A dilatometer test was conducted in a sand deposit at a depth of 6 m. The groundwater table was located at a depth of 2 m below the ground surface. Given, for the sand: γ_d = 14.5 kN/m³ and γ_sat = 19.8 kN/m³. The contact stress during the test was 260 kN/m². Estimate the soil friction angle, ϕ'.