Advanced Environmental Geology Exam 1

settlement

The process where soil settles via compaction and consolidation.

compaction

the loss of air

consolidation

the loss of water

3 Key Parts to Settlement

• shrinking and swelling

• slope failures

• premeability issues

What makes up soil?

mineral matter, organic matter, water, and air

Types of soils

granular, cohesive, and organic

Types of granular soils

gravel (>2mm), sand (0.75-2mm), silt(0.005-0.075mm)

Types of cohesive soils

clays (<0.005mm)

Origins of granular soils

residual- physical weathering
transported- many! shallow marine sands, river sands & gravels, dune sands, etc

Origins of cohesive soils

residual- chemical weathering

transported- many! river floodplain deposits, glacial lake deposits, deep sea, etc

Origins of organic soils

plant matter cannot fully decay due to low O2 & waterlogging. This forms soils like peat.

Soil Classification Systems

AASHTO (American Association of State Highway and Transportation Officials) & USCS (Unified Soil Classification System)

USCS

coarse-grained soils, fine-grained soils, and organic soils. Gravel (G), sand (S), silt (M), clay (C), and organic (O). Poorly graded (P, Uniform), Well graded (W, diversified particle sizes). High plasticity (H) and Low plasticity (L). CONSIDER GRAIN SIZE & SORTING as well as behavior of soil in presence or absense of water (Atteburg limits)

Atterburg Limits

Liquid limit (LL) & Plastic limit (PL)

Liquid Limit

Water content (in weight %) at which soils behave as liquids

Plastic Limit

Water content att which the soil behaves as a plastic (play doh)

AASHTO

grain size and Atteburg limits needed.

Grain size is determined by

sieve analysis to create a histogram where weight % is plotted against grain size. Narrow peak means well sorted and broad peak means poorly sorted. You will get statistics such as C_u and C_c.

C_c

coefficent of curvature. D₆₀/D₁₀

C_u

coefficient of uniformity. (D₃₀)²/(D₁₀*D₆₀)

D_x

diameter at which x% passes through the soil sieve. **You get this by looking at the graph**

Liquid Limit Test

Make a mudpie in a bowl, then make a groove in the mudpie. Then, the bowl will bounce off a rubber base using a crank, and then see how many drops are required to fill the gap you created. Then, you know if you’re beyond the liquid limit or not, 25 drops just close.

Plastic Limit Test

make “worms” out of clay, make a certain diamter→ if it breaks its too dry

Plasticity Index (P_I)

LL-PL

Shear strength of soils

soil needs strength, or everything would be flat

angle of repose

Pour the soil out of a bucket, and the angle of the pile is the respected angle

Gravity’s effect on slopes

Gravity works to eliminate slopes TPED→ tectonics proposes, erosion disposes

Soil has strength

its ability to resist stress, of which there are 3 kinds: compressive, tensional, and shear

Compressive stress

forces act towards each other, compressing

Tensional stress

forces act away from each other, tension.

Shear Stress

forces act towards each other on two planes of material, allowing them to slide past each other, shearing.

Coulumbs Equation

Shear Strength = cohesion + NF * tangent angle of internal friction

Where course-grained soils get their strength

friction

Where fine-grained soils get their strength

mainly cohesion but some friction

Tests for shear strength

unconfined compression test, direct shear test, and triaxial compression test

Direct Shear Test

plots shear strength vs normal force, granular soils start at the origin, and cohesive soils start with a higher y-value. The steeper the line, the more strength the soil gets from friction (angle of internal friction based off of the line)

Objectives of Direct Shear Test

1. Determines shear strength at varying normal forces

2. gives you they-intercept

Advantages of Direct Shear Test

easy, fast, cheap

Disadvantage of Direct Shear Test

forces the failure plane to be horizontal

Factors affecting shear strength of granular soils

compactness, particle surface roughness, water (pwp→directly opposes NF)

Modified Coulumb’s Equation

Shear Strength = cohesion + (NF-PWP) tan angle of internal friction

Factors affecting shear strength of cohesive soils

type of clay mineral and water content

soil settlement

the vertical component of reduction in soil volume that occurs when a load is applied (results from compaction and consolidation)

increased compaction and consolidation, increase engineering qualities

decrease settlement potential and increase shear strength

Predicting Settlement Potential

SPT (standard penetration test (FIELD)), Consolidometer (meters of consolidation (LAB), Nuclear Densiometer (FIELD).

Reducing Settlement Potential

Proctor Compaction Test (LAB) is easy and cheap

Standard Penetration Test (SPT) (FIELD)

Split Spoon Sampler→ pound into ground and see how much resistance. 140 lb hammer falls 30 inches to impact this device. Back of a drill rig: cable pulls the hammer up and lets it free-fall. # of drops or blows correlates to soil density. 3 trials and 6” at a time, the first 6” trial is thrown out, other two are kept.

Hollow Stem Auger Drilling (FIELD)

5’ in length with flutes going around (auger), cuttings come off when you drill into the ground, and they pile at the top of the hole. You stop when at the required depth, then insert a solid rod with a split-spoon sampler down the pipe. You can then remove the core and perform the SPT via SSS.

Geoprobe (FIELD)

An alternative to a truck-mounted rotary rig. It’s small and compact, maneuverable (track mounted), some can rotary drill, but more commonly they can do direct push without rotation. It includes a metal pipe with a plastic liner (pipe), which forces the sample into the plastic pipe. Commonly used for chemical spills and determines if the soil is poorly graded or well sorted.

Consolidometer (LAB)

Clay soils don’t consolidate immediately; granular soils do. Clay gives up water in the pores of the deposit. This device is applied to wet clays. Clay is put into a hockey puck core, with porous stones above and below the sample. Water doesn’t compress, so it needs to go somewhere. This is inside a bigger tube so a load can be applied in a constant even force.

Nuclear Densiometer (FIELD)

Field test to determine soil density. Gamma radiation to measure soil density, before you put the box down, you hammer a tube into the ground ~18”. Move the ND over the hole, and the ND has a probe that you put in the hole. In the probe, theres a gamma radiation source. The ND has detectors to pick up the “signal” from the radiation. Gives you the density, and is not so much for natural soils & landscapes. Used primarily for road work, laying new roads to make sure the substrate is compressed well enough to lay the road down. Typically used by independent agencies or consulting companies. Troxler Nuclear Densiometer

Proctor Compaction Test (LAB)

Determines optimum H2O content for compaction treatments. Example: Gravel from a dump truck + roller & vibration for compaction. (Presettlement) You need wet gravel, but not too much, because water reduces friction, but not enough to fill the pore space. Fill the cylinder just barely above the top. The piston is pushed down for a certain distance (# of drops) 5.5 lb hammer (25 drops per se), measure distance, put the sample in oven to determine dry density, you now have volume and mass (D=M*V), re-run trials to provide a compaction curve. The graph plots H2O content against dry density. GIVES YOU MAX DRY DENSITY and OPTIMUM H2O content for compaction treatments.

Water Weight

W_w = W_soil(mc_f -mc_i)

Clay Mineral

sheet or phyllosillicates

T Sheet

O and Si

O Sheet

O, OH, Al, Mg, Fe, etc

Clay minerals can stack as

TO or TOT, there are different metallic ions in the lattice (more common in TOT)

Adjacent TO sandwiches are held together

by hydrogen bonds

Adjacent TOT sandwiches are held together by

Van der Waals forces, theyre weak and water gets into the spaces

Each platy clay grain has a net negative charge via

Isomorphous substitution in oct. sheets (replacement of one atom by another of similar size in a crystal structure without disrupting or seriously changing the structure), unsatisfied ions on the crystal surface, pH (un)availability of H+ ions in the solution

Plasticity Index→swelling capacity

PI

0-10, low inherent swelling capacity

10-20, medium

20-35, high

>35, very high

Why is water attracted to clays?

hydrogen bonding to specific sites and general electrostatic attraction

Why do some clays swell more than others?

type of clay (TO vs TOT), geometry of the clay grain, and charge of cations in the lattice

Bentonite uses

drilling mud, plugging leaks, clumping cat litter, taconite pellets.

Why dont micas swell?

TOT clays with Al 3+ subs for Si +4

Identifying expansive soils

mineralogy via x-ray diffraction, consolidometer, free swell test, and PI (relate to chart)

Engineering Procedures for Preventing Swelling/Shrinking (clays)

Avoid them, remove them, control H2O content, and chemical stabilization (cation exchange)

Cation exchange

higher valence replace lower valence, for ions of the same charge, the smaller one is better absorbed

Lime injection

adding lime (CaO) to the soil where Ca +2 replaces the Na+

shear stress > shear strength

shear failure occurs

Rock Fall

A coherent mass that moves down (not in contact with the underlying failure plane) results in undercutting from differential weathering

Soil Fall

A coherent mass that moves down (not in contact with the underlying failure plane) results in undercutting

Rock Slide

A coherent mass moves over a predefined failure plane

Soil Slide (Slump)

A block (slump block) of soil lacks a ready plane of weakness so you get a rotational aspect which leaves a crescentic scarp.

Combination failure

very common*, slump-flow is very typical. Flow can remove basal support and allow for a soil slump or vice versa

Earth flow

not confined to a channel moving downslope moving similar to a viscous fluid incoherent mass

Mudflow

confined to a channel moving downslope, similar to a viscous fluid, moves quicker with soils mixed with water

Creep

VERY SLOW and SNEAKY! very slow flow-downslope movement from many mechanisms, freeze-thaw can cause this, freeze causes crystals to go up and thaw causes crystals to go down. Along a slope gravity pulls materials down.