Bio-Mediated CaCO3 Production as a Method for Strength Improved Soils

By: Justin Whitaker, Undergraduate Supervisor: Dr. Sai Vanapalli, Professor and Chair

Department of Civil Engineering, University of Ottawa

Rationale for Research

Previous Research

Objective

At present, grouting and ground improvement techniques to reduce liquefaction induced damages are diverse with respect to treatment, cost, environmental impact, site requirements, etc. With a focus on grouting, all man-made grouting chemicals, with the exception of sodium silicate, are toxic and/or hazardous. In addition, all grouting techniques lead to an irreversible loss in soil permeability, which limit their application to short range foundational strength improvements and seals. In search of alternatives, biomediated ground improvement techniques show promise in their ability to improve soil strength while maintaining soil porosity.

Research in Progress

Summary

Proposed Mechanism of Cementation

Methods and Results

Chile – 2010

Niigata – 1964

Liquefaction is a phenomenon in which the strength and stiffness of a soil is reduced by earthquake shaking or other rapid loading. Liquefaction is most often observed to occur in saturated , low-density (uncompacted) sandy soils with low-drainage (i.e. under seams of impermeable sediments, as foundations of buildings, rail tracks, bridges, etc). It has been responsible for tremendous amounts of damage in historical and recent earthquakes around the world.

The key objectives of this research are : 1) Verify that microbial precipitation can consolidate loose material into a binding, strength enhancing matrix using appropriate cementation media 2) Characterize a novel strain of ureolytic bacteria, confirming its potential as a model for CaCO3 precipitation 3) Validate the mechanistic role of bacteria in the biomineralization process, 4) Evaluate the economic benefits in other fields and cost effectiveness of biomediated soil improvement

Acknowledgements

The preliminary study has shown that significant strength and stiffness improvements can be obtained in sands using a microbial additive in a urea-CaCl2 solution base. The reagent, NiCl2 was shown to increase urease activity with results suggesting it as a necessary agent in effectual soil cementation. In addition, a novel strain, S. ureae has been studied and deemed suitable as a biocatalytic agent in the cementation of sandy soils. However, the control and predictability of the in-situ distribution of bacterial activity and reagents for suitable and homogenous CaCO3 production are not yet sufficient and pose the need for further research.

*S = (Ca²⁺) × (CO3²ˉ)/Ksp-max* ↔ S > Ksp-max = CaCO3(s)

(1) Cell-Ca²⁺ + Urea→

4-Step Process

(NH2)2CO

(NH2)2CO

(2) HCO³ˉ + NH3↔ NH4Cl +CO3²ˉ→ (3) Cell-Ca²⁺ + CO3²ˉ→(4) Cell-CaCO3

HCO3³ˉ

(NH2)2CO

(NH2)2CO HCO3³ˉ

NH3

HCO3³ˉ

NH3

H⁺ + NH3

(NH2)2CO

NH4⁺

CO3³ˉ

CO3³ˉ

CO3 ³ˉ + NH3 + H⁺

CaCO3

CaCO3 CaCO3

HCO3³ˉ

HCO3³ˉ

NH4⁺

NH3

(NH2)2CO

(NH2)2CO H⁺ + NH3 (NH2)2CO

(NH2)2CO

CaCO3

CaCO3

CaCO3 CaCO3

CaCO3

CaCO3

CaCO3

High pH = 9.5

Neutral pH = 7.4

CaCO3

CaCO3

CaCO3

CaCO3

NH4⁺ HCO3³ˉ

CO3³ˉ

CO3³ˉ

NH4⁺

CO3³ˉ

0.00%

20.00%

40.00%

60.00%

80.00%

100.00%

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Pe

rcen

t Fi

ner

Pas

sin

g (%

)

Grain Size (mm)

Sieve Analysis - Construction Grade Soil

D60 = 0.38

D10 = 0.12 mm

Uniformity Coefficient = D60 = 0.38 = 2.71 D10 0.14

Size Ratio = D95 = 0.98 = 9.8 D5 0.10 Curvature Coefficient = 1.98

FIGURE 3 A Focus on Geotechnics:

Hypothetical Urease Operon for S. Ureae (1)

Ure-R Ure-Structural (1) Up-regulating genetic expression of urease to increase cementation rate

(2) Tri-axial tests for determining undrained shear strength under cyclic and monotonic loads

(3) Shake-table test with large scale sand-box trial to simulate field conditions

(2)

(3)

(3)

This research was made possible with funding provided by the UROP grant. Special thanks to Dr. Vanapalli for supervising the project and Jean Celestine for aid in geotechnical preparation and testing. Collaborative efforts from members of Biochemical Engineering at CBY and from Technical Support Staff of the Biology Department at BSC were especially important and require additional thanks and recognition.

Extending Applications

Application of bio-mediated soil improvement to enhance weather resistance and increase load bearing capacity of roads and railroad embankments built on sands. Also to prevent soil erosion in economic areas such as agriculture or construction, while maintaining degrees of porosity in deposits for adequate drainage.

(1a) Selecting the Model Sand Type (1b) Selecting the Model Bacterial Strain

Requirements: • Novel species yet to be characterized in the literature • Low biohazard threat (ATCC biosafetyrating 1 or less) • Easily accessible, readily cultured and urease positive

Requirements: • Poorly graded but uniform sandy soil • Average pore volume between granules within range of bacteria size • Readily available and cheap

Hindered Movement of Bacteria (get trapped in pore-throats)

Unhindered Movement of Media and Bacteria

D10 = 0.9mm

S. ureae a common soil bacterium

(2a) Cell as a Nucleation Site (2b) Urease Activity (2c) Strength Enhancement

Reduce Slumping of Railroad Embankments

Enhancing Road Durability

Preventing Soil Erosion

2.0mm

1.5M Urea-CaCl2 + 35mM NiCl2

0.67X Zoom

1.5M Urea-CaCl2 + 35mM NiCl2

4.5X Zoom

0.2mm

0

0.2

0.4

0.6

0.8

1

1.2

0 10 20 30 40 50 60 70 80

Ure

ase

Act

ivir

t, U

(U

rea

Hyd

roly

zed

-mo

l/Lh

r)

Time, t (hr)

35mM NiCl2

40mM CaCl2

2ml/L H2O2

Control

Enzyme

0

0.5

1

1.5

2

2.5

3

Bacteria(3X+NiCl2)

Bacteria(1X+NiCl2)

Bacteria (1X) Bacteria (H2O2) Enzyme Control

She

ar S

tre

ngt

h, Ϯ

(M

Pa)

CaCl2-Urea Solution Variant

2(b)

2(c)

2(a)