Hydroelectric & mining projects are normally founded on hard rock. Power tunnels, penstock tunnels, water diversion tunnels & mine access tunnels are the common applications for drill blast tunnels. Highways & railways that penetrate under mountains also employ drill blast tunnels.

The alignment of the tunnel is identified by class of rock. There would normally by a minimum of 5 classes of rock defined from high quality to very poor. Rock support depending on class varies from no support, spot bolts, shotcrete on crown, patten bolts with shotcrete to steel sets at close centers. In high quality rock concrete lining may not be required. In poorer rock concrete paved inverts & concrete lining maybe necessary. When concrete lining is employed contact grouting behind the lining is required.

Refer to HCI 220 Drill Blast Tunnels

Tunnel boring machines are most used for transit projects & water supply projects. Different types of TBM’s with varying cutter & temporary support arrangements are used under different ground conditions. These would include unshielded Mainbeam TBM’s in good quality rock, Shielded TBM’s in fractured rock, Crossover TBM’s in mixed face conditions and Earth Pressure Balance (EPB) TBM’s under hydrostatic conditions.

Except for the Mainbeam machine, TBM’s are normally designed to assist in the installation of precast tunnels liners directly behind the advance. Annulus grouting is normally performed to fill the void between the liner & the ground.

Refer to HCI 230 TBM Tunnels

This method is usually employed for shorter tunnels, tunnels or crossovers of unusual shape, large size tunnels or underground stations.

Smaller sizes can be excavated with Roadheader equipment & employing temporary rockbolt & shotcrete support. Very large tunnel sections including under ground stations can be excavated using Sequential Excavation Methods, referred to as SEM, where small drifts are excavated in sequence. Refer to the Basis of Estimate FlipBooks for detailed illustrations.

Refer to HCI 240 Mined Tunnels

Refer to HCI 250 Underground Shafts & Caverns

Hard rock shafts can be excavated from the top down, referred to a sinking a shaft or from the bottom up, using raise bore machines. Refer to the Basis of Estimate FlipBooks for detailed means & methods.

Refer to HCI 250 Underground Shafts & Caverns

This type of construction is most employed for the excavation of underground powerhouses on hydro projects. The method includes providing access to & from the cavern by first driving a tunnel to the crown of the cavern, dropping a shaft to the invert then driving a mucking tunnel. The cavern itself is excavated using conventional drill blast & benching techniques. The muck is pushed down the drop shaft & hauled out of the mucking tunnel.

Refer to HCI 250 Underground Shafts & Caverns

This type of tunnel employs traditional surface construction techniques including support of excavation, earth excavation, dewatering, uplift anchors, waterproofing, reinforced concrete box & structural backfill.

Depending on the ground conditions & the level of the water table the support of excavation can vary from driven soldier piles & lagging in dry ground, drilled soldier piles & lagging in moderate conditions, drilled soldier piles with filler piles in wet ground & in high water table areas secant piles or diaphragm walls may be required.

Refer to HCI 180 Excavation & Backfill

Refer to HCI 210 Support of Excavation

Refer to HCI 270 Cut & Cover Tunnels

Refer to HCI-420-Cast in Place Concrete

Tunnels are normally constructed on a two or three shift basis.

Refer to HCI-010-Labor Rates for development of Overtime Rules

Tunnel projects are normally constructed by specialist contractors or joint ventures. Tunnel contracts normally include the procurement of specialized equipment like TBM’s requiring long lead times. Significant indirect facilities & services are required to service the underground work.

Refer to HCI-950 Urban Project Indirect Assemblies