River bank protection; tunnelling in hard soil

Methods of river bank protection

River bank protection prevents erosion and scour of the banks near bridge approaches and guides the flow to pass smoothly through the waterway (IRC:89 "Guidelines for Design and Construction of River Training and Control Works for Road Bridges"; Nepal DoR river training and bio-engineering guidelines). The main methods are:

1. Revetments (pitching): stone pitching (dry or grouted), concrete blocks or gabion mattresses laid on a filter layer over a graded bank slope, with a toe wall or launching apron to prevent undermining. Used for banks with moderate flow.
2. Gabion/crate walls and mattresses: wire-mesh boxes filled with boulders; flexible, free-draining, cheap and widely used in Nepal. Gabion mattress aprons adjust to scour.
3. Spurs/groynes: short structures projecting from the bank into the stream that deflect the flow away from the bank, cause silting between them and train the channel. Types: permeable (piles, timber) and impermeable (stone, boulders); by their angle to the flow they are attracting, repelling or deflecting.
4. Guide bunds: earthen embankments with a stone-pitched river face and curved heads at the ends, built on both sides of the bridge approach to guide the flow centrally through the waterway (IRC:89).
5. Retaining walls/embankment walls: masonry, RCC or gabion walls along the bank where space is limited.
6. Longitudinal dykes/marginal bunds: embankments parallel to the flow that confine the river.
7. Launching apron and toe protection: loose stones at the toe that launch down the slope when scour occurs, protecting the foundation of the protection work.
8. Bio-engineering: vegetation (grass, bamboo, shrubs, trees) planted on the bank, often combined with gabions; cheap and environment-friendly for smaller flows (DoR Roadside Bio-engineering Guideline).
9. Check dams/sills and permeable fences: reduce velocity and trap sediment in steep streams.

Tunnelling in hard soil/rock

Hard soil or rock is tunnelled by the full-face, heading and bench, or drift methods, depending on the size of the tunnel, the rock quality and its stability. The main methods are:

1. Full-face method: the entire section is excavated at once, using drill-and-blast or a TBM. It gives rapid progress but needs good, strong rock and large equipment; it is mostly used for small tunnels (up to about 3 m diameter) or larger tunnels in very good rock.
2. Heading and bench method: the upper part (heading) is advanced ahead and the lower part (bench) is excavated behind it. It is common for medium to large rock tunnels, makes the work flexible and allows early roof support.
3. Drift (pilot tunnel) method: a small drift is driven first, at the centre, side, top or bottom of the section, and then enlarged to the full section. The drift explores the rock, helps drainage and ventilation, and allows several working faces.
4. Drill and blast cycle: drilling, charging, blasting, ventilation, mucking, scaling and support (rock bolts, shotcrete, steel ribs). Controlled (smooth) blasting minimises overbreak.
5. Tunnel boring machine (TBM): continuous mechanical cutting for long tunnels in uniform rock, giving smooth walls and a high rate of progress.
6. New Austrian Tunnelling Method (NATM): the rock is made to support itself with rock bolts and shotcrete, with monitoring of deformation; widely applied in Nepal's Himalayan tunnels.
7. Roadheader/ripping: for softer rocks and weathered zones.