Soil Compaction PDF

# CVE 441: Soil Mechanics ## Sediment Treatment Sediment can be treated in confined disposal facilities (CDF) and then be used or disposed at designated site. The method of removal, means of transportation, availability of treatment location, disposal site or demand for reuse is key factors to con...

# CVE 441: Soil Mechanics ## Sediment Treatment Sediment can be treated in confined disposal facilities (CDF) and then be used or disposed at designated site. The method of removal, means of transportation, availability of treatment location, disposal site or demand for reuse is key factors to consider when planning for ex-situ stabilization. Treatment of sediments in CDF falls under ex-situ mass stabilization method, which can be accomplished in several ways depending on the natural of sediments and water contents. ## 3.0: Soil Compaction Soil compaction is the practice of applying mechanical compactive effort to densify soil by reducing the void space between soil particles. Compaction occurs when particles are pressed together to reduce the space between them. Highly compacted soils contain very few spaces, resulting in soil with a higher unit weight. Maximum density is achieved at an optimum moisture content, or OMC, for short. Compaction of soil is the pressing of soil particles close to each other by mechanical methods. Air during compaction of soil is expelled from the void space in the soil mass, and therefore the mass density is increased. Compaction of soil is done to improve the engineering properties of the soil. Compaction of soil is required for the construction of earth dams, canal embankments, highways, runways, and many other structures. The process of compaction decreases the likelihood of settlement after a building, roadway, runway, or parking lot is constructed. Settlement could result in premature pavement failure, costly maintenance or repairs. Soil consists of solid particles and voids filled with water and/or air. When subjected to stress, soil particles are redistributed within the soil mass, and the void volume decreases resulting in densification. The mechanical stress may be applied by kneading, or via dynamic or static methods. The degree of compaction is quantified by measuring the change of the soil's dry unit weight, $Y_d$. Compaction is a process that brings about an increase in soil density or unit weight, accompanied by a decrease in air volume. There is usually no change in water content. The degree of compaction is measured by dry unit weight and depends on the water content and compactive effort (weight of hammer, number of impacts, weight of roller, number of passes). For a given compactive effort, the maximum dry unit weight occurs at an optimum water content. Purposeful compaction is intended to improve the strength & stiffness of soil. Consequential (or accidental) compaction, and thus settlement, can occur due to vibration (piling, traffic, etc.) or self-weight of loose fill. ### Within the framework of engineering applications, compaction is particularly useful as it results in: 1. An increase in strength of soils 2. A decrease in compressibility of soils 3. A decrease in permeability of soils Those factors are crucial in structures and engineering applications such as earth dams, embankments, support of pavements, or support of foundations. The degree of the compaction depends on the soil properties, the type and amount of energy provided by the compaction process, and the soil's water content. For every soil, there is an optimum amount of moisture for which it can experience its maximum compression. In other words, for a given compactive effort, a soil is reaching its maximum dry unit weight ($Y_{d,max}$), at an optimum water content level ($W_{opt}$). The compressibility of a relatively dry soil increases as water is added to it. That is, for water content levels dry of optimum ($W_{opt}$), the water acts as a lubricant, enabling soil particles to slide relative to each other, thus leading to a denser configuration. Beyond a certain water content level (wet of optimum, $W>W_{opt}$), excess water within the soil results in pore water pressure increase that pushes the soil particles apart. A typical correlation between the dry unit weight and the water content is presented in Figure 3.1. Also, it is worthwhile to note that, as it can be seen in Figure 3.2, for a given soil, the highest strength is achieved just dry of optimum (Figure 3.2a), while the lowest hydraulic conductivity is achieved just wet of optimum (Figure 3.2b). The effect of the compactive effort on the maximum dry unit weight (yd,max), and the optimum water content level (Wopt) can be observed in Figure 4. With an increase in compactive effort, $Y_{d,max}$ increases, while $W_{opt}$ decreases. That is, a smaller water content level is sufficient to saturate a denser sample. ## 3.1: Importance of Soil Compaction 1. Increases the bearing capacity and stiffness of in-situ (natural state) or chemically modified soils. 2. Compaction increases the shear strength of soils by adding friction from the interlocking of particles. 3. Future settlement of soils is reduced by increasing the stiffness and eliminating voids creating a densified soil. 4. The removal of voids reduces the chance of the soil settling or shrinking or expanding, and it decreases water seepage that would lead to deleterious shrinking and swelling soil properties. 5. Shrink/swell properties compromise the pavement structure thereby leading to premature failure of the pavement structure. ## 3.2: Factors Affecting Soil Compaction? ### 1. Type of Soil Different types of soil respond differently with respect to compaction. Soils are classified by their particle size and, in some soil categories, by their critical water content values or Atterberg limits. Well-graded granular soils that contain a wide range of particles are preferred in construction applications because they can be easily compacted, thus eliminating voids by interlocking the particles and resisting moisture absorption, thereby allowing the soil to support heavier loads as a very dense soil. Poorly graded soils contain a narrow range of particle sizes, and are less conducive for construction purposes due to the fact that the soil lacks shear strength, not associated with the non-interlocking particles because of their similar sizes. ### 2. Moisture Content Water content plays a very important role in soil compaction. The maximum dry density is only achieved when the water content is at an ideal level. This point is known as optimum moisture content or OMC. Optimal moisture content and maximum dry density are determined in a laboratory and are then used as targets for on-site operations. If the soil is too dry, water trucks can be deployed to spread water in order to raise the water content within the acceptable range of optimal moisture content. Conversely, overly wet soils present their own set of problems. Recent rain, spring thawing, or soils that retain moisture can be handled in a number of ways. 1. Waiting on warm, dry weather conditions is the natural way to dry soils but can be time-consuming and often ineffective because of (additional) inclement weather. 2. Disking equipment, to help aerate soils, can reduce the amount of moisture but this method also opens the soil up to absorb even more moisture in the event of additional rain events. Moreover, disking will typically only reduce the moisture up to 5% and only to relatively shallow depths. 3. Cut and fill, also known as remove and replace, is a popular option but is expensive and time consuming. Borrow pits are becoming more and more scarce and disposal costs continue to escalate. 4. The most effective option is chemical drying. Portland cement can be used to dry soils but lime-based reagents are pound for pound the most effective chemical choice. Lime-based reagents contain a high amount of available calcium oxide, as high as 94-96 percent. Calcium oxide chemically combines with water, forming calcium hydroxide. More simply put, when lime is around water it absorbs it. This is an exothermic reaction that dries off additional moisture in the form of steam. Portland cement, in principle, will have almost no free lime as the CaO will combine to form other mineral phases. ### 3. Amount of Compaction Energy The compactive effort is the amount of energy applied on the soil. With a soil of given moisture content, if the amount of compaction energy increases, the Soil particles will be packed so that the dry unit weight increases. For a given compactive effort, there is only one moisture content which gives the maximum dry unit weight. If the compactive effort is increased the maximum dry unit weight also increases, but the optimum moisture content decreases. ### 4. Method of Compaction The dry density achieved depends on the method of compaction of soil being applied. ## 3.3: Effect of Compaction on Properties of Soil ### 1. Effect of Compaction on Soil Structure Soils compacted at water content less than the optimum generally has a flocculated structure. Soils compacted at water content more than the optimum usually has a dispersed structure. ### 2. Effect of Compaction of Soil on Permeability The permeability of a soil depends upon the size of voids. The permeability of a soil decreases with an increase in water content on the dry side of optimum water content. ### 3. Swelling ### 4. Pore water pressure ### 5. Shrinkage ### 6. Compressibility ### 7. Stress-strain relationship ### 8. Shear strength ## 3.4: Methods of Compaction of Soil ### 1. Standard Proctor Compaction Test (British Standard Light) (ASTM D-698) Proctor developed this test in connection with the construction of earth fill dams in California in 1933. It gives the standard specifications for conducting the test. A soil at a selected water content is placed in three layers into a mold of 101.6mm diameter, with each layer compacted by 25 blows of a 2.5kg rammer dropped from a height of 310mm, subjecting the soil to a total compactive effort of about 605.89 KN/m³, so that the resulting dry unit weight at optimum water content is determined. The test provides a relationship between the water content and the dry density. The water content at which the maximum dry density is attained is obtained from the relationship provided by the tests. Proctor used a standard mould of 4 inches internal diameter and an effective height of 4.6 inches with a capacity of 1/30 cubic foot. The mould had a detachable base plate and a removable collar of 2 inches height at its top. The soil is compacted in the mould in 3 layers, each layer was given 25 blows of 5.5 pounds rammer filling through a height of 12 inches. The mould recommended is of 100mm diameter, 127.3mm height and 1000ml capacity. The rammer recommended is of 2.5kg mass with a free drop of 310mm and a face diameter of 50mm. The soil is compacted in three layers. The mould is fixed to the detachable base plate. The collar is of 60mm height. ### 2. West African Compaction Test (West African Standard) (ASTM D-1557) This test method covers laboratory compaction procedures used to determine the relationship between water content and dry unit weight of soils, compacted in 5-layers by 101.6mm diameter mold with each layer compacted by 10 blows of a 4.5kg rammer dropped from a height of 457mm producing a compactive effort of 1008.71 KN/m³. ### 3. Modified Proctor Compaction Test (British Standard Heavy) (ASTM D-1557) This test method covers laboratory compaction procedures used to determine the relationship between water content and dry unit weight of soils, compacted in 5-layers by 101.6mm diameter mold with each layer compacted by 27 blows of a 4.5kg rammer dropped from a height of 450mm producing a compactive effort of 2,723.53 KN/m³. The modified Proctor test was developed to represent heavier compaction than that in the standard Proctor test. The test is used to simulate field conditions where heavy rollers are used. The test was standardized by American Association of State Highway Officials and is, therefore also known as modified AASHO test. In this, the mould used is same as that in the Std Proctor test. However, the rammer used is much heavier and has a greater drop than that in the Std Proctor test. The soil is compacted in five equal layers, each layer is given 27 blows. The compactive effort in modified Proctor test is 4.56 times greater than in the Std Proctor test. The rest of the procedure is same. ## 3.5: Laboratory Compaction Tests The variation in compaction with water content and compactive effort is first established in the laboratory. Target values are then specified for the dry density and/or air-voids content to be achieved on site. ## 3.5: Soil Compaction Test Equipment The equipment utilized to conduct the test includes: * 10-centimeter diameter cylindrical compaction mold equipped with a base and a collar * Proctor rammer weighing 2.5 kg or 4.5 kg depending on whether the standard of the modified test is conducted * No.4 Sieve * Steel straightedge * Moisture containers * Graduated cylinder * Mixer * Controlled oven * Metallic tray and a scoop * Extruder Typical cylindrical compaction molds and rammers are shown in Figure 3.3. ## 3.6: Test Procedure The procedure of the Proctor Compaction Test consists of the following steps: 1. Obtain about 3 kg of soil. 2. Pass the soil through the No. 4 sieve. 3. Weight the soil mass and the mold without the collar ($W_m$). 4. Place the soil in the mixer and gradually add water to reach the desired moisture content (w). 5. Apply lubricant to the collar. 6. Remove the soil from the mixer and place it in the mold in 3 layers or 5 layers depending on the method utilized (Standard Proctor or Modified Proctor). For each layer, initiate the compaction process with 25 blows per layer. The drops are applied manually or mechanically at a steady rate. The soil mass should fill the mold and extend into the collar but not more than ~1 centimeter. 7. Carefully remove the collar and trim the soil that extends above the mold with a sharpened straight edge. 8. Weight the mold and the containing soil (W). 9. Extrude the soil from the mold using a metallic extruder, making sure that the extruder and the mold are in-line. 10. Measure the water content from the top, middle and bottom of the sample. 11. Place the soil again in the mixer and add water to achieve higher water content, w ## 3.7: Calculations First, the compaction water content (w) of the soil sample is calculated using the average of the three measurements obtained (top, middle and bottom part of the soil mass). Subsequently, the dry unit weight ($Y_d$) is calculated as follows: $Y_d = \frac {W - W_m} {(1 + w) * V}$ where: * W = the weight of the mold and the soil mass (kg) * $W_m$ = the weight of the mold (kg) * w = the water content of the soil (%) * V = the volume of the mold (m³, typically 0.033m³) This procedure should be repeated for 4 more times, given that the selected water contents will be both lower and higher from the optimum. Ideally, the selected points should be well distributed with 1-2 of them close to the optimum moisture content. The derived dry unit weights along with the corresponding water contents are plotted in a diagram along with the zero-voids curve, a line showing the dry unit weight correlation with the water content assuming that the soil is 100% saturated. No matter how much energy is provided to the sample, it is impossible to compact it beyond this curve. The zero-voids curve is calculated as follows: $Y_d = \frac {G_s * Y_w} {(1 + w * G_s)}$, where: * $G_s$ = the specific gravity of soil particles (typically, $G_s$~2.70) * $Y_w$ = the saturated unit weight of the soil (kN/m³) Typical curves derived from the Standard and Modified Proctor tests, as well as the zero air voids curve are presented in Figure 3.4. ## 3.8: Dry-Density/Water-Content Relationship The aim of the test is to establish the maximum dry density that may be attained for a given soil with a standard amount of compactive effort. When a series of samples of a soil are compacted at different water content the plot usually shows a distinct peak. ## 3.9: Expressions for calculating density A compacted sample is weighed to determine its mass: M (grams) as seen in the Fig. 3.7 below The volume of the mould is: V (ml) Sub-samples are taken to determine the water content: w The calculations are: * Bulk density, $p = \frac {M}{V} [g/ml = Mg/m³]$ * Dry density, $p_d = \frac {p}{(1+w)} [g/ml = Mg/m³]$ ## Example 1: 1. A compacted soil sample has been weighed with the following results: * Mass= 1821 g Volume = 950 ml Water content = 9.2% Determine the bulk and dry densities. * **Solution:** * Bulk density $p = \frac {1821}{950}$ = 1.917 g/ml or Mg/mł * Dry density $p_d = \frac {1.917}{(1+0.092)}$ = 1.754 Mg/mł ## 3.10: Dry density and air-voids content A fully saturated soil has zero air content. In practice, even quite wet soil will have a small air content. $A_v = \frac {Volume of air}{Total volume}$ The maximum dry density is controlled by both the water content and the air-voids content. Curves for different air-voids contents can be added to the $p_d$ / w plot using this expression: $p_d = \frac{G_s * p_w * (1 - A_v)}{(1 + w * G_s)}$ The air-voids content corresponding to the maximum dry density and optimum water content can be read off the $ p_d$/w plot or calculated from the expression. ## Example 2: Determine the dry densities of a compacted soil sample at a water content of 12%, with air-voids contents of zero, 5% and 10%. ($G_s$ = 2.68). * **Solution** * For $A_v = 0$: * $ p_d = \frac{2.68 * 1.0}{(1 + 2.68 * 0.12)} = 2.03\ Mg/m^3$ * For $A_v = 5%$: * $p_d = \frac{2.68 * 1.0 * (1 - \frac{5}{100})}{(1 + 2.68 * 0.12)} = 1.93\ Mg/m^3$ * For $A_v = 10%$: * $p_d = \frac{2.68 * 1.0 * (1 - \frac{10}{100})}{(1 + 2.68 * 0.12)} = 1.83\ Mg/m^3$ ## 3.11: Effect of Increased Compactive Effort The compactive effort will be greater when using a heavier roller on site or a heavier rammer in the laboratory. With greater compactive effort: * Maximum dry density increases * Optimum water content decreases * Air-voids content remains almost the same. ## 3.12: Interpretation of Laboratory Data During the test, data is collected: 1. Volume of mould (V) 2. Mass of mould ($M_o$) 3. Specific gravity of the soil grain ($G_s$) 4. Mass of mould + compacted soil - for each sample (M) 5. Water content of each sample (w) Firstly, the densities are calculated ($p_d$) for samples with different values of water content, then $p_d$ / w curve is plotted together with the air-voids curves. The maximum dry density and optimum water content are read off the plot. The air content at the optimum water content is either read off or calculated. ## Table 3.1: Tabulated Data | S/NO (a) | VOL. OF MOULD (m³) (b) | WGT OF MOULD + BASE PLATE (Kg) (c) | WGT OF MOULD+BASE PLATE+WET SOIL (Kg) (d) | WGT OF WET SOIL (Kg) (e) | BULK DENSITY, $p_b$ (Kg/m³) (f) | WATER CONTENT, WC (%) (g) | DRY DENSITY, $p_d$ (Kg/m³) (h) | DRY DENSITY, $p_d$ (KN/m³) (i) | |---|---|---|---|---|---|---|---|---| | 4% | 0.001 | 4.20 | 5.85 | 1.65 | 1650 | 4.31% | 1,649.29 | 16.17952 | | 8% | 0.001 | 4.20 | 5.85 | 1.65 | 1650 | 8.90% | 1,648.53 | 16.17211 | | 12% | 0.001 | 4.20 | 6.25 | 2.05 | 2050 | 13.15% | 2,047.31 | 20.08409 | | 16% | 0.001 | 4.20 | 6.2 | 2.00 | 2000 | 16.77% | 1,996.65 | 19.58716 | | 20% | 0.001 | 4.20 | 6.2 | 2.00 | 2000 | 19.76% | 1,996.06 | 19.58131 | * **Wgt. Of Wet Soil (Kg)= (d-c)** * **Bulk Density, $p_b$ (Kg/m³)= (e/b)** * **Dry Density, $p_d$ (Kg/m³)= [f/(1+())]** * **Dry Density, $p_d$ (KN/m³)= [(1000) * 9.81]** ## Figure 3.4: Typical curves showing the zero air voids curve. A plot with "Dry Unit Weight" on the Y-axis and "Compaction Water Content" on the X-axis showing a curve for "Zero Air Voids Curve" and curves for "Low Effort - Standard Proctor" and "High Effort-Modified Proctor" ## Figure 3.3: Proctor molds and rammers (ASTM/AASHTO) An image of a typical cylindrical compaction mold and rammer used in laboratory compaction tests. ## Figure 3.5: Graph of the Compaction Results of Table 3.1 A plot of the compaction results from Table 3.1 with "Dry Density (KN/m3)" on the Y-axis, "Water Content (%)" on the X-axis, and the curve labeled, "Series 1". * **MDD = 20.28kN/m³** * **OMD = 14.10%** ## Figure 3.6: Graph of Dry-Density/Water-Content Relationship. A plot with "Pd" on the Y-axis and "W" on the X-axis, showing a peak curve with a maximum dry density, "Pd(max) = 1.86 Mg/m³" at an optimum water content "Wopt = 13%". ## Figure 3.7: Weighing of Compacted Soil with Mould An image of a compacted soil sample being weighed using a mold. ## Figure 3.8: Graph Showing Effect of Increased Compactive Effort. A plot with "Pd" on the Y-axis and "W" on the X-axis with curves for "2.5kg rammer" and "4-5kg rammer". The curve for the heavier rammer shows an increase in the maximum dry density and a decrease in the optimum water content.