CE 240 Soil Mechanics & Foundations Lecture 5.2. Permeability III (Das, Ch. 6) Summary Soil Index Properties (Das, Ch. 2-6)

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1 CE 40 Soil Mechanics & Foundations Lecture 5. Permeability III (Das, Ch. 6) Summary Soil Index Properties (Das, Ch. -6)

2 Outline of this Lecture 1. Getting the in situ hydraulic conductivity 1.1 pumping test 1. percolation test. Summary Soil Index Properties

3 Pumping test The purpose of pumping test is to get the information of permeability; Pumping test is the active way to get k, the permeability. The passive way is to do the percolation test.

4 Some fundamental concepts: 1 Drawdown: the amount of water level to be lowered by pumping in a well; Q q Time increase

5 Some fundamental concepts (cont.) Steady state: the equilibrium state when the drawdown keeps no change at one particular location to the well, no further drawdown develops as pumping continues. Specific yield: the quantity of water that a unit volume of the aquifer will give up when drained by gravity; Specific retention: those part not given up by gravity drainage; Specific yield+ Specific retention = porosity

6 (Figure 15.10, West, 1995) r

7 After the pump reaches a steady state ( the water table drawdown does not change with respect to time), for a given amount of time T, the amount of water has been pumped out is Q, in the unit of volume, like cubic meter, or gallons. T T dh dh Q = vadt = k Adt = k AT 0 0 dr dr where H is the head and x is the distance. The surface area A is really the surface area of the wall of a cylinder with a head of H, given the fact the groundwater flow is essentially horizontal in the aquifer to the well. So that A = rh π

8 then Transform it we get dh dh Q= k AT = kπ rh T dr dr r = π kt HdH dr dr r = π kthdh Q

9 Integration with respect to, respectively, horizontal distance r from the well, and the height of the drawdown cone, we have r r dr r = π kt Q 1 1 h h HdH The integration gives ln r π 1 Q r = kt H r1 h h 1

10 and r π ln r ln r1 = ln( ) = kt ( h h r Q 1 So that if we know the head h in each well and the wells locations, we can calculate the hydraulic conductivity k k r = ln( ) [ ] = [ ] πth ( h) r sec m sec 1 3 Q m m 1 1 q r k = q= Q T π ( h h ) r k = ln( ) / q r π ( h h ) r log 10( ) 1 1 )

11 If you use gallon per minute (gpm) for flow rate q, and feet for head h1 and h, then the equation is.303q r kcm ( / sec) = log ( ) 14.7 π ( h h ) r

12 Percolation Test Percolation test is a passive and cheap way to estimate soil permeability.

13 Percolation Test In the leaching field of the septic system: Need more than 6 holes, after test do the average; Each hole to certain depth, usually 3-5 ; Put thick of coarse sand or gravel into the hole; Fill 1 water above the sand/gravel; Stay overnight; In the next day before testing filling water by 6 above the sand/gravel; Measure the water level drop at 30 minutes interval for 4 hours. Every time, if needed, add 6 of water again; The drop occurred at the final 30 min. is used to determine the percolation rate; If the soil is highly permeable, use 10 min. for the duration of 1 hour.

14 The permeability is: k 1.4Q ( dh) with Q Q t = π π = = a h t h t 1 1 A a dh A h h1

15 Section 6.11: in situ hydraulic conductivity tests for compacted clay soils adapt similar approach as showed in the above percolation test. It is dry and straightforward, you can easily read through it.

16 Summary Soil Index Properties (Das, Ch. -6)

17 Elements of soil mechanics Index properties: Index properties mean the observable physical characteristics with significant influence on a soil s behavior. Index properties include the descriptions of: Soil particles; Soil density; Phase relationships; Soil consistency; Relative density; moisture content.

18 Soil mechanics is a fundamental subject of Geotechnical Engineering, which applies the geological knowledge into foundation engineering and highway engineering. Soil index property related subjects Soil moisture (Ch. 3); Soil consistency (Ch. 3); Soil classification (Ch. 4); Soil compaction (Ch. 5); Permeability/hydraulic conductivity (Ch. 6).

19 Lab tests 1. specific weight, moisture content, dry unit weight. Sieve test/hydrometer test 3. Atterberg limit test

20 Soil particles The description of the grain size distribution of soil particles according to their texture (particle size, shape, and gradation). Major textural classes include: gravel (> mm); sand (0.1 mm); silt ( mm); clay (< 0.01 mm). Furthermore, gravel and sand can be roughly classified as coarse textured soils, wile silt and clay can be classified as fine textures soils.

21 For engineering purposes, soils can also be divided into cohesive and non-cohesive soils. Non-cohesive means the soil has no shear strength if no confinement. Cohesive soil contains clay minerals and posses plasticity. In engineering practice, plasticity is defined as the ability to be rolled into thin thread before breaking into pieces. Clay is cohesive and plastic. For example, mud sticking on shoes in a rainy day when one walk in a field. Sand is non-cohesive and non-plastic.

22 Phase relationship: the phase diagram W a ~0 Air V a V v W w W T Water V T V w W s Solid V s Wt: total weight Ws: weight of solid Ww: weight of water Wa: weight of air = 0 Vt: total volume Vs: volume of solid Vw: volume of water Vv: volume of the void

23 Weight-Volume Relationships (Table 3.1) The 3 rd column is a special case of the 1st column when S = 1.

24 Example: If you have a soil sample with a weight of 150 g, after thorough sieving you get the following result. sieve# size(mm) W(g) % accum% 100-accum% The last column shows the percentage of material finer than that particular sieve size by weight.

25 D10 D10 D10 D60 West, Figure 7.1 D60 D60

26 Soil Consistency Soil consistency is defined as the relative ease with which a soil can be deformed use the terms of soft, firm, or hard. Consistency largely depends on soil minerals and the water content. Atterberg limits are the limits of water content used to define soil behavior. The consistency of soils according to Atterberg limits gives the following diagram.

27 Consistency of Soils

28 Increase permeability and decrease compressibility Lower compressibility Higher compressibility West, Figure 7.11 Plasticity Chart

29 Soil Properties Soil Properties Index properties (Soil Characteristics) Mechanical Specific Gravity Gradation Atterberg Limits Compaction Permeability Compressibility Strength (Shear) Moisture Cont. Unit Weight

30 In summary, course lectures in the past 5 weeks discussed in detail about the physical properties (the index property) of soil as a material. After these 5 weeks, we will change the theme to talk about the behavior of soil under a variety of load, i.e., we start to talk about the mechanics and the mechanical properties of soil.

31 Midterm Exam Open book, open notes Das, Chs. -6 Problems similar to homework in style ~4 problems.

Geology 229 Engineering Geology. Lecture 8. Elementary Soil Mechanics (West, Ch. 7)

Geology 229 Engineering Geology Lecture 8 Elementary Soil Mechanics (West, Ch. 7) Outline of this Lecture 1. Introduction of soil properties 2. Index properties of soils Soil particles Phase relationship