Understanding Consolidation Test on Cohesive Soil

1. Soil Mechanic LaboratoryAssist. Lec. Lubna Mohammed AbdEnvironmental Engineering Department Al- Mustansiriyah UniversityConsolidation Test on Cohesive Soil Sources: Soil Mechanics – Laboratory Manual, B.M. DAS (Chapter 17)

2. Class Outlines • Consolidation • Definitions & Introduction • Significance • Consolidation vs Compaction • Type of Consolidations • One-Dimensional Consolidation Test • Definition • Procedure • Graphs and results

3. Consolidation - Definition • Consolidation refers to the compression or settlement that soils undergo as a response of placing loads onto the ground • These loads produce corresponding increases in the vertical effective stress, sv’ • Consolidation is a time-dependent process, in some soils it may take long time (100 years ?) to achieve complete settlement

4. Significance • The amount of soil volume change that will occur is often one of the governing design criteria of a project • If the settlement is not kept to tolerable limit, the desire use of the structure may be impaired and the design life of the structure may be reduced • It is therefore important to have a mean of predicting the amount of soil compression or consolidation • It is also important to know the rate of consolidation as well as the total consolidation to be expected

5. Consolidation - Case Study Palacio de las Bellas, Artes, Mexico City Total settlement = 9ft The Leaning Tower of Pisa

6. Consolidation - Introduction • The compression is caused by: • Deformation of soil particles • Relocations of soil particles • Expulsion of water or air from void spaces • Most of the settlement of a structure on clay is mainly due to volumetric changes and rarely due to shear strain.

7. Consolidation vs. Compaction

8. Consolidation (cont.) • During consolidation, pore water or the water in the voids of saturated clay gets squeezed out – reducing the volume of the clay – hence causing settlement called as consolidation settlement The spring analogy to consolidation. Civil Engineering - Texas Tech University

9. Consolidation (cont.) z0′ z′ }z f′  c z0′ H z0′ }z f′ z0′ z′  e Voids Vv = eVs Vv = (e -  e)Vs Voids Solids Solids Vs Vs After Before

10. Types of Consolidation • There are three types of consolidation: • Immediate consolidation; caused by elastic deformation of dry soil or moist and saturated soil without change in moisture content • Primary consolidation; caused as a result of volume change in saturated cohesive soils due to exclusion of water occupied the void spaces • Secondary consolidation; occurs in saturated cohesive soils as a result of the plastic adjustment of soil fabrics

11. Types of Consolidation (cont.) • Clayey soils undergo consolidation settlement not only under the action of “external” loads (surcharge loads) but also under its own weight or weight of soils that exist above the clay (geostatic loads). • Clayey soils also undergo settlement when dewatered (e.g., ground water pumping) – because the effective stress on the clay increases • Coarse-grained soils DO NOT undergo consolidation settlement due to relatively high hydraulic conductivity compared to clayey soils. Instead, coarse-grained soils undergo IMMEDIATE settlement.

12. 1- D Consolidation Test • The main purpose of consolidation tests is to obtain soil data which is used in predicting the rate and amount of settlement of structures founded on clay. • The four most important soil properties determined by a consolidation test are: • The pre-consolidation stress, sp’, This is the maximum stress that the soil has “felt” in the past. • The compression index, Cc , which indicates the compressibility of a normally-consolidated soil. • The recompression index, Cr , which indicates the compressibility of an over-consolidated soil. • The coefficient of consolidation, cv , which indicates the rate of compression under a load increment.

13. Recompression Index Compression Index Pre-Consolidation Stress Test Results Cr B Cc sp Civil Engineering - Texas Tech University

14. Consolidation Test • Two types of consolidometers (oedometers) commonly used: • Floating-ring • Fixed ring • This lab uses the fixed-ring consolidometer • ASTM D 2435

15. Laboratory Consolidation Test

16. Consolidation Test Civil Engineering - Texas Tech University

17. Laboratory Consolidation Test v • Place sample in ring • Apply load • Measure height change • Repeat for new load Confining stress v Voids  V Vv Vv Voids Solids Solids Vs Vs Before After

18. Procedure • Measure the inner diameter and height of the consolidation cutter/ring and record its mass • Prepare a soil specimen for the test by trimming and placing the soil in the ring • Determine the mass of ring + soil • Collect some excess soil for moisture content • Assume Gs = 2.7 • Saturate the lower (larger) porous stone on the base of the consolidometer • Place the specimen and ring and place the upper stone/disk Follow the rest in your lab manual • Place 1.5 kg (1st day), 3kg (2nd day), 6kg (3rd day), 12kg (4th day)

19. Civil Engineering - Texas Tech University http://www.uic.edu/classes/cemm/cemmlab/Experiment%2011-Consolidation.pdf#search='consolidation%20test'

20. Calculations and Graphs - dv vs w(time)

21. dv vs wtimeGraph– Find t90

22. Calculation and Graph – dv vs log(time)

23. dv vs log(time) Graph – Find t50

24. Calculation • Determine the height of solids (Hs) of the specimen in the mold • Determine the change in height (DH) • Determine the final specimen height, Ht(f) • Determine the height of voids (Hv) • Determine the final void ratio

25. Calculation (cont.) • Calculate the coefficient of consolidation (cv) from t90 • Calculate the coefficient of consolidation (cv) from t50 • Plot e-log p curve and find: • sc, Cc, Cr • Plot cv – log p curves

26. Calculation Sample (Ex. pp.121) Eq 17.2 1(in) - Hs t90 Hv = Hi - Hs e = Hv / Hs Hi (0.848 x 0.99592)/(4 x 302) (1.0 + 0.9917) / 2

27. Cr R min Cc Plot e vs log p sc Civil Engineering - Texas Tech University

28. In Your Report • Plot all curves find t90 and t50 (10 plots) • Show your calculations in a table and find • e, cv (t90), cv (t50) • Plot e vs. log (p) and determine: • Pc • Cc • Cr • Plot cv vs. log (p) (2 plots)