Foundation in Difficult Ground Conditions

(1) Collapsible Soils

(2) Expansive Soils

(3) Peat

FOUNDATIONS ON LOESS AND COLLAPSIBLE SOILS

Collapsible soils are generally wind-blown (aeolian) deposits of silts, dune sands, and volcanic ash

Typically they are loose but stable

certain conditions of load + wetting produce a collapse of the soil structure with a resulting large settlement

Pressure (log)

V

o

i

d

r

a

t

i

o

Water added

Loess is a special case of a silt deposit characterized by having been deposited by wind

The grain distribution of loess deposits tends to be limited to approximately the range of 0.01 to 0.10 mm and is usually contaminated with clay and sand particles (< 0.05 mm) and also by organic leachates

Loess and other collapsible deposits are characterized by a complete absence of gravel or pebbles

In situ dry densities range from about 10 to 16.5 kN/m3.

Pressure (log)

V

o

i

d

r

a

t

i

o

Water added

(Eq. 11.2)

(Eq. 11.3)

1. Compact (excavate and replace) the soil to dry < 15.5 kN/m3.

2. Use an admixture during compaction. Admixtures may be lime, lime/fly ash, or Portland cement.

3. Use some means to ensure that the collapsible soil does not get wet (often not practical).

4. Use piles through the collapsing soils to a more competent underlying stratum

Expansive soils

Surface cracks Pop corn texture

-Characterised by

-swelling on absorption of water

-shrinkage on drying

Laboratory observation of behaviour of an expansive soil

Initial dry sample Soil sample after small amount ofwater was added

The same sample 48 hours later, after the sample has had time to shrink to a smaller volume.

Heaving caused by swelling soils Uneven distribution of expansive soils

Severe damages in a house basement

Cracks as a result of upward soil expansion

Major cracks in exterior walls at doors and windows

Crackedslabongradebuiltonexpansivesoils.Thisproblemwasexacerbatedbypoordrainageabouttheperimeteroftheslab.

Distribution of reported locations ofExpansive soils in Sri Lanka (Herath, 1991)

Murunkan

MihintaleAnuradhapura

Puttalam Dambulla

Mahiyanganaya

Uda WalaweButtala

Kataragama

Digana

Laboratory Measurement of Swell

Swelling Pressure Test

- Measures the maximum pressurerequired to prevent a sample from swelling.

- Free swell can also be determined

SWELLING PERCENTAGE VS SWELLING PRESSURE

00.5

11.5

22.5

3

0 20 40 60 80 100 120

SWELLING PRESSURE/(kPa)

S

W

E

L

L

I

N

G

P

E

R

C

E

N

T

A

G

E

/

(

%

)

Treatment of expansive soils

Techniques available for stabilising expansive soils before and after construction of structures and highways

- Soil replacement with compaction control

- Surcharge loading

- Chemical additives (eg. Lime stabilisation)

- Prewetting

- Moisture control

1. Compaction: Heave of expansive soils decreases substantially when the soil is compacted to a lower unit weight on the high side of the optimum moisture content (possibly 3-4% above the optimum moisture content).

Even under such conditions, a slab-on-ground type of construction should not be considered when the total probable heave is expected to be about 1.5 in. (38 mm) or more.

Figure shows the recommended limits of soil compaction in the field for reduction of heave. Note that the recommended dry unit weights are based on climatic ratings. According to U.S. Weather Bureau data, a climatic rating of 15 represents an extremely unfavorable climatic condition; a rating of 45 represents a favorable climatic condition.

2. Prewetting: One technique for increasing the moisture content of the soil is by ponding and hence achieving most of the heave before construction. However, this technique may be time-consuming because the seepage of water through highly plastic clays is slow. After ponding, 4-5% of hydrated lime may be added to the top layer of the soil to make it less plastic and more workable (Gromko, 1974).

3. Installation of moisture barriers: The long-term effect of the differential heave can be reduced by controlling the moisture variation in the soil. It is achieved by providing vertical moisture barriers about 1.5 m deep around the perimeter of slabs for the slab-on-grade type of construction.

Perimeter trench subdrains are an effective means of limiting subsurface flow beneath a home founded on expansive soils

4. Stabilization of soil:

Chemical stabilization with the aid of lime and cementhas often proved useful.

A mix containing about 5% lime is sufficient in most cases.

Lime or cement and water are mixed with the top layer of soil and compacted.

The addition of lime or cement will decrease the liquid limit, the plasticity index, and the swell characteristics of the soil.

This type of stabilization work can be done to a depth of 1-1.5 m. Hydrated high-calcium lime and dolomite lime are generally used for lime stabilization.

Another method of stabilization of expansive soil is the pressure injection of lime.slurry or lime-fly ash slurry into the soil, usually to a depth of 4-5 m and occasionally deeper to cover the active zone.

Construction on Expansive Soils

Waffle Slab

Under-reamed Piles

Soft Organic Soils/Peat

Soft organic soils/peat are characterised by veryhigh water contents (and void ratios).

Water contents as high as 500-600% are seen inpeaty soils. The void ratios can be as high as 10.

Peaty soils are formed by the decay of vegetationand other organic matter.

Fibrous peat- peats of low levels of humification

Amorphous peat- peats of increased degree ofhumification

Vertical

Horizontal

V

K0cu1 cu2

1

1

ArrangementofclaygrainsunderK0 consolidation

5m

Scanning electron microphotograph of (a) vertical section; (b) horizontal section of James Bay peat showing fabric with very large macropores (G. Mesri and M. Ajlouni , 2007)

(a)(b)

Over the years the decayed organic matter getsmixed with mineral soils.

Organic content of peat in Sri Lanka is found to varyfrom 20%-40% (could be as high as 98%).

Organic matter at a higher level of humificationmixed with a higher percentage of mineral soils istermed as organic clay.

Due to very high water contents and high void ratios, peats and organic clays have very high compressibility and very low shear strength.

-Immediate, primary and secondary consolidationsettlements are very high

Due to varying levels of humification, compressibility andshear strength could vary within the same site.

Often the sites with peat are low-lying land and will have tobe filled resulting in,

settlement of soft layers under the weight of the fill shear failure in some instances (it may be necessary todo the filling in stages)

Shallow foundation options available on sites having peaty soils

Construction on the filled groundConstruction on the improved groundConstruction after replacement of the soft clay/peat

Construction on the filled ground

Filling should be adequately compacted to atleast 95%Proctor density in layers of sufficiently small thickness ( 200mm).

The height of the safe fill height should be estimated, whichdoes not cause shear failure of the underlying soft soil.

This is done by analysing the stability of the embankmentusing an appropriate slope analysis method, using theundrained shear strength parameters.

A minimum Factor of safety of 1.2 on the potential failuresurface should be ensured.

The required total fill thickness should be estimated accounting for the settlement under the fill.

If the required fill thickness is greater than the safe fill height, the filling will have to be done in stages.

The safe fill thickness has to be placed initially and the soft layer is allowed to consolidate under the weight of the fill and gain shear strength.

The next stage can then be placed.

Time required for the consolidation can be estimated from the consolidation parameters.

Field monitoring of pore water pressures and settlementsshould be carried out to verify the degree of consolidationtaken place in the site.

Deep settlement plates and stand pipe piezometers canbe used for the above purpose.

Improvement of undrained shear strength with theconsolidation is given by,

=uc - total load increase due to filluc - shear strength gain = 0.2 for Sri Lankan peaty soils

Filling can also be done in more that two layers or byproviding a toe berm.

(a) Staged filling (b) Filling with a toe berm

Once consolidation of organic clay/peat is complete,shallow foundation can be placed on the fill.

If the foundation width is large, the stresses could transferto the bottom layer.

However, secondary consolidation of organic clay/peat layercontinues over the design life of the structure.

Therefore, steps should be taken to increase the structuralstiffness and to make settlements more uniform.

Inverted T-type strip foundations and Vierendeel girder typefoundations have incresed stiffness.

T-type Vierendeel Girder Type

Construction on improved ground

Construction after improvement of the soft soil underneath

Aimed at improving the stiffness and strength characteristicsof the soil by densifying the soil by reduction of void ratio.

Improvement of soft clay by pre-consolidation

Soft clay is consolidated under the weight of a fill of thickness imposing a stress equal to that is expected from the proposed structure.

The removal of the fill is an essential part of the preloading process. In addition to preloading, final ground level will have to be increased.

Stages of pre-consolidationInitial state of stress AStress state after placement of fill B (accompanied by settlement)Some thickness of fill is removed and stress state changes to C(accompanied by an increase of void ratio Over-consolidated clay)Proposed structure is constructed. The stress state changes to D. Therefore, the amount of settlement is significantly reduced if D is not going to exceed B.

Laboratory observations have shown that not only primary consolidation characteristics, but also secondary consolidation characteristics of peat can be significantly improved.

Full thickness or part of it can bereplaced.

Soft clay should be removed atleast to adepth of twice the width of foundation.

It is advisable to use a geotextile layer atthe boundary of the soft clay/peat andthe granular fill.

3. Replacement of soil

When the load transferred or the thickness of peat layer issmall replacement by a much stronger soil can be carriedout.

In case of framed structures, the excavations from the foundations of the columns can be supported by cylindrical hume pipes.

The sinking of the hume pipe could be done with the gradual removal of the soft soil under the pipe.

Individual columns can be tied up at plinth level using tie beams.

Braced Excavations

-Laying services, pipes-Basement floors

Lateral Earth Pressure in Braced Cuts

Slide Number 1Slide Number 2Slide Number 3Slide Number 4Slide Number 5Slide Number 6Slide Number 7Slide Number 8Slide Number 9Slide Number 10Slide Number 11Slide Number 12Slide Number 13Slide Number 14Slide Number 15Slide Number 16Slide Number 17Slide Number 18Slide Number 19Slide Number 20Slide Number 21Slide Number 22Slide Number 23Slide Number 24Slide Number 25Slide Number 26Slide Number 27Slide Number 28Slide Number 29Slide Number 30Slide Number 31Slide Number 32Slide Number 33Slide Number 34Slide Number 35Slide Number 36Slide Number 37Slide Number 38Slide Number 39Slide Number 40Slide Number 41Slide Number 42Slide Number 43Slide Number 44Slide Number 45Slide Number 46Slide Number 47Slide Number 48Slide Number 49Slide Number 50Slide Number 51Slide Number 52Slide Number 53Slide Number 54Slide Number 55Slide Number 56Slide Number 57