SPECIFICATION AND APPLICATION OF VOID SPACES BELOW

Michael Skoller, National Structural Engineering, Inc. 1

FPA-SC-11 Presentation 9 Jan 08

SPECIFICATION AND APPLICATION OF VOIDSPACES BELOW CONCRETE FOUNDATIONS

Document no: FPA-SC-11

Developed by: FPA Structural Committee

Committee chairs: Main Committee - Ron Kelm, P.E.Subcommittee - Michael Skoller, P.E.

Subcommittee: Mike Turner, Mohamed Moussaoui, Mari Mes,Al Maceiras, Jim Austin, Karl Breckon

Presented by: Michael Skoller, P.E., National Structural Engineering, Inc.

Presented to : Foundation Performance Association

Presented on: 9 January 2008

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Specification andApplication of Void

Spaces Below Concrete Foundations

Table of Contents


Michael Skoller, National Structural Engineering, Inc.

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PRESENTATION OUTLINE1. INTRODUCTION

2. VOID SPACE SYSTEM TYPES

3. ADVANTAGES/DISADVANTAGES OF VOIDSPACE SYSTEMS

4. DESIGN CONSIDERATIONS

5. DESIGN PROCEDURE

6. ONSITE PROTECTION & INSTALLATION

7. PHOTOGRAPHS



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VOID SPACESGaps designed to provide abuffer zone betweenexpansive soils and aconcrete foundation in orderthat heave can occur withoutimposing detrimental upliftpressures to the foundation



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HEAVE….The upward movement of an underlying supporting

soil stratum due to the addition of water to anunsaturated expansive soil within the moistureactive zone.

Commonly occurs within clayey soils with anavailable moisture source.

Without a void space, this movement may bedetrimental to foundations and superstructures.



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How Heave Happens:



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How Heave Happens:



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How Heave Happens:



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How Heave Happens:



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1. INTRODUCTION2. VOID SPACE SYSTEM TYPES3. ADVANTAGES/DISADVANTAGES OF VOID SPACE SYSTEMS4. DESIGN CONSIDERATIONS5. DESIGN PROCEDURE6. ONSITE PROTECTION & INSTALLATION7. PHOTOGRAPHS

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VOID SPACE SYSTEM TYPESDEGRADABLE VOID SPACE SYSTEMS• Only type to be used under slab areas

• Wax coated; will gradually absorb ground moistureand lose strength or degrade over a period of time

• Soil can heave into the resulting void space

• Material, such as gravel or plastic sheathing, undercarton forms should not be used because a capillarybreak may occur preventing moisture migration to thedegradable carton forms.

COLLAPSIBLE VOID SPACE SYSTEMSNON-COLLAPSIBLE VOID SPACE SYSTEMS



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VOID SPACE SYSTEM TYPESDEGRADABLE VOID SPACE SYSTEMS

COLLAPSIBLE VOID SPACE SYSTEMS• Non-degradable material designed to collapse under

heave pressures greater than the weight of thefoundation and superstructure dead loads

• Will not collapse during foundation make-up andplacement

• Not to be used under slab areas.

NON-COLLAPSIBLE VOID SPACE SYSTEMS



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VOID SPACE SYSTEM TYPESDEGRADABLE VOID SPACE SYSTEMSCOLLAPSIBLE VOID SPACE SYSTEMSNON-COLLAPSIBLE VOID SPACE SYSTEMS• Non-collapsible material designed to maintain its

original structural integrity throughout life of thefoundation

• Built-in void space• Materials include fully wax impregnated corrugated

paper, Styrofoam, plastics and other non-degradableproducts

• Use under grade beams only



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DEGRADABLE AND COLLAPSIBLEVOID SPACE SYSTEMS



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NON-COLLAPSIBLE VOID SPACE SYSTEMS


Grade Beam

Void space, sizedependent on width ofvertical supports and

PUM

3 Styrofoam pieces gluedtogether


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VOID SPACE SYSTEM(DEGRADABLE CARTON FORMS) UNDER SLAB AREAS

• Reduces foundation heave• Elevated slab areas are less affected by

surface water prior to concrete placement


Advantages


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VOID SPACE SYSTEM(DEGRADABLE CARTON FORMS) UNDER SLAB AREAS

• Creates path for water to migrate below slab• Termites may be attracted to carton forms

when paper products are used• Increases design and construction cost• Concrete may enter carton form during

placement if grade stakes are driven throughcarton forms


Disadvantages


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VOID SPACE SYSTEMS UNDERGRADE BEAMS

• Allows total isolation of the upperfoundation from the active soils

• Potential Upward Movement of foundationwill be reduced, assuming sufficient voidspace is maintained and deeply supportedfoundations are founded below the moistureactive zone.


Advantages


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VOID SPACE SYSTEMS UNDERGRADE BEAMS

• Void space below beams usually collectwater

• Prolongs installation time• Increases cost of design and construction• Due to inclement weather conditions, the

entire foundation makeup may need to beremoved in order to replace damaged or wetcarton forms


Disadvantages


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NO VOID SPACE UNDER GRADE BEAMS

Design procedure:

a) Determine the maximum uplift forces of the soil andcompare the capacity of the grade beams with thatforce applied to the bottom of the grade beams. Ifthe dead load on the beam is equal to or greaterthan the uplift force, no additional design isnecessary. If the uplift force is greater, the gradebeams need to be designed for this upward forceminus the dead load, and the piers should then bedesigned for the proper depth below the moistureactive zone to resist this uplift.



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b) There are two uplift forces to be considered:bearing on the bottom of the grade beams and sidefriction between the soil and sides of the beam. Onemethod of determining these forces is to provideswell tests to determine the surcharge needed forzero swell and use the shear capacity of the soil forthe side friction. The actual determination of theseforces is not in the scope of this document.

c) It would be a reasonable assumption that, if theabove items were accounted for and the actualforces are the same or lower than dead loads, a voidspace would not be necessary under the gradebeams.



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d) If the net uplift pressures on the grade beams aregreater than the dead weight of the foundation andsuperstructure one of two design choices should bemade:1) Design the foundation as a stiffened slab per

PTI, BRAB or WRI. Movement may occur.2) Use a void space under the grade beams in

conjunction with a void space under the slabarea.



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DESIGN CONSIDERATIONS• In order to determine the design height for

degradable Carton Forms & Collapsible VoidSpace Materials:

deformed/collapsed height of carton form + void space height required by geotech engineer Design Height of Void Space

• Non-Collapsible Void Space Materials: Excavatedvoid space should be large enough to compensatefor the volume of the permanent void space materialand the heaved soil



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TABLE 3.4-1 REQUIRED DESIGN LOADS FOR VOID SPACE SYSTEMSVOID SPACE SYSTEM MATERIAL

TYPE1

CRITERION CONDITION1, 3 NON-DEGRADABLEDEGRADABLE

COLLAPSIBLE2 NON-COLLAPSIBLE

Minimum Initial UltimateUniform Load CollapsePressure (as shipped, dry)[PSF]

Slabs with t ≤12” thick 600 Do not use Do not useSlabs with t > 12” 456 + 12t Do not use Do not useBeams with d ≤ 36” 1000 600 600Beams with d > 36” 568 + 12d 168 + 12d 168 + 12d

Maximum Initial UltimateUniform Collapse Pressure(as shipped, dry) [PSF]

Slabs with t ≤12” thick 1500 Do not use Do not useSlabs with t > 12” 1356 + 12t Do not use Do not useBeams with d ≤ 36” 1500 1000 No limitBeams with d > 36” 1068 + 12d 568 + 12d No limit

Maximum Final UltimateUniform Collapse Pressure(at 100% humidity for 7days) [PSF]

Slabs with t ≤12” thick 12t Do not use Do not useSlabs with t > 12” 12t Do not use Do not useBeams with d ≤ 36” 12d 1000 No limitBeams with d > 36” 12d 568 + 12d No limit

Notes:1. t = slab thickness [inches]; d = grade beam depth [inches], measured from the top of the slab to the bottom of the beam2. The foundation design engineer should verify the maximum ultimate collapse pressure.3. The slab values should also be used for pier caps.


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DESIGN CONSIDERATIONSFOR THE FORMS AROUND

THE TOPS OF PIERS



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FORMS AROUND THE TOPS OF PIERS

• Reducing contact area between top of pier andsoil reduces potential uplift pressures

• Less concrete waste

• Additional cost of materials• Additional labor to install


Advantages

Disadvantages


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RIGHT WAYMichael Skoller, National Structural Engineering, Inc.

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WRONG WAYMichael Skoller, National Structural Engineering, Inc.

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VOID SPACE SYSTEMMANUFACTURER TESTING



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VOID SPACE SYSTEM TESTINGFor a void space system to be considered acceptable underthis document, it should be tested by an independent laboratoryin accordance with the following minimum requirements:

a) Minimum of 3 randomly selected test samplesb) Minimum length = 2 times depth of test samplec) Repetitive cell pattern of at least two adjacent cells in each

horizontal directiond) Test pressure at failure is pressure where either deflection

increases without additional pressure, or total verticaldeformation > 5% of original vertical dimension

e) All samples fail at pressure ±10% of average failure pressuref) Test pressure shall be uniformly applied to test sample



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VOID SPACE SYSTEM DESIGN PROCEDURE

a) A geotechnical investigation and report should be inaccordance with the requirements of Document No.FPA-SC-04, Recommended Practice forGeotechnical Explorations and Reports and / or otherguidelines acceptable to the foundation designengineer.

b) The foundation design engineer decides whether ornot to utilize a Void Space System based onrecommendations from the geotechnical report anddiscussions with the client.



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VOID SPACE SYSTEM DESIGN PROCEDURE

c) The geotechnical report shall includerecommendations needed by the foundation designengineer to design the Void Space System, inparticular the net height of Void Space (PUM)required below the bottom of the slab and gradebeams.

d) The foundation design engineer specifies the VoidSpace height and Void Space System on the designdrawings and / or specifications based onrecommendations of the net height of Void Space(PUM) recommended by the geotechnical engineer.



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CONSIDERATIONS FORUNDER-SLAB UTILITIES



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Rebar or Cable

Pipe

Pipe Hanger

Assembly

Not to ScaleSoil

Slab and beams

Degradable

Void Space

System

material

Form board

UNDER-SLAB UTILITY CONSIDERATION--Piping must remain stationary with respect to the slab--


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UNDER-SLAB UTILITY CONSIDERATION--Piping must remain stationary with respect to the slab--


Rebar or Cable

Pipe

Not to Scale

Slab and beams

Soil

Form board

Degradable

Void Space

System

Material

Floor Drain

and Pipe

Pipe Hanger

Assembly

Piping May Be Subjected To

Bending


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TRAPEZOIDAL CARTON FORMS



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TRAPEZOIDAL CARTON FORMS

• Inappropriate for void space system• Designed to resist lateral pressures, but as

concrete on the sides is often thin andunreinforced, pressures are not resisted

• Lack sufficient interior vertical supports



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Trapezoidal carton forms that lack continuousconcrete to function as a soil retainer at the

bottom of grade beams



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1. INTRODUCTION2. VOID SPACE SYSTEM TYPES3. ADVANTAGES/DISADVANTAGES OF VOID SPACE SYSTEMS4. DESIGN CONSIDERATIONS5. DESIGN PROCEDURE6. ONSITE PROTECTION &

INSTALLATION7. PHOTOGRAPHS

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VOID SPACE SYSTEMSONSITE PROTECTION


•Store materials according to manufacturer’s recommendations•Store degradable Systems in an enclosed container or storage trailer•Protect Systems against the elements and other means of damage prior to installation•Do not install or use water sensitive void space materials that have been damaged


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VOID SPACE SYSTEMS INSTALLATION


a) Assemble products as recommended by the manufacturer todevelop designed strengths.

b) Degradable carton forms should have access to moisture inorder to properly deteriorate. Do not wrap degradable forms inpolyethylene. A moisture retarder (polyethylene) should not beused below degradable carton forms because this may not allowdeterioration from the subgrade below.

c) Place a protective covering over Void Space Systems asnecessary to distribute working load, bridge small gaps, andprotect the materials from puncture and other damage duringconcrete placement. A protective fiberboard may be used on topof all Void Space Systems, which helps distribute concentratedconstruction loads from rebar bolsters, personnel, etc. Do notuse protective fiberboard less than 1/8”, plywood or OSB board.


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Placement of fiberboard coverings


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d) Foundation design engineer should have an active roll in theconstruction procedures.

e) Use end caps to seal exposed ends adjacent to piers.

f) Protect degradablesystems from moisture.Replace wet or damagedmaterials before placingconcrete.


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g) Ensure positive drainage away from the foundation

h) Systems should be properly placed and anchored

i) Moisture retarder may beplaced over top of carton

forms. Collapsible void systems may be wrapped with moisture retarder as long as the entrapped air has a method of escaping.


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INSTALLATION COMMENTS/CONCERNS


• Selection of paper type• Void space system minimum/maximum strength (psf)• Pre-manufactured System eliminates field cutting• Fiberboard protection for System• Minimize openings to prevent concrete from flowing into System• Concentrated loads Systems such as from rebar chairs• Moisture retarder in wrong location• Onsite weather protection• Size of concrete pour• Labor force (size and skill level)• Weather conditions• Insufficient soil moisture for degradable System


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CARTON FORMS BENEATH BEAMS



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CARTON FORMS BENEATH BEAMS



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CARTON FORMS BENEATH WALL



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CARTON FORMS BENEATH SLAB & BEAM



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PRE-MANUFACTURED RADIAL CARTONFORM AT PIER INTERSECTION



56ONE-PIECE PIER CARTON FORM



57TWO-PIECE PIER CARTON FORM



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RADIAL SHAPES ARE FIELD-CUT AND PATCHEDMichael Skoller, National Structural Engineering, Inc.

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EXPOSED CARTON FORM INTERIORBY FIELD CUTTING


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FIELD-CUT PIECES AROUND PIERMichael Skoller, National Structural Engineering, Inc.

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WASTED CARTON FORM MATERIALMichael Skoller, National Structural Engineering, Inc.

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FIELD-CUT CARTON FORMS RANDOMLY PLACED AROUND PIER

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Michael Skoller, National Structural Engineering, Inc.PIER PERIMETER STUFFED WITH PAPER SCRAP

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PRE-MANUFACTURED CARTON FORMSFOR PIERS AND GRADE BEAMS

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Michael Skoller, National Structural Engineering, Inc.SLAB AND PIER CARTON FORMS

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VOIDED BASEMENT SLAB WITH FIBERBOARD



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FIBERBOARD BENEATH POST-TENSION CABLES



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VOIDED SLAB SECTIONSCOVERED WITH FIBERBOARD



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VOIDED SLAB SECTIONS COVERED WITHFIBERBOARD AND MOISTURE RETARDER



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VOIDED SLAB SECTIONS COVERED WITHMOISTURE RETARDER



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RESIDENTIAL VOIDED FOUNDATION SLAB COVERED WITH MOISTURE RETARDER



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DEGRADABLE CARTON FORMS AND TRAILERFOR STORAGE OF FORMS PRIOR TO USE



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IN CONCLUSION• USE VOID SPACE SYSTEMS WHEN HEAVE

IS A CONCERN• GEOTECHNICAL AND STRUCTURAL ENGINEERS BOTH RESPONSIBLE FOR THE DESIGN

OF VOID SPACE SYSTEMS• SPECIFY CARTON FORMS THAT HAVE BEEN

INDEPENDENTLY TESTED ACCORDING TO THEPROCEDURES IN THIS PAPER

• KEEP DEGRADABLE FORMS DRY AND ALL FORMSPROTECTED UNTIL USE

• REPLACE ANY DEGRADABLE FORMS THAT GET WET PRIOR TO CONCRETE PLACEMENT• COVER CARTON FORMS WITH A

PROTECTIVE BOARD PRIOR TO CONCRETEPLACEMENT



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REFERENCESSAVWAY Carton Forms

http://www.savwaycartonforms.com/

SureVoid Products Inc.http://www.surevoid.com

VoidForm International Ltd.http://www.voidforminternational.com/

FPA-SC-12 Presentationhttp://www.foundationperformance.org/committee_papers.cfm



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QUESTIONS?

Download “Specification and Application of Void Spaces Below Concrete Foundations” at:

http://www.foundationperformance.org/committee_papers.cfm


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SPECIFICATION AND APPLICATION OF VOID SPACES BELOW