Concept of NATM: Using thin-layer support to maintain surrounding rock strength, control surrounding rock deformation, leverage the self-bearing capacity of the surrounding rock, and guide tunnel design and construction through construction monitoring and measurement.

(I) Cross-section excavation methods
1. Full-face excavation method

2. Bench method

Includes: (1) Long bench method; (2) Short bench method; (3) Ultra-short bench method

3. Partial excavation method

(1) Ring excavation with core soil method (bench partial excavation method)

(2) Single-side drift method

(3) Double-side drift method (glasses method)

(4) Center drift method

(5) Center diaphragm method – CD method

(6) Center cross diaphragm method – CRD method
(II) Full-face excavation method
1. Concept – The full-face excavation method involves blasting the entire design excavation cross-section in one go.

Representation methods: cross-section procedure diagram, longitudinal-section procedure expansion diagram

Conventional labeling method: ① Arabic numerals 1, 2, 3... indicate excavation;                 ② Roman numerals I, II, III... indicate support;

2. Construction sequence

(1) Drilling jumbo drills holes, charges explosives, and connects fuses;

(2) The jumbo retreats, explosives are detonated, and the entire tunnel cross-section is excavated;

(3) Remove loose rocks (commonly known as "scaling");

(4) Spray arch concrete, and install arch rock bolts if necessary;

(5) The mucking machine loads rock debris into transport vehicles and hauls it out of the tunnel;

(6) Spray side wall concrete, and install side wall rock bolts if necessary;

(7) Spray a second layer of concrete and tunnel invert concrete as needed;

(8) Measure and judge the deformation of the surrounding rock and initial support to provide a basis for modifying support parameters;

(9) Begin the next cycle.

3. Applicable conditions:

1) Grade III to I surrounding rock;

2) Large construction machinery (drilling jumbo: four-boom, two-boom, etc.);

3) Economic considerations: the longer the excavation distance, the more economical.

4. Construction characteristics:

1) Fewer procedures and less interference, facilitating construction organization and management;

2) Large excavation face, facilitating mechanized operations, good blasting effect (deep-hole blasting), and fast progress;

3) One-time excavation forming reduces the number of blasts and causes less disturbance to the surrounding rock;

4) Narrow geological applicability; requires good geological conditions, otherwise it is difficult to change the construction method when geological conditions change.

5. Three main operation lines for mechanized construction:

(III) Bench excavation method

1. Characteristics:

1) The cross-section is divided into an upper bench and a lower bench – excavated separately;

2) Currently the most widely applicable construction method;

3) With adjustment of bench length, it can be used in almost all strata – it is the dominant method.

2. Selection conditions

1) Time requirement for the initial support to form a closed ring: the worse the surrounding rock, the shorter the required closure time, and the bench must be shortened;

2) Construction machinery efficiency: if efficiency is high, the support closure time can be shortened, and the bench can be lengthened;

These two conditions reflect one principle: namely, the desire for the initial support to close as soon as possible.

For weak surrounding rock, the first condition dominates.

When the surrounding rock is good, the second condition dominates.

Why?

Answer: To leverage machinery efficiency and ensure construction economy.

3. Long bench method

Bench length: L ≥ 5B, where B is the width of the tunnel opening. It is even possible to drive the upper bench through first, which is the "half-section method."

Characteristics: Fast progress, second only to the full-face method.

Applicable to: Grade III to I surrounding rock.

Operation sequence:

(1) Excavate the upper bench: drill – charge – blast – anchor and spray – push muck to the lower bench;

(2) Excavate the lower bench: haul out the pushed-down muck – drill – blast – muck removal – side wall anchoring and spraying;

(3) Complete the entire cross-section.

4. Short bench method

Bench shortened: (1–1.5)B ≤ L < 5B

Characteristics: ① Faster support closure time, increased surrounding rock stability; ② Shortening the bench increases interference with lower bench construction. Applicable to: Grade V and IV surrounding rock.

Question: What adverse effects does shortening the bench have on construction progress?

Answer: The muck storage area on the upper bench decreases, and the number of times muck is pushed to the lower bench increases.

Solution: Increase time for parallel operations, which helps speed up progress.

① Belt conveyor

② Transition ramp

I. Steel structure springboard (invert constructed first)

II. Muck ramp (invert constructed later) ~ see figure

5. Ultra-short bench method

The upper bench length is only 3–5 m.

Characteristics: More conducive to controlling surrounding rock deformation, but the upper and lower benches are too close, causing great construction interference.

Applicable to: Weak and broken surrounding rock (Grade V and VI surrounding rock)

Precautions for short bench construction:

① Lower bench excavation should proceed only after the initial support of the upper bench is basically stable;

② Spray concrete promptly after excavation; the longer the exposure time, the less safe;

③ Measure promptly; when displacement rate accelerates, take immediate measures.

Question: Can the lower bench be advanced first?

Answer: Reverse bench method: fast but unsafe, obsolete.
(IV) Partial excavation methods

(1) Ring excavation with core soil method (bench partial excavation method)

(2) Single-side drift method

(3) Double-side drift method (glasses method)

(4) Center drift method

(5) Center diaphragm method – CD method

(6) Center cross diaphragm method – CRD method    ====================================   1. Ring excavation with core soil method

Characteristics:

① Retaining the core soil to support the working face provides better stability than the ultra-short bench method.

② Slow progress, many blocks, unfavorable ventilation.

Applicable to: Soil or easily collapsing weak surrounding rock; small construction machinery.

2. Single-side drift method

Characteristics:

① Reduced excavation span of the tunnel opening, conducive to surrounding rock stability;

② The drift must be closed, increasing material consumption;

③ Slow progress.

Applicable to: Weak strata with large cross-section span and control requirements for surface subsidence.

3. Double-side drift method

Characteristics:

① Double drifts reduce the maximum excavation span of the tunnel opening, increasing stability;

② Increased material consumption;

③ Slow progress.

Applicable to: Tunnels with higher surface subsidence requirements or larger spans.

Questions:

1) Should the excavation of double-side drifts be simultaneous or staggered? Why?

Answer: It should be staggered, with a spacing of 7–10 m.

To avoid excessive total span when excavating simultaneously on the same cross-section.

2) How should the drift dimensions be determined?

Answer: They should meet construction requirements, but the width should not exceed 1/3 of the maximum cross-section span.

4. Center drift method

Characteristics: First excavate the central rock/soil mass, construct the center wall concrete, then excavate both sides.

Applicable to: Double-arch tunnels.

Construction requirements:

1) The excavation height of the center drift should be 1 m greater than the center wall height, and the excavation width should be greater than 5 m;

2) The excavation length of the center drift depends on tunnel length, width, and geological conditions, generally 50–80 m;

3) After center drift excavation, apply initial support promptly, and pour the center wall concrete in sections;

4) After the center wall concrete reaches design strength, remove formwork and install temporary transverse bracing.

5. Center diaphragm method – CD method – Center Diaphragm

Characteristics:

1) Excavate from top to bottom in 2 or 3 parts along one side.

2) After each part is excavated, promptly apply anchor and shotcrete support, install steel frames, and construct the center diaphragm.

3) Install a temporary invert at the bottom.

4) Connect the center diaphragm wall part by part in sequence.

5) Excavate the other side of the center diaphragm wall, with the same steps as above.

Applicable to: Shallow-buried double-track tunnels in Grade V–VI surrounding rock.

Construction requirements: During excavation of each part, the peripheral contour should be as smooth as possible to reduce stress concentration; the bottom elevation of each part should be consistent with the steel frame joints; the excavation height of each part should be about 3.5 m; the excavation of the rear side should be closed in a full cross-section promptly; the longitudinal spacing between left and right sides is generally 30–50 m; the center diaphragm should be arc-shaped or circular-arc-shaped.

6. Center cross diaphragm method – CRD method – Center Cross Diaphragm

Characteristics: Excavate from top to bottom in 2 or 3 parts along one side; after each part is excavated, promptly apply anchor and shotcrete support, install steel frames, and construct the center diaphragm; install a temporary invert at the bottom; after completing parts 1–2, begin excavation and temporary support of parts 1–2 on the other side; forming a situation where excavation and support on the left and right sides cross each other.

Applicable to: Shallow-buried double-track tunnels in Grade V–VI surrounding rock.

Comparison of CD and CRD: