SAND DRAIN DAN PVD YULVI

SAND DRAIN Tujuan dibuatnya sand drain adalah untuk mempercepat penurunan akibat konsolidasi dari tanah lunak jenuh dengan memperpendek jalan air Konstruksi sand drain dibuat dengan membuat lubang bor sampai dengan lapisan kedap air. Lubang bor kemudian diisi dengan agregat kasar. Sand drain dengan ujung lapisan kedap air dianggap “half closed drain”. Bila lapisan ujungnya merupan lapisan tembus air maka air akan mengalir kedua arah,

Lateral drainage Sand drained Fill Impervious Settlement profile After consolidation s s 2R R=0,53s rd R=0,56s 2R Pola segitiga Pola segiempat

Sand drain Diameter sand drain berkisar 200 – 600 mm Jarak antar sand drain 2m – 5m pola segiempat maupun segitiga. Jari- jari pengaruh dinyatakan dalam R Konsolidasi terjadi dalam 2 arah yaitu.. Arah drainase vertical dan radial Persamaan diferensial parsial arah radian dinyatakan: 𝜕𝑢 𝜕𝑡 =𝐶ℎ 𝜕 2 𝑢 𝜕 𝑟 2 + 1 𝑟 𝜕𝑢 𝜕𝑟

PDP Persamaan diferensial parsial arah radian dinyatakan: 𝜕𝑢 𝜕𝑡 =𝐶ℎ 𝜕 2 𝑢 𝜕 𝑟 2 + 1 𝑟 𝜕𝑢 𝜕𝑟 Kondisi Batas: Pada r=rd: u=0 ketika t>0 r=R ; du/dr =0 Kondisi awal t =0 : u=u0

Solusi PDParsial Kasus free strain : beban terdistribusi merata (pondasi fleksible) yang menghasilkan penurunan yg tidak rata. Kasus timbunan tanah di atas tanah ekspansif. Kasus equal strain: bila permukaan penuruan dibuat merata (pondasi kaku) dan menghasilkan beban yang tidak merata. Richart (1959) mengatakan bahwa beda kedua kasusnya kecil dan solusi utk kasus equal strain sering digunakan dalam pelaksanaan.

Time Vactor Time vactor for consolidation in radial direction 𝑇 𝑟 = 𝐶 ℎ 𝑡 4 𝑅 2 The permeability of soil in the horizontal or radial is sometime much greater (2 to 10 times) for many soils) than in vertical direction. Consequently Ch/Cv  1,2 to 2 During drilling of the borehole instalation of the drain, a thin layer of the soil at the interface of the drain is often remolded. This is thin layer names a smear zone in which Cv and Ch are often much lower

Average Degree of consolidation vertical and horizontal The average degree Uvr dissipation of pore water pressure 𝑈 𝑣𝑟 =1− 1−𝑈 1− 𝑈 𝑟 U is everage degree of consolidation for vertical drainage

Problem A foundation for structure is to be constructed on a soft deposit of clay. Below the soft clay is a stiff overconsolidated clay 20 m thick. The calculated settlement can not be tolerated and it was decided that the soil should be preconsolidated by an embankment equivalent to the building load. The available data are: Cv= 6 m2/yr; Ch=10m2/yr, mv=0,2m2/MN. The foundation size is10m x 10m and qs=400kPa. The drain diameter is 300 mm. Determine is spacing of a square grid of the sand drains to achieve 90% consolidation in 4 month.

Calculated the primary consolidation Calculated the primary consolidation. Determine the average increase in applied vertical stress at mid-depth of soil under the center of the foundation B/2= 10/2=5m; L/2=5m; z= 20/2=10m m= 5/10=0,5; n= 5/10=0,5……Iz=0,085 =4x 0,085 x 400= 136 kPa Sc = mv. H0 . =0,2/(103)x 20 x136=0,544m Determine the required degree of consolidation Uar=90% 0,9 x 544 mm= 489,6 mm Determine TV, The drainage is one way (stiff clay layer at bottom is assumed impervious) 𝑇 𝑣 = 𝐶 𝑣 𝑡 𝐻 2 = 6 𝑥 4 12 20 2 =0,005

Determine U From Figure U=8% for Tv= 0,005 Determine Ur; Uvr=90% 𝑈 𝑣𝑟 =1− 1−𝑈 1− 𝑈 𝑟 Ur=0,89 Determine Tr; n=R/rd 𝑇 𝑟 = 𝐶 ℎ 𝑡 4 (𝑛 𝑟 𝑑 ) 2 = 37 𝑛 2 Determine n try n=5 Tr= 1,48….Fig…slide 7…..Ur =1 > 0,89 try n=10 Tr=0,37……Ur= 0.85 <0.89 Use n=10 Calculate the spacing s= 1,8 n rd =2,7

Mochtar (2012)

Pola pemasangan dan jarak PVD sama dengan Sand drained Derajat konsolidasi dan Th ( semua vertical drained)

Kenaikan Daya Dukung karena vertical drained