**5. Curing process and bender elements**

The progress of the cementation process during curing was monitored using bender elements by recording the shear wave velocity change over time. This was achieved during the preparation of samples for triaxial testing. Split mold using PVC material fitted with a rubber membrane inside was designed to produce identical cylindrical samples with length of two times the diameter. Bender elements in the end platens transmitted waves vertically through the samples, and the waveforms and travel times were monitored using a semiautomated procedure [28]. The typical responses in **Figure 5** happen during the curing of gypsum-cemented samples. The hardening process occurs rapidly for gypsum contents above 10% and that curing is essentially complete after 1 h. This is consistent with the setting time reported by the gypsum supplier of 55 minutes. However, for low gypsum contents, there is some variability and longer setting times have been recorded. This is believed to be because

**Figure 5.** *Shear wave velocity changes during curing of gypsum-cemented specimens.*

#### **Figure 6.**

*Shear wave velocity changes during curing of bio-cemented specimens.*

the setting of gypsum involves a hydration reaction that can be affected by changes in temperature and humidity. The samples were effectively sealed with limited supply of oxygen during the curing reaction, and with low gypsum contents, temperature rises associated with the exothermic reaction would be limited, hence limiting the reaction rate. After the hardening phase, the samples were left unattended overnight, and during this time no significant change in shear wave velocity was captured.

The variations of shear wave velocity during the calcite precipitation and hardening of the bio-cemented samples are shown in **Figure 6**. Small step changes in the shear wave velocity shown in **Figure 6** are a consequence of manual intervention in the semiautomated interpretation procedure and do not reflect the material response. For the range of final calcite values shown, the reaction time is very similar. In all cases there is a lag of about 1 hour before the cementation process begins, and the process is complete in about 12 hours after which the shear wave velocity remains constant. Samples were left for 24 hours before commencing the triaxial tests, and during this time the stiffness remained essentially constant. **Figure 6** also shows that the initial 100 m/s value increases over time, which is proportional to the stiffness and tends to increase with the calcite content as expected.

The comparative study on shear wave signal responses during curing for selected gypsum-cemented and bio-cemented sand samples is projected in **Figure 7**. The trend in the responses are similar for 5% gypsum and 1.88% calcite, whereas UCS tests have shown that the strength associated with 1.88% calcite is equivalent to about 10% gypsum. However, the rates of the cementation reactions depend on the chemistry of the hydration and MICP processes and are not expected to influence strength and stiffness. Nevertheless, it may be noted that the ratio between strength and stiffness varies with the cement type. The calcite-cemented samples have lower stiffness (shear wave velocity) than gypsum samples with the same strength. A likewise pattern was seen in the UCS tests and was inferred to be a simple consequence of the low amount of calcite cement.

No triaxial tests were performed with calcite contents lower than 1.88%, and thus it is unclear whether with lower calcite contents the reaction time will increase, which occurred for low gypsum contents. In other tests [19, 29], when the cementation occurred underwater, the time required for curing was greater than 24 hours, and it is expected that the curing time will depend on the chemical and

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**Figure 8.**

*Response of uncemented Sydney sand.*

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

environmental conditions. It may also be noted that the lag at the start of the cementation process is beneficial for both injection and mixing approaches at field scale.

To enable the influence of the cement to be appreciated, triaxial tests have been performed on loose Sydney sand. Samples with various relative densities have been subjected to standard drained (CID) and undrained (CIU) tests with different confining pressures. Test results as in **Figure 8** are presented in terms of stress ratio (q/p′) against the axial strain. The results indicate that in all tests the stress ratio rises to a peak before gradually reducing toward a critical state value at large strain. Where the stress ratio dropped rapidly post peak, the samples had formed pronounced shear planes. The dotted line in **Figure 8** shows the estimated critical state

stress ratio, M = 1.35, which corresponds to a friction angle of 32°.

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

**6. Triaxial stress: strain responses**

*Comparison of curing for gypsum and bio-cemented specimens.*

**6.1 Uncemented sand**

**Figure 7.**

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

**Figure 7.** *Comparison of curing for gypsum and bio-cemented specimens.*

environmental conditions. It may also be noted that the lag at the start of the cementation process is beneficial for both injection and mixing approaches at field scale.
