CE/ENVE 320 – Vadose Zone Hydrology/Soil Physics Spring 2004 Copyright © Markus Tuller and Dani...

CE/ENVE 320 – Vadose Zone Hydrology/Soil PhysicsCE/ENVE 320 – Vadose Zone Hydrology/Soil PhysicsSpring 2004Spring 2004

Copyright © Markus Tuller and Dani Or 2002-2004Copyright © Markus Tuller and Dani Or 2002-2004

Soil Bulk Density and Compaction

Hillel, pp. 12 & 112 4

Copyright© Markus Tuller and Dani Or2002-2004

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 till the mass remains constant (usually after 24–48 hrs).


Core Method

Brass InnerCylinder

SoilSurface

CuttingCylinder

“Undisturbed”Soil Sample

hrVV 2tc

Volume Cylinder

Dry Mass Sample

Oven105oC


Core Method - Example

322t cm3391214.33hrV

r = 3 cm

h = 12 cm

Ms = 480 g (oven-dry mass)

r

h

33

t

sb cm/g42.1

cm339g480

VM


Excavation Methods – Sand Funnel

A quantity of soil is excavated in the field, dried at 105 oC and weight.

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 Balloon

Water Towerwith Scale

In 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


Bulk Density – Clod Method

The clod method is based on measuring volume and mass of a wet clod sampled in the field according to the following steps.

1. Sample clod and put it into a nylon hairnet of known mass. Weigh clod +hairnet.

2. Coat the clod with Saran solution to make it waterproof. Apply 2-3 layers and allow 15-30 min drying time for each layer (solvent evaporates). Weigh container with saran before and after coating to determine the mass of saran covering the clod.

3. Weigh clod with coating and hairnet. Weigh clod again when it is suspended in water at known temperature (Archimedes principle).

4. To correct for water content of the soil, break open the clod and take a small sub-sample. Weigh sub-sample before and after oven drying.


Bulk Density – Clod Method

CLOD METHOD:


Clod Method5. Calculate oven-dry mass of clod:

where Mw is the wet weight of the clod (corrected for the hairnet), and m is gravimetric water content determined for the sub-sample.

6. Calculate bulk density:

)1(

MM

m

ws

Saran

Saran

w

susair

sb MMM

M

Ms.......... Mass of dry clod Mair........ Mass of wet clod and Saran Msus ....... Mass of wet clod and Saran suspended in water MSaran .... Mass Saran w .......... Density of water at certain temperature Saran...... Density of Saran


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)


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 Rays

SCATTERING 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.


Gamma RaySCATTERING 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.


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


Soil Compaction

Low 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 reduced water infiltration.

forest ecosystems are extremely sensitive to soil compaction!

Compaction associated with timber harvest operations could disturb ecosystem for many years.


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

Agricultural Soil Compaction – Causes


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

Soil Compaction – Indicators


Soil Compaction - Agriculture


Compaction hampers plant growth and decreases crop yield.

The extent of soil degradation due to compaction exceeds an area of 6.8x104 km2 worldwide (Oldeman 1991).

Soil Compaction – Effects


Visualization of effect of compaction on flow

Water flow pathways are visualized using dye tracer experiments

The resulting distribution of dye tracer in the soil reflects the degree and patterns of soil compaction.

(Kulli 2001)


Soil Compaction – Effects on pore spaces


Modeling effects of soil compaction on pore space

rm

m

m

m

m

m

Results from uniaxial compression tests of soil samples at different initial water potentials (Berli, 2002) were in agreement with pore closure model (Ghezzehei & Or, 2002).


Soil Compaction – agricultural practices

Subsoiler


Mechanical impedance to root growth

A tillage pan has been mechanically broken by a subsoiler.

Roots penetrate into the subsoil through the subsoiler slot to access water and nutrients.


Mechanical impedance to root growth

Loose zoneSubsoiling Chisel

Plowpan

Root distribution of a cotton plant


Soil Compaction – Gas flow (aeration)Surface compaction can be partially reduced with an aerator


Soil Compaction – AerationThe aerator cuts and removes small cores from the top few centimeters, thereby creating vertical channels for improved infiltration and aeration.


Soil Compaction – Aeration

CE/ENVE 320 – Vadose Zone Hydrology/Soil Physics Spring 2004 Copyright © Markus Tuller and Dani...

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Transcript of CE/ENVE 320 – Vadose Zone Hydrology/Soil Physics Spring 2004 Copyright © Markus Tuller and Dani...