Physical Characteristics of Expansive Clay Soil of the Bobonaro Formation at Km 68 Takari, Kupang Regency
Keywords:
Bobonaro formation, Expansive soil, Index properties, Moisture content, Swelling potentialAbstract
Periodic damage to road infrastructure on Timor Island, particularly along the Takari Highway in Kupang Regency, is generally triggered by the poor geotechnical characteristics of the subgrade. This study aims to isolate and analyze the basic physical characteristics and initial swelling potential of the expansive clay soil from the Bobonaro Formation at KM 68 Takari. The laboratory experimental method was conducted to test soil index properties based on SNI standards, followed by swell testing at five variations of initial moisture content (17.05% to 18.85%) using two compaction energy levels (35 and 65 blows). The physical results show that the soil is classified as CH (Fat Clay) according to USCS and A-7-6 according to AASHTO, featuring a high plasticity index of 35.06% and a clay fraction of 47.69%. In the swell test, higher compaction energy consistently generated larger swelling percentages. This study successfully identified a critical finding in the form of a non-linear fluctuation pattern, where the maximum peak swelling of 5.77% (High category) occurred at a moisture content of 18.40% (wet side of OMC). In conclusion, conditioning at the critical moisture content of 18.40% must be avoided during field compaction to minimize the risk of pavement deformation. Contribution: This study contributes a site-specific physical and swelling characterization of the previously undocumented Bobonaro clay along the Takari corridor and identifies a novel non-linear swelling-versus-moisture pattern that departs from the conventional monotonic model reported in prior expansive soil literature. These insights provide local infrastructure practitioners and pavement designers in Kupang Regency with an actionable critical moisture threshold to avoid during subgrade compaction, directly supporting the journal's focus on practical, locally grounded civil engineering solutions.
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