Biogrouting of Aggregates Using Premixed Injection Method with or without pH Adjustment

To increase the efficiency of biogrouting for coarse sand or gravel, a premixed injection method with or without pH adjustment was developed and adopted for biogrouting of aggregates with grain sizes ranging from 0.75 to 10 mm. The premixed injection method is to inject premixed bacterial suspension and cementation solutions together. This method can reduce the biochemical treatment cycles as compared to the traditional treatment. The shortcoming of this method is that the premixed bacterial suspension and cementation solution tends to form bacterial flocs before the injection. However, the bacterial flocs can be inhibited by adjusting down the pH of the solution and thus enhancing the groutability and efficiency of biogrout. The effect of bacterial flocs on the strength improvement of aggregates was investigated. Based on the testing results, both premixed injection methods with or without bacterial flocs are effective for increasing the strength of coarse aggregates (with grain sizes larger than 2 mm). However, for fine aggregates with grain sizes within 0.75–2 mm, a large difference in the strength was observed between samples treated with and without bacterial flocs. Thus, only the premixed injection method without bacterial flocs (i.e., with pH adjustment) is suitable to be used for aggregates with different grain sizes.