Section 7 - Compaction, Dewatering, Stress and Settlement, Pilings

Section 7
  1. Compaction
  2. Dewatering
  3. Stress and Settlement
  4. Foundations
  5. Pilings

Compaction - Lecture Notes

Soil is extensively used as a basic material of construction.  For example: dams, dikes, embankments, ramps, etc.  The advantages of using soil are that it (1) is generally available everywhere, (2) is durable - it will last a long time, and (3) has a comparatively low cost.

It is typically placed in layers (sometimes called lifts) with each layer being compacted to develop a final elevation and/or shape.

Why Does It Need Compacted?

Compaction increases a soils density.  This produces the following effects:
  1. increases the soil's shear strength
  2. decreases future settlement
  3. decreases the soil's permeability (also a function of soil type)
  4. stable against volume change as water content or other factors change
  5. relatively durable and safe against deterioration

It is most appropriate to talk about a compaction energy.  The compaction energy given to a soil is proportional to the pressure, speed of rolling, and the number of times it is rolled.  A unique aspect of soil is encountered when one wants to maximize the density but minimize the compaction energy - which makes good business sense.  For a given compaction energy, there is an optimum water content that will obtain a maximum dry density.  Too little or too much water content will cause a smaller dry density.  The water acts as a lubricant and allows the soil particles to squeeze together more easily.

The Standard Proctor Test is a laboratory test used to determine the optimum water for a given compaction energy, for a given soil.  The graph below illustrate the results obtained from a Standard Proctor test:


Quick glance at the Standard Proctor test procedures (ASTM D 1557): (1) dry sample until friable (easily crumbled) with trowel (2) prepare at least 4 samples using the same soil but different moisture contents (3) wait for a specified curing time (4) compact (gives a standard energy/vol) (5) measure g and w.

Types of compactors (machine images courtesy of Bomag GmbH)


Hand Compactor
(motorized and non-motorized)

Walk Behind Roller

Walk Behind Vibratory Plate


Walk Behind Double Smooth Roller


Towed Single Roller
(Vibratory or non-vibratory)




Smooth Roller
(Many times vibratory)



Smooth Roller

Pneumatic Roller (smooth rubber tires)

(Protrusions stick out from smooth roller, can supply pressures in excess of 600 psi or 4200 kN/m2)


(Not a compactor, but often used in conjunction with compactors)


Heavy Compactor/Bulldozer
(also a "Sheepsfoot" compactor)


Soil type, water content, and type of compactor are factors that need to be considered when compacting.  Compaction is often used when fill (disturbed soil from another location and transported) is used at a construction site.  This implies you may be using self-propelled scrapers (earth movers), bulldozers, and graders.  An earthcut or "borrow" is popular to use.  A borrow is simply a hole dug (usually near the construction site) so that soil from this hole is used elsewhere as fill.


Borrows and fill dirt being used at a construction site.  Notice the darker top soil
with the lighter subsoil.


Rules of Thumb For Compacting Soils:
I.  Granularsoils can be compacted in thicker layers (or "lifts) than silt or clay.
II.  Fill placed underwater (or requiring good drainage properties) should consist of granular or coarse material.
III. Check to make sure natural soil is adequate for supporting compacted fill.  This can be tested by rolling over it with a heavy piece of equipment and observing compaction characteristics (called "proof-rolling").
IV.  Cohesionless soils usually need kneading, tamping, vibratory compacting.  (Note: kneading is defined as working by folding.)  Cohesive soils usually need kneading, tamping, or impact.  Heavy cohesive soils can sometimes require dynamic compacting that uses large weights dropped from heights or underground dynamite with directed explosions.

Compaction Control Field Testing:
1.  Sand Cone - requires hole excavated, weigh the soil removed and determine the volume of the hole with sand.  This is done by filling the hole with a sand of known density.
2.  Washington Densometer - requires hole excavated
3.  Oil Replacement - requires hole excavated, weigh the soil removed and determine the volume of the hole with a device with an expandable rubber membrane
4.  Nuclear Densometer - uses a radioactive source and "counter" to determine soil density.  This method has fast results with the potential for a large number of tests in a short time.  It is usually calibrated with the Sand Cone method.

Link to nuclear densometer page.

Compaction Characteristics and Soil Grouping in USCS

Group Symbol

Compaction Characteristics

Compressibility and Expansion

Value as Embankment Material

Value as Subgrade Material



Very Little

Very Stable




Very Little

Reasonably Stable

Excellent to Good




Reasonably Stable

Excellent to Good




Reasonably Stable




Very Little

Very Stable




Very Little

Reasonably Stable when Dense

Good to Fair




Reasonably Stable when Dense

Good to Fair


Good to Fair

Slight to Medium

Reasonably Stable

Good to Fair


Good to Poor

Slight to Medium

Poor, gets better with high density

Fair to Poor


Good to Fair



Fair to Poor


Fair to Poor


Poor, Unstable

Poor to Not Suitable


Compaction - Related Web Links

Compaction Meter - A new device on the market that measures the strength of impulses transmitted through the soil as a function of compaction.
Marcel Equipment Limited (company that sells used compactor equipment)
Bomag GmbH (company that sells compactor equipment)
Standard Proctor test equipment
Google - Search for Soil Compaction

Dewatering - Lecture Notes

When soil is excavated below or near the water table, pumps will usually be used to dewater the site.  This involves creating a drawdown curve (or cone of depression) that is below the base of the excavation.  Factors that are important include soil permeability, depth of water table, depth (and geometry) of excavation.


Single stage dewatering

The above diagram illustrates a dewatering technique using small trenches dug around the perimeter of the excavation.  One can estimate the pumping requirements based upon the formula


(reference: Soils In Construction, W.L. Schroeder, S.E. Dickenson, Prentice Hall 1996, pg. 162).  The value D represents the radius of influence, H is the depth to an impermeable layer from the original water table, ht is the height of the water level in the interceptor ditch with respect to the impermeable layer, k is the soil's permeability, and q is the pump per unit length of ditch.  A more elaborate two-stage dewatering technique is shown in the diagram below.


Multi-stage dewatering

As a general rule, when the excavation is deep (with respect to the water table) and the soil is very permeable (i.e. gravel or sand), a high pumping rate will be required.  For an excavation that extends just slightly below the water table and the soil is somewhat impermeable (i.e. clay or silt), a lower pumping rate is required.  Be careful, the depth of a water table varies as a function of time for any given site!  This means that the depth of the water table varies with seasons or possibly local precipitation.

Drawdown curve for an excavation site with two pumps.

Create a 3-dimensional graph of a drawdown surface using with an Excel spreadsheet with up to 20 dewatering pumps.  (This requires you to have Microsoft Excel 97 on your computer.  A fast computer is also preferable.)

Dewatering - Related Web Links

Griffin Dewatering Corporation (lots of dewatering techniques and explanations)
Google - Search for Dewatering Excavation

Stress and Settlement - Lecture Notes

A strange case of Palace of Fine Arts in the Alameda area of Mexico City.  Built sometime between 1900 and 1934, it was a magnificent and strongly built structure.  It was built on grade, level with the square and other buildings nearby.  But because of loose sand permeated with water in the subsurface, the massive structure sunk 6 ft into the ground!  (Luckily, it settled evenly minimizing structural damage.)  Believe it or not, in the 1960's the building moved again.  This time it moved 12 ft up!  The weight of skyscrapers being built around the Palace had pushed the subsurface water and soil around sufficiently to raise the building.
(Source: Why Buildings Fall Down, M. Levy and M. Salvadori, WW Norton & Company, 1992)

The Milwaukee Metropolitan Sewerage District (MMSD) agreed to a $24 million settlement in a claim against the engineering firm CH2M Hill.  MMSD claimed the engineering firm mis-judged the weak bedrock and potential for flooding in the designs of a 5.3-mile North Shore deep tunnel project.  This project was designed to store raw sewage during rain-storms and snow melts, preventing the polluted water from fouling the area's rivers and Lake Michigan.  MMSD also agreed to pay $3.5 million to settle claims from downtown businesses.  These businesses claimed water pouring into the tunnel drained ground water under downtown businesses, causing building foundations, walls, sidewalks and sewer connections to crack.
(Source:  Milwaukee Journal Sentinel, December 5, 1998)

Worlds oldest building code, the Code of Hammurabi.

Settlement and Consolidation of Soils

Any structure built on soil is subject to settlement. Some settlement is inevitable and, depending on the situation, some settlements are tolerable. When building structures on top of soils, one needs to have some knowledge of how settlement occurs and predict how much and how fast settlement will occur in a given situation.

Important factors that influence settlement:

  • Soil Permeability
  • Soil Drainage
  • Load to be placed on the soil
  • History of loads placed upon the soil (normally or over-consolidated?)
  • Water Table

Settlement is caused both by soil compression and lateral yielding (movement of soil in the lateral direction) of the soils located under the loaded area. Cohesive soils usually settle from compression while cohesionless soils often settle from lateral yielding - however, both factors may play a role. Some other less common causes of settlement include dynamic forces, changes in the groundwater table, adjacent excavations, etc. Compressive deformation generally results from a reduction in the void volume, accompanied by the rearrangement of soil grains. The reduction in void volume and rearrangement of soil grains is a function of time. How these deformations develop with time depends on the type of soil and the strength of the externally applied load (or pressure). In soils of high permeability (e.g. coarse-grained soils), this process requires a short time interval for completion, and almost all settlement occurs by the time construction is complete. In low permeable soils (e.g. fine-grained soils) the process occurs very slowly. Thus, settlement takes place slowly and continues over a long period of time. In essence, a graph of the void ratio as a function of time for several different applied loads, provides an enormous amount of information about the settlement characteristics of a soil.


Pressure (or load) is defined as the amount of weight being distributed over an amount of area. 

Mathematically: eq1_4.

Overburden pressure is the effective pressure (sometimes referred to as effective weight) of the overlaying soil. This can be calculated according to the formula P=gh where g is the unit weight of overlaying soil and h is the depth.

Normally consolidated clay has never been subjected to any loading larger than the present effective overburden pressure. The height of the soil above the clay has been fairly constant through time.

Overconsolidated clay has been subjected at some time to a loading greater than the present overburden pressure. This type of clay is generally less compressible.

Coefficient of consolidation, cv, is a measure of how fast and how much a sample of soil will deform under a load. A large value indicates a fast consolidation and a low value indicates a slow consolidation.

Estimating Settlement in Clay and Sand

How fast does the soil settle?

The process of obtaining a quantitative prediction of how much a soil will settle and how fast begins with examining a plot of soil deformation as a function of time for a given load. The soil deformation will correspond to a void ratio. Figure 1 shows such a plot.


Figure 1

Primary consolidation of the soil happens before point A on the graph. The secondary consolidation happens after point A and is characterized by a very slow settlement. The coefficient of consolidation, cv, for a particular loading is related to the shape of this graph and is defined as

eq2_1                                           (1)

where H is the thickness of the test specimen at 50% consolidation, and t50 is the time to 50% consolidation. One can use this parameter to calculate the time rate of settlement with equation 2 and figure 2. The time, t, to reach a particular percent of consolidation is

eq3_1                                               (2)

where H is the thickness of the consolidating layer, Tv is a time factor that depends of the percent consolidation and is obtained from figure 2, and cv is the coefficient of consolidation.


Figure 2

How much will the soil settle?


Figure 3

Now, to calculate the total settlement due to primary consolidation, we need to introduce the equation (derived from figure 3):

eq4_1                              (3)

= total settlement due to primary consolidation,
eo = initial void ratio of the soil in situ,
e = void ratio of the soil when subjected to a total pressure (p),
H = thickness of the consolidating clay layer (if the cohesive soil layer is underlain by sand and gravel then use ½H for the thickness and use H if underlain by bedrock) ,
p = total pressure acting at midheight of the consolidating layer,
po = present effective overburden pressure at midheight of the consolidating layer.

The constant Cc is the compression index and is equal to the slope of the curve indicated in figure 3. Its value can be calculated by

eq5_1                                        (4)

with the variables defined the same as in equation 3.

Example: Consider an 8 ft clay layer beneath a building that is overlain by a stratum of permeable sand and gravel and is underlain by impermeable bedrock. The total expected consolidation settlement for the clay layer due to the footing load is 2.5 in. It is also known from laboratory tests that cv=2.68x10-3 in.2/min.

Find: (1) How many years it will take for 90% of the total expected consolidation settlement to take place? (2) What amount of consolidation settlement will occur in 1 yr.?

t = (Tv/cv)H2
Tv = 0.848  (using U = 90% in figure 2)
H = 8ft(12in/1ft) = 96 in
t90 = ( (0.848)(96in)2 )/(2.68x102 in2/min) = 2.9x106 min
2.9x106 min (1hr/60min)(1d/24hr)(1yr/365d) = 5.5 years

Work part one in reverse:

t = (1yr)(365d/1yr)(24hr/1d)(60min/1hr) = 5.26x105 min
Tv = (tcv)/H2 = ( (5.26x105 min)(2.69x10-3 in2/min) )/(96in)2 = 0.15
Tv = 0.15 corresponds to U = 43% (figure 2)
Thus, S1yr = (2.5in)(0.43) = 1.08 in.

In sandy soils, settlement occurs fast (soil is usually settled before construction is done) and the amount of settlement is determined in a different way than cohesive soils. The maximum settlement on dry sand can be calculated by


where smax is the maximum settlement (inches), q is the applied pressure (tsf), B is the width of the footing, and Nlowest is a number of blows required to drive a rod while following a standard set of procedures. It should be noted that this equation has a correction factor if the groundwater table is close to the footing.

Example of a settlement analysis with a high water table and multiple soil layers

p1 Time of Soil Settlement Animation
Illustrates primary and secondary rates of consolidation.

p2 Soil Compression Characteristics Animation
Soil does not behave like a spring (i.e. it does not follow Hooke's Law). Once compressed it rebounds only slightly upon un-loading.  This animation demonstrates the different behavior for normally consolidated and over-consolidated soil.

Settlement cracks that have developed in the masonry near the the Stout physics department offices in Jarvis Hall before the remodeling of 2009.

Same crack line but on the opposite side of the wall.  The crack goes right into the floor tiling.



Differential settlement of the Soil Retaining wall at UW-Stout during the summer of 2001.

Stress and Settlement - Related Web Links

An Engineering Ethics Cases With Numerical Problems (an example with soils) from Texas A&M
Geotechnical Engineering Hall of Fame (lots of famous people that developed the field of soil mechanics)
Soil Testing Services and Suppliers
Soil Settlement and Its Effect on Buildings
Legal company that specializes in expansive soil claims
Google - Search for

Foundations - Lecture Notes

"On account of the fact that there is no glory attached to the foundations and that the sources of success or failure are hidden deep in the ground, building foundations have always been treated as step children and their acts of revenge for the lack of attention can be very embarrassing."
~ Karl Terzaghi  [source: Lundin, T., 2001, Are you saving nickels or dollars? Hanson Insight newsletter, May]

Important aspects to be aware of:

I.  In the design plans, the depth of the footings should be indicated with respect to the final grade around the house.  Foundation footings should be no less than 4 feet deep (Wisconsin Standard) and should not be placed onto disturbed soil.  The footings need to be below the frost line.  The frost line is the depth to which soil freezes during the winter.  The soil above the frost line is subject to large amounts of fost heaving and shrinking (when ice melts) and can cause extreme cracking for too shallow of foundations.

II.  Foundation footings should be placed upon good soil.  This information can be obtained by soil exploration and laboratory testing.  One could also ask neighbors about their foundations and the extent of cracking in their walls.

III.  The sewer pipe should enter the house below the footing (sometimes at a depth of 8 inches from the bottom of the footing to the top of the pipe).  The sewer pipe should have a slope of about 1/8in. every foot causing contents to move away from the house.

Bearing Capacity for Shallow Foundations

Structure foundations are subject not only to settlement but also to shear failures. First of all, foundations usually have the design of an inverted T. Where columns or walls are resting on a footing and the footing has an enlarged area to reduce the pressure exerted on the soil for a given load. In general, foundations must be designed to satisfy the following criteria:

  1. They must be located properly (both vertically and horizontally orientation) so as not to be adversely affected by outside influences.
  2. They must be safe from excessive (or non-uniform) settlement.
  1. They must be safe from bearing capacity failure (shear failure).

There are three modes of shear failure: general shear failure, local shear failure, and punching shear failure.  These modes characterize the stress-strain dynamics that happen in certain soil types.

General shear failure is identified by a well-defined wedge beneath the foundation and slip surfaces extending diagonally from the side edges of the footing downward through the soil, then upward to the ground surface. The ground surface adjacent to the footing bulges upward. Soil displacement is accompanied by tilting of the foundation (unless the foundation is restrained). The load-settlement curve for the general shear case indicates that failure is abrupt.


Punching shear failure involves significant compression of a wedge-shaped soil zone beneath the foundation and is accompanied by the occurrence of vertical shear beneath the edges of the foundation. The soil zones beyond the edges of the foundation a little affected, and no significant degree of bulging occurs. Aside from a large settlement, failure is not clearly recognized.


Local shear failure has elements of both general and punching shear failure. It has well-defined slip surfaces that fade into the soil mass beyond the edges of the foundation and do not carry upward to the ground surface. Slight bulging of the ground surface adjacent to the foundation does occur. Significant vertical compression takes place beneath the foundation.


Terzaghi has developed a theory that predicts the ultimate bearing capacity a soil has in regards to shear failure. Before working with the formula’s, it is important to understand the terms "ultimate bearing capacity" (qult) and "allowable bearing capacity" (qa). The ultimate bearing capacity of a soil refers to the loading per unit area that will just cause shear failure in the soil. Allowable bearing capacity refers to the loading per unit area that the soil is able to support without unsafe movement. Such that, (qult) = (safety factor)x(qa). The formulas for calculating the qult are:

Continuous Footings (width B):

qult = cNc + gamma symbolDfNq + 0.5gamma symbolNg

Circular Footings (radius R):

qult = 1.2cNc + gamma symbolfNq + 0.6gamma symbolNg

Square Footings (width B):

qult =1.2cNc + gamma symbolfNq + 0.4gamma symbolNg    

qult = ultimate bearing capacity,
c = cohesion of soil (measured with a shearvane - as a rule of thumb, the unconfined compressive strength is about twice the cohesion of the soil), 
gamma symbol= effective unit weight of soil,
Df = depth of footing, or distance from ground surface to base of footing,
B = width of continuous or square footing,
R = radius of circular footing,
Nc, Ng, Nq = soil-bearing capacity factors, dimensionless terms, whose values relate to the angle of internal friction, j.  These values can be calculated when j is known or they can be looked up in the table below.

Nq = epitanjphitan2(45o + phi/2)    
Nc = (Nq - 1)cot(phi) when phi> 0o or Nc =5.14 when phi = 0o.
Ngamma symbol = 2(Nq + 1)tanphi




Ngamma symbol





































































As an example of using these equations, consider a strip of wall footing 3.5 ft wide and is being supported in a uniform deposit of stiff clay.  (The stiff clay implies j = 0o.) The unconfined compressive strength (by a pocket penetrometer) of this soil is 2.8 kips/ft2 (1 kips = 1000 lbs). The unit weight is 130 lb/ft3. There was no groundwater encountered and the depth of the wall footing is 2 ft.

Find the ultimate bearing capacity of this footing and the allowable wall load, using a factor of safety of 3.


qult = cNc + gamma symbolDfNq + 0.5gamma symbolBNg
c~qu/2 = (2.8 kips/ft2)/2 = 1.4 kips/ft2
gamma symbol = 0.130 kips/ft3
Df = 2 ft
B = 3.5 ft
from the table above: Nc = 14.0, Nq = 3.9, Ngamma symbol = 2.6

qult = (1.4 kips/ft2)(14) + (0.130 kips/ft3)(2 ft)(3.9) + (0.5)(0.130 kips/ft3)(3.5 ft)(2.6) = 21.2 kips/ft2

qa = qult/3 = (21.2 kips/ft2)/3 = 7.1 kips/ft2

The Terzaghi equations above do not consider eccentric (torques or non-vertical forces) loads, inclined foundation base, or footings on or near slopes. The bearing capacity of footings placed into sloping ground is less than if the footings were on level ground. In fact, the bearing capacity of a footing is inversely proportional to ground slope. Modifications to the Terzaghi equations do exist and enable one to calculate the ultimate bearing capacity under eccentric loads, inclined foundations, and sloped ground.

Foundations - Related Web Links

Septic Systems and Soil Failure
Foundation Cracking Slide Show (Funded by FEMA)
Google - Search for Soil Stress
Google - Search for Soil Settlement

Pilings - Lecture Notes

During the 1950's, a large hotel was to be built along the coast in Florida.  After performing soil explorations, the geotechnical engineers recommended 30 ft long friction piles to support a 25 story hotel.  During the last drop of a pile driver's weight, one of the piles disappeared!  It had suddenly busted through to a very weakly supporting soil layer called "Florida pancake" that was not identified in original explorations.  The piles had to be lengthened to 140 ft.
(Source: Why Buildings Fall Down, M. Levy and M. Salvadori, WW Norton & Company, 1992)

Pile and Caisson Foundations

When an extended layer of soil is unsuitable to build upon because of bearing capacity failure or excessive settlement, a Pile or Caisson foundation can be used to support structures. These foundations are designed to transmit the load of a structure to firmer soil, or rock that exists deep below the structure.

Pile foundations consists of a long and slender "member" that is forced or driven into the soil. It is driven until it rests on a hard, imprenetrable layer of soil or rock, the load of the structure is transmitted primarily axially through the pile. This type of pile is an end-bearing pile. If the pile cannot be driven to a hard stratum of soil or rock, the load of the structure must be borne primarily by skin friction or adhesion between the surface of the pile and adjacent soil. This is a friction pile. Piles can be made of timber, concrete (precast or cast-in-place), or steel (pipe-shaped or eye-beam shaped). Sometimes piles are a combination of these materials.

Caisson foundations usually consist of a structural box or chamber that is sunk in place or built in place by systematically excavating below the bottom of the unit, which thereby descends to the final depth. The drilled caisson is another type (less extensive in scope than the box type) that is constructed by using an auger drill to forma hole in the soil in which concrete is eventually poured.

Illustrations of different piling

I-Beam Piling

Concrete Bulb Piling

      (material removed from inside)

Larger Shaft Caisson For Performing Work Within the Caisson

Pile type


Description of use and availability

Range of Maximum Load


Depends on wood (tree) type. Lengths in the 50 to 60 ft range (15 to 18 m) are usually available in most areas; lengths to about 75 ft (25 m) are available but in limited quantity; lengths up to the 100 ft range (30 m) are available, but supply is very limited.


Steel H and pipe

Unlimited length; "short" sections are driven and additional sections are field-welded to obtain a desired total length.


Steel shell, cast-in-place

Typically to between 100 and 125 ft (30 to 40 m), depending on shell type and manufacturer-contractor.

250- 700

Precast concrete

Solid, small cross-section piles usually extend into the 50 – 60 ft length (15 to 18 m), depending on cross-section shape, dimensions, and manufacturer. Large-diameter cylinder piles can extend to about 200 ft long (60 m).


Drilled-shaft, cast-in-place concrete

Usually in the 50 – 70 ft range (15 to 25 m), depending on contractor equipment.


Bulb-type, cast-in-place concrete

Up to about 100 ft (30 m).



Related to available lengths of material in the different sections. If steel and thin-shell cast-in-place concrete are used, the length can be unlimited; if timber and thin-shell cast-in-place concrete are used, lengths can be on the order of 150 ft (45 m).

250- 600


Pilings - Related Web Links

Geopiers - New type of pier for intermediate soils
, Co. (pictures of piling and pile testing)
Timber Pilings
Piling Rigs
Sheet Piling
Google - Search for Piling or Caissons
Google - Search for Piling or Caissons Pictures