Tunneling on Soft Soils

TUNNELING ON SOFT SOILS

1.0 Three

Introduction 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 Shortening of vertical dia. = 2δmax

1 p r4 = ---- -----6 Ej

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 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

c)

a)

Belt conveyors

b)

Loador cars

c)

Traction engines

d)

Truck etc.,

Shield advancement : Accurate propulsion & direction of shield is very delicate part of shield tunneling because only

it determines not

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. 2. 3. 4. 5.

Cement grout, cement ………….. Lean mixture of various types of cement & plasticising agents. Bitumen e………. Hot bitumen 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.