GROUND WATER FREEZING
By:-Humaira AtharMahesh Sharma
CONTENTS:Introduction
Principle
Purpose
Process
Equipments
Characteristics
Advantages
Application
Case Studies
Steps in Ground Freezing
Conclusion
“Nothing in the world is more flexible and yielding
than water. Yet when it attacks the firm and the
strong, none can withstand it, because they have
no way to change it. So the flexible overcome the
adamant, the yielding overcome the forceful.
Everyone knows this, but no one can do it.”
ROLE OF WATER
ROLE OF WATER
DEWATERING
“The separation of water from the soil”
Construction below ground water level
Costly issue if overlooked
Common methods—
Sumps
Wells
Well points
Other techniques—
Ground freezing
Electro osmosis
Fig 1.1 Steps of Dewatering
GROUND FREEZING
Principle:—
To change the water in the soil into
solid wall of ice, which is completely
impermeable.
Water inflow can cause enormous problem in excavation and tunneling projects so the best way to stop water is to freeze it.
Frozen ground is twice as strong as concrete and it is essentially impermeable.
Groundwater cutoff
Earth support
Temporary underpinning
Stabilization of Earth for tunnel excavation
Arrest landslides
Stabilize abundant mine shafts
PURPOSE
GROUND FREEZING (CONTD.)
Process—
It is analogous to pumping ground water from wells.
Arrow of freeze pipes placed vertically in the soil and
heat
energy is removed through these pipes using chilled
brine.
Isotherms move out from freezepipes with time.
GROUND FREEZING (CONTD.)
Fig 1.2 Freezing Process
GROUND FREEZING (CONTD.)
Water in soil pores turns to ice at 32°F
In granular soil, ground water in pores freezes readily.
Further decrease in temperature increases strength
marginally.
In cohesive soil, below freezing temperature ,only
some portion of pore water freezes to stiffen the clay.
Further reduction in temperature enables it to gain more
strength.
A temperature of +20°F may be sufficient in sand,
whereas
-20°F may be required in soft clay.
GROUND FREEZING (CONTD.)Formation of frozen Earth barrier is governed by
thermal and hydraulic properties of each stratum.
Typically, rock and coarse-grained soils freeze
faster than clays and silts.
Fig 1.3 Formation of frozen earth barrier in different soils
Referring to the figure, the frozen earth first forms in
the shape of vertical cylinders surroundings the freeze
pipes.
As cylinders gradually enlarge they intersect, forming a
continuous wall. Once the design thickness is achieved
freeze plant is operated at a reduced rate.
GROUND FREEZING (CONTD.)
GROUND FREEZING (CONTD.)
Fig 1.4 Steps of freezing
FREEZING EQUIPMENTS Most common method is by circulating brine (Calcium
Chloride).
Chilled brine is pumped to the bottom of the freeze pipe and
flows up drawing heat from the soil.
Fig.1.5 Portable twin 60 ton brine refrigeration unit
FREEZING EQUIPMENTS (CONTD.)
The other method is using liquid nitrogen (LN2).
Cost per unit of heat extracted is much higher.
Occasionally competitive for small, short term projects.
Fig. 1.6 Typical LN2 system for ground freezing
CHARACTERISTICS OF GROUND FREEZING
Not limited by soil types irrespective of complex
geologic and hydrological conditions.
Strength of freezing soil can be controlled.
Equipments can be recycled and reused.
No frost heaving and freezing-thawing settlements for
gravel soil whereas for fine grained soil settlements are
predictable.
Can be used in congested areas.
Method is relatively fast.
ADVANTAGES
Lesser risk to surrounding structures particularly in built-up areas.
No shuttering is required.
Environment friendly process.
Unlimited depth can be frozen from the surface.
Can be removed easily and completely.
Allows frozen soil barriers can be easily monitored.
Can be used either for temporary containment or as long term
barrier.
It freezes loose ,wet sand to prevent liquefaction during an
earthquake.
It bonds soil and waste together to prevent dangerous mixing during
removal
APPLICATIONS1) Circular excavation supported by freeze wall:
Freezing can perform dual function of water cutoff
and earth support, eliminating sheeting and
bracing.
Penetration of freeze does not vary readily with
permeability, so more effective as a cutoff than
grout.
Fig 1.7 Circular excavation
2) Excavation supported by gravity wall of frozen earth:
A combination of vertical and inclined freeze pipes is
typical, to achieve the shape illustrated.
APPLICATIONS (CONTD.)
Fig:- 1.8
APPLICATIONS (CONTD.)3) Shaft sinking :
When it is applied to inclined shaft construction, the inclined
shaft freezing drilling method can be adopted as shown in Fig.
Fig 1.9 Shaft sinking
APPLICATIONS (CONTD.)
4) Tunnels and subways:
Mainly applied with side channel and shield entrance.
Two construction plans—
Top-down vertical layout Horizontal layout
Overlaying soil is not too Thick overlaying soil
thick
CASE STUDY: IGround Freezing for Tunnel Support
No. 7 Line Subway Extension, New York City
Project consist of two 6.1m dia. Tunnels connecting the
Javitz Center with the Times Square subway station.
Two tunnels boring machine (TBMs) were designed for
bedrock conditions.
The first construction was in a bedrock depression that
contain unconsolidated, water bearing soil.
To prevent excessive inflow of water into the TBMs the soil
was artificially frozen.
Fig :1.9 Freeze pipe layout in 3D
CASE STUDY I: (CONTD.)
CASE STUDY: IIAquarius Project – Artificial Ground Freezing in
Timmins, Ontario
Water table was high and the site
was rich in gold concentration.
To extract gold, while preventing
inflows of groundwater to the mine
excavation, a frozen earth barrier
was constructed around the
perimeter of the Aquarius open pit
gold mine.
The frozen barrier is the most
environmentally compatible
technique.
CONCLUSION
The viability and versatility of ground freezing
for construction purposes is thus demonstrated
through this presentation including few case
studies along with mentioning of some of its
many applications. In addition, given that the
current interest in ground freezing is becoming
increasingly focused on environmental
remediation.
REFERENCES
Winter Quarter 2007 .Professor Kamran M. NematiCM 420 TEMPORARY STRUCTURES Lesson 7: Construction Dewatering and Ground Freezing .
Arz, P.;Semprich, S. (1993) Modern Methods of Tunnel Support in NATM Tunnelling. Proc. Symp.Taipei Rapid transit Systems, Taipei, Taiwan, Vol. C, 677-686.
Borkenstein; Jordan; Schäfers (1991) Construction of a shallow tunnel under protection of a frozen soil structure, Fahrlach Tunnel at Mannheim. Proc. 6th Int. Symp. on Ground Freezing, Beijing, China.
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