LABORATORY CONSOLIDATION TEST

Transcription

1 1 LABORATORY CONSOLIDATION TEST Application Settlement problems are actually two problems in one. Both the magnitude of settlement and time rate at which this process occurs must be estimated on the basis of laboratory tests. This laboratory will focus on the time rate of settlement as it is often the most important aspect of the problem. This lab also focuses on the pressure-settlement relations. For instance, suppose it is proposed to build a highway embankment over soft soils. The embankment places a load on the soil and depending on the thickness of soft soil and size of the loaded area, the total magnitude of settlement computed may take many tens of years to occur. Thus the project must wait for the settlement to occur before construction of pavement sections can take place. There are technologies available that can speed up the rate of consolidation. These are discussed in courses on Ground Modification. Recall that a complete consolidation test involves loading a soil specimen in a series of increments. From these data, a void ratio (e) vs. log pressure curve is developed and estimates of the total magnitude of settlement are made using the parameters C c, C r and σ p. Estimates of the time rate of settlement are made using the Coefficient of Consolidation, C v, that is interpreted from plots of dial gage reading (compression) vs. log time during individual load increments of a consolidation test. C v is not a constant but varies with applied stress. As the stress level increases, the void ratio decreases and permeability decrease thereby increasing the time required for water to flow from the soil voids. Therefore, in an actual project, C v should be estimated for the actual range of effective stress expected in the field. Equipment Consolidometer set Filter paper Stop watch Balance sensitive to 0.01 lb Moisture cans Drying oven Procedure You are provided with a trimmed soil specimen. Measure height of specimen (Subtract the gap between top of the ring and specimen). Take weight of the specimen with ring.

2 Set a lower porous stone in the consolidometer. Set a filter paper on it. Then set the soil specimen into the consolidometer. Set a filter paper and upper porous stone on top of the specimen. Set the upper platen on top of the porous stone. Pour water from outer jacket and saturate the specimen for about 15 minutes. This water level should be kept higher than the porous stone during the test. Set a steel ball on top of the upper platen and the loading device on top of the ball. All equipment are set with vertical displacement transducers. Record the initial LVDT reading for your equipment. Apply load to the specimen which will give vertical stress of about 7 psi. Record the deflection dial gauge reading at 0 min, 0.5 min, 1 min, min, 4 min, 6 min, 9 min, 1 min, 0 min, 5 min, 36 min, 60 min, 10 min, 40 min, 480 min, and 1440 min. Increase the load to about 14 psi, and 8 psi, 56 psi, and 11 psi in every 4 hours and repeat the same procedure. But just take the reading at 0.5 and 1440 minute for those readings. Next Thursday, increase the load to psi and record the deflection at 0 min, 0.5 min, 1 min, min, 4 min, 6 min, 9 min, 1 min, 0 min, 5 min, 36 min, 60 min, 10 min, 40 min, 480 min, and 1440 min. After that reduce the load slowly and record deformation during unloading at the end of 11 psi, 56 psi, 8 psi, 14 psi, and 7 psi. You don t need to note down intermediate values for the vertical loads other than 7 psi, and 4 psi. For other loads, just note the initial and final deflection. Remove the soil specimen after 4 hours of the application of the vertical stress of 4 psi and measure final height of the specimen. Measure the weight of the specimen and put the entire specimen into the oven and oven-dry them for 4 hours. Measure the weight after 4 hours. That will help you to get void ratio. Calculations The lab reports shall include the following: Sample calculations. A plot of settlement versus the logarithm of time for the vertical stress of 7 psi and psi. Calculate settlement amount and time for 50% consolidation. Calculate time for 50% consolidation. A plot of settlement vs. square root of time for the vertical stress of 7 psi and psi. Calculate settlement amount and time for 50% consolidation. Calculate time for 90% consolidation.

3 Determine the coefficient of consolidation, C v from the time rate data collected in the lab. The calculations are presented below and most textbooks cover this in detail. Using the settlement data for 100% consolidation, calculate the void ratio (e) for the corresponding stresses. Plot a e-log σ' graph and find out the pre-consolidation pressure. 3 EQUATION TO BE USED H M s s = (1) AGs ρ w H H s e0 = () H s Where, H s = Height of soil solid M s = Dry mass of the specimen A = Area of the specimen G s = Specific gravity of the soil solid (take.68) ρ w = Density of water H = Initial height of the specimen = Initial void ratio e 0 For the first incremental loading, Likewise for the second load increment, e e 1 ΔH1 = e (3) 0 H s ΔH = e (4) 0 H s Shown in figure 1 is an example of e-longσ curve.

4 4 Figure 1 e-logσ curve (Source: BM Das, 006) Determination of preconsolidation pressure (σ p or P p ) Figure Method to determine pre-consolidation pressure (Source: BM Das, 006)

5 Choose by eye the point of minimum radius of curvature on the e-logσ curve (point A). Draw a horizontal line through point A. Draw a line tangent to the curve at point A. Bisect the angle made by steps and 3. Extend the straight line portion of the virgin compression curve up to intersect the bisecting line from step 4. The intersection point gives the best estimate of preconsolidation pressure. 5

6 ONE-DIMENSIONAL CONSOLIDATION TEST (ASTM D435) LABORATORY DATA SHEET 6 I. GENERAL INFORMATION Specimen prepared by: Ishwar Dhungana Date: 3/3/008 Lab partners/organization: USUF Client: CSUF Project: EGCE 34L Boring no.: N/A Recovery depth: N/A Recovery date: N/A Recovery method: N/A Soil description: II. TEST DETAILS Load frame type/serial no.: Scale type/serial no./precision: Consolidation ring diameter: Initial specimen height, H o : Consolidation ring mass: Specimen volume, V o: Specific gravity of soil solids, G s : (take.68) Notes, observations, and deviations from ASTM D435 test standard: III. MEASUREMENTS AND CALCULATIONS Before Test After Test Mass of moist soil + porous stone + Ring Mass of moist soil M To = M Tf = Mass of porous stone + Ring Mass of dry soil M d = M d = Mass of moisture Moisture content w o = w f = Void ratio e o = e f = Degree of saturation S o = 100% S f = 100% IV. TEST DETAILS Scale type/serial no./precision: Load no.: Load increment, σ : Filter paper type: Porous stone type, weight and thickness: Machine deflection: Deformation indicator type and conversion factor K (if applicable): Notes, observations, and deviations from ASTM D435 test standard:

7 ONE-DIMENSIONAL CONSOLIDATION TEST (ASTM D435) TIME-DEFORMATION MEASUREMENTS LABORATORY DATA SHEET V. MEASUREMENTS AND CALCULATIONS σ = 7 psi Date (mm/dd/yy) σ = 14 psi σ = 8 psi Clock Time (hh:mm:ss) Elapsed Time (min) Raw Deformation ( ) Deflection-Corrected Deformation ( ) 7

8 V. MEASUREMENTS AND CALCULATIONS Civil & Environmental Engineering Department 8 Date (mm/dd/yy) σ = 56 psi Clock Time (hh:mm:ss) Elapsed Time (min) Raw Deformation ( ) Deflection-Corrected Deformation ( ) σ = 11 psi σ = 4 psi Deformations while reducing load: 11 psi 56 psi 8 psi 14 psi 7 psi

9 ONE-DIMENSIONAL CONSOLIDATION TEST (ASTM D435) TIME-DEFORMATION PLOTTING USING THE LOG TIME METHOD I. TEST DETAILS Load no.: Load, σ : Initial specimen height, H o : Deflection units: Dial gauge conversion factor, K: Notes, observations, and deviations from ASTM D435 test standard: 9 II. MEASUREMENTS AND CALCULATIONS σ : d 100 : t : d : t 1 : d 1 : Δd: d o : d 50 : t 50 : H D50 : c v : CALCULATION SPACE: III. EQUATIONS From figure 3, t 1 = t /4 Δd = d d 1 d 0 = d 1 Δd d 50 = (d 0 + d 100 )/ H d ( K ) H o 50 D50 = or H D50 = H o d 50 c v = 0.197( th 50 D50 ) d 0 Δd d 1 Deformation, d d 50 d Δd d 100 t 1 t t 50 Time, t (log scale) Figure 3 Deformation-Log time plot for the consolidation data

10 10 Civil & Environmental Engineering Department ONE-DIMENSIONAL CONSOLIDATION TEST (ASTM D435) TIME-DEFORMATION PLOTTING USING THE ROOT TIME METHOD I. TEST DETAILS Load no.: Load, σ : Initial specimen height, H o : Deflection units: Dial gauge conversion factor, K: Notes, observations, and deviations from ASTM D435 test standard: II. MEASUREMENTS AND CALCULATIONS CALCULATION SPACE: σ : d 0 : X: 1.15X: d 90 : t 90 : d 100 : H D50 : c v : III. EQUATIONS From figure 4, d = d ( d90 d o ) c v = ( th 90 D50 ) t 90 Time, t (minutes; root scale) d 0 Deformation, d d 90 d 100 d-t curve x = (linear scale) X 1.15X Figure 4 Deformation-square root of time plot for the consolidation data

11 11 ONE-DIMENSIONAL CONSOLIDATION TEST (ASTM D435) TIME-DEFORMATION PLOTTING USING THE ROOT TIME METHOD PLOTTING PAPER Elapsed time t (min) Deformation Settlement ( S (division) ) 0 1 cm

12 Calculation Example 1 Civil & Environmental Engineering Department A specimen of the fine-grained soil, 75 mm in diameter and 0 mm thick, was tested in an oedometer in a laboratory. The initial water content was 6% and G s was.7. The vertical stresses were applied incrementally each increment remaining on the specimen until the pore water pressure change was negligible. The cumulative settlement values at the end of each loading steps are shown in table 1. The time settlement data when the vertical stress was 40 kpa are shown in table. a) Determine the pre-consolidation pressure. b) Calculate and plot the settlement-time curve. Table 1 Vertical stress (kpa) Settlement (mm) Table Please check the excel spread sheet for the detailed calculation. time settlement sqrt 0 0 time time settlement H 0 0 mm d 0 0 d c v 4.07 mm /min d d t H D H 0 0 mm d c v mm /min d d t H D

13 13 Civil & Environmental Engineering Department stress settlement e cc cr Pre-consolodation pressure = 60 kpa Time-settlement curve d50= t1 = 1 time (min) t50 = 1. t = d0 = 0.04 Settlement (mm) t100 = 3 min d Deformation log time graph

14 14 Civil & Environmental Engineering Department Time-settlement curve sqrt (t) = 1.4 a =.38 square root of time (min) 0 t = 1.96 b = settlement (mm) Deformation time graph pressure-void ratio curve Void ratio Effective vertical stress (kpa) e log σ graph