Drained and Undrained Soil Condition

21 04 2013

setelah membaca beberapa literatur mengenai kondisi tanah drained dan undrained, dimana kondisi undrained-lah yang sering digunakan dalam perancangan desain, prinsip mengenai kondisi ini cukup kritikal, sebab secara bahasa pengertian drained = dry or emptied (id : kering), undrained : not dry or not emptied, tetapi pengertian tersebut tidak mengambarkan apa yang dimaksudkan dalam mekanika tanah.

definisi drained and undrained dalam mekanika tanah terkait dengan kecepatan air masuk / keluar dengan waktu terhadap tanah yang diberikan beban (ini pengertian menurut saya, dari bukunya M. Duncan),

menurut M. Duncan di dalam bukunya Soil Strength and Slope Stability Chapter 3 Page 19, di jelaskan pengertiannya sebagai berikut :

Drained is the condition under which water is able to flow into or out of a mass of soil in the length of time that the soil is subjected to some change in load. Under drained conditions, changes in the loads on the soil do not cause changes in the water pressure in the voids in the soil, because the water can move in or out of the soil freely when the volume of voids increases or decreases in response to the changing loads.

Undrained is the condition under which there is no flow of water into or out of a mass of soil in the length of time that the soil is subjected to some change in load. Changes in the loads on the soil cause changes in the water pressure in the voids, because the water cannot move in or out in response to the tendency for the volume of voids to change.

berikut penjelasan/kesimpulan terkait drained and undrained

  • The difference between undrained and drained conditions is time.
  • Undrained signifies a condition where changes in loads occur more rapidly than water can flow in or out of the soil. The pore pressures increase or decrease in response to the changes in loads.
  • Drained signifies a condition where changes in load are slow enough, or remain in place long enough, so that water is able to flow in or out of the soil, permitting the soil to reach a state of equilibrium with regard to water flow. The pore pressures in the drained condition are controlled by the hydraulic boundary conditions, and are unaffected by the changes in loads.

source : Soil Strength and Slope Stability_Michael Duncan, Chapter 3.

Cimone, April 21 2013

[update 25 Juni 2013]

masih membahas perilaku tanah pada saat drained dan undrained,

sepemahaman saya sampai saat ini, drained condition itu menggunakan analisa effective stress dimana tekanan air pori diketahui kondisi tanahnya yaitu memungkinkan air terdrainse atau sifat tanahnya permebialitas (umumnya pasir), sementara undrained condition dalam analisanya menggunakan total stress (sigma t) dimana total stress adalah effective strees (sigma ‘) + tekanan air pori (u).

umumnya dalam analisa geoteknik dipilih kondisi mana yang akan di pilih dalam analisa ada beberapa kriteria yang menentukannya, dalam desain deep excavation, beberapa ahli merekomendasikan seperti ini :

Suggestion by Vermeer & Meier (1998) for deep excavations:

T < 0.10 (U < 10%) use undrained analysis
T > 0.40 (U > 70%) use drained analysis

formula :
t= (k E oed/gw D^2 ) x t

dimana : k = Permeability, Eoed = Oedometric modulus = 1/mv, gw = Unit weight of water, D = Drainage length,t = Construction time
T = Dimensionless time factor, and U = Degree of consolidation.

lalu pertanyaan berikutnya Dalam kondisi apa tanah berada pada keadaan kritis?

jawabannya ada di buku [Atkinson]_Mechanics_Soil_and_Foundations_2nd Ed_2007, saya sedang masih mempelajarinya.

1st update (25 Juni 2013)


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