ALQUEVA DAM ( PORTUGAL) FAULT 22 TREATMENT -REPLACEMENT WITH CONCRETE

The geological fault of Alqueva (Fault 22) was discovered in the decade of 70, in the course of a geological survey conducted on the initial design studies for the dam.

Later, the promoter (EDIA) insisted on the safety of the dam, reaffirming that the fault had no seismic activity, this was corroborated by two studies elaborated at the same time, one by the Portuguese National Laboratory of Civil Engineering (LNEC) and other an American researcher at the University of California. In these terms, EDIA decided to maintain the construction of the dam in the place where it was originally designed.

The dam/ foundation interface consists of green schist of good quality on the right bank and in the river bottom and of quite good phyllite on the left bank.

The area of the phyllite is more fractured and is crossed by several faults, the most important being fault 22, along which the green schist/ phyllite interface occurs. Various sets of sub-vertical and sub-horizontal rock joints were identified at the dam site.

Numerical analysis of both concrete dam and rock mass was carried out based on a discrete element model.

THREE-DIMENSIONAL MODEL :

MAIN WALL OF THE DAM - DOWNSTREAM-SIDE - FAULT LOCATION :

FAULT MODEL - CEILING AND WALL :

CROSS SECTIONS – JOINTS 2 AND 3E :

As a result, it was necessary to consolidate the aforementioned fault - The solution consisted in the execution of tunnels to access the inside of the fault, creating galleries to excavate the materials existing between the hanging wall and the footwall of the fault and replace it with concrete.

During the excavation and sustaining of the tunnels and galleries several tunneling techniques were used, namely:

• Rock anchors - Swelex
• Forepoling (special measures ahead of the face)
•  Steel ribs (metallic arcs)
• Shotcrete Lining reinforced by steel mesh

Rebar Rock Bolt Reinforcement - Swellex rock bolts:

Rock anchor bolt is a long anchor bolt, for stabilizing rock excavations, which may be used in tunnels or rock cuts. It provides high tensile property and good ductility performance. In the following figures, can be seen how the rock anchor bolt performs in the rock body:

? Support surrounding rock

? Reinforce surrounding rock

? Raise the friction between the rock layers

The Swellex system consists of a rock bolt made of a circular steel tube, which has ken folded to reduce its diameter, a face-plate and a high-pressure water pump.

The Swellex system provides immediate axial friction throughout the entire bolt length and mechanic anchoring is provided by a radial mechanical lock.

The installation is a straightforward process. The Swellex bolt is placed in a borehole with a diameter of 32-39 mm. A water pump is used to inject high-pressure water into the Swellex bolt. The high internal water pressure then inflates the tube. When the preset pressure has been reached, the mechanic interlock with the rock is achieved

During the expansion process the Swelled bolt compresses the material surrounding the hole and adapts its shape to fit the shape of the hole. The combination of fictional and mechanical locking effect is generated throughout the entire length of the bolt.

Forepoling - Arch Canopy System (pipe roofing) :

Forepoling, also known as tube umbrella, is an application used to strengthen tunnel roof in broken rock conditions.

The principles of pipe roofing in implementing the method, consists of steel pipes that are installed ahead of the tunnel face, arranged like an umbrella or canopy around the excavation line. The umbrella stabilizes and protects the ceiling and face of the tunnel by increasing the load bearing capacity of the ground. Thanks to the rigidity of the steel pipes, distribution of loads will be improved and the critical spot will be moved further ahead of the working face.

The drilling system consists of casing tubes wich are drilled trought the overburden as an and migh or not be filled with grout :

1) Forepoling with Metal pipes

2) Metal pipes with high-pressure grout injection (horizontal Grouting)

Structural steel ribs (Arch Supports)

Structural steel ribs (arch supports) are used for roof support or ground control where severe conditions are anticipated or to maintain critical areas of the tunnel where roof bolting is not sufficient or practical. Used in conjunction with forepoling, metallic arches can solve difficult conditions on a long-term basis.

Sprayed concrete (shotcrete) :

In this case the supporting system, consists of bolting, rebar and/or arch supports combined with shotcrete. Shotcrete is a common reinforcing measure in tunnels construction

Shotcrete in Tunnel Construction:

- Shotcrete process :

? Spraying surface treatment

? Check steel frame

? Mark the laying of shotcrete layer thickness

? Spray concrete

? Curing

- Main advantages :

? Support surrounding rocks

? Transfer and unload the deformation energy

? Fill and reinforce the surrounding rocks

? Cover the surface of surrounding rocks

? Prevent from the surrounding rock loose

? Distribute the external forceBibliographic references

Bibliographic references :Hydromechanical analysis of dam foundations -application issues and case studies - José V. Lemos (LNEC)

Paulo Calico

Project Manager - Senior Civil Engineer