TUNNELING ON SOFT SOILS

1.0 Introduction

Three basic considerations as t the construction methods and

corresponding costs should be clarified during preliminary proposals.

1. As the material hard or easy for tunneling and will be explosives be

needed.

2. What part of tunnel needs supports and what type of supports are

needed.

3. Is water likely to encounter and if so in what portion of the tunnel

and in what quantities.

Most of the hard massive rocks can be excavated readily and may stand

without support unless disturbed by geological considerations.

Though workability of tunneling improves with diminished harness, their

stability to stand without support decreases, as in case of many shales,

clays etc. Similarly presence of near by water basins at higher levels

indicates possibility of water inflow into the tunnel. Similarly presence of

surface springs from which warm water issues may indicate deep seated

water flows.

Incase of tunneling in soft sound the stand up time after excavation will

be shorter than incase of hard/firm rocks. The stand up time varies from

zero in sands upto one day in stiff days.

On the major differences between rock and soft ground tunnels is the

influence of water table on tunnel construction. In soft ground tunneling

fight with water may be a continuous construction problem.

2.0 Methods of Tunneling in soft soil :

The methods employed for tunneling vary with the type of strata

encountered i.e. hard rock or fully self supporting strata and soft strata

requiring supports during and after construction.

The soft strata or soft soils are classified as below :

Strata needing instant support after excavation - sands or other cohesion

less soils.

Soft ground needing instant support for roof i.e. soft clay soil.

Firm ground wherein roof will stand for a short period but sides for a

much longer period - Firm clay, dry earth etc.

Self supporting ground when soil stands unsupported for a short period

and for length upto 4-5 mtrs.

2.1 Some of the common methods of tunneling in soft strata are :

Fore poling method

Shield tunneling method

New Austrian tunneling (NATM) method

Needle Beam method

Liner plate method

American method etc

Compressed air tunneling

Mechanical pre cutting method

2.01 Fore poling method :

This method is used under running ground and similar soil. The method

is slow and requires skilled miners. For tunnels of small dimensions at

smattes depths this method is applied.

Needle beam method :

This method is suitable for soils in which roof could stand unsupported

for a very short period (a few minutes).

Mechanical Pre cutting method :

This method was developed to over come the difficulties of tunneling

particularly in urbane areas where ground is mostly alluvial, where level

of ground water is high, building density, traffic volume not allow easy

working.

This method consists in creating in the ground, previous to any

excavation, a groove of various - thickness depending on ground

characteristics, the lie of which follow theoretical extrudes profile of the

tunnel, with a length which may vary from 1.5 m to 4 m. This method

allows to place a previous protection in the ground before excavation,

which prevents any precompression in the upper ground.

Precutting in soft ground :

In soft ground a groove is bored and this groove is filled with concrete, in

order to for a shell to shelter laterally the earth and the vault concreting.

The groove excavation and equipment used are some as that for rock

except that the groove thickness is larger so that the "prevault" gets

sufficient strength.

The m/c used included a gantry like moveable frame, with a free passage

in the middle or a boring machine. The boring system includes a chain

provided with picks, moving on a supporting arm, the length of which

can allow to bore a groove up to 2.5 m. The chain is rotated at an

adjustable speed of about 1 m/sec the boring assembly is moved on the

two tracks. The gantry and the extrudes - line of tunnel are concentric so

that groove perfectly envelopes the tunnel.

The groove begins where the vault starts and follows vault extrudes. With

progress of ground cutting (2m/minutes) the groove can be filled with

quick setting shotcrete, early enough to avoid decompression of the

ground. As soon as compressive strength of vault concrete is sufficient

(80 bars) the trench excavation can be full face made by Mechanical

machines.

To absorb local dissymetric pressures, which are encountered due to

variations in the geological structure and seepage water, arches are

positioned under each trench which can be totally or partially recovered

when final is placed. The arches suit the shape of the vault.

With reference to traditional tunneling methods in soft ground,

mechanical precutting offers the following advantages.

Natural ground properties are retained and any alteration is avoided by

means of immediate filling of groove with concrete.

Almost total suppression of ever breaks due to uniform precutting.

Improved safety for staff on work site and safety of adjutant buildings

ensured by low settlement.

Progress ............ can be increased.

(Adopted France - sub ways)

Shield Tunneling Method :

In USSR, hydraulic tunnels in soft soils are constructed, using shield

driving method. Tunnels are built upto 150-200 km in soil with

compressive strength varying from 5 to 50 Mpa, with the help of shields

of conventional and mechanical types 2 to 6 m dia. Prefab R.C. elements

and cast insitu pressed concrete are used for lining. (Tunneling rates

using prefab. R.C. lining are 700m/MO or more - rate of tunneling about

1.8 m / shift to 2.4 m shift).

In shield tunneling the excavated hole should be well supported until

permanent lining is constructed.

A full cycle of shield tunneling of composites of

a) Excavation & temporary support at the front face at an appropriate

depth

b) adjoining shield, taking support on previously erected,

c) Placing another course of ring of the permanent lining,

The shield ………. Of cutting edge, trunk, tail The design of shield depends on the skin resistance, …. Unfavourable

loading conditions. The shield ………….. with I section provides to

counter full over burden weight with equal section at bottom. Further

leading ……………. the B.M. ……..upon the ring wall (both for ……………)

M = (pr2)/4 cos 2θ Axial force = N = pr sin2θ Max. B.M. @ crown & supporting M = +

(pr2)/4

1 p r4 Radial deformation = δ = ---- -------- cos 2θ 12 E j 1 p r4 Shortening of vertical dia. = 2δmax = ---- ------ 6 E j The choice of shield of suitable depth is basic problem. It is generally

based on dimensions of jacks & lining segments. The ration of shield

length to diameter (L/D) governs steerability, mobility & steadiness of its

direction. The L/D is also called objective length.

The relative length generally 0.40 <L/D < 1.4. In present condition L/D > 0.75

…………. & Mayo gave following formula. Approximate steel weight of a tunnel shield W = 15 ( D-10) where W = in tons

D = in feet The main working procedures of shield tunneling will be a) Excavation,

b) Mauling (Haulage)

c) Shield advancement

d) Erection of tunnel lining

e) Grouting, caulking & drainage

Excavation: It is toughest part involves: a) Application of roof shield after partial excavation

b) Supporting by breast boards which inturn supported by the

stiffener rings.

c) Reduction of load due to saturated water in the over burden by

application of compressed air,

d) Artificial solidification if required by injection of pipes radially,

e) Grouting to create arch action and create binding in the

surrounding

f) Provision of bulkhead wall at the face to counter possibility of

gushing of liquid soil.

g) Development of blind push i.e. to close the face by mechanical

excavators.

h) Rotary excavator /sheet cutters are attached to cross members

half in scooping material and transfer the same by buckets.

………….. (Haulage) Effective ………… haulage & one of the major problems of efficient tunnel

driving. In shield tunneling it is performed in two steps.

i) Immediate removal of soil from the ……………. body

ii) Its conveyance to the ventilation of working shaft generally belt

conveyors are installed to facilitate disposal of muck.

This is generally accomplished by

a) Belt conveyors

b) Loador cars

c) Traction engines

d) Truck etc.,

c) Shield advancement : Accurate propulsion & direction of shield is

very delicate part of shield tunneling because it determines not

only how far the designed alignments & gradients will be

conformed to but it also seriously effects the construction. Extra

………….. may have to be placed at bottom to prevent the shield

from “nosing down” i.e., wobbling.

The correct direction of the shield must be checked by geodetic

instruments for every push. For this diopter is hung from top of tail

section in the adjustable side lines. For correct alignment the spider

lines should strictly coincide with those of the ………… hanging from the

completed sections of the tunnel.

d) Erecting of Tunnel lining. : Simple mechanical device with hoisting

winch the rope which is led on a moving boom, both mounted on a

traveling platform used for placing lining segments. This will be adopted

for tunnel for smaller diameter. For tunnels of larger diameter the lining

segments are erected with hydraulically operated erector arm which can

be directly mounted on axis of the shield tail of on a traveling platform

following closely behind.

e) Grouting . Caulking & Drainage Lining segments must be followed as soon as possible after placing the

almost all kinds of ground, by the grouting the back space left behind

the shield tail after the push ………. to prevent considerable surface

subsidence & increase in saturated muck/soil/…………….. pressure. The

usual …………… left between respective surface of circular shield thin

and smoother dia. Lining ring, its max. height at the crown being equal

to sum of the thickness tail shin (max. 60mm), width of the safety gap

(50mm) left between extruders of the lining of intrudes of the tail of any

………. anticipated in the tunnel, horizontal bow string bars (turn buckle

rods) at the …………. Line may be employed.

The grouting has several function such as

Establishment of tight buck fill

Water sealing

Stabilization of surrounding ground so contributing to reduction

by ground pressure acting on ring,

Generally i) Primary grouting intended to fill the back space ( p = 0.5 to

0.6 MPa)

ii) Secondary grouting intended for sealing of stabilizing (p = 1 to 2.5 Mpa)

The grout plugs are 40 to 50 mm dia. Minimum 4 grout holes are

provided in each lining ring in staggered position.

……….. et al. extension …… of grouting in homogeneous material can

be completed from formula

where h = Pressure head t = duration of grouting n = void content k = Coeff. Of permeability The secondary grouting can be carried out with …………. The following grout materials 1. Cement grout, cement ………….. 2. Lean mixture of various types of cement & plasticising agents. 3. Bitumen e………. 4. Hot bitumen 5. Various chemical agents But due consideration shall be given for ……….. of material and ground

water content. Deformation due to grout pressure shall be constantly

checked.

Highest permissible grouting pressure which remains within safety

margin to avoid upheaval of the layer can be calculated.

From Terzaghis such pressure theory

For homogeneous soil: Where

C = C……….. (t/m2)

ν = ………… density (t/m3)

ϕ = friction angle

h = height of over burden

λ = ph/pv

d = 36 A ( A = vol. Of ……….. grout in m3)

Lining Segments:

It is essential to have lining segments instantaneous bearing capacity, to

be water tight and allow for rapid & simple positioning. The main

requirement for such lining segments are:

Instantaneous bearing capacity against external earth & water pressure

without detrimental deformation & leakage.

Resistance to impact stresses due to rough handling, transport &

erection operations.

Resistance to high axial stresses produced during advancement (push) of

the shield by propulsion jacks.

Resistance to moisture and ground water effects of the segment itself

of its joints and water temperature also to ensure resistance against …

action.

Economy in construction of maintenance.

Types of lining :

a. Brick lining : being and in exterior days.

b. C.I.Lining : easy operation by heavy .. demand weight, …

appearance of surface. Not much is practice

now

c. Structural steel : easy operation suitable for further dia tunnels

symmetric for corrosion fabrication difficult

expensive.

d. concrete segments : cost effective though involves multiple operations

for errections

achieve good aesthetic appearance.

No. of types of segments can be cast to suit the

requirement.

Access of Ventilation shafts.

1. provided by … vertical shaft in rock by mining method.

2. In soil shaft seeking done by partial excavation and repeating the

process successively after inserting rings.