Civil Engineering Department College of Engineering Course: Soil and Rock Mechanics (CE 260) Lecturer: Dr. Frederick Owusu-Nimo

Introduction What do you do when the soil at a site is not appropriate in terms of its engineering properties? o Avoid the potential soil problem o Adapt the design to the site conditions or o Improve the soil (Soil Stabilization) Soil improvement involves altering the soil properties to improve its engineering performance One of the most important soil improvement methods is densification and it is achieved through o Compaction o Preloading o De-watering

Compaction What is Compaction? Is a deliberate process of pressing soil particles tightly together by expelling air from its void space. Involves application of energy to bring about densification arising from the expulsion of air from the soil-water-air system Purpose of compaction is to produce a soil mass with controlled or improved engineering properties. Compaction of soil increases its density producing o An increase in its shear strength o A decrease in compressibility (future settlement) of the soil o A decrease in its permeability

Compaction

Compaction Dry soils can be best compacted if for each soil a certain amount of water is added to it Water acts as a lubricant and allows soil particles to be packed together better When too much water is added, a lesser density results For a given compactive effort, there is a particular moisture content at which dry unit weight is greatest and compaction best

Compaction Curve Upper limit of dry unit weight

Laboratory Compaction Compaction Equipment o Base plate o Removable collar o Mould o Hammer Specifications of component dependent on standardized compaction test

Laboratory Compaction Obtain soil sample from the field and allow to dry Prepare specimen for compaction by adding water and mixing it thoroughly Place the specimen in the mold and compact in layers by dropping the hammer a specified number of uniformly distributed blows per layer The wet unit weight and moisture content of the compacted specimen is determined The process is repeated by increasing the moisture content The dry density at each moisture content can be determined from its measured wet unit weight and moisture content A compaction curve is plotted to determine the MDD and OMC

Laboratory Compaction Mould Dimension (mm) No of layers Blows per layer Hammer Mass (kg) Hammer Drop (mm) Designation 101.6ф x Standard AASHTO 152ф x Modified AASHTO 101.6ф x PROCTOR

Example The combined weight of a mould and the specimen of compacted soil is 4kg Mould’s volume is 9.44 x m 3 Mould’s weight is 2kg Specimens water content is 10% Determine o Bulk density o Wet unit weight o Dry Density o Dry Unit weight

Factors affecting Compaction The water content The amount of compaction energy used The type of soil involved

Factors affecting Compaction

The Type of Soil o For a given compaction effort, the MDD and OMC depends The grain size distribution of soil The shape of solids Specific gravity of solids Type and amount of clay mineral present o Higher dry unit weight is associated with well-graded granular materials o Uniformly graded sands, clays of high plasticity, organic silts and clays respond poorly to compaction

Factors affecting Compaction

Properties of Compaction Structure of compacted cohesive soil Flocculated structure o Reduced interparticle repulsion o Random particle orientation o Few but larger voids (higher permeability) Dispersed structure o Increased interparticle repulsion o Greater degree of particle orientation ( more parallel orientation)

Properties of Compaction Strength and Compressibility Dry side compaction yields a structure that is o brittle and of low compressibility o Higher strength Wet side compaction o Flexible material of low strength o Reduction in permeability of soil o Appropriate for core of earth dam

Properties of Compaction

Compaction Specification Work Type SpecificationPerformance Specification Tells contractor what to do and how to do it Engineer specifies o Type of compaction equipment o Water content o Maximum lift of loose material o Number of passes of compaction equipment Relieves contractor of liabilities Tells contractor what he must achieve The relative compaction (RC) for cohesive soils and relative density for cohesionless soils is specified The acceptable range of moisture content is also specified Contractor responsible for achieving required specification

Compaction Specification Relative Density – Cohesionless Soils Applicable to clean, free draining granular soils Loosest possible condition (max void ratio or min dry density) Densest possible condition (min void ratio or max dry density)

Compaction Specification Relative Compaction – Cohesive Soils

Field Compaction Compaction Equipment The type of equipment selected depends on The type of soil to be compacted The degree of compaction required The space available for compaction

Compaction Equipment EquipmentTypeSuitable for RollersSmooth wheel Pneumatic tyred Sheep foot All soil types except wet clay and uniformly graded sand Most soil types particularly wet cohesive soils Cohesive soils RammersDropping WeightSmall jobs e.g. trenches VibratorsVibratory rollers Vibrating plate Granular soils Most soil types

Compaction Control Compaction control tests involve determination of insitu dry density and water content of the fill o The sand replacement method The sand replacement method o Core cutter method o Rubber balloon method Rubber balloon method Non destructive methods include o Nuclear method Nuclear method Emits gamma rays through the soil Some absorbed; others reflects and reach a detector Soil unit weight inversely proportional to radiation reaching detector o Intelligent compaction systems Intelligent compaction systems Continuous compaction control Measures acceleration of the drum and calculates compaction meter value ( soil modulus or stiffness)

Field vrs laboratory Compaction Laboratory compaction can be different from the actual field compaction characteristics because o Compactive effort used in lab may be different from that used in field o Particle size distribution may be different e.g. for modified AASHTO, particles larger than 20mm are not used in lab o Rigid mould used in lab imposes strict confinement whiles in the field there is a certain degree of movement Despite these limitations, lab tests are very useful. Information from the lab are used as a guide to o The selection of equipment o Possible level of compaction attainable in the field o Most desirable water content