Void ratio has traditionally been used as a state variable for predicting the liquefaction behaviour of soils under the critical state soil mechanics framework. Recent publications show that void ratio may not be a good state variable for characterizing sand with fines. An alternative state variable referred to as the equivalent granular void ratio has been proposed to resolve this problem. To calculate this alternative state variable, a b parameter is needed. This b parameter represents the fraction of fines that actively participate in the force structure of the solid skeleton. However, predicting the value of b is problematic. Most, if not all, of the b values reported were determined by case-specific back-analysis, that is, the b value was selected so that a single correlation between equivalent granular void ratio and the measured steady state strength (or cyclic resistance) could be achieved. This paper examines the factors affecting the b value based on published work on binary packing. This leads to a simple semi-empirical equation for predicting the value of b based on fines size and fines content. Published data appear to be in support of the proposed equation. A series of experiments were conducted on a specially designed sand–fines type to provide additional validation of the proposed equation and to reinforce the use of equivalent granular void ratio in a more generalized context.

Microbial-induced calcite precipitation (MICP) is a promising new technology in the area of Civil Engineering with potential to become a cost-effective, environmentally friendly and sustainable solution to many problems such as ground improvement, liquefaction remediation, enhancing properties of concrete and so forth. This paper reviews the research and developments over the past 25 years since the first reported application of MICP in 1995. Historical developments in the area, the biological processes involved, the behaviour of improved soils, developments in modelling the behaviour of treated soil and the challenges associated are discussed with a focus on the geotechnical aspects of the problem. The paper also presents an assessment of cost and environmental benefits tied with three application scenarios in pavement construction. It is understood for some applications that at this stage, MICP may not be a cost-effective or even environmentally friendly solution; however, following the latest developments, MICP has the potential to become one.

Enzyme-induced carbonate precipitation (EICP) is a relatively new bio-cementation technique for ground improvement. In EICP, calcium carbonate (${\mathrm{CaCO}}_3$) precipitation occurs via urea hydrolysis catalysed by the urease enzyme sourced from plants. EICP offers significant potential for innovative and sustainable engineering applications, including strengthening of soils, remediation of contaminants, enhancement of oil recovery through bio-plugging and other in situ field applications. Given the numerous potential applications of EICP, theoretical understanding of the rate and quantity of ${\mathrm{CaCO}}_3$ precipitation via the ureolytic chemical reaction is vital for optimising the process. For instance, in a typical EICP process, the rate and quantity of ${\mathrm{CaCO}}_3$ precipitation can depend significantly on the concentration, activity and kinetic properties of the enzyme used along with the reaction environment such as pH and temperature. This paper reviews the research and development of enzyme-catalysed reactions and its applications for enhancing ${\mathrm{CaCO}}_3$ precipitation in EICP. The paper also presents the assessment and estimation of kinetic parameters, such as the maximal reaction velocity (${\mathrm{V}}_{\max }$) and the Michaelis constant (${\mathrm{K}}_{\mathrm{m}}$), that are associated with applications in civil and geotechnical engineering. Various models for evaluating the kinetic reactions in EICP are presented and discussed, taking into account the influence of pH, temperature and inhibitors. It is shown that a good understanding of the kinetic properties of the urease enzyme can be useful in the development, optimisation and prediction of the rate of ${\mathrm{CaCO}}_3$ precipitation in EICP.

Void ratio has been used as a state variable for predicting the liquefaction behaviour of soils under the critical state, sometimes also referred to as the steady state, framework. Recent publications show that void ratio may not be a good parameter for characterising sand with fines because the steady state line (or curve) in the e-log(p′) space moves downward with increase in fines content until it reaches a threshold value referred to as the threshold fines content (TFC). Recently, an alternative state variable, referred to as the equivalent granular void ratio, has been proposed to resolve this problem. To calculate this alternative state variable, an additional parameter ‘b’ is needed. This parameter ‘b’ represents the fraction of fines that actively participate in the force structure of the solid skeleton. However, predicting the ‘b’ value is problematic. This paper examines the factors affecting the ‘b’ value based on published work on binary packing. This leads to a simple semi-empirical equation for predicting the ‘b’ value based on fines size and fines content. The proposed equations were evaluated with published data sets. Then, the concept of an equivalent granular steady state line is proposed. This concept was used to predict the location of SSLs for sand with different fines content from either the SSL of clean sand or the SSL of sand with a given fines content. The predictions agree well with experimental results.