# Stabilization of Road Embarkments on Peat Soils Using Oil Shale Ash and Pozzolanic Additives
## 1 Introduction  
Since the 1970s, scientists in many parts of the world have explored ways to build \"floating\" foundations, roads, and railway embankments by mixing unstable soils with binders on site instead of excavating all the soft soil beneath the road embankment down to the bedrock and replacing it with mineral fill. This makes it possible to significantly reduce the volume and cost of excavation and ensure the linear movement of the construction front. This approach is economically attractive and more environmentally friendly than traditional methods. The lifetimes of the bases obtained in this way are generally several times longer than those of filled bases [1]. The most problematic scenario is the stabilization of peat soils. This is being actively investigated (especially in Malaysia, Indonesia, and other peat-rich countries), but current solutions are expensive and require the use of large amounts of high-quality cement [2]. The use of lime (including lime-containing fly ash) is not possible on peat soils due to their low silicate and aluminate contents. Low pH values caused by organic acids (incl. humic and fulvic acids) can prevent the dissolution of clay minerals [3]. Additionally, the effect of humic acids on the setting process of cement is not fully understood. In Estonia, improvements in the geotechnical properties of organic-rich soils have been investigated mostly by mixing or covering organic soils with filling soils and by compacting the organic soil. In 1979, a 4 m thick peat layer at a construction site was covered with a 4 m thick filling layer. Within 3 months, the thickness of the peat layer decreased by 60%, the undrained shear strength Cu doubled, and the water content decreased twofold. The 5 m high partition walls built on the peat subsided 3-6 mm, and the floors subsided 1-2 mm in the subsequent 5 years. In Estonia, the most attractive binding agent for stabilizing road embankments on soils containing humic substances is oil shale ash [3; 4], which is domestically produced in power and oil plants in excess of 7 million tons per year (before the reduction in power generation by the incineration of oil shale) [5]. Since 2014, geotechnical studies on soil stabilization to build road embankments have been carried out on the Vobbu polygon. In a particular experiment, organic-rich soil was loaded with a pressure of 42-97 kPa. At maximum pressure, the relative consolidation was 51%, and at lower pressure, it was 35%. The Cu\({}_{\text{u}}\) of peat soil increased by 7 kPa to 28 kPa. After removing the load, the secondary consolidation ranged between 23 and 44 mm. In laboratory tests, soil samples taken from the Vobbu polygon were mixed with cement, oil shale ash, limestone screenings, and sand. Satisfactory results were only obtained when the soil was mixed with cement at a volumetric cement ratio exceeding 10%, and additional compaction under the influence of vertical loads was used. This made the methodology expensive. Mixing oil shale ash and peat generated no satisfactory results and even caused a reduction in the compressive strength of the peat [6]. This phenomenon has been observed before; floors made of oil shale ash have decayed due to the action of organic matter in previous studies. However, road construction projects which utilize the removal and replacement of organic soil with backfilling showing an adequate load-bearing capacity or with the dense compacting of organic soil are time-consuming and costly as well as ecologically unsustainable [2]. The promoters of this project have previously (KIK 11048 \"Reuse of secondary polymer waste and oil shale ash as building material raw material. Peat-based composite material with spatially prestressed polymer reinforcement\") and successfully solved all problems related to chemical processes and developed a composite material of peat and oil shale ash [4]. A similar methodology can also be used to stabilize road embankments on peat soil because the chemical processes taking place are analogous.