Chapter 10 — Design of Large Underground Caverns¶

10.1 Geometry & Excavation Sequence in Underground Caverns¶

Large underground caverns (hydroelectric powerhouses, transformer halls, underground mine crusher chambers, and defense installations) feature span dimensions exceeding \(20 - 35\text{ m}\) and wall heights reaching \(40 - 60\text{ m}\).

At these dimensions, single-pass excavation is impossible. Caverns must be developed through multi-stage sequential heading and benching:

Step 1: Top Heading Pilot Tunnel (Advance with crown rockbolts & shotcrete)
Step 2: Crown Slashings (Enlarge crown to full span width; install cablebolts)
Step 3: Bench 1 Excavation (Central bench down 3-5 m; sidewall pattern support)
Step 4: Bench 2 to N (Sequential benching downwards, maintaining sidewall support)
Step 5: Invert Excavation & Drainage Gallery Connection

10.2 Cavern Axis Orientation Relative to In-Situ Stresses & Joint Sets¶

Cavern orientation is the single most important design decision:

Alignment with Major Horizontal Stress (\(\sigma_H\)):
The longitudinal axis of the cavern should be aligned parallel (or within \(15^\circ - 20^\circ\)) to the direction of the maximum horizontal principal stress (\(\sigma_H\)).
This minimizes the high compressive stresses acting across the cavern sidewalls, preventing massive sidewall buckling and deep shear zones.
Alignment Relative to Dominant Joint Sets:
The cavern axis must avoid striking parallel to steeply dipping persistent joint sets or faults.
If a fault strikes parallel to a long high sidewall, huge multi-thousand-tonne wedge blocks are formed that require uneconomically heavy cable anchor reinforcement.

10.3 Pre-Stressed Cablebolt Design for Caverns¶

Standard rockbolts (\(3 - 5\text{ m}\) length) are insufficient to reinforce the deep plastic yield zones (\(6 - 15\text{ m}\) depth) surrounding large caverns. High-capacity pre-stressed cablebolts (\(15 - 25\text{ m}\) length, \(500 - 1,000\text{ kN}\) capacity) are mandatory:

Cable Anchor Components:¶

Tendon: Multiple 7-wire high-tensile steel strands (\(15.2\text{ mm}\) diameter, ultimate capacity \(260\text{ kN}\) per strand).
Bond Length (Anchor Zone): Resin or cement grouted fixed length (\(5 - 8\text{ m}\)) anchored into virgin, elastic rock beyond the yield zone.
Free Length: Sheathed or unbonded length allowing elastic elongation during tensioning.
Anchor Head & Bearing Plate: Large steel bearing plate distribution stress across reinforced shotcrete pads.

\[\text{Design Working Load } T_{\text{work}} \le 0.60 - 0.70 T_{\text{ult}}\]

10.4 3D Instrumentation Arrays in Large Caverns¶

Monitoring during cavern benching provides the feedback loop validating numerical models (boundary element and finite element analyses):

Instrument	Location & Array	Primary Monitoring Objective
Multi-Point Borehole Extensometers (MPBX)	Radial fans from crown and high sidewalls (\(10, 20, 30\text{ m}\) depth)	Measures depth of rock dilation and verifies that cable anchor bond lengths remain anchored in stationary rock
Borehole Stressmeters (Vibrating Wire)	Biaxial cells installed in sidewall rock pillars	Measures stress concentrations between adjacent powerhouse and transformer caverns
Load Cells on Cablebolts	Under bearing plate anchor heads	Continuously tracks whether rock dilation is overloading cable tendons toward tensile rupture
Convergence Targets (Optical RTS)	Grid points across cavern crown and sidewalls	Measures 3D inward closure vectors during successive lower bench blasts

10.5 Canonical Terminology¶

English Term	Canonical Vietnamese Translation	Definition
Underground cavern	Buồng ngầm quy mô lớn	Large underground excavation with high span and wall height
Pre-stressed cablebolt	Cáp neo dự ứng lực	Long high-capacity steel strand anchor pre-tensioned against rock
Free length	Đoạn tự do của neo	Unbonded section of an anchor that elongates elastically
Bond length	Đoạn neo bầu / Chiều dài đoạn ngàm	Grout-anchored fixed section transferring load to the rock mass
Sequential benching	Đào hạ tầng giật cấp	Top-down excavation method removing rock in progressive horizontal benches