Constant Rate of Strain (CRS) Test

Experimental Setup:

1. 1. Triaxial loading frame
      
   2. Load cell
      
   3. LVDT
      
   4. Piston
      
   5. Triaxial cell
      
   6. CRS setup

               a) CRS base arrangement
               b) Consolidation ring
               c) Top Platen
               d) Sample Holder
               e) Bolts and Washers

   7. Pore pressure transducer

Testing procedure (ASTM D 4186-06):

• The soil specimen with consolidation ring is pushed out of the UDS tube (undisturbed) with the help of sample extruder and extra soil is trimmed off. The top and bottom surfaces of the soil specimen in the consolidation ring are levelled with a cutting tool and the outer surface of the consolidation ring is cleaned. The CRS setup is then assembled.
  
• The assembled CRS setup is placed inside the confining cell, which is then filled with water. A cell pressure of 100 to 200 kPa is generally applied to saturate the soil specimen.
  
• Axial load is applied on the CRS specimen at appropriate strain rate using triaxial loading frame. Data acquisition system records the axial load, axial displacement and pore pressure values at the predetermined logging interval.
  
• After finishing the loading phase, unloading phase is started (removal of load) at a strain rate 2-3 times lower than that during initial loading. It is important to find e-logσ curves for loading and unloading phases of the test. The compression (C_c) and re-compression index (C_r) are calculated using loading and unloading curves, respectively.
  
• According to ASTM D 4186-06, the selection of strain rate is based on pore-water pressure ratio. The strain rate should be chosen in a manner that pore-water pressure ratio during the CRS test should lie in the range of 3% to 30%. Pore-water pressure ratio is the ratio of base excess pore water pressure to the total axial stress.

Constant Rate of Strain (CRS) Test

Observation Sheet:

Weight of Sample:___________________ In-situ Density: __________________
Initial Water Content(w_i):_______________Initial void ratio (e₀):_______________
Initial height of the sample (H₀):___________ Specific gravity (G_s):_______________.
Diameter of specimen (D₀) = _____________ Area of specimen (A₀) = _____________
Cell pressure (kPa):__________________
Loading Strain rate:_ _________________ Unloading Strain rate:_ _________________

Calculations:

• Excess base pore pressure (u_b) = Pore pressure (u) – Initial pore pressure (u₀)
  
• (1 kPa = 1 kN/m²)

• Sample height at a given time (H_n)= Initial height (H₀)- axial displacement
  
• H_n = H₀ – ΔH

• Height of solids (H_S):
  
• H_S = W_S/(G_S.γ_w.A₀), where γ_w is unit wt of water (10 kN/m³)

• Void ratio at given time (e_n):
  
• e_n = (H₀ -H_S)/H_S

• Axial strain at given time (e_n):
  
• ε_n = (ΔH /H₀)x100

• Total Axial stress (σ_n):
  
• σ_n (kPa) = Axial load/A₀

• Effective Axial stress (σ’_n):
  
• σ’_n (kPa) = σ_n – 0.677. u_b

Constant Rate of Strain (CRS) Test

Graphs:

• Void Ratio versus Effective Axial Stress curve for Loading
  
• Void Ratio versus Effective Axial Stress curve for Unloading
  
• Base pore pressure versus Time for Loading and Unloading

Example:

CRS test was performed on soft marine clayey soil. Specimen was consolidated under 0.005mm/min strain rate during loading and 0.0025 mm/min strain rate during unloading. Cell pressure was kept to be 100 kPa. This loading condition showed a pore-water pressure ratio of 14% in soil’s normally consolidated range. As per the ASTM code, the pore pressure ratio for CRS test should lie in the range of 3% to 30% for normally consolidated state.

Information about soil specimen:
D₀ = 75 mm
H₀= 25 mm
G_s = 2.77
B value = 0.95 (saturation = 95%)
e₀ = 1.95
Initial water content (w_i) = 70%
Final water content (after CRS consolidation) = 64%

Results:

Compression index (C_c) = 0.72 (average of both the tests)
Re-compression index (C_r) = 0.13 (average of both the tests)

General Remarks:

• Unloading strain rate must be half or less than half of the loading strain rate for proper pore pressure dissipation while consolidation load is subjected to the CRS specimen.
  
• This test assumes a high permeability porous disk is used in the base pressure measurement system. Use of low permeability porous disk or high air entry (greater than 1 atm) disk will require modification of the equipment.

Constant Rate of Strain (CRS) Test

Theory:

In CRS test, the specimen is sandwiched axially between two parallel, rigid platens and laterally constrained such that cross-sectional area remains constant. Drainage is provided along one boundary (typically at top) and the water pressure is measured at the other sealed boundary (typically at base). A back pressure is applied to saturate both the specimen and the base pressure measurement system. The specimen is deformed axially at a constant rate while measuring the time, axial displacement, load and base pressure. CRS consolidation test typically includes one loading phase and one unloading phase. the rate of deformation is selected to produce a pore pressure ratio in between 3% to 15% at the end of the loading phase. During loading and unloading, the measurements are first evaluated in order to be sure transient effects are small. Steady state equations are then used to compute the one-dimensional stress versus strain relationship. During the loading phase, when base excess pressures are significant, the measurements are used to compute both the coefficient of consolidation and permeability.

CRS test is very useful for thick clays as it is a quick test as compared to conventional 1-D consolidation test takes more than a week to perform the test.

The information concerning magnitude of compression and rate of consolidation of soil is essential in the design of earth structures and earth supported structures. The results of this test can be used to estimate one-dimensional settlement, rate of settlement associated with the dissipation of excess pore water pressure, and rate of water transport due to hydraulic gradients. This test cannot be used measure the properties of partially saturated soils because it requires the soil to be saturated (by applying back pressure) prior to consolidation.