# 1 soil bulk density compaction - university of assignments/soil...soil bulk density compaction 1...

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Hydrologic Measurement Techniques These are slides provided by Markus Tuller for students in Hydrologic

Measurement Techniques at the University of Idaho.

Do not distribute these notes.

Soil Bulk Density & Compaction1

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Dry Bulk Density

Ratio of the mass of oven-dry soil and total sample volume

The dry bulk density is primarily affected by soil texture and structure, including aggregation and particle size distribution.

If the pore space is half of the bulk volume the dry bulk density b is about half of the particle density s (1300 to 1350 kg/m3)

Fine textured soils commonly have lower bulk densities than coarse textured soils.

tsb VM=

Determination of Bulk DensityTo determine bulk density we need to measure the dry mass and the total volume occupied by the soil sample.

CORE METHOD:

A cylindrical metal sampler is driven into the soil to remove a known volume (core).

The core (soil + brass cylinder) is oven-dried at 105 oC to remove non-structural soil water until the mass remains constant (usually after 2448 hrs).

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Core Method

Brass InnerCylinder

SoilSurface

CuttingCylinder

UndisturbedSoil Sample

hrVV 2tc ==

Volume Cylinder

Dry Mass Sample

Oven105oC

Core Method - Example

322t cm3391214.33hrV ===

r = 3 cm h = 12 cmMs = 480 g (oven-dry mass)

r

h

33t

sb cm/g42.1cm339

g480V

M ===

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Excavation Methods Sand FunnelA quantity of soil is excavated in the field, dried at 105 oC and weighed.

The volume is determined by filling the excavated hole with a well defined standard sand of which the volume per unit mass is known. (SAND-FUNNEL Method)

Valve

StandardSand

Base Plate

Excavation Methods Rubber BalloonIn the RUBBER BALLOON Method the volume is determined by inserting a balloon into the excavation and filling it with water or an other fluid with known density.

RubberMembrane

Valve

Water Towerwith Scale

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Bulk Density Clod MethodCLOD METHOD:

Bulk Density Gamma Rays

GAMMA RAY TECHNIQUES:

Gamma ray techniques are based on attenuation and diffraction of gamma rays emitted from a 137Caesium or 241Americium source due to collision with other atoms of the soil phases.Attenuation and diffraction are dependent on bulk density and other soil properties (e.g., water content)

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Gamma Rays

TRANSMISSION TECHNIQUE:Two probes at a fixed spacing are lowered into previously prepared openings in the soil. One probe contains a Geiger tube, which detects the attenuated radiation transmitted through the soil from the gamma source located in the second probe.

Gamma RaysSCATTERING TECHNIQUE:A single probe contains both, detector and source separated by shielding. Can be used on the surface or placed in a hole dependent on design of the equipment.

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Gamma Rays

In agriculture and forestry soil compaction is undesirable.

For many engineering applications a well compacted soil is crucial for safe foundations (the Leaning Tower of Pisa is an example of building on soft soil).

Soil Compaction Desired or Not?

Image: Opera Primaziale Pisana

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Soil CompactionLow porosity (n) or high bulk density (b) are indicators for soil compaction.

Soil compaction results in mechanical impedance to plant root growth, poor aeration, and restrictions to water infiltration.

Forest ecosystems are extremely sensitive to soil compaction!

Compaction associated with timber harvest could disturb ecosystems for many years.

Operation of heavy vehicles (e.g. harvesters, construction machines) on agricultural land can cause soil compaction.

Agricultural Soil Compaction Causes

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Compaction alters soil hydraulic and gaseous exchange properties and increases mechanical impedance to plant roots.

Soil Compaction Indicators

Compaction hampers plant growth and decreases crop yield.

The extent of soil degradation due to compaction exceeds an area of 6.8x104 km2worldwide (Oldeman1991).

Soil Compaction Effects

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Soil Compaction - AgriculturePotato yield on a clay loam in Minnesota

Soil Compaction Effects on Pore Spaces

uncompacted compacted

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Soil Compaction - Agriculture

Subsoiler

Soil Compaction - Agriculture

The tillage pan has been mechanically broken by a subsoiler. The vertical slot allows roots to penetrate into the subsoil to access water and nutrients.

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Soil Compaction - Agriculture

Loose zoneSubsoiling Chisel

Plowpan

Root distribution of a cotton plant

Soil Compaction - AgricultureSoil compaction can be reduced by spreading the applied weight.

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Soil CompactionSurface compaction can be partially reduced with an aerator

Characterization of the Liquid Phase

2

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Characterization of the Liquid PhaseThe two most important characteristics of the liquid phase are: The amount of water in the soil (soil water content) The forces by which water is held in the soil matrix (matric potential)These two soil attributes are related through a function known as the SOIL WATER CHARACTERISTIC (SWC).

Changes in soil water content and matric potential affect many soil transport and mechanical properties, such as (1) ability to transfer liquid and gases; (2) mechanical properties such as soil strength, compactibility, penetrability, and bulk density in swelling soils.

SOIL WATER CHARACTERISTIC

0.01

0.1

1

10

100

1000

0 0.1 0.2 0.3 0.4 0.5

Volumetric Water Content [m3/m3]

Mat

ric P

oten

tial [

-m]

Soil Water Content Measurement Methods

soildryovenmass)soildryovenmass()soilwetmass(

soildryofmasswaterofmass

m

==

volumesamplewaterofdensity

waterofmass

soilofvolumebulkwaterofvolume

v

==

GRAVIMETRIC WATER CONTENT:Samples obtained by digging, augering, or coring are weighed (moist sample), and weighed again after oven drying (105 oC).

VOLUMETRIC WATER CONTENT:Samples with known volume (core samples) may be processed the same way as in the gravimetric water content method.

The conversion between gravimetric and volumetric water content requires knowing the dry bulk density.

w

bmv

=

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Soil Water Content Gravimetric Determination

Soil Water Content Volumetric Determination

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Nondestructive Methods - Neutron Scattering

rrww

rrDDDryDry

WetWet

NeutronNeutronProbeProbe

AccessAccessTubeTube

Sphere ofSphere ofInfluenceInfluence

ProbeProbe(Source and(Source and

Detector)Detector)

Neutron Scattering (or Neutron probe) is a widely used field method for repetitive measurement of volumetric soil water content.

It is based on the propensity of water molecules to slow down (thermalize) high energy fast neutrons emitted from a radio active source (Americium-241 Beryllium).

Thermalized neutrons are counted by a detector present in the access tube (along with the source).

Nondestructive Methods - Neutron Scattering

DryDry

WetWet

Fast neutrons are emitted radially into the soil and collide with various atomic nuclei. Collisions with most nuclei are virtually elastic with only minor loss of kinetic energy.

Collisions with hydrogen nuclei causes significant loss of kinetic energy and slow down of the fast neutrons (thermalization).

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Neutron Scattering Method Calibration of the Neutron Probe is necessary to account for

background hydrogen sources and other local effects like bulk density.

Calibration is achieved by simultaneously measuring soil water content and count ratio CR - ratio of slow neutrons to standard count obtained with the radiation source in the shield.

)CR(bav +=

Limitations of Neutron Scattering Method

Radiation hazardsRequires site specific calibrationVariable volume of measurementNot suitable for near-surface measurementsProvides snap shots, difficult to automate Installation and measurements are labor intensiveLimited accuracy

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SOIL 415 SOIL 415 Soil

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