Geotechnical Mapping of Najran Soils for Safe Urban Expansion, Najran Region, Saudi Arabia

One hundred eighty-three rotary-drilled boreholes from fifteen districts in Najran area, Kingdom of Saudi Arabia, were sampled and tested in the field and in laboratory to analyze the spatial variation in their geotechnical properties. Number of samples varied with the performed test including grain size distribution (668), Atterberg limits (24), moisture content (432), free swelling (49), direct shear strength (51), unconfined compression strength (37), and soil water chemistry of 168 samples were analyzed for Cl⁻, SO₃⁻², and CO₃⁻), and 120 samples for pH. Descriptive statistics were appraised for clarifying the geotechnical variations among the districts and various AASHTO-classified soil types. GIS-based geostatistical kriging with minimum error achieved was used for preparing the spatial maps of the geotechnical parameters. Results showed that majority of soil samples were non-plastic. Hubuna district showed the larger LL (42%), PL (25%), and PI (20%) while Mefraq Al-Khamis recorded the lowest values of LL (23%), PL (19%), and PI (2%). Atterberg limits showed a northwestward increase to reach its maxima at Thar. Moisture content was larget in Khobash and showed local highs at Najran Al-Fahd and Al-Mashaliya.Fine sands prevailed north and east of Abu Shadad with local highs in Al-Mashaliya and Najran University. Silt and clay soils dominate in Najran Al-Fahd district. Local highs of free swelling were vivid at Abu Shadad, Khobash, Mefraq Al-Khamis, and Najran Al-Fahd districts. UCS showed largest in Najran University. Shear test friction angle showed lowest at Khobash and Abu Shadad districts dominated by coarse to medium sands and gravels and largest of 35° in Najran University and extends to Thar attaining the largest fine sand and gravel contents. Largest Cl⁻ and SO₃⁻² contents characterized the districts of Thar, Al-Hosayniya and Hubuna while CO₃⁻ showed local high at Najran Al-Fahd district, the soils were acidic with pH lower than 5.5. Three main soil types according to AASHTO classification formed about 91% of the samples: A-1-b (56%), A-2-4 (23.5%), and A-3 (11.4%), were mapped and their geotechnical properties were characterized. Results can help efficient decision-making process to set proper management zones and prevent geotechnical risks where measures in the design and construction stages can be decided for future expansions and subsurface tunneling based on the district- and soil type-specific.