## 3 Light Embankment Piling  
Light embankment piling is often used along the northern coast of Sweden as a foundation method for roads and railways on sulphide soil. The sulphide soil types range mostly from clay to silt and can be found worldwide in coastal floodplains and intertidal swamps [14]. Sulphide silts and clays pose a geotechnical problem, as they are loose to very loose and highly compressible, with a low shear strength of 10-20 kPa [15]. The organic material binds water, typically increasing the water content above 100%. In the early 1990s, the method of timber piled embankments had an upswing as a competitive measure for reducing settlements along the northern coast of Sweden. A simple and cost-effective foundation method was needed to counter the sulphide soil's high compressibility and its impracticality to handle, considering that the soil oxidises if it is excavated and placed above the groundwater table. Since sulphide soil is environmentally hazardous to excavate and the large coastal forests provide a renewable source of the softwoods Scots pine (_Pinus sylvetris_) and Norway spruce (_Pinus resinosa_), timber pile embankments proved to be one of the most sustainable foundation methods for roads and railways in the area. Figure 2: Illustration of the load distribution in a geosynthetic-reinforced pile-supported embankment (GRPSE). The light embankment pile groups contain numerous piles. According to the current Swedish design criteria for road and railway construction, TK Geo 13 [16], centre-to-centre pile spacing (\(s\)) should be 0.8-1.2 m for lightly piled embankments. For a two-lane road (16 m wide embankment at the ground) this would amount to 9000-20,000 piles per kilometre. There is an interest in the possibility of reducing the number of piles to save natural and economical resources. A main concern, however, is whether the piles should be installed in a square or a triangular arrangement (Figure 3). TK Geo 13 states that a triangular pile arrangement should be used, instead of the square pile arrangement prescribed in the previous standard TK Geo 11 [17], as the shorter diagonal distance between the piles results in more stable arches. The diagonal distance between the piles (\(s_{d}\)) in the square pile arrangement, \(s_{d}=\sqrt{2}\cdot\)s, is longer than the longest distance between two piles in the triangular pile arrangement, \(s_{d}=\sqrt{s^{2}+\left(s/2\right)^{2}}=\sqrt{1.25}\cdot s\). There is, however, a lack of research supporting this statement, and the question also involves the optimum value of \(s\). However, the tributary area \(A_{s}\) is equal to \(s^{2}\) for both the studied square and triangular pile arrangement, as the triangular arrangement is comprised of isosceles triangles. [PERSON] and [PERSON] [12] and [PERSON] and [PERSON] [13] studied triangular arrangements comprised of equilateral triangles, for which \(A_{s}=\sqrt{3}/2\cdot s^{2}\). A geotextile filter fabric can be fitted along the timber piles to increase the vertical drainage of the less permeable subsoil [18]. For this purpose, a triangular arrangement is preferrable as it provides a more uniform consolidation between drains than a square arrangement [19]. Timber is highly suitable as a pile material due to its high strength-to-weight ratio. The piles can be easily handled and trimmed to a preferred length. In comparison with steel and concrete, however, a timber pile risks fracturing or brooming (splitting at the toe) due its low strength if driven in too hard during installation. To avoid this, the piles are either driven with a low practical refusal blow count limit or, if possible, pushed down to a known firmer soil layer, which is most often a glacial till. The firm soil layer should allow the pile group to settle to some extent, so that there is enough mobilised shaft friction to maintain a lower resulting force at the toe. This keeps the timber from fracturing or brooming.