Hazard, Risk, and the Steelhead Landslide.

The Oso, Washington landslide was predicted and preventable, and that doesn’t matter.

Journalists, stop snarking about how they never should have built there — you live somewhere risky and are apparently clueless about it.

Geologists, the community listened to their scientists and mitigated the hazard. We rarely relocate towns due to risk, and the risk was deemed inside acceptable levels. Maybe the various reports missed something, but this town was not foolish for merely existing.

I wrote more over on io9.

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Characterizing Density

Measuring density is a simple method of measuring the mass, measuring the volume, then calculating the density as mass divided by volume.

Dry density, wet density, and grain density can all have geotechnical utility. All three are calculated by dividing mass by volume, but use different techniques for determining which mass and which volume to use in the calculation. Dry density is determined by drying the sample before weighing. Wet density assumes that all available pore space is filled with water, so is calculated by measuring the porosity and dry mass, then adding the mass of water that will fill in the pore volume:
mwater = Vpore x ρwater

For grain density, only the solid mass is included and pore space is neglected, so dry mass is divided by the volume of only the solid material. This is calculated by subtracting the pore volume from the total volume:
Vsolid = Vtotal − Vpore

The total volume of the material is determined by either dumping a loose material (such as gravels, soils, and aggregates) into a pycnometer filled with air or water, or by preparing an intact rock to a right cylinder of measurable dimensions.

Density can also be measured in-situ through digging a test pit, weighing the material removed, and filling the with a water, sand, or another material to measure the volume, or through driving a cylinder of fixed volume into soil to remove material for weighing.

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Density of Geological Materials

The density of a rock is closely tied to its mineralogy. Sedimentary rocks have a lower density than igneous rocks, with most sandstones having a density close to the density of quartz (ρ = 2.65 g/cm3), suggesting that they are composed primarily of quartz grains and cement. Sandstones with higher densities are composed of quartz mixed with denser minerals such as calcite (ρ = 2.71 g/cm3) and dolomite (ρ = 2.8 to 3.1 g/cm3).

Amongst igneous rocks, mafic rocks contain the high-density minerals pyroxene and olivine, and are thus more dense than felsic rocks which contain more of the lower-density minerals quartz and feldspar. As oceanic crust is primarily mafic rocks like basalt and peridotite, while continental crust is primarily felsic rocks like granite, therefore oceanic rocks are generally more dense than continental rocks.

Similarly, metallic ores are usually more dense than quartz-rich crustal rocks due to a larger component of heavy metallic minerals. The large portion of light ions like hydrogen and sodium in evaporites like salt mean that evaporites have a lower density than crustal rocks. As salt caps are associated with petroleum deposits, this density difference makes gravity surveying effective for petroleum fields. More complex environments such as glacial deposits exhibit a substantial range in density: fluvial-glacial deposits of fine, dry sands are less dense than glacial moraines of cobbles and rocks.

Water content impacts density, both by permeating void space or by absorption into the material structure, potentially leading to substantial changes between dry and wet densities. Porous rocks above the water table are less dense than the same rocks below the water table, as water infills the pores. Clay can absorb substantial amount of water, swelling in volume while also increasing in density.

The grain density of a material is an indicator of the mineralogy. A rock like shale is composed of several minerals of different densities including dolomite (ρ = 2.8 to 3.1 g/cm3), calcite (ρ = 2.71 g/cm3), and clays like illite (ρ = 2.6 to 2.9 g/cm3) and kaolinite (ρ = 2.6 g/cm3). The grain density of shale (ρ = 2.65 to 2.8 g/cm3) is then a combination of these mineral densities at different proportions.

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Density (ρ) is the mass (m) of a material divided by its volume (V), given in units of mass per unit
volume (kg/m3): ρ = m/V
The density of a material is a function of the density of the individual grains or crystals, the porosity of the material1, and the fluid filling the pore space. Even so, the variation in density of natural geologic materials is quite small, with only a 16% density increase from limestone to basalt.

Measurements of dry and wet samples are taken to calculate the dry density, wet density, and the grain density of a material. The dry density is the density of the material when the pore space is full of air, and is closely linked to porosity. The wet density is the density of the material when the pore space is filled with fluid (water or oil), and is related to the porosity and the fluid density. The grain density is the density of the solid material excluding porosity, and is closely linked to the mineralogy.

The density of a particular material will remain relatively stable under different circumstances, with little variation. Variation depends on saturation, the degree of compaction, cementation, or fracturing, and degree of metamorphism. Water has a higher density than air, so as water infills pore space, saturated materials are more dense than unsaturated materials. Materials that are more compact will be more dense as pore space is reduced, and intact materials will be more dense than those riddled by fractures or voids. Cementation can also infill void space, as can recrystallization, so more cemented or metamorphosed rocks are more dense than even compacted grains.

The critical density of a soil is the point at which a soil begins to demonstrate dilative behaviour when under high strain. This density will change with respect to the confining stress, the fabric of the material, the stress history, and the type and duration of loading, so is not constant for a particular geologic material under different conditions.

Specific gravity is the density of a soil normalized to the density of water, calculated as the material mass divided by the mass of an equal volume of water. It is a unitless measurement:
s.g. = ρsample/ ρwater

Because water density fluctuates with temperature, density and specific gravity measurements are made with respect to a reference temperature (typically 20◦C).

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Pancake Rocks

In a decidedly inedible celebration of Pancake Day, I’ve written about the Pancake Rock formation over on io9.

(If the link isn’t live yet, I’ve mucked up my cross-timezone scheduled posts. It’ll be out at 1pm EST.)

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Polar Bear Day!

It’s polar bear day! This year, have some adorable photos of a curious exploring bear, or learn about Google mapping polar-bear turf.

Polar bears are threatened by habitat loss, climate change, and trophy hunting. They are way too vicious and charismatic to die out that way. Plus, the males embrace alternative culture by getting their lips tattooed for tracking.

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Detecting Porosity & Permeability

Effective porosity can be measured with electromagnetic induction probes. Hydraulic conductivity can be measured in-situ by observing water-level fluctuations in drill-holes. For a system at equilibrium, the system needs to be disturbed to produce fluid flow by either pumping water out and observing the recharge rate, or injecting a slug of water and observing how quickly it dissipates out of the hole.

The purpose and environmental conditions of the test will impact what method is most appropriate. If permeability is being measured as part of contamination monitoring and modelling, the pumping or injection tests may further spread the contaminant. In some cases, it is best to substitute gas as the fluid, using equipment like the Core Laboratories Portable Probe Permeameter. The permeameter forces gas into an exposed rock face at a fixed initial pressure, then measures how quickly the gas dissipates into the outcrop as the pressure decays.

Borehole Logging

Flow zones may also be delineated through borehole surveys, usually involving temperature differentiation. A Temperature/Fluid Resistivity probe can be used to assess the temperature gradient, identify zones of variable water quality or salinity, and delineate features. An Impeller Flow Meter delineates water flow zones, while at a Heat Pulse Flow Meter can do the same thing but for lower flow rates.

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Characterization of Porosity & Permeability

Porosity and permeability are closely related, so it is unsurprising that measuring the properties share common techniques that involve determining interconnected pore volume.


Porosity (n) is measured as the ratio of the volume of voids within a material to the total volume of the material. In a laboratory setting, this requires careful measurement of sample volume, and of pore volume. A right cylindrical core sample is extracted using a core drill press, rock saw, and surface grinder. X-ray CT scanners may be used to identify undamaged full-diameter sections for sampling. The dimensions of the sample – length (l) and diameter (d) – are measured using calipers. The volume (V ) of a right cylinder is then a simple calculation: Vtotal = πdl

Next, the sample is dried by baking it in an oven for 24 hours to ensure that no water remains in the pore space.

The pore space is filled with helium gas, which is both nonreactive (thus not altering the sample) and has a small nucleus to ensure the gas can quickly penetrate even small pore spaces provided the pores are interconnected and not isolated. The cylinder is placed in a helium pycnometer: a sample chamber and a reference chamber, both at a known volume and at a fixed temperature. The reference and sample chambers are pressurized with helium gas. Once the sample is inserted, the two chambers are connected, allowing the gas to flow out of the reference chamber into the sample chamber. The ratio of the initial and final pressures is used in conjunction with Boyle’s Law (P1V1 = P2V2) to calculate the solid volume of the sample: Vsolid = V2 = P1V1/P2

Finally, the pore volume is calculated as the difference of the total volume (determined by dimensional measurement) and the solid volume (determined by the helium pycnometer): Vpore = Vtotal − Vsolid

This technique is limited to materials with interconnected pore spaces. Isolated pores are not penetrated by the helium gas, and thus are not measured by this technique.

Alternately, liquid mercury can also be used following a similar process, where the size of infiltrated pores is proportional to the exerted pressure. The measured pore volume has the the same limitations, with the added risk of working with a neurotoxin. In the future, when helium resources have been bled away in party balloons, mercury will be the primary option, and careless graduate students will be the new mad hatters.


Permeability is measured as the hydraulic conductivity (k), which is the the ratio of flow velocity (v) to the hydraulic gradient (i): k = v (3.5) i

Hydraulic conductivity is measured in a laboratory setting by placing a sample under standard temperature conditions, then measuring the rate of discharge of water through a cross-sectional area of the medium. The water must be under laminar flow conditions so that turbulence does not complicate the flow rate. The hydraulic gradient and the cross-sectional area are coordinated to produce unit measurements. A typical example of the laboratory equipment are the Matest Hoek cells to measure the flow of water through a rock specimen of the specified diameter. A similar process can be followed using flowing air instead of water for dissolvable materials

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Porosity and Permeability

Porosity is the number of pores per unit of volume in soil. Porosity may decrease through packing or compaction, increase through dilation during shear, or change when fines are transported from one area to another. Materials with higher porosity have a higher storage capacity to hold fluids within the void space. Porosity, in conjunction with the pore fluid properties, directly impacts resistivity.
Permeability is the capacity of a material to conduct a fluid. High permeability materials are less resistant to fluid flow and require less pressure to force the fluid through than lower permeability materials. Permeability is dependent upon the geometry of fissures, pores, and cracks.

Relationship with Geological Materials

Porosity and permeability are highest in coarse-grained poorly-consolidated sedimentary rocks, with decreasing porosity with finer grained and more compacted materials. Igneous rocks typically have low porosity, excepting extrusive tuffs and pumice. Metamorphic rocks usually have intermediate porosity, where compaction and recrystallization infill pore spaces. Weathering increases porosity and permeability, where increasing cracks, fractures, and void spaces all allow for greater pore fluid mobility.

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Geotechnical Properties

Some physical properties of geologic materials are important in geotechnical engineering, but are not useful for geophysical interpretation. These properties are the dry strength, dilatancy plasticity, and toughness of the material. All four properties may be easily quantitatively categorized in the field by handling the materials.

Dry Strength

Dry strength is how strong the material is when it is dry. The dry strength is categorized in the field by the engineer modelling a small ball, adding water if necessary until the material has the consistency of putty, then applying finger pressure. If the ball crumbles with the pressure of handling, it has no dry strength. Categories progress through low, medium, and high, with very high dry strength indicating a material that cannot be broken with the pressure applied by squeezing the sample between a thumb and a hard surface.


Dilatancy is how the volume of a cohesion less soil will expand under loading or shear deformation. The dilatancy of a sample can be qualitatively categorized in the field using basic tools. The field technician will need to mould the specimen into a small ball, adding water until it is soft, then smoothing the surface with a blade or spatula. Then, he or she will shake the ball horizontally, striking the side of one hand against the other, and observing how quickly water appears on the surface of the ball. Then the engineer squeezes the balled sample by closing his or her hand, or by pinching the material, and observing if and how quickly the water disappears back into the ball. The dilatancy is then categorized as none, slow, or rapid depending on how quickly the water appears and disappears when the ball is manipulated.


Plasticity is how far a material may be deformed under constant stress, without cracking or dilatancy. The plastic limit can be determined by rolling the material into a thread, then folding and re-rolling the material until the thread crumbles. After reaching the plastic limit (when the thread crumbles), the field technician kneads the material into a lump, continuing to knead until the material crumbles. The plasticity is qualitatively categorized by how the material behaves with more or less water than the plastic limit, particularly how long it must be rolled to form a thread, then how a lump of the material behaves as it dries out.


Toughness is the ratio of the plasticity index to the flow index. It is tested concurrently to the plasticity test. Qualitatively, it is categorized into low, medium, and high by how much pressure is required to rolling the material into a thread and kneading it into a lump during the plasticity test, and the stiffness of the thread and lump.

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