Jagadanand Jha Guru Nanak Dev Engineering College, Ludhiana 141006, India & Sanjay Kumar ShuklaEdith Cowan University, Perth, WA 6027, Australia
CompactionLaboratory TestField CompactionCase StudyDerivation for Field Compaction
The most commonly used ground improvement technique,where the soil is densified through external compactiveeffort/mechanical means by reducing volume of air.CompactiveEffort + water =
•To refill an excavation, or a void adjacent to a structure (such as behind a retaining wall.)•To provide man-made ground to support a structure•As a sub-base for a road, railway or airfield runway.•As a structure in itself, such as an embankment or earth dam, including reinforced earth Improvement Effect on mass fill Higher shear strength Greater stability Lower compressibility Less settlement under state load Higher CBR value Less deformation under repeated Lower permeability Less tendency to absorb water Lower frost susceptibility Less likelihood of frost heave
Zero Air Compaction Curve Void Curve Sr =100% ρ d, max Load optimum water content Air Air water WaterSoil Compressed SoilMatrix Solid Solid Vol. = VT2 Vol. = VT1 γsoil (2) > γsoil (1)
Soil Compaction in theLab:1- Standard Proctor Test2- Modified Proctor Test3- Gyratory Compaction Standard Proctor Test Modified Proctor Test
Gs γw Soil Compaction in the Lab: γZAV = Gs γ w 1+ Wc Gs 1- Standard Proctor Test γ dry = Sr 1+ e Dry Density Zero Air Void Curve Sr =100% 5.5 pound hammer γ d max 3 H = 12 in 4 2 5 1 25 blows per layer Compaction wc1 wc2 wc3 wc4 wc5 Dry to Wet to Optimum Optimum Curve γd1 γd2 γd3 γd4 γd5 (OWC) Water Content Optimum Increasing Water Content Water Content γwet4 inch diameter compaction mold. γdry = 1+ Wc%(V = 1/30 of a cubic foot) 100
Soil Compaction in the Lab: Zero Air Void Curve Sr = 60% Dry Density1- Standard Proctor Test Zero Air Void Curve Sr =100%ASTM D-698 or AASHTO T-99 γ d maxEnergy = 12,375 foot-pounds per cubic foot Zero Air Void Curve γ d max Sr < 100% Compaction Curve for2- Modified Proctor Test ModifiedASTM D-1557 or AASHTO T-180 Proctor Energy = 56,520 foot-pounds per cubic foot Compaction Curve for Standard Proctor (OMC) Moisture (OMC) Content Number of blows per layer x Number of layers x Weight of hammer x Height of drop hammer Energy = Volume of mold
•Type of soil•Compactive effort•Effect of soil Structure / water Content•Organic content
Type of clayEffect of clay contenton density (Das 2006) Proctor compaction test on Sand
Effect of Energy on Soil Compactio (Compactive Effort) Increasing compaction energy Lower OWC and higher dry density Higher Dry Density Energy In the field increasing compaction energy ZA = increasing number of passes V or reducing lift depth In the labincreasing compaction energy= increasing number of blows Water Content
Dry side of optimum- Flocculated structure and wet side of optimum- Dispersed structure Higher compactive effort or water content give more dispersed fabric
Cohesive Soil: Attractive force -Van der waals force acts between two soil particles; Remains same in magnitude Repulsive force – Due to the double layer of adsorbed water tending to come into contact with each other; directly related to the size of double layers If net force is attractive – Structure is Flocculated If net force is repulsive – Structure is Dispersed
Low Water Content: Repulsive force is small because double layer is not fully developed; net force is attractive. Makes difficult for particle to move when compactive effort is applied: Result low dry unit weight
High Water Content: Interparticle repulsive force increases since double layer expands Particle easily slide over one another and get packed more easily : Result high dry unit weight
Double layer expansion is complete at Optimum Moisture Content (OMC): Result maximum dry unit weight at this stage Beyond OMC; water does not add to expansion but replaces the soil grains by water: Result a decrease in dry unit weight
First Decrease in dry unit weight with increase in water content Reason:Capillary tension in pore water prevents soil particle coming close together (Phenomenon- Bulking of Sand- maximum bulkking occurs at 4-5% water content) Further increase in water content : Menisci are broken and particles move and adopt to a closer packing
Permeabilty higherwhen compacted dryof optimum than whencompacted wet ofoptimum
At relatively low stress level clays compacted wet of optimum are more compressibleAt relatively high stress levelclays compacted dry ofoptimum are morecompressible
Organic content Effect of drying history andMaximum dry unit weight Vs. organic content on optimumOrganic content for all compaction moisturetest content (Das 2006)(Das 2006)
Shallow Compaction: Compaction depends onfollowing factorsThickness of liftArea over which the pressure is appliedIntensity of pressure applied to the soilType of rollerNumber of roller passesEffect of number of passes oncompaction of lean clay
Smooth Wheel RollerProvide a smooth finished gradeUsed for pavingEffective only upto 20-30 cm,[Therefore place the soil in shallow layers (Lifts)]
Greater compaction pressure,Provides kneading action,“walk out” after compactionEffective for compacting fine-grained soil / Clays
Effective for compacting clayey soil and silty soils
Compacted unit weight for 8ft (2.44m)lift height for 2,5,15 and 45 vibratoryroller passes
Provides deeper compaction (2-3 m) eg. Air fields
Suitable for granular soils, land fills andkarst terrain with sink holes.(Solutioncavities in lime stone) Pounder (Tamper) Crater created by the impact (to be backfilled)
Pounder (Tamper)Mass = 5-30 tonneDrop = 10-30 m
Suitable for granular soils Practiced in several forms: vibro–compaction stone columns vibro-replacementVibroflot (vibrating unit)Length = 2 – 3 mDiameter = 0.3 – 0.5 mMass = 2 tonnes(lowered into the ground and vibrated)
vibrator makes a hole backfilled ..and compacted Denselyhole in the weak compactedground stone column
For densifying granular soils Aftermath of blasting Fireworks
Site: Anpara Thermal Power Plant, Uttar PradeshExpansion of existing thermal power plant:Unit D of 2x 500 MW CapacitySite allocated for Expansion: An abandoned Ash Pond of area app. 5400 acres.Depth of Site: 3m to 13mState of Denseness: Loose to Medium dense in conditionExisting bearing capacity of the flyash deposit: < 10 t/m2Site falls under Zone III – IS 1893 (Part1) 1982- Susceptible to liquefactionMethod adopted for improvement of the Ash Pond: Vibro Stone Column (Dry bottom feed method)
Soil Strata:Ash deposit 3-13mClayey silt/Silty clay upto 23mDense sandy silt or Hard clayey silt withoccasionally weathered rock (Granitic gnesis) Density within Ash deposit: Considerable variationSPT value of Ash deposit – Range of N 2 to 30, but on an average 3 to 8SPT value of Hard Clayey Silt : N ranges between 9 and 30
Vibro Stone Column (Bottom feed method):Method does not require water for penetration thus avoiding the disposal of large quantities of muck and also making environmental friendlyRig used: Vibrocat, operational avantage is it is able to exert a pull down force improving penetration speedVibrocat feeds the Coarse granular material to the tip of vibrator with the aid of pressurized airInstallation method consists of alternate step of penetration and retractionDuring retraction gravel runs into the annular space created and then compacted using vibrator thrusts and compressed air
Improving Bearing Capacity of open foundationVibro stone column of dia 0.9mat 2m centre to centre spacing ina triangular grid pattern resultedthe bearing capacity value10t/m2
Vibro stone column enhanced the densityof Fly ash deposits, which inturn improvedLateral load carrying capacity.After Improvement, Result Reported:Design lateral load capacity = 7 tUltimate Load = 20 t
Typical detail of stone column installed surrounding the piles
The selection of right depth, right diameter and proper compaction is essential. Computerised monitoring of penetration depth of vibrator. Sensor within the depth vibrator indicates the compaction effort of depth vibrator.
General Procedure in Compaction Tests Depending on the size of the compaction mould, a fraction of the soil sample having particle size larger than a specific value, say d0, is discarded For example, in the standard Proctor compaction test, the soil particles coarser than 19 mm are discarded before compacting soil in the standard 101.6 mm- diameter laboratory mould; IS270 (Parts 7 and 8) recommends 100-mm diameter mould (BIS, 1980, 1983); AS12126.96.36.199 (Standards Australia, 2003) recommends 105-mm diameter mould
If the fraction removed is significant, the laboratory optimum moisture content and the maximum dry unit weight determined for the remaining soil are not directly comparable with the field values. To make laboratory values more representative, the following approaches can be used:
In the laboratory soil sample for conducting the test, the coarse fraction larger than d0, say 19 mm, is replaced by an equal amount of material between 19 mm and the next smaller sieve size, say 4.75 mm; The water/moisture content and dry unit weight of the discarded coarse fraction (larger than d0) are estimated and the field values are computed as weighted averages of those of the discarded coarse fraction and of the remaining soil.
The field optimum moisture content is calculated using water content of coarse Zero Air fraction (larger than d0) as Void Curve described above in second Sr =100% approach, and then the maximum dry unit weight is calculated assuming that the ρ d, max saturation of the soil in field is equal to that achieved in the laboratory test. This treatment is equivalent to shifting the compaction curve upward along a saturation line. It requires optimum water content knowledge of the specific gravity of the soil particles.
First step: To calculate the saturation from the laboratory values of Zero Air Void maximum dry unit weight, Curve optimum moisture content Sr =100% and specific gravity of soil particles. ρ d, maxSecond step: The equivalent field unit weight is then computed from the laboratory degree of saturation, field optimum moisture content and specific optimum water content gravity of soil particles.
Field Compacted Sample Laboratory Compacted SampleWhen the coarser fraction, larger than size d0 (e.g. 19 mm), is removed, it also takes away some water associated with its water content.In addition, there is also possibility of some change in the air void volume when the soil is compacted without this coarse fraction.
1/γdF=(1-p)(1+β)/γdL+p/Gcγw+(pWc-(1-p)βWL)/γw-(1-p)β/(Gfγw) WF = (1-p)WL+pWc Gf = specific gravity of the fine soil particles (smaller than d0) in the field/laboratory soil sample Va = volume of the air in voids of the field soil sample VF = total volume of field soil sample VL = total volume of the laboratory soil sample wc = water content of the coarse soil particles in the field soil sample Ws = weight of the soil particles in the field sample Wwc = weight of the water with coarse soil particles in the field soil sample Wwf = weight of the water with fine soil particles in the field/laboratory soil sample α = ratio of volume of the air in voids of the laboratory sample to that in the field soil sample Gcγw = unit weight of the coarser fraction of soil particles in the field soil sample Gfγw = unit weight of the finer fraction of soil particles in the field/laboratory soil sample .
The authors wish to acknowledge all the sources (journals/books/photographs) used for the preparation of this presentation. Thank you.