Tanah adalah lapisan permukaan bumi yang berasal dari material induk yang telah mengalami proses lanjut karena perubahan alami di bawah pengaruh air, udara dan organisme baik yang masih hidup maupun yang telah mati. luasnya area tanah lempung lunak menjadi salah satu masalah besar dalam pengembangan infrastruktur di kota tarakan. hal ini disebabkan sifat fisik dan karakteristik tanah lempung yang sangat lunak dan tidak stabil. metode yang digunakan adalah metode perbaikan tanah iyalah metode stabilisasi menggunakan limbah biji plastik dan fly ash. penelitian ini bertujuan untuk mengetahui nilai maksimal, peningkatan stabilizer yang terjadi dan sifat fisis maupun sifat mekanis tanah sebelum maupun sesudah distabilisasi terhadap masa peram 14 hari, 21 hari dan 28 hari. pengujian sifat fisis yaitu pengujian kadar air, uji berat jenis, analisa saringan, uji atterberg limit (batas cair dan batas plastis). sifat mekanis yaitu pengujian pemadatan dan uct (uncofined compression test). penambahan biji plastik sebesar 5%, 10%, 15% dan fly ash sebesar 10% dari berat tanah. hasil pengujian sifat mekanis pada nilai rata-rata kuat tekan bebas tanah asli sebesar 0,297 kg/cm2. nilai rata-rata kuat tekan bebas terbesar tanah asli yang telah di stabilizer 15% masa peram 28 hari sebesar 1,39 kg/cm2 dan nilai terkecil 25% masa peram 14 hari sebesar 0,54 kg/cm2. peningkatan maksimal yang terjadi pada tanah sebesar 1,09 kg/cm2 dan peningkatan minimal sebesar 0,26 kg/cm2. pengujian sifat fisis dan nilai kuat tekan bebas menunjukkan pengaruh paling maksimal terjadi pada massa peram 28 hari, hasil ini menunjukkan bahwa semakin lama masa peram maka semakin optimal terjadinya peningkatan pada nilai kuat tekan bebas. kata kunci: sifat fisis, uct, tanah lempung lunak, stabilisasi, biji plastik, fly ash

Soil is the surface layer of the earth that originates from parent material that has undergone significant changes due to natural processes influenced by water, air, and living and decaying organisms. the extensive area of soft clay soil poses a significant challenge to infrastructure development in tarakan city due to the physical properties and characteristics of the very soft and unstable clay. this study's method used for soil improvement is stabilization using plastic seed waste and fly ash. this research aims to determine the maximum values, the increase in stabilizer effects, and the physical and mechanical properties of the soil before and after stabilization over curing periods of 14 days, 21 days, and 28 days. the physical property tests include moisture content, specific gravity, sieve analysis, and atterberg limits (liquid and plastic limits). the mechanical properties assessed include compaction and unconfined compression test (uct). plastic seed waste was added at rates of 5%, 10%, and 15%, along with 10% fly ash by weight of the soil. the mechanical property test results indicated that the original soil's average unconfined compressive strength was 0.297 kg/cm². the highest average unconfined compressive strength of the stabilized soil with 15% stabilizer after 28 days of curing was 1.39 kg/cm², while the lowest was 0.54 kg/cm² after 14 days with 25% stabilizer. the maximum increase in soil strength was 1.09 kg/cm², and the minimum increase was 0.26 kg/cm². the tests on physical properties and unconfined compressive strength show that the most significant improvements occur at the 28-day curing period, indicating that longer curing times lead to more optimal increases in unconfined compressive strength. keywords: physical properties, uct, soft clay soil, stabilization, plastic waste, fly ash