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Soft-ground tunneling methods are commonly used for urban services (subways, sewers, and other utilities). The tunnel structure in soft ground is generally designed to support the entire load of the ground above it, partly because the ground arch in soil deteriorates with time and as an allowance for load changes resulting from future construction of buildings or tunnels.
Soft-ground tunnels are typically circular in shape to inherit greater strength and readjust to future load changes.
Types of Tunneling Methods in Soft Soil
- Forepoling method
- Needle beam method
- Army method
- American method
- English method
- Belgian method
1. Forepoling Method
This is probably the only system advocated for running ground and similar soils. However, the process is slow, time-consuming, and requires skilled miners. Nevertheless, tunnels of small dimensions required for laying sewers, gas pipes, etc., at ordinary depths could be constructed through this method.
It is essential that the sequence of operations strictly adheres to the correct order. For example, the series of operations for a 5' X 5' tunnel in running ground is explained below:
- A shaft is sunk from the surface to the grade level and adequately protected with timber sheeting.
- A wooden bent properly braced is set up a few inches from the sheeting.
- Small holes at close intervals are drilled through the sheeting to facilitate sections being cut out later, 3 inches apart above the cap and another line of holes below the cap.
- A piece of sheeting above the cap is cut out along the top line of holes.
- Fore poles or 'spiles' consisting of planks 5.6" X 6" X 2" with wedge ends are entered one at a time and driven through the cut into the ground for half their lengths with an upward inclination of 2" per foot.
- This inclination is essential to prevent fouling of spiles. A few spiles are also started on the sides, flaring out with a slope of 1.5" per foot.
- The roof and part of side spiles are driven to half their lengths, a timber is laid across the back ends of the spiles, and by wedging this down, the front ends of spiles are cantilevered up.
- The face sheeting is now cut across the lower line of holes, which removes the sheeting between the two rows of holes, and the loose soil is allowed to run into the tunnel till the face assumes a natural slope.
- A 'horse head' is set as temporary support about 2 feet from the sheeting, and the spiles are driven to their full length.
- The earth beneath the forward end is scooped out for a depth of 18", and the face is supported by a breast board, placed underneath the point of the spile.
- The next cap supported on a bridge is set and temporarily supported on a single post.
- Meanwhile, the side spiles are also driven for their full-lengths.
- A heavy horizontal beam 6" X 8" is pushed forward to support the forward cap.
- This facilitates clearing the forward bench, setting the new bent for the bold cap, and relieving the temporary supporting beam.
2. Needle Beam Method
This method is suitable for soils where the roof could be depended upon to stand for some minutes without support. This method could be advanced by 10' to 12' length per day. The needle beam consists of a stout timber beam or a composite flitched beam and forms the temporary primary support during the excavation.
The sequence of operations is as follows:
- A monkey drift for a short distance of 3' is driven beyond the day's work, on the working face.
- The roof of the drift is supported by lagging carried on wooden segments, which are, in turn, supported by two trench jacks set in hitches cut in the sides of the monkey drift.
- After this drift is completed, the needle beam, which is about 16' long, is slowly skidded forward into the monkey drift.
- The front end of the needle beam rests on planks on the drifting floor, while the rear end is carried on stout posts resting on the floor lining of the tunnel.
- A trench jack is placed on the centerline of the needle beam to support the segment, thus transferring the roof load to the needle beam.
- The other trench jacks are removed, and the drift is widened side-ways and supported as before by laggings, segments, and trench jacks supported on the sides of the needle beam.
- If necessary, compressed air could be used initially for roof support, at a pressure of 12 lbs/sq.
3. Army Method or Case Method
The United States Army devised this method for constructing small tunnels at reasonably shallow depths. This was mainly used for laying underground sewers.
The advantage of this method lies in its simplicity and economy as only a few timber planks, with 1-2 trench jacks, form the leading equipment.
The process is as follows:
- To advance the work, the top breast board is removed and the ground excavated for a short distance of 8" to 10" ahead; the breast board is reset in the new face and braced back.
- The next cap board is set and held in position by a 'crutch' or trench jack.
- The breast boards are removed one at a time and reset in a line below the top breast board after removing the earth behind each breast board, thus forming a new advanced face.
- The sill boards are now advanced after checking the grade level.
- The side posts are now fixed between the sill board and top cap correctly.
4. American Method
This method is suitable for large-sized railway or highway tunnels. The process involved is as follows:
- A top drift shown by a dotted line in fig. 8 is first driven and supported by laggings, segments, and two posts.
- Sides of the drift are now widened and supported on shoulder segment timbers and struts from the sill; widening is thus carried up to the springing.
- Wallplates of 16 feet in length are introduced at the springing, supporting the arch set composed of the segments connected suitably by dowels at their ends.
- The wall plates are then pinned by introducing props or vertical posts at certain intervals.
- The timber arch segment and roof load are thus transferred to the wall plate and posts for support, relieving the timber support in the top drift, which are now removed.
5. English Method
- A central top heading about 16 feet ahead of the existing arch lining is driven.
- This is supported by crown bars, which are supported on posts in front and blocked by the face of the completed arch ring in the rear.
- Widening of the heading is then done as in the American method, and the sill piece is extended right across the tunnel.
- The extended sill is underpinned, and supports are introduced, the entire arch now being carried on the longitudinal crown bars.
- This method involves using a lot of timber, and the most significant disadvantage is the frequent shifting of heavy timber logs back and forth.
6. Belgian Method
This is a popular method and is suitable for all classes of moderately firm or hard soils. Fig. 10 indicates the sequence of operations.
- A top center heading for the whole arch's rise a b c d is driven and supported by crown bars, posts, and laggings, similar to the English method. The posts are kept on sills.
- The heading is widened sideways and supported by additional crown bars and posts supported from the same sills.
- The arch lining is built, and a horizontal brace is fixed between the ends of the arch at the springing.
- A trench M N O P is excavated to clear the benching down to grade level. Pockets are cut at intervals in the trench sides to insert shores to underpin the arch.
- The alternating spaces between shores are then cleared, and the supporting side masonry is built. The shoring is now removed, and the space is filled with masonry. The invert is then constructed.
The advantage of the Belgian method lies in lighter timber sections, as the timber is placed closely. But the disadvantage is due to the system of the underpinning of the built arch, mainly when the avoidable subsidence of the soil may occur, causing settlement and cracks in the arch masonry.
1. Forepoling method
2. Needle beam method
3. Army method
4. American method
5. English method
6. Belgian method
Soft-ground tunneling methods are commonly adopted for urban services such as subways, sewers, and other utilities.