Microbial-Induced Carbonate Precipitation for Strengthening Soft Clay

J Z Xiao,Q H Wang,H Cai,Y Q Wei,Z W Wang,T Yang,S F Wu

doi:10.1155/2020/8140724

Abstract

Currently, calcite produced in sediments by microbial-induced carbonate precipitation (MICP) is mainly used as a strengthening binder in sand because sands are porous and have good permeability. Conventional wisdom does not consider MICP to be suitable for use in soft clay because of the clay particles’ small size and its minimal porosity. Because of the clay’s high water content and complex chemical composition, very little research has been done and not much is known about the use of MICP in soft clay for strength enhancement. For this paper, soft clay specimens were prepared by mixing a solution containing Sporosarcina pasteurii bacteria, solutions with different concentrations of nutrient salts, and soft clay. Unconfined compressive strength tests were carried out on these specimens after they had cured for 28 days in a moisture-controlled environment. These laboratory tests were used to study the chemical reactions, the clay’s strength, and other influencing factors. The results are as follows: (1) directly mixing a S. pasteurii solution, nutrient salts, and soft clay considerably improves the uniformity of the spatial distribution of the bacteria and the nutrients in the soft clay. Directly mixing these constituents promotes the formation of calcium carbonate and greatly simplifies soft clay sample preparation. (2) It is feasible to use MICP to increase the strength of soft clay. Compared to control specimens cured under the same conditions but without introduced nutrients and bacteria solution, the unconfined compressive strength of MICP-treated specimens can be increased by as much as 2.42 times to an unconfined compressive strength of 43.31 kPa. The water content in MICP-treated specimens was significantly reduced by the MICP reactions and in one case decreased from 40% to 30.73%. (3) The strength enhancement of microbially solidified soft clay is the result of two processes: urea hydration catalyzed by enzymes consumes water in the clay and the bacterially precipitated calcite forms in the sediment’s pores. (4) The micro-organism-produced calcite in the soft clay increases the calcite abundance from 0% to as much as 3.5%. (5) The MICP-treated strength of soft clay varies with the concentration of the nutrients provided. For the experimental conditions used for this paper, the optimum concentration of the CaCl2·2H2O and CH4N2O nutrients is 0.5 mol/L.

Highlights

Most soft clay is deposited in still or very slowly flowing water
Nutrient specimen (Figure 5) but no bacteria growing on the specimens with only water added to the clay (Figure 6). e different appearances indicate that the different nutrient concentrations significantly influence the growth of microorganisms in soft clay cured at room temperature and 75% humidity
Average stress (g) 17.89 43.31 28.89 30.19 formerly cultivated land have been converted to clay storage areas. is greatly increases the cost of projects that must remove large quantities of soft soil. erefore, selecting an appropriate method for strengthening soft clay so it can be used as a construction material is important

Summary

Introduction

Most soft clay is deposited in still or very slowly flowing water. With the rapid development of the Chinese economy over the past few decades, more and more construction projects involving soft clay have been initiated such as construction for water conservation, land reclamation, and port and waterway hydraulic filling and dredging. ese projects produce large amounts of soft clay. Commonly composed of unconsolidated particles of illite and montmorillonite, has many undesirable engineering properties including high water content, high compressibility, low permeability, and low strength. It is deformed and commonly leads to stability problems [1]. E differentiation, accumulation, migration, and formation of mineral deposits of many elements on earth are closely related to microbial activity Because of their rapid rate of reproduction, flexible metabolism, low energy consumption, variety, abundance, and ecofriendliness, the microorganisms used in microbial-induced carbonate precipitation (MICP) can have an important impact on the mechanical and engineering properties of soil [12, 15, 16]. Because of the clay’s high water content and mechanical properties, only a few soil engineering studies have used MICP on soft clay. Two control specimens and soft clay specimens that contained only the same weight water, with no nutrient salts and no bacteria solution added, were prepared

Specimen Preparation and Curing

Test Results

Discussion

Conclusions