Soils Exam 2

what two maps are superimposed to produce a grading plan

existing topographic maps and proposed topographic maps

net import

extra soil needs to be hauled to the site

net export

extra soil needs to be hauled away from the site

balanced earthwork

neither import not export

positive shrinkage factor

net volume decreased

negative shrinkage factor

net volume increased

fluid/water flows from

high total head to low total head

energy is dissipated in overcoming the soil resistance and hence is

the head loss

energy equations used in fluid flow

Bernoulli’s equation, darcy’s law equation

Beroulli’s equation defines

total head

Darcy’s law relates

flow velocity to head loss

what is potential energy from

due to elevation above datum

what is the strain energy from

due to pressure in water

what is the kinetic energy from

due to velocity of water

For flow of water through soils, the _____ is very small.

flow velocity

The the ____ can be neglected

velocity head

water flows only when there is

a total head difference or energy difference

hydraulic gradient

change in total head per unit distance in the direction of flow over which change occurs is called

hydraulic conductivity

coefficient of permeability, k, describes ability of water to flow through soils

factors that hydraulic conductivity depend on

fluid viscosity, grain-size distribution, void ration, soil type, and degree of saturation

four possible systems with repect to spatial and directional variability

isotropic and homogeneous, anisotropic and homogeneous, isotropic and heterogeneous, anisotropic and heterogeneous

most natural systems are

anisotropic and heterogeneous

constant head test is used for

coarse-grained soils

falling head test is used for

fine-grained soils

permeability is the function of

porous media only

hydraulic conductivity is the function of

porus media and the fluid

applications for 2D flow analysis

seepage calculation through earth dams, levees, and other hydraulic structures

seepage quantity

how much water is lost through/under a structure

flow net analysis allows for the evaluation of

seepage quantity, uplift pressures, and piping/erosion failure

soil is completely

staurated

soil is ___ and ____

homogeneous and isotropic

flow line/stream line

line along which water molecules travel from upstream to downstream

equipotential line

line along which the head is constant

Compaction is defined as

densification of soil by removal of air using mechanical energy

on a standard proctor compaction curve, dry unit weight is max at

the optimum moisture content

How does compaction densify soil

by expelling air using mechanical energy, it increases shear strength and erosion resistance while decreasing compressibility and hydraulic conductivity

for a given compaction effort there is an optimum water content at which what is achieved

maximum dry unit weight

dry of optimum, added water lubricates particles and ____ unit weight

increases

wet of optimum, added water displaces solids and ____ unit weight

decreases

compared to the standard proctor test, the modified proctor test uses

a heavier hammer with a larger drop, producing more compactive energy

the modified proctor test uses a heavier hammer and larger drop with the same mold in order to

better represent field conditions for heavy loads like aircraft and trucks (4.5 times the energy of the Standard Proctor test)

As compaction effort increases the compaction curve

shifts up and to the left (higher dry unit weight, lower optimum moisture content)

The zero-air-voids (ZAV) line on a compaction plot represents:

S=100%

For cohesive soils, a typical compacted lift thickness in the field is about

200 mm (8 in)

Going from the borrow (in-situ) condition to the final compacted fill condition, total volume typically

decreases (shrinkage)

In borrow-fill-voulme- change calculations, which quantity is assumed to remain constant across the borrow, transport, and fill states?

weight of solids

borrow soil is typically compacted to a ____ state than its natural condition, so the net change from borrow to fill is a volume decrease “shrinkage”

denser

For flow through soils, which component of total head (Bernoulli’s equation) is typically neglected?

velocity head, because flow velocity through soil is very small, making it negligible

i

hydraulic gradient

within the laminar flow regime, Darcy’s law states that discharge velocity is

proportional to hydraulic gradient

since water can only travel through the void space, the actual seepage velocit through the pores is always ___ than the average discharge velocity

greater

As void ratio (e) decreases, for example, due to compaction, hydraulic conductivity (k) generally:

decreases

what soil type generally has the lowest hydraulic conductivity

clays

For flow perpendicular to soil layering, which statement is true?

The layer with the smallest k has the largest head loss across it

the key difference between permeability (K) and hydraulic conductivity (k) is that

K depends only on the porus media, k depends on both the media and the fluid

why cant darcy’s law be applied directly to solve most 2D groundwater flow problems

Hydraulic gradient (i) and area (A) vary throughout the flow domain

Assumptions underlying Laplace’s equation for 2D flow

soil is homogeneous, soil is isotropic, darcy’s law is valid

A flow net is composed of two orthogonal families of curves. What are they?

Flow lines and equipotential lines

In an isotropic soil, flow lines and equipotential lines intersect at

Right angles (90 degrees), forming curvilinear squares

The boundaries of an impervious layer or impervious structure in a flow net are

flow lines

If the total head loss across a flow net is 9m and there are 10 equipotential drops, the head loss per drop is

0.9 m

Uplift (pore water) pressure at a point beneath a hydraulic structure is calculated as

pressure head (hp) times unit weight of water

If the total uplift force beneath a dam exceeds the structure’s weight (pplus any anchoring resistance), the primary risk is

the dam can float or heave, threatening structural stability