Fook-Hou Lee

The ultimate vertical pullout capacity of single-helix anchors in sand was explored using the coupled Eulerian–Lagrangian (CEL) technique to account for large deformation processes. A semifluidized bounding surface plasticity model was incorporated in a novel manner to improve the continuum representation of highly disturbed sand around the anchor, particularly under very low confinement. The model was shown to be necessary for reproducing the soil backflow beneath the helix and the experimentally measured load-displacement response. A series of CEL analyses was conducted to investigate the effects of helix pitch size, embedment depth, sand relative density, and helix-to-shaft ratio on the pullout behavior of single-helix anchors in sand. The pitch size was found to have little influence on the ultimate pullout capacity, consistent with available centrifuge observations. Increasing sand relative density contributes to greater capacity but changes the relative contributions of the shaft and helix. The helix resistance becomes more dominant in dense sand, whereas the shaft and helix provide nearly equal contributions in loose sand. The numerical results were used to extend an analytical solution for estimating the ultimate pullout capacity. The proposed solution provides excellent predictions across different sand densities and anchor geometries, suggesting its potential use in practical design.

Coarse sand cannot sustain pozzolanic reaction owing to the small surface-area-to-volume ratio arising from their large particle sizes. However, natural sandy soils often have small amounts of fines, which may contain pozzolanic materials such as silica. Asghari et al. (2003) reported pozzolanic hardening of admixtures of lime and a sandy alluvium containing about 6% fines. In addition, Nakarai and Yoshida (2015) reported that sand–lime admixtures exposed to the atmosphere may undergo long-term hardening well beyond 28 days, owing to carbonation of the lime by carbon dioxide in the air. However, this is inapplicable to deep mixing where the lime is not exposed to the atmosphere. The most commonly used hardening agent for sand is Ordinary Portland Cement (OPC). Unless some pozzolans are also added, the primary hardening reaction is hydration reaction. The microstructural changes of cement-treated sand are not dissimilar to those of concrete. Subramaniam et al. (2018) noted that 1-day cured samples of OPC-treated sand comprised loosely packed assemblages of calcium silicate hydrates with small amounts of needle-shaped ettringite crystal. The ettringite is likely to have been produced by the reaction between tricalcium silicate and the small amount of gypsum (calcium sulphate), which is added to prevent “flash setting.” After 28 days, the hydration reaction is largely completed, and the calcium silicate hydrate assemblages are much more densely packed. By this time, the ettringite has been almost completely converted to monosulphate. Hence, the hardening effect of OPC on sand is derived mainly from the infilling of the large voids by cementitious products, mainly calcium silicate hydrate. This is similar to the gel-space theory (Powers 1958), which states that the strength of hardened cement paste is proportional to the power of the proportion of space infilled by calcium silicate hydrate.

A primary result of deep soil mixing and jet-grouting processes is to create hardened columnar inclusions in the ground. These columnar inclusions posed challenges to conventional geotechnical analysis and design using manual calculations. In order to address these challenges, analytical and design methodologies were developed. These include the use of stress concentration ratios as well as guidelines for analysing the effects of columnar inclusions in soft soil. Many of these analytical methodologies were developed in the 1980s and 1990s and they are largely applicable to foundation and embankment stability design (e.g. Kitazume and Terashi 2013).

The goal of binder–soil mixing processes is often to produce a treated mass which is uniform in terms of strength and stiffness. In general, however, chemically stabilised soil mass invariably has a certain degree of non-uniformity. The parameters which are typically used to quantify the departure from a perfectly uniform mix are the coefficient of variation and the scale of fluctuation. Such non-uniformity typically arises from two sources. Firstly, the in situ soil may be non-uniform. Spatial variation in soil strength has been widely studied (e.g. Phoon and Kulhawy 1999; Kim 2005). However, Liu et al. (2019) noted that the strength of in situ soil is not readily correlated to the stiffness and strength of the soil after treatment, since the soil would have been remoulded and admixed with binders. Instead, they proposed that a more appropriate parameter for characterising spatial variation within a layer of deposited soft clay to be improved is the in situ water content. For instance, Liu et al.’s (2019) data from the Marina Bay Financial Centre site appears to show higher water content in the mid-layer of the soft clay, which is consistent with the fact that the soil is still consolidating. This is also correlated to the variation in strength of the cement-treated clay.

The idea of using chemicals to improve and harden soil has been known for a few thousand years. Lime mortar mixed with sand and clay were discovered on the floors of houses at the archaeological site of Lepenski Vir in Serbia, dating back to 7500 BC (Radić et al. 2008). Lime was also used in the construction of the pyramids at Shersi in Tibet (Miller 2000), as well as in Egypt, China, Greece, Rome and other ancient civilisations. In many of these construction-related applications, lime was commonly used in the admixture with soil in order to bring about pozzolanic reactions, which led to the production of cementitious products, that comprise mainly calcium silicate hydrate and calcium aluminate hydrate.

Stabilisation of soil using chemical binders has been studied and has been used extensively in various civil engineering works since the 1970s. These include road and highway construction, stabilisation of marginal land with soft and contaminated soils, land fill and reclamation as well as excavations and underground construction. The chemicals that are most widely used include cement, lime, fly ash, ground granulated blast furnace slag (GGBS). Other binders and binder components include sulphoaluminate cement (e.g. Chen et al. 2020), gypsum (e.g. Zhu et al. 2018; Chen et al. 2018, 2020; He et al. 2021), cement kiln dust (e.g. Miller and Zaman 2000), bentonite (e.g. Katsioti et al. 2008; Lin et al. 2009) and mining waste products such as jarosite and alunite (e.g. Boura et al. 2003; Katsioti et al. 2009). Some of these are used as components in a cocktail of binders. Additives are sometimes required to predispose the conditions to initiate as well as to balance the stoichiometry for the hardening reactions. Non-cementitious binders such as asphaltic emulsion (Lucena et al. 2014) have also been studied. However, as they have not been used on a wide scale, they will not be discussed herein.

The parameters which are assessed for acceptability in chemical treatment of soil depends primarily on the aim of the improvement process. For instance, if the objective of improvement is to stabilise and solidify contaminated soil and prevent leakage of toxic chemicals such as heavy metals and hydrocarbons, then hazardous leachate contents will be an important parameter (e.g. Conner 1994). If the objective is to provide seepage cut-off, such as in cut-off walls, then (low) permeability and continuity of the cut-off structure are important parameters. However, if the objective is to stabilise soft and marginal soils for construction, then the important parameters are strength, stiffness and spatial uniformity. An important aspect of chemical soil mixing is spatial variability. Indeed, this is an important aspect which makes adequate monitoring all the more important. This issue will be discussed in more detail in this as well as in the next chapter.