Tunnel driving in hard and soft strata

Methods of Tunneling in Soft Soil

i. Fore poling 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:

1. A shaft is sunk from the surface to the grade level and adequately protected with timber
sheeting.
2. A wooden bent properly braced is set up a few inches from the sheeting.
3. 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.
4. A piece of sheeting above the cap is cut out along the top line of holes.
5. 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.
6. 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.
7. 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.

ASHWINI P, Department of Civil Engineering, MyCEM

 8. 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.
9. A 'horse head' is set as temporary support about 2 feet from the sheeting, and the spiles
are driven to their full length.
10. 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.
11. The next cap supported on a bridge is set and temporarily supported on a single post.
12. Meanwhile, the side spiles are also driven for their full-lengths.
13. A heavy horizontal beam 6" X 8" is pushed forward to support the forward cap.
14. This facilitates clearing the forward bench, setting the new bent for the bold cap, and
relieving the temporary supporting beam.

ii. 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.

ASHWINI P, Department of Civil Engineering, MyCEM

 iii. 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.

iv. 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.

ASHWINI P, Department of Civil Engineering, MyCEM

  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.

v. 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.

ASHWINI P, Department of Civil Engineering, MyCEM

 vi. Belgian Method

 This is a popular method and is suitable for all classes of moderately firm or hard soils.
 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.

ASHWINI P, Department of Civil Engineering, MyCEM

 vii. Austrian Method

 The method is also called as Sequential Excavation Method. A centre cut is taken for full
height. Cut is widened to full face. Permit short section of masonry to be completed.

viii. Italian Method

The Italian tunneling method is also known as packing method. It is used to overcome very
difficult geological conditions. Excavation is done in parts with immediate installation of
packing material and later broken out after ring closure.

ix. German Method

In this method 3 drifts are used to support the roof and side. One at the crown and rest two are at
bottom along the wall. Also called Core Leaving Method.

Other Methods:

x. Liner Plate Method:

 In this method pressed steel liner plates (91cm x 41cm) are used to support the soil during
excavation work.
 First about 40cm deep hole is excavated at the crown and a liner plate is placed in
position to support the roof.
 Now the excavation is done on the side and liner plates are provided and bolted with
central plate.
 The excavation is done in the lower side and the trench jacks are provided to support the
liner plates.
 After further excavation wall plates and wedges are provided at the ends of the lower
liner plate and the load of the jacks is transferred to them. Now the trench jacks are
removed and the excavation is started further in the above sequence.

ASHWINI P, Department of Civil Engineering, MyCEM

 xi. Shield Method:

 This method is used for driving a tunnel through water bearing strata. It is a speedy
method.
 Shield is an equipment which acts as a bridge with roof for workers
 A shield is movable frame and it is used to support the face of tunnel
 The excavation and lining of tunnel can be carried out under protection of shield.
Method of operation

 The ground is excavated ahead of the shield of 0.45. to 0.75m

 The shield is jacked forward
 Primary lining is the heavy cast iron lining used in conjunction with the shield.
 Secondary lining is 5 to 7cm concrete placed over the flange of iron.

xii. Compressed Air Method:

 Most modern method used for tunneling in soft grounds having water bearing strata.
 Timber supports is not required, support is provided by compressed air into enclosed
space to prevent collapse of roof and sides of tunnel.
 Air pressure adopted is approximately 1kg/cm2
 If pressure will be more than working hour will reduce which will increase the cost of
labors and the construction cost as well.

Methods of Tunneling in Hard Soil

1. Full Face Method :

ASHWINI P, Department of Civil Engineering, MyCEM

  This method is suitable for small tunnels through stable and self-supporting rocks. In this
whole section of tunnel is drilled at the same time. Since the full section is tackled in one
continuous operation, additional units of tunneling equipment and man power is needed.
 It is suitable for small cross sectional area about 3m dia. Bars are installed whose width is
less than height. Columns are installed vertically whose height is less than its width.

 The present day method of shield tunneling and Tunnel Boring Machine (TBM) are very
suitable for full face method. The full face method is a particularly useful for small
tunnels having diameter below 6 m and face area below 19 m2.
 The entire section is drilled, and the holes are loaded and the explosives are discharged.
The debris or broken rocks are collected through mucking equipment
 In case of larger diameters the full face area increases above these limits and the
equipment costs and support problems make the operation uneconomical. In the case of
poor ground conditions the problem becomes all the more difficult.

This method has the following advantages:

 The operation is simple and minimum quantity of equipment is needed.

 Total ground disturbances and settlement are minimum.
 Simplicity of work.
 In situations where multi-face excavation cannot be adopted this system provides definite
advantage.
 Mucking truck can be positioned once and for further mucking the position is shifted in
according to the progress of the work.

2. Heading and Bench Method:

It is used when tunnel is very large and the quantity of rock is not satisfactory. If rock is hard
and self suppoting, heading is advanced ahead by one round of the bench. The bench
provides platform for timber supports. Heading is usually 3 to 3.5m ahead of bench.

ASHWINI P, Department of Civil Engineering, MyCEM

 3. Drift Method:

A drift is a small tunnel driven through all or a portion of the length of the tunnel prior to
excavating full bore.

Types:
 Centre drift
 Side drift
 Top drift
 Bottom drift

 Centre Drift: Generally 3mx3m to 2.5mx2.5m centre drift is driven through portal to
portal. Holes for enlargement are ring drilled with a drill set upon the axis of tunnel.
 Side Drift: Two drifts are advanced along the sides of tunnel. Breakups are then made in
form of arch and permanent timber is erected to support the roof.
 Top Drift: When the drift selection chosen lies in top of the tunnel section, it is called top
drift.
 Bottom Drift: In this, the lower part is first blasted and cleared for full length.

4. Cantilever Car Dump Method:

This method is primarily for providing an improved system of throwing the muck from the bench
to the floor of the tunnel. The characteristics features of the system are:

ASHWINI P, Department of Civil Engineering, MyCEM

  Provision of two plate girders about 23m in length fixed at I .8 m centre to centre
distance.
 Belt conveyor lined with a number of jacks runs on the plate girders.
 The plate girder ends project beyond the full face of the bench.
 Front ends of the girders supported by means of struts from top of the tunnel.
 The blasted material (muck) is removed to sites outside the tunnel entrance.
 The quick disposal of the muck makes the system economical as in other system disposal
of muck accounts for considerable expenditure making the operation costly.

ASHWINI P, Department of Civil Engineering, MyCEM