handout of soil mechanics, Lecture notes of Soil Mechanics and Foundations

Download handout of soil mechanics and more Lecture notes Soil Mechanics and Foundations in PDF only on Docsity! Fundamentals of Geotechnical Engineering - Il Chapter 5 Soil Compaction ine General Outl Introduction Let’s reexamine the equation for dry unit weight, that is, _(_4&s _ v _ Gs Ya~\t4e)™~T4o \14+0G,/s)™ How can one increase the dry unit weight? Examination of the equation reveals that we have to reduce the void ratio; that is,w/S must be reduced since Gs is constant. The theoretical maximum dry unit weight is obtained when S = 1 (S = 100%); that is, Cmin = WGs Introduction Consider a plot of the theoretical dry unit weight versus water content for different degrees of compaction is shown in figure below. 30 ho on $= 100% ho oa ao on Theoretical maximum dry unit weight (kKN/m?) — on o 0 5 10 15 20 25 30 Water content (%) Introduction a The theoretical dry unit weight decreases as the water content increases because the soil solids are heavier than water for the same volume occupied. a The theoretical dry unit weight decreases as the degree of saturation decreases. a The mass of air is negligible, so as air replaces water in the void space, the volume of soil remains constant but its mass decreases. Thus, the dry unit weight decreases. The curve corresponding to S=100% is the saturation line, sometimes called the zero air voids curve. Introduction cntd Principles of Compaction -Compaction of soils is achieved by reducing the volume of voids. -It is assumed that the compaction process does not decrease the volume of the solids or soil grains. ET ABE: Bes uncompacted compacted uncompacted compacted 10 Introduction cntd Principles of Compaction cntd e The degree of compaction of a soil is measured by the dry unit weight of the skeleton. e The dry unit weight correlates with the degree of packing of the soil grains. Recall that yd= G.y,, /(1+e) - e The more compacted a soil is: >» the smaller its void ratio (e) will be. > the higher its dry unit weight (yd) will be. 11 Introduction cntd e Due to [J aN | | 4 compaction, loose Ne Sy structu re of th e Loose Angular —** Angular soil mass will become denser. 12 Laboratory Compaction cntd Proctor Compaction Test: deliver a standard amount of mechanical energy (compactive effort) to determine the maximum dry unit weight of a soil. Proctor showed that v There exists a defined relationship between the soil moisture content and the degree of dry density to which a soil may be compacted. ¥ That for a specific amount of compactive energy applied on the soil there is one moisture content termed Optimum Moisture Content (OMC) at which a particular soil attains maximum dry density. 15 Laboratory Compaction cntd Purpose: LJ The purpose of a laboratory compaction test is to determine the proper amount of mixing water to use when compacting the soil in the field and the resulting degree of denseness which can be expected from compaction at this optimum water LJ Proctor proposed tests to determine’ the relationship between moisture content, dry density or void ratio of a compacted soil in a manner to determine the OMC for the soil. 16 Laboratory Compaction cntd =" Compaction = f[dry density, compactive effort and soil type, gradation, presence of clay mineral, etc...] =" Compactive effort is a measure of mechanical energy applied to a soil mass. Impact compaction: >» The proctor test is an impact compaction. >» A hammer is dropped several times on a soil sample ina mold. >» The mass of the hammer, height of drop, number of drops, number of layers of soil, and the volume of the mold are specified. 17 Laboratory Compaction cntd Variables of Compaction Proctor established that compaction is a function of four variables: (1) Dry density (py) or dry unit weight (y, ) (2) Water content (w) (3) Compactive effort (energy, E) (4) Soil type (gradation, presence of clay minerals, etc.) : Height of Number of Weight of drop of X blows per X Number of hammer layers E- hammer layer Volume of mold EL (24.4.N)(0.305 m)(3 layers)(25 blows / layer) For standard 0.944 x 1073 m? Proctor test, = 591.26 KJ/m? 20 Laboratory Compaction cntd e Factors Influencing Compaction ES -—!—, mE ee 21 Laboratory Compaction cntd e Factors Influencing Compaction 1. Moisture Content o At low moisture contents the strength of clay soils and friction between granular particles is high so a given compactive effort will not be able to remove all air voids leaving the soil in an overall compressible state when it is subjected to stresses from further layers of fill or a structure, and in a potentially collapsible state. o At higher moisture content clay soils become weaker and friction between granular particles reduces so the air voids are more easily removed during compaction. o At moisture content greater than the OMC the soil particles cannot move any closer together because even though most of the air has been expelled there is more water present in the voids. 22 Laboratory Compaction cntd 2. Soil Type 24 22 Zero air voids—100% saturation _ Silt with sand a 20 = = = she fm ; Plastic cl 3 Maxiraum astic clay ry uni = 1°) weight 7 > 5=80% oa 14 Poorly graded sand 12 10 C} 4 6 & 10 12 14 | 16 18 20 Water content (%) Optimum water content 2. Laboratory Compaction Soil Type Lee & Suedkamp (1972) studied compaction curves for 35 soil samples. They observed that 4 types of compaction curves can be found. Type A: single peak. Generally found for soils that have a liquid limit between 30 & 70. Type B: one and one-half peak Ya Type C: double-peak curve Type D: does not have a definite peak, termed odd shaped. Type B & Can be found for soil that have a LL < 30. Type C & D might be exhibited by soil having LL > 70. cntd Laboratory Compaction cntd 3. Effect of Compaction Effort i Amount of Compactive Effort . Maximum dry unit weight increases with increasing compactive effort. . OMC decreases to some extent with increase in compactive effort. SX 7 Increasing compaction (SX energy /\ stn Line of optimum 27 Laboratory Compaction cntd Standard Proctor Test o Four or more tests are compacted on the soil using different water contents. o The last test is identified when additional water causes the bulk unit weight of the soil to decrease. o The results are plotted as dry unit weight Vs water content. o At water content below optimum, air is expelled and water facilitates the rearrangement of soil grains into a denser configuration. o At water content above optimum, the compactive effort cannot expel more air and additional water displaces soil grains. 30 Laboratory Compaction Standard Proctor Test Se Cy Dro Laboratory Compaction cntd Procedures A dry soil specimen is mixed with water and compacted in a cylindrical mold of volume 9.44 x 104-4 m43 (standard proctor mold) by repeated blows from a hammer 2.5kg, falling freely from a height of 305mm. The soil is compacted in 3 layers, each of which is subjected to 25 blows. 2.5 kg (5.5 Ib) hammer 25 blows per layer Compactive effort 16 778 Nm (12,375 ft-lbs) 42" Soil sample 0,001 m (0.03 ft?) 3 layers 25 Blows/Layer 32 Laboratory Compaction cntd Results -The peak point of the compaction curve: » the point with the maximum dry density Py max: a Corresponding to the maximum dry density py ma, iS a water content known as the optimum water content Wopt (also known as the optimum moisture content, OMC). a Note that the maximum dry density is only a maximum for a specific compactive effort and method of compaction. This does not necessarily reflect the maximum dry density that can be obtained in the field. 35 Laboratory Compaction cntd -Line of optimums: A line drawn through the peak points of several compaction curves at different compactive efforts for the same soil will be almost parallel to a 100 % S curve, it is called the line of optimums. (Wop Pa, max) Pa 36 Laboratory Compaction cntd Results "Below w,,; (dry side of optimum): as the water content increases, the particles develop larger and larger water films around them, which tend to “lubricate” the particles and make them easier to be moved about and reoriented into a denser configuration. BAt Wop: the density is at the maximum, and it does not increase any further. "Above w,,, (wet side of optimum): water starts to replace soil particles in the mold, and since p,, << p, the dry density starts to decrease. 37 Laboratory Compaction cntd Standard Proctor Test Mold size: 944 cm3 (1/30 ft?) 305 mm (12 in) height of drop 24.4 N (5.5 lb) hammer 3 layers 25 blows/layer Modified Proctor Test Mold size: 944 cm? (1/30 ft?) 457 mm (18 in) height of drop 44.5 N (10 lb) hammer 5 layers 25 blows/layer 40 (OhMC} (OMC) IMoistu ie Laboratory Compaction cntd Interpretation of Compaction Test o For construction specification of soil improvement by compaction usually call for a minimum of 95% Proctor maximum dry unit weight. Relative compaction, RC = Ya(field) Yd(max) o This level of compaction can be attained at two water contents; = Before the attainment of maximum dry unit weight — dry of optimum » After the attainment of maximum dry unit weight — wet of optimum o Normally the former one is used o The latter one is used for projects where soil volume changes from changes in moisture condition are intolerable. 42 Laboratory Compaction cntd EXCERCISE 5.2.2 — Standard Proctor Test Below is the results of a standard compaction test. Water content (%) 6.2 8.1 9.8 11.5 12.3 13.2 Bulk unit weight (kN/m3) | 16.9 18.7 19.5 20.5 20.4 20.1 a) Determine the maximum dry unit weight and optimum water content. bo) What is the dry unit weight and water content at 95% standard compaction, dry of optimum? <) Determine the degree of saturation at the maximum dry density. d) Plot the zero air voids line. 45 3. Effects of Compaction > On Clay Structure > On Swelling > On Soil Permeability > On Soil Compressibility > On Soil Strength 46 Effects of Compaction Done Right (Benefits) Not Done Right (Consequences) Increased soil strength Increased load-bearing capacity Reduction in settlement (lower compressibility) Reduction in flow of water (water seepage) Reduction in soil swelling (expansion) and collapse (soil contraction) Increased soil stability Reduction in frost damage Structural distress from excessive total and differential settlements Cracking of pavements, floors and basements Structural damage to buried structures, water and sewer pipes and utility conduits Soil erosion 47 Effects of Compaction : On Permeability =" Increasing the water content results in a decrease in permeability on the dry side of the optimum moisture content and a slight increase in permeability on the wet side of optimum. "Increasing the compactive effort reduces the permeability since it increases the dry density, thereby reducing the voids available for flow. Permeability (cm/s) Dry density (Ibf/ft?) T —> Shows change in moisture and density from permeation _| 15 16 Water content (%) 17 2.0 Dry density (Mg/m?) Effects of Compaction: On Compressibility At low stresses the sample compacted on the wet side is more compressible than the one compacted on the dry side. Dry compacted or undisturbed sample Void ratio, e Wet compacted or remolded sample © 0 Pressure, natural scale Low pressure consolidation. 51 51 Effects of Compaction : On Compressibility At the high applied stresses the sample compacted on the dry side is more compressible than the sample compacted on the wet side. Void ratio, e Dry compacted or undisturbed sample _ Wet compacted or remolded sample Rebound for both samples 0 Pressure, log scale High-pressure consolidation. 52 Field Compaction Introduction o The soil mass is compacted in layers called lifts. o The stress imparted by compactors, especially static compactors, decreases with lift depth. Consequently, the top part of the lift is subjected to greater stresses than the bottom and attain a higher degree of compaction. oLower lift thickness is preferable for uniform compaction. o Different types of materials will require different lift thickness and a suitable type of field compactors. 55 Field Compaction o 1. Compaction is accomplished by Static vertical force Applied by a dead weight that imparts pressure and/or kneading action to the soil mass. E.g. sheep foot rollers, grid rollers, rubber-tired rollers, drum rollers, loaders and scrapers. Vibratory vertical force Applied by engine-driven systems with rotating eccentric weights or spring/piston mechanisms that impart a rapid sequence of blows to the soil surface. E.g. vibrating plate compactors, vibrating rollers and vibrating sheepfoot rollers. 56 Field Compaction Drum-roller Rubber-tired rollers Vibrating plate compactor Vibrating roller 57 Field Compaction cntd Control Parameters : Dry density and water content correlate well with the engineering properties, and thus they are convenient construction control parameters. - Since the objective of compaction is to stabilize soils and improve their engineering behavior, it is important to keep in mind the desired engineering properties of the fill, not just its dry density and water content. This point is often lost in the earthwork construction control. 60 Field Compaction cntd Control Parameters - Laboratory tests are conducted on samples of the proposed borrow materials to define the properties required for design. - After the earth structure is designed, the compaction specifications are written. Field compaction control tests are specified, and the results of these become the standard for controlling the project. 61 Field Compaction cntd Relative compaction or percent compaction R.C,=—Parfield yy 190% Pmax-laboratory Correlation between relative compaction (R.C.) and relative density (D,.) R.C.= 80 + 0.2D, NB.This is a statistical result based on 47 soil samples. Typically R.C. = 90%~95% is required. 62 Field Compaction cntd Sand cone method procedures 1. Fill the jar with a standard sand—a sand with known density— determine the weight of the sand cone apparatus with the jar filled with sand (W,). 2. Determine the weight of sand to fill the cone (W,). 3. Excavate a small hole in the soil and determine the weight of the excavated soil (W3). 4. Determine the water content of the excavated soil (w). 5. Fill the hole with the standard sand by inverting the sand cone apparatus over the hole and opening the valve. 6. Determine the weight of the sand cone apparatus with the remaining sand in the jar (W,). 65 Field Compaction cntd Sand cone method procedures 7. Calculate the unit weight of the soil as follows: Weight of sand to fill hole = W, = W, — (W, +W,) Ws Volume of hole = V = —————_ (Yaottawa sand Weight of dry soil = Wy =— eight of dry soil = d=Tho oe Wa Dry unit weight = yq = Tv 66 Field Compaction cntd » The measuring error is mainly from the determination of the volume of the excavated material. - For example, for the sand cone method, the vibration from nearby working equipment will increase the density of the sand in the hole, which gives a larger hole volume and a lower field density. - If the compacted fill is gravel or contains large gravel particles, any kind of unevenness in the walls of the hole causes a Significant error in the balloon method. » If the soil is coarse sand or gravel, none of the liquid methods works well, unless the hole is very large and a polyethylene sheet is used to contain the water or oil. 67 Field Compaction EXERCISE 5.3.4 SAND CONE METHOD cntd A sand cone test conducted during the compaction of a roadway embankment gave the following data. Calibration to find dry unit weight of the standard sand Mass of Proctor mold 4178 grams Mass of Proctor mold and sand 5609 grams Volume of mold 0.00095 m* Calibration of sand cone Mass of sand cone apparatus and jar filled with sand 5466 grams Mass of sand cone apparatus with remaining sand in jar 3755 grams Sand cone test results Mass of sand cone apparatus and jar filled with sand 7387 grams Mass of excavated soil 1827 grams Mass of sand cone apparatus with remaining sand in jar 3919 grams Water content of excavated soil 4.8% 70 Field Compaction cntd EXERCISE 5.3.4 SAND CONE METHOD (a) Determine the dry unit weight. (b) The standard Proctor maximum dry unit weight of the roadway embankment soil is 16 kKN/m3 at an optimum water content of 4.2%, dry of optimum. The specification requires a minimum dry unit weight of 95% of Proctor maximum dry unit weight. Is the specification met? If not, how can it be achieved? 71