## 2 Geological Conditions and Geotechnical Engineering Issues of Sabkha Deposit  
Several in-situ tests, including borings, test pits, electrical resistivity tomography, crosshole seismic tests, and cone penetration tests (CPT), were carried out in the study area. The sabkha sediments are predominantly sandy soil classified mainly as silty sand (SM), poorly graded sand with silt (SP-SM), or poorly graded sand (SP). The sabkha deposit in the study area is composed of a medium dense sand and silt layer with a depth of 1-2 mm overlying loose to very loose sabkha deposit extending down to a depth of 7.5-10.5 m. Thin layers of fine-grained soil are also found intermittently at around GL-7 m.  
Figure 1 shows a typical grain size distribution curve of the sabkha deposits. Generally, sabkha soil is poorly graded fine sand with 5%-15% of fines, and thin silty sand layer with more than 40% of fines is occasionally found at a depth of 6-7 m. Sabkha does not possess any plasticity, and the soil is classified as nonplastic. Laboratory compaction and California Bearing Ratio (CBR) tests were conducted on selected soil samples. Table 1 indicates compaction and CBR characteristics, and Figure 2 shows a typical compaction curve. The groundwater table was measured at least 24 hours after borehole drilling to measure the stabilized groundwater level. The groundwater table was observed at about 0.5-1.5 m below ground surface in boreholes and test pits, and perched groundwater table, which seems to have been collected on top of the impervious clay layer, was observed in some boreholes. It should also be noted that the groundwater level may experience seasonal and tidal fluctuations. \begin{table}
\begin{tabular}{c c c c} \hline
**Parameter** & **Maximum Value** & **Minimum Value** & **Average** \\ \hline CBR (soaked) @ 95\% of MDD (\%) & 14.50 & 6.55 & 10.5 \\ Max. dry density (g/cm\({}^{3}\)) & 1.99 & 1.83 & 1.89 \\ OMC (\%) & 15.80 & 9.70 & 12.80 \\ Field dry density (g/cm\({}^{3}\)) & 1.88 & 1.58 & - \\ Degree of compaction \% & 97.80 & 83.60 & - \\ \hline \end{tabular}
\end{table} 
Table 1: Compaction and California Bearing Ratio (CBR) characteristics of sabkha deposits. Figure 1: Sieve analysis results of sabkha soil. High groundwater level, high compressibility, and presence of dissoluble materials in sabkha soil are likely to cause various geotechnical and construction problems during the construction and operation of railways in the sabkha deposit. The precipitation of salts below the groundwater table is ascribed to the increase in the salt concentration above its saturation limit. The possible construction problems that can occur during railway construction are summarized as follows:  
* excessive total and differential settlements due to varying compactness of sabkha deposit;
* high collapse potential because of dissolution of sodium chloride, leaching of calcium ions, and soil grain rearrangement;
* possible long-term settlement;
* trafficability problem due to saturation and loss of strength in sabkha sites; and
* corrosion of steel and deterioration of concrete because of highly concentrated chloride and sulfate salts present in sabkha sediments. The top layers of sabkha may exhibit firm and stiff characteristics in its dry state. However, when dampened with water, the strength is highly reduced as the cementing salts are susceptible to leaching and dissolution or softening, thereby leading to rapid settlement in wet conditions. Therefore, ground improvement or ground reinforcement is required in order to remove the problem of unacceptable future long-term settlement. [PERSON] et al. [1] carried out a series of large-scale direct shear tests on the shear strength characteristics of reinforced sand-clay soil mixed with natural fibers, and it was found that the shear strength parameters of the reinforced sand-clay mixture were significantly improved. [PERSON] et al.