# Stabilization of Road Embarkments on Peat Soils Using Oil Shale Ash and Pozzolanic Additives
## 1 Introduction
In this case, only the upper geotechnically necessary minimum part of the subsoil (down to 1-2 m) is stabilized. The rest of the subsoil (deeper than 1-2 m) is left untouched without disturbing the existing water regimes in ecosystems [7]. Initially, lime and materials containing it were mainly used to harden floating roadbeds, but in the 1980s, it was found that cement or cement-lim mixtures where cement has partially been replaced with lime are considerably more effective (but also more expensive). Soil stabilization with lime is possible only if its mineral part contains a sufficient amount of pozzolanic components (silicate clay particles) [1]. The stabilization of soils containing peat is expensive and requires the use of large amounts of high-quality cement [2]. The common use of lime (including lime-containing fly ash) is not possible on peat soils due to its low silicate content and the humic acids present in soil [1]. Humic acids have a strong chemical affinity for group II metal ions (Ca\({}^{2+}\) Mg\({}^{2+}\), etc.) [8], so they react with lime to form insoluble calcium and magnesium humates which can be adsorbed on the surface of the cementing particles, preventing the hydration of silicates and pozzolanic reactions [9]. Similar problems have also been described in studies dealing with the stabilization of peat soils with cement:  
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2. Humic acids actively bind Ca\({}^{2+}\), forming insoluble humates and thus decreasing the amount of available Ca\({}^{2+}\) ions to form C-S-H gel, while fulvic acids more selectively attack Al-containing particles, covering them with an adsorbed layer of fulvates and inhibiting further reactions. Fulvic acids can also disrupt the crystallization of calcium aluminate hydrate, calcium sulfate aluminate hydrate, and calcium ferrite aluminate hydrate [9]. 2. The dissolution of acidic compounds causes the pH level in the pore water to drop close to or below the threshold, above which secondary cementation products can form (pH-9), thus slowing down or preventing hardening [9]. 3. A high water-binding capacity, as well as high moisture content, slows down the hardening processes [9]. In Estonia, studies have been carried out to strengthen road embankments using oil shale ash [10; 11; 12], but their implementation was based on incomplete assumptions. These studies aimed to achieve the stabilization of soil at the expense of the hydraulic hardening of oil shale ash, but due to its low pozzolanic content, solidification occurs only when lime reacts with atmospheric carbon dioxide; in the deeper layers, it would require hundreds of years [13]. Additionally, the inhibitory effect of humic acids on hardening processes and their lime-degrading effect were not taken into account [6]. However, it has been proven that these problems can be successfully addressed with relatively cheap additives and a suitable treatment regime, which allows the bearing capacity of a road embankment to exceed 100 kPa. The aim of this study is to apply the results of earlier chemical studies performed at the University of Tartu [3; 4; 14] for the in situ mass-stabilization of peat soil areas, allowing the retarding effect of humic acids to be minimized.