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  • 1

    Copyright Markus Tuller 2002-2006

    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

    Copyright Markus Tuller 2002-2006

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    Copyright Markus Tuller 2002-2006

    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=

    Copyright Markus Tuller 2002-2006

    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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    Copyright Markus Tuller 2002-2006

    Core Method

    Brass InnerCylinder

    SoilSurface

    CuttingCylinder

    UndisturbedSoil Sample

    hrVV 2tc ==

    Volume Cylinder

    Dry Mass Sample

    Oven105oC

    Copyright Markus Tuller 2002-2006

    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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    Copyright Markus Tuller 2002-2006

    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

    Copyright Markus Tuller 2002-2006

    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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    Copyright Markus Tuller 2002-2006

    Bulk Density Clod MethodCLOD METHOD:

    Copyright Markus Tuller 2002-2006

    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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    Copyright Markus Tuller 2002-2006

    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.

    Copyright Markus Tuller 2002-2006

    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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    Copyright Markus Tuller 2002-2006

    Gamma Rays

    Copyright Markus Tuller 2002-2006

    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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    Copyright Markus Tuller 2002-2006

    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.

    Copyright Markus Tuller 2002-2006

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

    Agricultural Soil Compaction Causes

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    Copyright Markus Tuller 2002-2006

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

    Soil Compaction Indicators

    Copyright Markus Tuller 2002-2006

    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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    Copyright Markus Tuller 2002-2006

    Soil Compaction - AgriculturePotato yield on a clay loam in Minnesota

    Copyright Markus Tuller 2002-2006

    Soil Compaction Effects on Pore Spaces

    uncompacted compacted

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    Copyright Markus Tuller 2002-2006

    Soil Compaction - Agriculture

    Subsoiler

    Copyright Markus Tuller 2002-2006

    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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    Copyright Markus Tuller 2002-2006

    Soil Compaction - Agriculture

    Loose zoneSubsoiling Chisel

    Plowpan

    Root distribution of a cotton plant

    Copyright Markus Tuller 2002-2006

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

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    Copyright Markus Tuller 2002-2006

    Soil CompactionSurface compaction can be partially reduced with an aerator

    Characterization of the Liquid Phase

    2

    Copyright Markus Tuller 2002-2006

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    Copyright Markus Tuller 2002-2006

    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]

    Copyright Markus Tuller 2002-2006

    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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    Copyright Markus Tuller 2002-2006

    Soil Water Content Gravimetric Determination

    Copyright Markus Tuller 2002-2006

    Soil Water Content Volumetric Determination

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    Copyright Markus Tuller 2002-2006

    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).

    Copyright Markus Tuller 2002-2006

    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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    Copyright Markus Tuller 2002-2006

    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 +=

    Copyright Markus Tuller 2002-2006

    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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