ENCE4610FoundationDesignandAnalysis

Expansive and Collapsible Soils

OverviewofCollapsibleSoils

CollapsibleSoilsTypes of Collapsible Soils Loess Deposits Quick clays (highly

sensitive) Loose sands subject to

liquefaction Loose sands held

together by apparent cohesion

Saprolites with high void ratio

IdentificationofCollapsibleSoils

DealingWithCollapsibleSoils

Other Foundation Solutionso Shallow Foundationsfurnish a

system of grade beams to distribute the load and mitigate the effects of uneven collapse of the underlying soils

o Deep foundationsavoid the effects of collapsible soils altogether by transferring the structure load to a more stable stratum

Solutions for Pavementso Ponding water over the region

of collapsible soils.o Infiltration wells.o Compaction - conventional

with heavy vibratory roller for shallow depths (within 0.3 or 0.6 m (1 or 2 ft))

o Compaction - dynamic or vibratory for deeper deposits of more than half a meter (a few feet) (could be combined with inundation)

o Excavated and replaced.

ExpansiveSoilsDefinition Location and Nature

Expansive soils are soils which undergo significant volume change with the addition or deletion of pore water

Generally the result of the chemical structure of certain types of clay soils (usually montmorillonite)

In the United States, most expansive soils are located in the central and western parts of the countryo Shrinking and swelling are

aggravated by wide swings in the water content, which in turn are due to climactic conditions and wide varieties in rainfall during the year

RegionsofExpansiveSoils

DamageDuetoExpansiveSoils

FoundationHeaveduetoMoistureChanges

PotentialSwell Use: a method of

making a preliminary, qualitative evaluation of a soil to determine if measures to deal with soil expansion need to be taken

Can be determined either directly or through index properties

Formal definition: percentage of swell of a laterally confined sample in a consolidometer test which is fully saturated under a surcharge load of 7 kPa (1 psi) after being compacted to maximum dry density at the optimum moisture content

PotentialSwellWES Method of Determining

Potential Swell Other Indirect Methods Based on extensive field

tests Soils where LL < 40 and PI

< 15 generally are not expansive

Knowledge of soil composition is important but not absolutely necessary

DirectDeterminationofVolumeExpansion

Consolidometer Swell Test Simplified Swell Test Procedure Described in Appendix VIII of EM

1110-2-1906. (Consolidation test) The swell test may be performed to

predict vertical heave H of soil thickness H when the vertical overburden and structural pressures on thickness H are known prior to the test.

The total vertical heave at the ground surface is the sum of the H for each thickness H in the soil profile. Figure 5-4 illustrates the application of swell test data. The swell pressure test is performed to evaluate the swell pressure and swell index required for prediction of vertical heave. The confining pressure required to restrain heave is defined as s.

An initial loading pressure, simulating field initial (preconstruction) vertical pressure, should be applied to determine the initial void ratio eo, point 1 of Figure 4-2 (following slide), then removed to the seating pressure (i.e., the lowest possible load) prior to adding distilled water, point 2. The specimen is allowed to expand at the seating pressure until primary swell is complete, point 3, before applying the consolidation pressures.

The swell test of Figure 4-2 can eliminate the need for additional tests when behavior is different than that anticipated (e.g., the specimen consolidates rather than swells following addition of water at loading pressures greater than the seating pressure). The void ratio-log pressure curve for final effective pressures, varying from the seating to the maximum applied pressure, can be used to determine heave or settlement with respect to the initial void ratio. Net settlements will occur for final effective pressures exceeding the swell pressure. Figure 4-2 illustrated how the percent swell or heave may be found with respect to the initial vertical pressure.

The s in Figure 4-2 is defined as a confining pressure that must be applied to the soil to reduce the volume expansion down to the (approximated) in situ eo in the presence of free water. Consolidometer tests in appendix VIII of EM 1110-2-1906 tend to provide lower limits of the in situ swell pressure, while the simple swell test, Figure 4-2, tends to provide upper limits. The maximum past pressure is often a us

SimpleSwellTest

ApplicationofConsolidometerResults

DeterminationofActualSoilExpansion

VanderMerwe Method

BasisforCalculations

FoundationsforExpansiveSoils

ShallowFoundationsforExpansiveSoils

UseofDrainageTechniques

BelledBasesforDrilledShafts

z Use of an enlarged base, in conjunction with the shaft resistance of the straight portion, is a common way of dealing with expansive soils for major structures

z Enlarged bases offer additional uplift capacityz Only applicable to clays, since

sands would usually collapsez Difficult to quantify

UpwardLoadCapacityofBelledShafts

z Variables for uplift capacityz (Pupward)a = allowable upward

load capacityz su = average undrained shear

strength of the cohesive soil between the base of the bell and 2Bb above the base

z zD = total stress at the bottom of the base

z Bb = diameter of the enlarged base

z Bs = diameter of the shaftz Db = depth of embedment of

enlarged base into bearing stratum

z Uplift of base only

( ) ( )

90.7

Clays) (

93.5

Clays) (4

22

b

bu

b

bu

uusb

aupward

BD=N

FissuredBD=N

Unfissured

NsBB=P

ExampleofUpliftCapacity

z Givenz Drilled Shaft as shown

z Findz Uplift capacity of shaft

z Assumez Factor of safety = 5z Very stiff clay at toe is fissuredz Include weight of foundation

ComputeShaftFrictionandNu

LayerLayer Start

Layer End

Thickness, ft.

su, psf alpha

fs, psf

As, sq. ft.

fsAs, kips

1 0 5 5 1600 0 0 31.416 02 5 12 7 1600 0.55 880 43.98 38.73 12 37 25 1400 0.55 770 157.08 1214 37 50 13 4000 0.48 1920 81.68 156.85 50 60 10 4000 0 0 62.83 0

Total 316.5

Perimeter 6.28319Top Segment to be Ignored 5Toe Segment to be Ignored 10

Different from compressionEqual to twice the diameter of thebelled toe

93.225

230.70.7 ==BD=Nb

bu

ComputeUpliftCapacity Compute Weight of

Foundation and Total Uplift Capacity

Compute Ultimate Uplift Capacity of Bell

( ) ( )( ) ( )

( ) kips 212)22.3)(4(

4254

22

22

=P

=P

NsBB=P

aupward

sbaupward

uusb

aupward

( ) kips=+=Pkips=W

+

+=W

aupward

f

f

965

2120.7531730 30

0.1501.542

52

58.542

2

2

+

Questions