Kajian tahanan gesek pada interface tanah lempung tiang meruncing dengan tegangan vertikal yang divariasikan

JATMOKO, Heru Dwi, Dr.Ir. Hary Cristady Hardiyatmo, M.Eng.,DEA

2006 | Tesis | S2 Teknik Sipil

Tiang pada tanah kohesif mengandalkan lekatan dalam mendukung beban. Secara umum perilaku tanah kohesif sangat dipengaruhi oleh kadar air dan kepadatanya. Keruncingan tiang adalah parameter yang belum banyak dipelajari terhadap peningkatan kapasitas dukungnya. Perilaku distribusi gaya yang terjadi disepanjang tiang merupakan satu hal yang penting. Atas dasar inilah penelitian ini dilakukan. Penelitian dilakukan dengan tiang berukuran kecil berbentuk meruncing dan seragam sebagai pembanding. Ukuran tiang meruncing diameter atas 5 cm dan bawah 4 cm sedang untuk tiang seragam berdiameter luar 4,5 cm masingmasing dengan panjang efektif 20 cm serta ketebalan tiang 0,3 cm. Sebagai batasan tanah uji pengujian dilakukan di dalam mould dengan diameter 30 cm. Untuk mengetahui perilaku distribusi gaya maka dipasang strain gauge pada tiang dan mould dengan 3 posisi. Media tanah digunakan tanah asli, lempung tak organik dengan plastisitas tinggi (CH) yang diambil dari Desa Kedung Sari, Sentolo, Kulonprogo, DIY, yang dijenuhkan pada berat volume kering, γ d = 10,9 kN/m3 dan kadar air, w = 52%. Untuk mengkondisikan kedalaman tiang tanah diberikan tekanan vertikal, σ v sebesar 163 kN/m2, 326 kN/m2 dan 489 kN/m2. Setelah tiang diposisikan dan tanah dimasukan dalam mould pengujian geser dilakukan sampai runtuh. Analisa dilakukan tentang perilaku distribusi gaya, tahanan gesek, displacement, faktor keruncingan serta sudut gesek interface. Hasil pengujian menunjukan dengan bertambahnya takanan vertikal, confining pressure akan naik, pada tiang seragam diperoleh tekanan confining pressure lebih besar dari tiang meruncing. Saat uji geser (P) akan terjadi peningkatan confining pressure, walaupun pada awalnya cofining pressure pada tiang meruncing lebih kecil namun dengan bertambahnya gaya gesek (P) akan terjadi peningkatan lebih besar sehingga melebihi confining pressure pada tiang seragam, hal ini terjadi karena fenomena pemampatan akibat keruncingan tiang. Pada tiang meruncing pada tekanan vertikal 163 kN/m2, 326 kN/m2 dan 489 kN/m2 akan runtuh pada beban 90 kg, 100 kg dan 110 kg, sedang pada tiang seragam 90 kg. Tinjauan terhadap faktor keruncingan, Fw diperoleh hasil pada tekanan vertikal 163 kN/m2, 326 kN/m2 dan 489 kN/m2 akan diperoleh nilai Fw = 1; 1,125 dan 1,25. Pada tiang seragam dan meruncing diperoleh nilai sudut gesek interface, δ sangat kecil bahkan mendekati nol.

Pile foundation in cohesive soil, receives the majority of its support by adhesion from the soil along its shaft. Generally, the behaviour of cohesive soil strongly influenced by its water content and density. The “tappered” shape of a pile is a parameter that haven’t been studied well, according to its ability for improving the bearing capacity of soil. The behaviour of force distribution along a pile foundation is an important thing. Based on this fact, the research was conducted. The research was conducted with a little steel pile, “tappered” on its shape, and another steel pile, with “uniform” shape, as a comparison. The tappered steel pile, has 5 cm on its upper diameter and 4 cm on bottom diameter, whereas another pile with uniform shape has 4.5 cm on its diameter. Both of piles have 20 cm effective length, with 0.3 cm thickness. The research was performed in a mould with 30 cm of diameter. To find out the behaviour of force distribution, the strain gauges were placed on the pile and mould in 3 different positions. The soil used in this research was non-organic, high plasticity clay (CH), which was taken from Desa Kedung Sari, Sentolo, Kulon Progo, DIY, tested in saturated condition, with the dry density, γd=10.9 kN/m3, and water content, w=52%. In order to simulated the pile depth condition, given some vertikal pressures, σv = 163 kN/m2, 326 kN/m2, and 489 kN/m2. After the pile was positioned and the soil was poured in mould, a shear test was conducted until failure and analysis was performed on its force distribution, friction resistance, displacement, tappered factor and the angel of interface friction. The result indicated that with the increasing in vertical pressure, there was also an increasing in confining pressure. The confining pressure on uniform pile was bigger than which was occured on tappered pile. When the shear test (P) was conducted, the confining pressure was increases, although at the beginning the confining pressure which was occured on tappered pile was smaller, but during the increment of shear force (P), the confining pressure on tappered pile increase and the increase took place moreover compared to that occured on uniform pile. It occured because of the solidity phenomenon caused by tappered pile. The tappered pile with vertikal pressures σv = 163 kN/m2, 326 kN/m2, and 489 kN/m2 , reached the failure condition at 90 kg, 100 kg, and 110 kg of loads, whereas the uniform pile reached the failure at 90 kg. Review on the tappered factor (Fw), for vertikal pressures 163 kN/m2, 326 kN/m2, and 489 kN/m2, we obtained Fw = 1, 1.125, 1.25 respectively. Uniform pile and tappered pile has the angel of interface friction, δ unit very small that can be neglected .

Kata Kunci : Perencanaan Fondasi,Tanah Lempung,Tiang Meruncing,Confining pressure, friction unit, cohesive soil, tappered pile, interface