FOUNDATIONS WITH FROST HEAVE By Derek Xia M

FOUNDATIONS WITH FROST HEAVE By Derek Xia, M. Sc. , P. Eng. , M. CSCE, M. ASCE , M. AISC derek. xia@ualberta. ca

Contents q Introduction q Soil Classification q Freezing in Ground q Engineering Issues q Summary

Introduction

Introduction q Many engineering projects are located in areas where the ground experiences sub-zero conditions. q Frozen ground is also caused by artificial freezing such as chilled gas pipelines, refrigerated cold storage rooms, and freezing ground in civil and mining engineering. q Freezing and subsequent thaw induce engineering problems for most projects in areas with frost condition. q It is geotechnical engineer’s responsibility to solve most frozen ground problems. However, it’s always good for a civil engineer working on other areas to know some basic issues about frozen ground.

Soil Classification

Soil Classification Soils Fine grained soils Silts, Clays According to particle size Coarse grained soils Sands, Gravels, Cobbles, Boulders

Soil Classification Frost Susceptible soils Response to Frost Non-frost susceptible soils Soils Fine grained soils Silts, Clays According to particle size Coarse grained soils Sands, Gravels, Cobbles, Boulders

Freezing in Ground

Freezing in Ground Temperature profiles in ground Seasonal frost Area Permafrost Area

Freezing in Ground Phases of Un-frozen and Frozen Soils

Freezing in Ground Unfrozen water in coarse-grained soil Soil Grain Unfrozen water Ice q Most water exists in pore as free water in unfrozen soil. q Unfrozen water exists in pore surrounding by ice in frozen zone. q Therefore, no path for water migration to frozen zone.

Freezing in Ground Freezing in coarse-grained soils q Most pore water freezes in frozen zone as frost penetrates into soil. q Pore water expands by ~9% volumetrically when freezing. q This 9% expansion usually pushes the water downward into the unfrozen zone and drains away. q Normally, it does NOT change the soil structure. No frost heave takes place in these soils.

Freezing in Ground Unfrozen water in fine-grained soil q Water in fine-grained soil exists simultaneously as Ø free water in bulk, Ø capillary water, Ø film water, Ø and hygroscopic water. q Each has different freezing point due to different stresses. q Water coexists with ice as thin films in frozen soil through which moisture migration may occur.

Freezing in Ground Unfrozen water in fine-grained soil

Freezing in Ground – Zones in Frozen Soil

Freezing in Ground Freezing in fine-grained soils q Freezing of a fine-grained soil is a very complicated process including both mass and heat transfer. q Ice crystals initiate in frozen zone with frost penetrating into a fine-grained soil. q While frost penetrates into soil, water is drawn from the unfrozen zone towards the ice crystals through the frozen fringe, forming ice lenses. Frozen Fringe q The warmest ice lens grows until the next ice lens forming during transient heat flow. q Final ice lens can grow forever during thermal steady state if there is enough water supply.

Freezing in Ground – Transient Freezing Top plate: -5 o. C, Bottom plate 2 o. C 02 h 19

Freezing in Ground – Transient Freezing Top plate: -5 o. C, Bottom plate 2 o. C 02 h 36

Freezing in Ground – Transient Freezing Top plate: -5 o. C, Bottom plate 2 o. C 02 h 52

Freezing in Ground – Transient Freezing Top plate: -5 o. C, Bottom plate 2 o. C 03 h 08

Freezing in Ground – Transient Freezing Top plate: -5 o. C, Bottom plate 2 o. C 03 h 24

Freezing in Ground – Transient Freezing Top plate: -5 o. C, Bottom plate 2 o. C 03 h 40

Freezing in Ground – Transient Freezing Top plate: -5 o. C, Bottom plate 2 o. C 03 h 56

Freezing in Ground – Transient Freezing Top plate: -5 o. C, Bottom plate 2 o. C 04 h 12

Freezing in Ground – Transient Freezing Top plate: -5 o. C, Bottom plate 2 o. C 04 h 28

Freezing in Ground – Steady State Top plate: -5 o. C, Bottom plate 2 o. C 21 h 45

Freezing in Ground – Steady State Top plate: -5 o. C, Bottom plate 2 o. C 32 h 00

Freezing in Ground – Steady State Top plate: -5 o. C, Bottom plate 2 o. C 42 h 00

Freezing in Ground – Steady State Top plate: -5 o. C, Bottom plate 2 o. C 52 h 00

Freezing in Ground – Steady State Top plate: -5 o. C, Bottom plate 2 o. C 62 h 00

Freezing in Ground – Steady State Top plate: -5 o. C, Bottom plate 2 o. C 72 h 00

Freezing in Ground – Steady State Top plate: -5 o. C, Bottom plate 2 o. C 82 h 00

Freezing in Ground – Frozen Soil Structure Frozen Soil (fine) MC redistribution Frozen zone No visible ice lens Vertical ice lens Final ice lens Frozen fringe Frost front Unfrozen zone

Freezing in Ground – Frost Penetration Latest Ice Lens

Freezing in Ground – Final Ice Lens Growth

Engineering Issues

Engineering Issues Saturated fine grained soil Freezing Uplifts and Frost-jacking Engineering Problems Bearing Capacities Settlements Frost heave Thaw Others Stabilities

Engineering Issues Shallow Foundation q In Ft. Mc. Murray, Alberta the typical amount of frost heave is 50 -75 mm. q Uplift force due to frost heave can be up to 2 MPa if the heave is confined. q Burying the foundation below the frost depth (approximately 4 m in Ft. Mc. ) is a normal approach. q This approach can also solve the problem due to thaw weakening and extensive settlement.

Engineering Issues Shallow Foundation for Heated Buildings Exterior Heated building interior q Shallow foundations for heated buildings need to be insulated to eliminate frost penetrating below the foundations. q Therefore, avoid frost heave.

Engineering Issues Uplift force through voidform on piles q Uplift force due to frost heave can be up to 2 MPa if the heave is confined. q Voidform, an expanded polystyrene product, should be used to allow this upward movement. q For a known thickness of voidform and a predicted amount of frost heave, the uplift pressure on the bottom of the pile cap can be calculated.

Engineering Issues Adfreeze forces on piles q Adfreeze is a bond of frozen moisture between the foundation sides and the soil. q The value of adfreeze is typically 55 -140 k. Pa. q Using bond breakers can eliminate the adfreeze forces around the cap. q Adfreeze forces can induce frost jacking on a pile.

Engineering Issues Frost Jacking q Frost jacking occurs when a pile is not embedded well. q Pile goes up with frozen soil during winters. q Soil settles down during summers when thawing q The pile can be jacked out of the ground after several cycles.

Engineering Issues Frost Jacking q Frost jacking occurs when a pile is not embedded well. q Pile goes up with frozen soil during winters. q Soil settles down during summers when thawing. q The pile can be jacked out of the ground after several cycles.

Engineering Issues Frost Jacking q Frost jacking occurs when a pile is not embedded well. q Pile goes up with frozen soil during winters. q Soil settles down during summers when thawing q The pile can be jacked out of the ground after several cycles.

Engineering Issues Permafrost area q Frozen soil is very strong material. q Embed piles into permafrost and elevate structures above the grade to minimize heat transfer into soil. Active layer Permafrost q Allow convection between structure and grade to cool down the piles. q Key issue is to avoid thawing permafrost to preserve its strength.

Summary Frost Heave Requires Ø Freezing temperatures (temperature gradient) for a prolonged period of time. Ø Fine grained soil particles allowing ice lenses initiation and moisture migration. Ø Water supply. Engineering Issues Ø Frost heave and subsequent thaw weakening and settlement. Ø Uplift forces through voidform and adfreezing. Ø Frost-jacking. Ø Engineering in permafrost.