Pengaruh Strength Reduction Tanah Clay-Shale Akibat Pelaksanaan Pemboran Terhadap Nilai Daya Dukung Pondasi Tiang di Jembatan Suramadu Berdasarkan Analisis Hasil Tes OC

Masyhur Irsyam, Andhika Sahadewa, Atyanto Boesono, Soebagyo Soebagyo

Abstract


Abstrak. Jembatan Suramadu yang akan menghubungkan kota Surabaya dengan Madura memiliki bentang total sepanjang 5.438 m. Mengingat lokasi pembangunan yang berada di laut dan beban rencana yang besar pada tiang pondasi, maka uji pembebanan statik konvensional tidak mungkin untuk dilakukan di jembatan utama. Alternatif tes pembebanan tiang yang dapat dilakukan adalah Tes OC yang mengikuti  prosedur Osterberg Cell Test. Tes ini dilengkapi dengan load cell, telltale, dan strain gauge sehingga dapat diperoleh kurva load-displacement untuk tahanan ujung dan tahanan selimut pada berbagai kedalaman. Hasil penyelidikan tanah menunjukkan keberadaan lapisan tanah clay-shale yang mengandung montmorillonite pada lokasi tiang bor. Jenis tanah ini sensitif terhadap berkurangnya tegangan lateral akibat pemboran dan terhadap weathering process sehingga dapat terjadi strength reduction. Strength reduction tanah menyulitkan predikasi tahanan ujung dan tahanan selimut pondasi tiang bor. Hasil Tes OC digunakan sebagai acuan dalam menentukan pengaruh strength reduction terhadap tahanan ujung dan tahanan selimut dengan cara membandingkan hasil pengukuran dengan hasil prediksi berdasarkan korelasi empiris yang umum dipakai. Untuk tahanan ujung, hasil Tes OC menunjukkan harga yang relatif dekat dengan hasil prediksi sehingga strength reduction untuk tahanan ujung akibat pemboran relatif kecil selama kondisi dasar pondasi bersih. Sedangkan untuk tahanan selimut, hasil pengukuran nilainya jauh lebih kecil dibanding hasil prediksi. Untuk meningkatkan daya dukung, maka dilakukan grouting melalui ujung tiang. Hasil pengujian menunjukkan bahwa setelah dilakukan grouting pada pondasi terjadi kenaikan nilai tahanan ujung dan tahanan selimut. Dibandingkan dengan tahanan ujung, tahanan selimut memiliki peningkatan kekuatan yang lebih besar dan setelah dilakukan grouting nilainya mendekati nilai prediksi.

Abstract. Suramadu Bridge, which will connect Surabaya and Madura, is designed to have 5,438 m total span. Considering bridge construction that is above sea and huge design load on its pile foundations, conventional static loading test can not be performed at the main bridge. Alternative pile load test that can be performed is OC Test, which followed Osterberg Cell Test procedure. Equipped with load cell, telltale, and strain gauge, this test can provide load-displacement curve of end bearing and skin friction at any depth. Soil investigation result shows existence of clay-shale layer, which contains montmorillonite, at bore pile’s location. This soil is sensitive to lateral pressure reduction caused by boring and weathering process so that strength reduction can occur. Strength reduction of soil makes end bearing and skin friction difficult to be predicted. Strength reduction effect toward end bearing and skin friction are determined by means of comparing OC Test result to prediction result based on common empirical correlation. For end bearing, OC Test result shows value that close to prediction result’s. Therefore, strength reduction of end bearing caused by boring is relative small. On the other hand, skin friction measurement result shows that its value is much smaller than prediction result’s. For increasing bearing capacity, grouting through the pile base was conducted. Test result shows increasing of value either end bearing and skin friction after grouting through the pile base. Compared to end bearing’s value, skin friction’s value rise more and close to prediction value.

Keywords


Osterberg cell test; Pondasi tiang bor; Tahanan ujung; Tahanan selimut; Clay-shale; Strength reduction; Grouting.

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DOI: http://dx.doi.org/10.5614%2Fjts.2007.14.2.1

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