GROUND FREEZING 3/27/2020
Department of Civil Engineering, NIT, Jamshedpur INTRODUCTION • Ground freezing is a process of making water- bearing strata temporarily impermeable and to 3/27/2020 increase their compressive and shear strength by transforming joint water into ice.
• Freezing is normally used to provide structural underpinning; temporary supports for an excavation or to prevent ground water flow into an excavated area
• The technique is time tested and is over a hundred years old. • Successful freezing of permeable water-bearing ground affects simultaneously a seal against
water and substantial strengthening of incoherent 3/27/2020 ground.
• No extraneous materials need to be injected and apart from the contingency of frost heave, the ground normally reverts to its normal state.
• It is applicable to a wide range of soils but it takes considerable time to establish a substantial ice wall and the freeze must be maintained by continued refrigeration as long as required.
AREAS OF APPLICATION 3/27/2020 • Ground freezing may be used in any soil or rock formation regardless of structure, grain size or permeability. • However, it is best suited for soft ground rather than rock conditions. • Freezing may be used for any size, shape or depth of excavation and the same cooling plant can be used from job to job. • As the impervious frozen earth barrier is constructed prior to excavation, it generally eliminates the need for compressed air, dewatering, or the concern for ground collapse during dewatering or excavation. PRINCIPLES OF GROUND FREEZING • The effectiveness of freezing depends on the
presence of water to create ice, cementing the 3/27/2020 particles and increasing the strength of the ground to the equivalent of soft or medium rock. • If the ground is saturated or nearly so it will be rendered impermeable. • If the moisture does not fill the pores, it may be necessary to add water. • The strength achieved depends on freeze temperature, moisture content and the nature of the soil.
STAGES OF GROUND FREEZING 3/27/2020 Two stages
Active stage. freezing of ground water to form ice.
Passive stage. maintain the established thickness of ice wall against thawing.
SHAFT EXCAVATION BY GROUND FREEZING 3/27/2020 SECTION OF FREEZE PIPE 3/27/2020
TIMELINE OF FREEZING 3/27/2020 Pipes installed
Initiation of freezing
Propogation of freezing
Steady state freezing 3/27/2020 TOP VIEW OF A FROZEN GROUND 3/27/2020 prof. A.K.Choudhary
TYPES OF GROUND FREEZING 3/27/2020 Open process Closed process Combined process
OPEN PROCESS 3/27/2020
Refrigerant fluid-liquid nitrogen.
Transported by tankers kept at temperature -196 degree Celsius, (pressure-0.5MPa).
Venting nitrogen gas to atmosphere after circulating through freezing pipes. 3/27/2020
Ground freezing by liquid nitrogen OPE PROCES -
N S SETUP 3/27/2020 VENTIN OF GA
G NITROGEN S 3/27/2020 LIQUID NITROGEN With this method a large portable refrigeration plant is
not necessary, and the temp is much lower and therefore 3/27/2020 quicker in application The nitrogen under moderate pressure is brought to site in insulated containers as a liquid which boils at –196°C at normal pressure and thereby effects the required cooling. It can be stored on site. There is a particular advantage for emergency use, i.e quick freezing without elaborate fixed plant and equipment. This may be double advantageous on sites remote from power supplies. In such conditions the nitrogen can be discharged directly through tubes driven into the ground, and allowed to escape to atmosphere. Precautions for adequate ventilation must be observed. When there is time for preparation, an array of freezing tubes is installed for the nitrogen circulation, including
return pipes exhausting to atmosphere. 3/27/2020 The speed of ground freezing with N2 is much quicker than with other methods, days rather than weeks, but liquid nitrogen is costly.
The method is particularly appropriate for a short period of freezing up to about 3 weeks. It may be used in conjunction with the other processes with the same array of freezing tubes and network of insulated distribution pipes, in which liquid nitrogen is first used to establish the freeze quickly and is followed by ordinary refrigeration to maintain the condition while work is executed. This can be of particular help when a natural flow of ground water makes initial freezing difficult.
CLOSED PROCESS 3/27/2020 Refrigerating circuit liquefies the primary fluid by condensers and compressors. This cools the refrigerant liquid which circulates the freezing tube in closed circuit Primary fluid-ammonia or freon. Refrigerant liquid-brine with operating temp –25 to –30 degree Celsius.
CLOSED PROCESS - SETUP 3/27/2020
Ground freezing with super cooled brine BRIN COOLIN
E G 3/27/2020
COMBINED PROCESS 3/27/2020 Rapid cooling of nitrogen combined with economical continued operation of brine.
advantage : economy of refrigerants (less losses), and increased safety. COMPARISON OF LIQUID NITROGEN AND BRINE
COOLANT 3/27/2020
FACTORS AFFECTING FREEZING DESIGN 3/27/2020 Thermal conductivity of the ground. Geological and hydro geological conditions. Soil type present. Water content. Time taken in freezing. Ground temperature. Growth of ground freeze GROWTH OF GROUND FREEZE with with varying dry WITH NO: OF DAYS density 3/27/2020
TYPICAL APPLICATION PROCESS 3/27/2020
Assembling freezing pipe
Installation of freeze pipe
Application of freeze with refrigeration plant. 3/27/2020
Frost development on freeze pipe headers.
Excavation following completion of freeze wall. EXCAVATIO O FROZE GROUN
N F N D 3/27/2020
TYPICAL SECTION OF FROZEN GROUND 3/27/2020 FROZEN SOIL SHORING WALL AROUND A
10-FOOT-DEEP BASEMENT EXCAVATION 3/27/2020 Frozen soil shoring wall around a 10-foot-deep basement excavation for a condominium in Madison Par k. 3/27/2020
APPLICATION S SHALLOW EXCAVATION SMALL SITE 3/27/2020 •Can be designed to completely cut off groundwater into the excavation •Uplift is controlled by freezing deeper than the excavation depth. •Very cost-effective shoring method when contaminated groundwater is present. •Can be effectively used to depths of 40 feet or more. SHALLOW EXCAVATION
LARGESITE 3/27/2020 •Allows large, shallow excavations to be accomplished in the "dry" with no dewatering. •Freeze pipes are installed nearly horizontal using directional drilling methods. •Completely seals the site from groundwater. •Can also be used to cut off groundwater beneath existing structures. •Cost effective to depths of 60 feet or more. 3/27/2020 DEEP EXCAVATION •Circular-shape frozen soil wall provides very strong shoring without internal bracing. •Has been used on hundreds of shafts worldwide to depths of 800 feet. •Completely cuts off ground water to eliminate dewatering. •Generally costs less than other deep shaft methods (secant concrete piles, jet grouting, etc.). •A frozen soil seal can be created at sites where no natural groundwater cutoff is present. •May be used in nearly all soil and groundwater conditions. •Designed to resist uplift without expensive tie-downs. 3/27/2020 3/27/2020 3/27/2020
LANDSLIDE STABILIZATION 3/27/2020
•Stabilizes a failed hillside b y creating zones of frozen soil reinforcement. •Can be used as a temporary fix to allow safe mobilization of construction equipment. •Can also be used as a long- term solution where other methods are not feasible. LIQUEFACTION/CONTAINMENT
MITIGATION 3/27/2020 •Stabilizes loose, wet sand so that it will not liquefy during an earthquake. •Allows the use of shallow footings, instead of expensive piling, for 30-year design life. •Where contamination is present, ground freezing prevents off site migration indefinitely. •Maintenance costs are relatively low and electricity costs are part of building operation costs. STABILIZATION OF SOFT GROUND 3/27/2020
•An environmentally friendly method to stabilized very soft soil or peat. •Creates a temporary frozen soil road to support very heavy construction equipment. •Freeze pipe and insulation can be laid down by hand with little disturbance to the ground. •When the project is complete, the freeze system is removed and the soil is allowed to thaw out TUNNELING 3/27/2020
•Provides stable shoring to allow tunneling in wet, loose soils or fractured bedrock. •Effective for cutting off ground water into tunnel excavation. •Frozen soil shoring can also be used to stabilize ground entrance and exit access shafts.
GROUND FREEZING FOR TUNNEL EXCAVATION 3/27/2020 GROUND FREEZING IN TUNNELING 3/27/2020 3/27/2020 prof.A.K.Choudhary 3/27/2020 3/27/2020 UNDERPINNING EXISTING STRUCTURE
•Prevents vertical and horizontal movement of adjacent structures during excavation. •Effective in wet sands and gravel where frost heave is not a concern. •Employed for the Washington D.C. subway project to protect historic buildings GROUND WATER CUTOFF 3/27/2020
•A frozen soil cutoff can be used toprevent groundwater from entering open pit mines, •gravel pits, landfills. •Because frozen soil has very low permeabilit •Eliminates or greatly reduces the need for dewatering. •Can also be used to stop the migration of contaminated groundwater near streams or lakes. •Maintenance of a buried frozen soil cutoff is relatively inexpensive making it competitive f long-term cutoffs 3/27/2020
GROUND FREEZING AT FUKUSHIMA, JAPAN 3/27/2020 ADVANTAGES
Effectively cuts off groundwater.
- Can eliminate dewatering and/or concrete seals for many projects. 3/27/2020 Works in all types of soil and groundwater conditions. -Running sand, clay, peat, cobbles, gravel, bedrock. Freezing can be particularly effective in stabilizing silts, which are too fine for injection of any ordinary grouts.
very low vibrations and noise during installation. - Excellent at sensitive sites near buildings or residences.
Easily installed around and below existing buried utilities. - Allows continuous use of existing utilities without disruption of service.
Can be completely removed after construction, if needed. - This returns site to pre-construction conditions after the soil is thawed.
Less affected by power outages than dewatering. -Unlike dewatered sites, frozen shoring can remain stable for weeks without power.
LIMITATIONS 3/27/2020 Soil should be saturated or near saturated Cost factor for shallow depths The chemicals used for ground freezing (calcium chloride, anhydrous ammonia, and liquid nitrogen) require extreme care. In rock and clay ice lenses may build up and enlarge fine fissures so causing increase in permeability after thaw. A typical ground freezing system using a circulating coolant can take 6 to 8 weeks for a smaller diameter shaft and 10 to 12 weeks for larger areas. Large rate of groundwater flow. REFERENCES F.G. Bell “Engineering treatment of soil”chap-3,”Ground
Freezing”; pp55-67 3/27/2020 Raj Purushothama “ground improvement techniques”chap- 9;pp-237. “Improved thermal computations for artificially frozen ground”. By John T Holdon.Journal of geotechnical and environmental engg,1997. “IN SITU GROUND FREEZING”. By B. A. Hofmann, D. C. Sego, and P. K. Robertson. Journal of geotechnical and environmental engg,Nov 2000. www.cryocell.com www.djc.com/special/design98/10047020.html www.groundfreezing.com/artificial_ground freezing. www.Ground freezing technology.htm www.imws.tuwien.ac.at/index.php 3/27/2020
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