CHAPTER TWO SOIL COMPRESSION SOIL COMPRESSION This refers to a process that describes the decrease in soil volume under an externally applied load. Soil compression can involve removal of air from soil pores called compaction or expulsion of water from soil pores called consolidation. Soil compaction is more usual in agricultural fields since soils are normally worked at unsaturated states. MEASUREMENT OF SOIL COMPRESSION Soil compression can be measured in the laboratory using uniaxial confined compression test in an oedometer, the triaxial compression cell, or direct shear test. Stone and Ekwue (1995, 1996) described a simple method to measure the compression of unsaturated agricultural soils. MEASUREMENT OF SOIL COMPRESSION The soil at a known moisture content is packed at the required bulk density into a cylinder. A steel plate with perforations are then placed on top of the soil in the cylinder and the cylinder placed on the load cell of a compression machine. The steel plate serves to spread the load from the plunger of the machine to the soil. The perforations on the steel plate provide an exit for excess pore pressures to leave the soil sample during compression if present.. MEASUREMENT OF SOIL COMPRESSION CONTD During the test, force (F) exerted by the loading plunger is continuously measured as a function of decrease in sample height () due to plunger movement. The vertical stress () on the sample is then F/A where A is the area of the soil cylinder Soil Compression Machine Derivation of Soil Compression Equations The initial dry bulk density on soil packing (with no strain), given by: M Mass i Volume s H0 . A is ........................................(1) Where: Ms is the dry mass of sample; A is the area of the soil cylinder and Ho is the original height of soil in the cylinder. . Strain ( ) at any applied stress, Ho H Ho Change in soil sample height Original height ...............................(2) Where: H is the new height of the sample at any applied stress (see figure below) Soil Compression Test F Ho H Derivation of Compression Equations Contd. Note: Since there is no lateral strain on the sample as it is confined, axial strain is equal to volumetric strain. From Eqn. (2), H = Ho (1 - ) .......... (3) Dry bulk density, b at any stress M M V H. A ...... (4) Substituting Eqn (3) into Eqn (4), b M H0 (1 ) A From Eqn (1), b i 1 and 1 i b These equations were first derived by Stone and Ekwue (1995). Void Ratio Void Ratio: Void ratio, e is defined as e = Vp / Vs Where: Vp is the volume of voids = total soil volume (V) - Volume of solids (Vs) V Vs V e 1 .........................(6)Also Vs Vs Soil particle density is: s Ms M and dry bulk density , b s Vs V s Ms V V x b Vs M s Vs From Eqn (6), e s 1 b i. e. ....(7) Note: Soil particle density can be taken as 2.65 gm/cm3 for most mineral soils. Soil Compression Index 2.1.3 SOIL COMPRESSION INDEX Soil compression index, Cc is defined as: Cc (e2 e1 ) log( 2 / 1 ) Where: e1 and e2 are void ratios at two applied stresses 1 and 2 . Example: The following results were computed for the Piarco sandy loam soil in a laboratory experiment. Plot the strain/stress curve; and the soil compression curves. The initial soil bulk density before soil compaction was 0.89 gm/cm3 and the so particle density is 2.65 gm/cm3. Example Contd. Applied stress Bulk density Strain (kPa) (gm/cm3) 10 1.09 0.21 40 1.20 0.28 60 1.24 0.31 80 1.27 0.32 100 1.29 0.33 150 1.34 0.36 200 1.37 0.37 400 1.45 0.41 500 1.47 0.42 600 1.49 0.42 800 1.52 0.43 1000 1.55 0.45 Solution: Using stresses of 10 kPa and 100 kPa Cc (1.05 1.43) log (100/ 10) 0.38 Void ratio 1.43 1.21 1.14 1.09 1.05 0.98 0.93 0.83 0.80 0.78 0.74 0.71 Bulk Density and Applied Stress Stress/Strain Relations Void Ratio and Applied Stress 2.2 SOIL COMPACTION Soil Compaction is defined as the volume change produced by momentary load application caused by rolling, tamping or vibration. It involves the expulsion of air without significant change in the amount of water in the soil mass. The most common causes of agricultural soil compaction are trampling by livestock and pressures imposed by vehicles or tillage equipment. Soil Compaction Contd. While Soil compaction is desirable in most engineering situations, it is undesirable in agricultural fields. Improvements of engineering properties of soils through compaction lead to advantages such as: i) Reduction or prevention of detrimental settlement of soil. ii) Soil strength increases and improvements of slope stability. iii) Improvement of bearing capacity of pavements and iv) The control of undesirable volume changes caused by frost action, swelling and shrinkage. Compaction Contd. Compaction in agricultural fields leads to Excess soil hardness, Reduced soil permeability to water and airflow and a resulting loss of crop yields. It is not possible to remove water from the voids by compaction, but the addition of water to a slightly moist soil increases compaction by reducing surface tension. Compaction increases to a limit called the optimum moisture content above which further addition of water causes an increase in voids, leading to reductions in soil compaction. State of Compaction The State of Compaction of a Soil can be Measured by Dry Bulk Density, Shear Strength, Penetration Resistance or Reductions in Soil Permeability. To determine compaction of a soil in terms of dry density, it is necessary to find the bulk density and moisture content. This is usually done using the Standard Proctor test. PROCTOR TEST The Standard Proctor test is a method of finding the optimum moisture content for compaction of a soil. A cylindrical mould 0.001 m3 in volume is filled with a sieved soil sample in three equal layers, each layer being compacted by 25 or 27 blows in a standard hammer, weight 2.5 kg, dropped from a height of 300 mm for each blow. Proctor Test Contd. The mould is then trimmed and weighed, to determine the bulk density of the soil. Moisture content of the soil is then determined to obtain the dry density. The test is carried out with soil at different moisture contents and a graph of dry density against moisture content is plotted. A heavy compaction test uses a greater compactive effort from a 4.5 kg hammer dropping 450 mm on to five soil layers in the mould. Typical Proctor Test Curve Proctor Test APPLICATIONS OF PROCTOR TEST IN AGRICULTURE Proctor Compaction Soil mechanics Test can be used to index and predict with reasonable accuracy, the compaction behaviour of agricultural soils over a wide range of soil moisture contents and single or multiple passes of tyres of mechanical equipment with varying contact pressures. The knowledge of the moisture content and pressure changes on dry density of a soil could be provided in order to make recommendations to the farmer or machine designer. APPLICATIONS OF PROCTOR TEST IN AGRICULTURE CONTD. The Proctor compaction test has hitherto been reserved for earthwork engineering. In agricultural practice, it is advisable to limit soil working below the optimum moisture content in order not to cause maximum soil compaction. Factors that Affect Soil Compaction (1)The Magnitude and Nature of Compacting forces: The higher the Compactive effort, the higher the maximum dry density but the optimum moisture content reduces. i) d Higher compactive force Lower compactive force % Moisture Content The extent of soil compaction also varies according to whether the force acts by impact, kneading action or vibration etc. Factors that Affect Soil Compaction Contd. ii) Moisture Content of the Soil (see diagram above). iii) The Degree of Compaction of the Soil at the time of compaction. iv) Soil properties eg. texture, density, and organic matter content: Sandy soils are more compactible than clays but clays have higher optimum moisture contents. Organic matter reduces the maximum dry density and increases the optimum or critical moisture content. This increases soil workability since it can be worked over a wider range of moisture content without achieving maximum compaction. Example Example: Standard Proctor Compaction test carried out on a Piarco sandy soil yielded the following results: Bulk density(kg/m3) 1860 Moisture content(%) 24.7 1700 1880 2010 1940 5.1 10.4 14.4 19.6 Plot the curve of dry density against moisture content and hence find the maximum dry density and the optimum(critical) moisture content. Solution Solution: where: rd = dry bulk density, m = Moisture Content m 0.051 r 1.70 (gm/cm3) rd 1.62 (gm/cm3) r = 1+m 0.104 1.88 1.7 rd r = Wet Density, 0.144 0.196 0.247 2.01 1.94 1.76 1.62 1.86 1.49 Solution Concluded Dry bulk density (gm/cm 3) Compaction Curve For Piarco Sand 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1 0 10 20 30 Moisture Content (%) From graph, Maximum Dry Density = 1.76 gm/cm3 and Optimum(critical) Moisture Content = 14.5%.