using a 40 t crane, while for mucking a secondary 10 t crane was used.

The construction procedure used was:

· Slope excavation and reinforcement: 1.5 months· Shaft excavation and support; including the break with the Tailrace tunnel: 2 months.· Construction of the Discharge Portal Structure: 3 months· Excavation and concrete of the discharge channel: 1 month

Photos 6, 7 and 8 shows different phases of the construction of the Discharge structure.

So the total construction time was 7 months and a half.

It this period it was only necessary the maintenance of the ecological ow 1.5 months (from 10thof August to 24th of September) with the resulting economical savings, minimizing the affectionscaused to the normal dam’s explotation, and also minimizing the environmental impact to a verytouristic and specially protected area.

Photo 6, 7 and 8. Construction of the Discharge structure.

Documents

San Esteban II