An Evaluation of Using Void Box for Slab-on-Grade Foundation on Expansive Clay

Presented By Gausul Hasan

(Master of Science in Civil Engineering)

Committee Members: Dr. Sazzad Bin-Shafique

Dr. Jie Huang Dr. Arturo Montoya

April 26, 2017

Outline of the Presentation • Introduction

• Background

• Objectives

• Materials

• Methods

• Results

• Conclusions

Introduction: Expansive Clay Texas soil • Metropolitan city like Houston, San

Antonio, Austin, and Dallas in Texas have high frequency of expansive soil.

• Soil shrink/Swell due to seasonal change

• According to ASCE, 1/4 of all homes in

the US have some damage caused by expansive soils.

• In a typical year, expansive soil causes a greater financial loss to property owners than earthquakes, floods, hurricanes and tornadoes combined.

Introduction: Damage from expansive soil

Background: Foundation on Expansive Clay Slab-on-grade foundation Concrete placed on ground leaving no place between ground and structure.

Center Lift

Edge lift

Background: Pier Foundation in Expansive Soil

• Low bearing capacity of the soil

• Full depth high plasticity soil and

• Stronger clay layer at the bottom

Pier foundation is often required in expansive soil due to:

How can you cast your concrete foundation keeping void at the bottom?

Void

Background: Void Box

• Void box is the new technology to isolate foundation from the soil.

• Reduce the effect of volume change of soil on slab-on-grade foundation.

• Temporary/permanent support formwork for the weight of concrete until it has reached a specified strength.

Permanent Void Box Temporary Void Box

Installation of Molded Paper Void Form .

Photographs were taken from a field site in Austin.

Objectives of the Research

• Determining the strength and stiffness of void boxes at different moisture content

• Observing the water absorption of void boxes with time.

• Measuring the amount of swelling underneath the void box due to soil heave

• Simulating the void box system using the Geo-Studio software and comparison of the experimental and simulated results.

This study focused on two types of Recycled Molded Paper Void Form

Slab Void Box Beam Void Box

Materials

Properties Slab void box Beam void box

Length x Width x Height 60cm x 60 cm x 20cm 60cm x 60cm x 20cm

Surface Contact Area 18%(cones down) 27%(cones up)

Dry weight 1.36 kg 2.27 kg

Material Recycled Waste Paper Recycled Waste Paper

Pulp Color Tan Brown

Asphalt Not used Used

Materials Soil

Test Method Test Name Test results IBC Criteria for Expansive Soil

ASTM D4318 Liquid Limit 72 N/A

ASTM D4318 Plastic Limit 21 N/A

ASTM D4318 Plasticity Index 51 >15

ASTM D1140 Materials finer than 75 µm 93% >10

ASTM D422 Materials finer than 2 µm 64% >10

ASTM D854 Specific gravity 2.69 N/A

ASTM D 4829 Expansion Index 280 >20

Materials Soil

13.6

13.8

14

14.2

14.4

14.6

10 15 20 25 30 35

Dry

Uni

t Wei

ght (

KN/m

^3 )

Moisture Content (%) 0.1 1 10 100 1000 10000 100000

Perc

ent S

wel

l

Time (min)

Water Content (17%)

Water Content (21%)

Compaction Curve Expansion Index graph

Methods Determination of Compressive Strength and Stiffness of Void Box at Different Water Contents

First, the void boxes were subjected to humidity chamber at 100% humid condition for allowing absorption of water.

Water absorption (%) was measured from final weight and initial weight of void forms.

A set of void boxes were also tested at dry condition

Humidity Chamber

Methods Relationship among Temperature, Relative Humidity, and

Water in Air Void boxes were put in 100%

humidity in humidity chamber and water absorption(%) was measured for 1 day, 3 days, 7days, 14 days, and 28 days.

This procedure was done at 23°c and 27°c to observe how fast the void boxes absorb water and get wet. The air sprayed by humidity chamber is a function of temperature and relative humidity.

Methods Determining Compressive Strength and Stiffness at different

water content

After measuring the water absorption (%), the samples were subjected to compressive strength tests using UTS machine

Minimum three samples were tested for each type of void box

Load was uniformly applied to the test samples

Compressive Strength test set-up

Methods Determine Compressive Strength and Stiffness at different

water content

0

10

20

30

40

50

60

70

80

90

0 0.05 0.1 0.15 0.2 0.25 0.3

Stre

ss (K

Pa)

Strain

Stress vs Strain Curve for Slab Void Box at dry condition

Strength

Methods Swelling underneath the void boxes due to soil

heave

• A chamber of 30cm x 30cm x 45 cm was built with thick wood.

• Geo-fabric was glued around the four walls.

• Three rows of hole have been created at depths of 5 cm, 15 cm, and 25 cm.

• Chamber was filled up to 30 cm with soil

• Soil was compacted at 17% moisture content (50% degree of saturation) to achieve a dry unit weight 13.8 kN/m3.

Test set up after adding water

• Initial reading of soil depth was taken at 4 different places.

• Void box was put on top of soil and top of chamber was nailed with wood.

• Water is added and after 90 days wood and void box were removed to measure swelling underneath void box.

• Final reading of soil depth was taken.

Methods Swelling underneath the void boxes due to soil

heave (contd.)

Methods Simulated Swelling data using the Geo studio software

• A model was done using geo studio to observe the swelling.

• Sigma and Seepage tools were used to observe volume change

and stress developed in the contact area.

Section 1 Section 2

1

2

Results Compressive Strength of Void Box at Different Water Contents

0

20

40

60

80

100

0 20 40 60 80 100 120 140 160

Stre

ngth

(KPa

)

Water absorption (%)

Slab Void box

Beam void box

Results Stiffness of the Void Box at Different Water Contents

0

200

400

600

800

1000

0 20 40 60 80 100 120 140 160

Stiff

ness

(KPa

)

Water Absorption(%)

Slab void box

Beam void box

Results Water Absorption with Time

Slab void box Beam void box

0

20

40

60

80

100

120

0 10 20 30

Wat

er A

bsor

ptio

n (%

)

Time (Days)

23 Degree Celsius27 Degree Celsius

0

10

20

30

40

50

60

0 10 20 30

Wat

er A

bsor

ptio

n (%

)

Time (Days)

23 Degree Celsius

27 Degree Celcius

Results Swelling of Soil Underneath the Void Box

The swelling reading was measured in 4 points. Points 1 2 3 4

Swelling, m (in) 0.007 (0.28) 0.024 (0.94) 0.012 (0.45) 0.035 (1.37)

1 1 2 3

4

Results Comparison Between Simulated Data vs Experimental Data

0

0.01

0.02

0.03

0.04

0.05

0 0.01 0.02 0.03 0.04 0.05

Sim

ulat

ed d

ata

(m)

Experimental data (m)

Results Stress developed at different points

Section 1 Section 2

05

101520253035

0 0.1 0.2 0.3

Stre

ss (k

pa)

Distance from chamber wall (m)

0

10

20

30

40

0 0.1 0.2 0.3

Stre

ss (

kpa)

Distance from chamber wall (m)

Conclusions

Dry strength of Beam void box is 82 kPa and slab void box is 80 kPa.

Both type of void boxes loose its strength at a rapid rate from dry point to 30% water absorption. After this point, both void box loose its strength at very lower rate.

Slab void box absorbs water faster than the beam void box at the same ambient condition. Using asphalt to manufacture beam void box might be the reason of slower water absorption.

At any particular moisture content, beam void box loose strength slightly lower than that of the slab void box. Again, using asphalt to manufacture beam void box might be the reason of slight higher strength

The strain of failure point of void boxes are in the range of 10-12%.

Stiffness of beam void box is 911 kPa and slab void box is 855 kPa at dry situation.

Conclusions

Stiffness changes with water content follow the same trend like strength.

From the lab test (box), the maximum deformation occurred at the middle of the void (1.15 in or 10%) and the minimum deformation occurred at the support of the void box (0.36 in or 3%).

The deformation occurred at the support of the void box is probably due to higher modulus of soil than that of the void box.

The maximum water absorption of slab void box is 135%. The strength at this point is 7.68 KPa. After this point, void box loose its maximum strength and compression test can not be performed.

The beam void box is tested up to 11.04 kPa (equivalent to 18” load of beam). At this point water absorption of beam void box is 105%.

The displacement at void spaces in experimental data is little higher than the simulated data. In simulation 2D model is considered. This is the reason of higher value of experimental data.

Thank You!

QUESTIONS ?

ACKNOWLEDGMENTS This research was supported by the funds provided by Consolidated

Reinforcement Inc.