Step perhitungan desain PVD
2. Step perhitungan desain PVD
Alur perhitungan pendesainan PVD adalah sebagai berikut : Menghitung besarnya konsolidasi tanah dasar sebelum dilakukan pemasangan
PVD. Menghitung waktu terjadinya konsolidasi (t) sebelum dilakukan pemasangan
PVD. Perhitungan nilai t sesuai dengan hasil breakdown persamaan 21. Biasanya waktu terjadinya konsolidasi tanpa adanya PVD adalah sangat lama hingga mencapai ratusan tahun tergantung nilai Cv pada tanah. Apabila tanah dasarnya berlapis dan nilai Cv pada setiap lapisan adalah berbeda maka perlu dilakukan perhitungan nilai Cv gabungan dengan rumusan sebagai berikut :
(23) rata rata
h
Menghitung panjang kedalaman pemasangan PVD sesuai dengan umur rencana
dengan melakukan perhitungan nilai rate of settlement. Nilai rate of settlement yang diharapkan adalah 1.5 cm/tahun. Perhitungan nilai ini dilakukan dengan menghitung nilai Tv, Uv dan settlement pada waktu yang kita inginkan. Contoh perhitungan dapat dilihat pada tabel berikut.
RATE OF SETTLEMENT Total
TEBAL LAPISAN TERKONSOLIDASI
SETTLEMENT AKIBAT TIMBUNAN
SETELAH 20 TAHUN
sett(m) ( cm/ tahun ) (m)
PVD(m)
settlemen(m)
Berdasarkan perhitungan nilai pemampatan diatas kemudian dibuat grafik nilai rate of consolidation sesuai dengan gambar berikut.
hub.kedalaman pemasangan PVD dng Rate of settlement lap. tanah
di bawah PVD
Kedalaman pemasangan PVD (m)
Berdasarkan grafik diatas, nilai rate of settlement 1.5 cm/tahun diperoleh dengan pemasangan PVD sedalam kira-kira 15-16 meter.
Menghitung jarak pemasangan PVD. Tentukan dimensi dari PVD dan pola pemasangan kemudian hitung nilai De, F(n) dan D dengan jarak yang bervariasi. Hitung nilai Urata-rata pada masing- masing jarak pemasangan PVD. Rumus perhitungan nilai Uh dan contoh perhitungan nilai Urata-rata dapat dilihat pada persamaan dibawah.
Tabel nilai F(n) pada masing-masing jarak pemasangan. Jarak PVD
D a b Dw
F (n)
S (m) (mm)
Tabel perhitungan nilai U rata-rata pada jarak antar PVD (S) = 0.8 meter
Untuk S =
U total
Berdasarkan data pada tabel U rata-rata tersebut kemudian dibuat grafik hubungan antara nilai U rata-rata (Derajat konsolidasi gabungan) dengan T (waktu).
b 70 s=0.8
T(minggu)
Berdasarkan grafik diatas, dapat ditentukan jarak pemasangan PVD yang dapat menghasilkan nilai derajat konsolidasi gabungan mendekati nilai 100% atau paling tidak diatas 90%. Keputusan pemilihan jarak pemasangan antar PVD tersebut tergantung perencana dilihat dari factor biaya dan factor efektifitas pemasangan.
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