The neighbouring bores on the Pando-Monte Lirio-El Alto hydro cascade scheme in Panama were set to present the possibilites for unsual challenges for TBMs. The geology consists of highly variable lahar volcanic rock, an uncommon barrier for shields to penetrate. There was potential for quite different experiences within as well as between the drives, and that has proved to be so as excavation has started.
Where the lahar so far has been good, on Pando, there has been the benefit of consistent progress for tunnelling contractor and TBM manufacturer Seli in the west of the country.
The Italian firm supplied three TBMs and crews for the three projects in the Chiriqui Viejo Valley where Pando and Monte Lirio are being built just upstream of El Alto, which has a different developer (see figure 1). All three projects are being constructed under very long, 50-year, concessions, awarded by Autoridad Nacional de los Servicos Publicos (ASEP).
Seli supplied three mixed-face EPBMs under two separate contracts for the projects, which will see a total of almost 16.5km of tunnel boring through lahars.
Further downstream, another hydropower plant in the cascade in the early construction phase is Bajo Frio, which is being developed by Credicorp and Aqua Imara, which formerly had the name of an SN Power subsidiary and is majority held still by the firm. They are operating as Fountain Intertrade on the project. However, because the topography in the lower reaches of the valley opens out so much, no ore underground works in lahars are called for.
The lahar tunnelling challenges are upstream in the tight, snaking valley. However, Seli has other tunnelling work in the area, being called in as part of a group to help repair a tunnel collapse on the Esti hydropower project .
The trouble with lahars
Volcanic rocks are always a challenge but lahars especially so. They derive mainly from volcanic slurries and can contain significant amounts of water, depending on the local geology and hydrology. They can be formed due to eruptions – flows or explosions – or seismically-induced shifts and mixing of the ground.
As mudflows with such different possible origins, the slurries have quite varied characteristics which further change with progress of the flows, both dropping out material as well as gaining inclusions. Then, solidified, each flow is termed a lahar, and the properties of each matrix are highly varied, even locally.
Prior to excavation, the local lahars in Chiquiri Viejo valley were classed into two types: matrix- and clast-supported. There was no information then to help split classification with respect to cohesiveness.
The lahars showed uniform residual and peak friction characteristics. There were also groundwater risks, depending on insufficient fines in the mix or water-bearing strata being met. Generally, permeability was seen as low in at least the matrix-supported rock which was the poorer in terms of fines and less well graded of the two classes.
Mix at Pando-Monte Lirio
Awarded an engineering, procurement and construction (EPC) contract for the Pando and Monte Lirio underground works in late 2009, Seli is constructing a total of 13.1km of headrace tunnel for the schemes which are being developed by Electron Investment. The projects are to be commissioned by the end of 2013.
The Monte Liro project has the larger of the two tunnels – almost 7.9km in length with a finished diameter of 3.2m. The tunnel will link Monte Lirio with El Alto to the west. The tube is being bored by a 3.92m diameter EPBM, and the lining comprises 1.2m-long rings constructed of 250mm thick precast concrete segments in a five plus one arrangement. Spoil removal is by locos and wagons.
Despite a positive start in early 2011, the TBM soon met with difficult wet ground conditions and advance rates slowed. In one period of two and a half months it had advanced approximately 780m.
Shield standstill
Encountering loose lahar and large water inflows, the machine was stuck, it adds. Seli says that a top heading of approximately 20m in length was excavated to free the shield. The cutterhead had been trapped by a large, uncemented boulder and there were significant inflows, "recognised by the geologists as unforeseen ground conditions," according to Seli.
After four months of standstill the drive was able to continue late last year. By the beginning of this year the EPBM had constructed 1,550m, or approximately one-fifth, of the headrace.
For the Pando project, Seli supplied a 3.72m diameter but otherwise virtually identical EPBM with 17 inch discs as well as rippers on the cutterhead. The TBM is driving an almost 5.2km long headrace of 3m i.d. but with the same arrangement of lining. The bore will link Pando with Monte Lirio in the west. Spoil removal is by the same method as the Monte Lirio project.
The TBM set off early last year and met with a cemented portion of the headrace tunnel, enabling it to achieve good progress, encountering few difficulties until near the end of 2011. By October, and including the learning curve, the machine had achieved a 1,600m advance in some five months.
However, at the end of the year, the first notable problem was met. This was ‘very similar’ to that encountered at Monte Lirio, says Seli. Part of the work performed to get through the problem included conslidating the front face and draining the groundwater to reduce pressure. By then, the machine had bored 1,900m, or just over a third, of the headrace.
Over both projects, Monte Lirio and Pando, the ground conditions met ranged from good to poor. Where it was good, the rock mass was tough at the face, holding highly cemented basalt boulders and having low permeability, which allowed tunnelling with only minor difficulties.
But, where it is poor, the mass is loose with uncemented small stones, gravel and sand with accompanying large inflows of groundwater. In those sections the drive progresses with full pressure face control.
On the projects, the EPC contractor for the main civil works is Cobra.
El Alto experience
Under construction down the valley, the El Alto hydropower plant will also have extensive underground structures for the relative scale of the firm power of the scheme. The prime structures are the 3.24km-long headrace plus the surge tanks, a 96m-high shaft and the penstock of just over a third of a kilometre in length. El Alto is being developed by a JV of Hydro Caisan and Panama Power Holdings.
Scheduled for commissioning next year, excavation of the El Alto headrace began in the third quarter of 2011. A 6.79m diameter EPBM was ordered from Seli under another EPC contract.
The crew driving the EPBM have had a challenging time taking the TBM through loose lahar from the outset. The particular mix of lahar strata is made up of rounded blocks of uncemented rock and there is
"considerable water pressure", says Seli.
However, it notes further that otherwise there have been no major problems on the drive. By the beginning of 2012 the machine had constructed 970m, or approaching a third of the length of the headrace tunnel.
Early in the drive, the machine advanced 350m in three months, which includes the learning curve as well as necessary pauses for installation of a continuous conveyor belt for spoil removal.
