**6.3 Bio-cemented sand specimens**

During the preparation of the UCS and triaxial samples, dry sand was mixed with equal amounts by mass, of powdered calcium chloride and urea, and then with water containing the bacteria (see **Figure 15**). The amounts of calcite precipitated, measured after the mechanical tests, are shown in **Figure 16**. For UCS specimens, there is a clear relationship between the amount of urea added and the amount of calcite. Triaxial samples, on the other hand, show considerable dispersion and variability in the precipitated amount of calcite quantified using acid-wash test. Nevertheless, when the uniformity of calcite precipitation is verified after the test, there is a variability of less than 5% of the amount of calcite in each sample. The calcite concentration in the UCS samples and some typical data were included in **Table 2**.

**Figure 16** shows the amount of calcite. However, the amount of calcite in the triaxial specimens has increased. In addition, the variability of the data could be the result of a change of procedure during saturation. In some initial tests, calcite was removed from the samples when pumping water, which could explain some of the lower results. However, in the majority of tests, it is simply a question of pumping water into the samples, and there is no need to lose nutrients or bacteria. The variability of the amount of calcite obtained from the UCS and triaxial tests can be motivated by several reasons. In addition to the hardening time and access to the air causing the drying of the samples, it was the same during triaxial tests; no particular action was taken. Differences in soil temperature and pH during sample preparation may also have contributed to differences in calcite precipitation.

**Figure 15.** *Procedure of (a) mixing and (b) molding bio-cemented samples for triaxial test.*

**123**

1.5 to 2.3%.

**Figure 18.**

**Figure 17.**

*Geomechanical Behavior of Bio-Cemented Sand for Foundation Works*

*Deviator stress, axial strain responses for bio-cemented specimens (1.5–2.3% calcite).*

*Volume strain, axial strain responses for bio-cemented specimens (1.5–2.3% calcite).*

To study the effects of confining pressure and calcite, different ranges of calcite sample were prepared. In all cases, these differences were prepared in the same way. **Figures 17** and **18** show the stresses, deformations and volume stresses, and axial strains, resulting from a series of CID drained tests with different confinement constraints for the lowest calcite contents, ranging from

The set of tests includes a sample for which the membrane leaks because the cell and the back pressures were equal. This has been done effectively on a totally saturated sample. The UCS tests are shown in **Figure 4**. **Figure 4** suggests a UCS strength of about 300 kPa for a calcite content of 2%. Past research [17] claims that the strength of the UCS in bio-cemented sample is influenced by the level of saturation. However, [17] reported that the degree of saturation causes an increase in UCS, which is the opposite trend of the current study. Previous research [13] has therefore focused on the localization of calcite, with lower degrees of saturation leading to precipitation only at particle contact. In this chapter, all samples have been prepared so that there is no significant saturation effect on the results. The results shown in **Figures 16** and **17** indicate a general tendency toward strength and stiffness increase with the confining constraint. However, at a

*DOI: http://dx.doi.org/10.5772/intechopen.88159*

**Figure 16.** *Relation between amount of urea in mixture and calcite measured posttest.*

*Geomechanical Behavior of Bio-Cemented Sand for Foundation Works DOI: http://dx.doi.org/10.5772/intechopen.88159*

#### **Figure 17.**

*Sandy Materials in Civil Engineering - Usage and Management*

During the preparation of the UCS and triaxial samples, dry sand was mixed with equal amounts by mass, of powdered calcium chloride and urea, and then with water containing the bacteria (see **Figure 15**). The amounts of calcite precipitated, measured after the mechanical tests, are shown in **Figure 16**. For UCS specimens, there is a clear relationship between the amount of urea added and the amount of calcite. Triaxial samples, on the other hand, show considerable dispersion and variability in the precipitated amount of calcite quantified using acid-wash test. Nevertheless, when the uniformity of calcite precipitation is verified after the test, there is a variability of less than 5% of the amount of calcite in each sample. The calcite concentra-

tion in the UCS samples and some typical data were included in **Table 2**.

**Figure 16** shows the amount of calcite. However, the amount of calcite in the triaxial specimens has increased. In addition, the variability of the data could be the result of a change of procedure during saturation. In some initial tests, calcite was removed from the samples when pumping water, which could explain some of the lower results. However, in the majority of tests, it is simply a question of pumping water into the samples, and there is no need to lose nutrients or bacteria. The variability of the amount of calcite obtained from the UCS and triaxial tests can be motivated by several reasons. In addition to the hardening time and access to the air causing the drying of the samples, it was the same during triaxial tests; no particular action was taken. Differences in soil temperature and pH during sample preparation may also have contributed to

**6.3 Bio-cemented sand specimens**

differences in calcite precipitation.

**122**

**Figure 16.**

**Figure 15.**

*Relation between amount of urea in mixture and calcite measured posttest.*

*Procedure of (a) mixing and (b) molding bio-cemented samples for triaxial test.*

*Deviator stress, axial strain responses for bio-cemented specimens (1.5–2.3% calcite).*

**Figure 18.**

*Volume strain, axial strain responses for bio-cemented specimens (1.5–2.3% calcite).*

To study the effects of confining pressure and calcite, different ranges of calcite sample were prepared. In all cases, these differences were prepared in the same way. **Figures 17** and **18** show the stresses, deformations and volume stresses, and axial strains, resulting from a series of CID drained tests with different confinement constraints for the lowest calcite contents, ranging from 1.5 to 2.3%.

The set of tests includes a sample for which the membrane leaks because the cell and the back pressures were equal. This has been done effectively on a totally saturated sample. The UCS tests are shown in **Figure 4**. **Figure 4** suggests a UCS strength of about 300 kPa for a calcite content of 2%. Past research [17] claims that the strength of the UCS in bio-cemented sample is influenced by the level of saturation. However, [17] reported that the degree of saturation causes an increase in UCS, which is the opposite trend of the current study. Previous research [13] has therefore focused on the localization of calcite, with lower degrees of saturation leading to precipitation only at particle contact. In this chapter, all samples have been prepared so that there is no significant saturation effect on the results.

The results shown in **Figures 16** and **17** indicate a general tendency toward strength and stiffness increase with the confining constraint. However, at a

**Figure 19.**

*Deviator stress, axial strain responses for bio-cemented specimens (2.8–3.4% calcite).*

**Figure 20.** *Volume strain, axial strain responses for bio-cemented specimens (2.8–3.4% calcite).*

confining stress of more than 200 kPa, the resistance becomes more obvious. This could be due to the lower calcite content in the more heavily stressed sample. It can also be noted that the cumulative response of the sample subjected to higher stresses shows less compression and more gradual expansion, which corresponds to a lesser effect of cementation. Thus, the calcite content is not only low but also the level of increased stress. Nevertheless, the general behavior patterns correspond to those expected for cemented specimens and are similar to gypsum cement.

**Figures 19** and **20** show the effects of confining stress for a series of triaxial CID tests with calcite contents between 2.8 and 3.4%. Another UCS test is available for a saturated test in this cement content range, as previously following the rupture of the membrane. The UCS resistance of 820 kPa is again significantly higher than expected in the UCS tests of **Figure 4**, giving a value of 450 kPa for a calcite content of 3.4%. Reasonably consistent, all bio-cemented specimens showing increased in strength and stiffness as containment stress increases. For lower calcite levels, the rate of expansion tends to decrease as the level of stress increases, although the effect is less pronounced for those more cemented specimens. The trends are generally similar to those of the lower calcite content.

**125**

*Geomechanical Behavior of Bio-Cemented Sand for Foundation Works*

The following concluding remarks are made based on the performance and

However, there are limits to mixing with the content of the mixture.

fully demonstrated as feasible at the laboratory scale.

to predict the degree of cementation.

ficulties in obtaining reliable data.

during the research belong to the University of Sydney.

**Acknowledgements**

• Bio-cemented samples were prepared by mixing sand, bacteria, and nutrients, as well as samples cemented with gypsum. It has been found that it produces no damage when produced by the sample preparation method. A mixing technique is recommended to study the response of a weakly cemented material.

• As shown in several other studies, calcite is an extremely effective cementing agent, and, for a given amount of cement, it offers higher strength and stiffness than other cementing agents. The results show that the strength in the UCS tests is similar to, or slightly higher than, the samples treated with injection techniques. At the same time, the problem of injection site obstruction was avoided by using an ex situ mixing technique, and this has been success-

• The patterns of behavior observed in bio-cemented Sydney sand in triaxial tests are very similar to those of gypsum-related specimens. The results were reasonably consistent throughout the laboratory tests. The results of the triaxial tests were obtained with the amount of calcite produced, and it is difficult

• The use of automated shear wave velocity measurement has enabled variations in stiffness, and hence degree of cementation, to be monitored throughout the processes of curing, stress application, and shearing. However, the large changes in shear wave velocity associated with curing have caused some dif-

This research was conducted in the best interest of the corresponding author's PhD research work. His entire studies were fully sponsored by the Ministry of Higher Education of Malaysia. All the research facilities and the instruments used

*DOI: http://dx.doi.org/10.5772/intechopen.88159*

behavior of bio-cemented Sydney sand:

**7. Conclusion**

*Geomechanical Behavior of Bio-Cemented Sand for Foundation Works DOI: http://dx.doi.org/10.5772/intechopen.88159*
