EAT 212 SOIL MECHANICS TOPIC COMPACTION CO 3

EAT 212 SOIL MECHANICS TOPIC: COMPACTION CO 3: Ability to solve calculation problem using mechanics involving physical properties, compaction and effective stress

SOIL COMPACTION �Compaction: increase soil density by reduce the volume – expel air from the void space �Important effects of compaction: 1. 2. 3. Increase in soil’s shear strength, Decreased in future settlement of the soil, Decreased in its permeability. � Beneficial for various types of earth construction (eg. Highway, airfield, earthdam etc) � Greater the compaction, greater these benefits will be � Cheap and effective way to improve the properties of a given soil.

�Compaction is quantified by soil’s dry unit weight, γd: γd = Where: γ 1+w γ - Wet unit weight w - Moisture content �In most cases, dry soil can be best compacted is a certain amount of water in added to it → act as lubricant → allows soil particles to be packed together �Too much water is added, a lesser density result �For a given compactive effort, there is a particular moisture content (optimum moisture content, OMC) at which dry unit weight is greatest (maximum dry unit weight, γd) and compaction is best

Dry density ( d) or dry unit weight ( d) Maximum dry density ( dmax) or maximum dry unit weight ( dmax) Zero air voids (ZAVC) Optimum moisture content (OMC) Moisture content, w (%) Typical compaction test results showing relationship between the moisture content and dry density.

�Usual practice in a construction project: 1. Perform laboratory compaction tests on representative soil samples from the construction site. 2. Determine the soil’s optimum moisture content , OMC and maximum dry unit weight, γd 3. Max. dry unit weight is used by designer in specifying design shear strength, resistance to future settlement and permeability characteristics. 4. Then, the soil is compacted by field compaction methods until laboratory maximum dry unit weight (or an acceptable % of it) has been achieved. 5. In-place soil unit weight tests are used to determine if and when the max. dry unit weight has been reached.

FACTORS AFFECTING COMPACTION 1. Moisture content: � As discussed before 2. Compaction effort: � Quantified in terms of compaction energy per unit volume � Can be function of: no. of blow per layer, no. of layer, weight of hammer, height of drop of hammer, volume of the mold � Greater the compaction energy per unit volume, greater will be the compaction � Compaction energy per unit volume is changed, the Proctor curve (unit weight vs. moisture content) will change

Effect of compaction energy on the compaction of a sandy clay

3. Types of soil: � The grain-size distribution of soil, shape, specific gravity of solids, type and amount of clay mineral present → affect max. dry unit weight and optimum moisture content for a given compactive effort and compaction method � Higher optimum moisture content → lower dry unit weight � Higher dry unit weight associated with well graded granular material � Uniformly graded sand, clay of high plasticity, organic silts and clays typically respond poorly to compaction

Moisture vs. density relations for various types of soils as determined by ASTM Method D 698.

FIELD COMPACTION 1. Soil is compacted in layers 2. Approximately 8 -in loose horizontal layer is spread from trucks 3. Moisture content can be: ü ü increased by sprinkling water over the soil if soil is too dry and/ or, reduced by aeration (eg. spreading the soil in the sun and turning it) if soil is too wet. 4. Then, soil is compacted to a thickness of 6 -in 5. Accomplished with a maximum of 6 to 10 complete coverage by compaction equipment (eg. tampers and/or rollers) 6. Surface of each compacted layer is scarified by disk plowing or other means to provide bonding between layers.

Tamper: Smooth wheel roller:

Sheepsfoot roller: Pneumatic roller:

IN-PLACE SOIL UNIT WEIGHT TEST �In order to ascertain the compacted soil achieved the max. laboratory dry unit weight. �If max. dry unit weight (or an acceptable %) has not been attained, additional compaction effort is required. �To determine max. dry unit weight of in-place soil, γd, weight and volume have to be known. → Weight, W - direct measurement → Volume, V - have to be determined using several methods: 1) 2) 3) 4) Cohesive soil - Density of soil in-place by the drive cylinder method Cohesionless soil or low plasticity soil - Unit weight of soil in-place by the sand-cone method Unit weight of soil in-place by the rubber-balloon method Unit weight of soil in-place by the nuclear methods

Cohesive soil - Density of soil in-place by the drive cylinder method (ASTM D 2930 or AASTHO T 204) 1) The thin wall cylinder driven into the soil to remove a sample 2) The sample volume is known from the cylinder’s volume Cohesionless soil or low plasticity soil - Unit weight of soil in-place by the sand-cone method (ASTM D 1556 or AASTHO T 191) 1) 2) 3) 4) A hole is dug in the ground or compacted soil Removed soil sample is weighed and tested for water content The volume of soil removed, which same with the volume of the hole can be determined by filling the hole with loose, and dry sand of uniform unit weight Cont. . next slide

4) Volume of the hole can be determined by measuring the weight of sand required to fill the hole and knowing the sand’s unit weight V= W/γ 5) By knowing the sample’s weight, volume and water content, its dry unit weight can be computed using: γd = γ 1+w �Procedure to determine unit weight of soil in-place is shown in next slide.

EXAMPLE During construction of a soil embankment, a sand-cone in-place unit weight test was performed in the field. The following data were obtained: 1) 2) 3) 4) 5) Mass of sand used to fill test hole and funnel of sand-cone device = 867 g Mass of sand to fill funnel = 319 g Unit weight of sand = 98. 0 lb/ft 3 Mass of wet soil from the test hole = 747 g Moisture content of soil from test hole as determined by Speedy Moisture Tester = 13. 7 % Determine dry unit weight of the compacted soil.

SOLUTION

FIELD CONTROL OF COMPACTION �In theory, procedure of compaction, starting from determination of max. lab γd and OMC of soil until in-place soil unit weight test (as discussed before) is simple enough to do as long as the percentage of compaction meet the requirement as stated in contract document. �However, in practical, application of compaction needs to take account of some considerations. For example: Type of soil, or compaction characteristics of soil taken from borrow pits may vary from one location to another, 2. Degree of compaction may not be uniform throughout. 1.

�To deal with problem of example 1: 1) Conduct compaction test in the laboratory to establish the max. lab. γd and OMC for each type of soil encountered in the project, then 2) as soil is transported from the borrow pit, placed and compacted in the fill area, each in-place soil unit weight test must be checked against the max. lab. γd of the respective type of soil. � To deal with problem of example 2: 1) Specify a minimum number of field unit weight test. For instance, a dam embankment might be specified that one test must ne made for every 2400 yd 3 of fill placed

�To ensure the required field unit weight is achieved by the field compaction, a specific contract between the owner and the contractor is prepared �The contract normally specify: 1) 2) 3) 4) 5) The required percent of compaction, Minimum no. of field unit weight tests required Maximum thickness of loose lifts (layers) prior to compaction Method to obtain max. γd Method to determine in-place unit weight and so on.