Foundations on Expansive Clays

Dr. Ajjarapu Srirama RaoFormerly Principal & Prof. of Civil Engineering,

JNTU College of Engineering, Kakinada-533001, Email: srajjarapu@yahoo.com

ABSTRACT : Expansive soils are considered problematic in view of the alternate swelling and shrinkage they undergo due to seasonal moisture variations. Structures built in these soils are subjected to distress as a result. Therefore, special foundations have to be adopted in these soils. Sand cushion, CNS cushion and under-reamed piles are some of the foundations that are being used in this country to overcome the problems posed by these soils. However, each one of the above practices suffers from one drawback or the other. The author proposes two alternative foundation techniques which, to a great extent, can overcome the limitations of the foundation practices in vogue.

INTRODUCTIONExpansive soils, popularly known as black cotton soils in India, undergo alternate swelling and shrinkage due to seasonal moisture fluctuations. The volume changes of the soil are restricted only to a certain depth which is called the active zone. Thus, even if an expansive soil extends to a depth of 10m below the ground surface, soil in the entire thickness is not susceptible to volume changes. It will be limited only to a some depth, which, on an average, is 3.0 to 3.5m in India.

Unlike in ordinary soils, in which settlement or shear strength is the governing criterion, heave and swelling pressure govern the design of foundations in black cotton soils. Therefore, foundations in black cotton soils should be designed with greater care and circumspection. Different from the structures founded in all other types of soils, it is the light structures that are prone to damage, when founded in expansive clays. But, before taking up the design, identification of expansive clays is very essential. All clays that are black in colour are not expansive in nature. Clays that are truly expansive in nature can be identified from some routine classification tests, particularly the Free Swell Index (FSI) test.

EXISTING FOUNDATION PRACTICES

Some of the existing foundation practices in black cotton soils, which are in vogue in India, are discussed below:

Sand CushionProvision of a sand cushion beneath the foundation footing is normally believed to arrest heave of the black cotton soil lying under it. The actual design philosophy of a sand cushion is that sand undergoes bulking during the summer, which offsets shrinkage of the expansive soil while the settlement undergone by it during the monsoon compensates for the swelling of the expansive clay. But, arriving at the thickness of the sand cushion is not easy. It cannot normally be fixed arbitrarily at 300mm or 450mm as is normally done. An unscientifically and inadequately designed sand cushion may be a case of cure being worse than the disease, as it accelerates the process of swelling of the expansive clay lying underneath, by serving as an effective drain.

Under-Reamed Pile FoundationThe most popular foundation technique adopted in this country, the under-reamed pile is a bored cast-in-situ pile with an enlarged bottom and connected at the top to a plinth beam. A multi-underreamed pile provides greater frictional resistance along an enlarged perimeter, if the bulbs along the stem are spaced sufficiently close. Field engineers are particularly cautioned against using the tables in I.S. code (I.S.:2911, part III, 1980) for arriving at the safe loads, for, the properties of soils, which are very vital, are not taken into account while giving the safe load in both compression and uplift. Besides, the under-reamed pile suffers from the following disadvantages:

(i) It is not suitable in all soil profiles. It should be anchored only in the inactive zone, that too only in soils possessing cohesion. It cannot be installed in sands because sands cannot take negative slope.

(ii) Construction requires special skill. Untrained labour cannot install the pile properly. An under-reamed pile that is discontinuous fails to serve the purpose.

(iii) Provision of under-reamed piles as foundations for light structures, whose cost is normally less, might not be economically viable.

CNS CushionCohesive Non-swelling Soil (CNS) technique, developed by Katti (1979), is being increasingly used to arrest swelling and shrinkage and consequential cracking of canal linings and slopes of summer storage tanks. Few cases have been reported about its use in building foundations. Development of cohesive bonds, on saturation, in a non-swelling cohesive soil cushion, provided between the footing and the expansive clay bed, has been adduced as the reason for arresting heave. Besides the difficulties encountered in meeting the specifications of a CNS material (Katti, 1979), research carried out at I.I.Sc. (Subba Rao, 2000) has pointed out the chinks in the technique which has been claimed to be a sound technique. Alternate wetting and drying cycles have been found to render the CNS ineffective (Fig.1).

Fig.1 Swell-shrink behaviour of CNS-cushioned Expansive Clay (after Subba Rao, 2000)

THE PROPOSED TECHNIQUES

The foregoing discussion on the foundation practices, being adopted in India, currently, focuses on their short-comings and limitations. The upcoming sections show how some of the above disadvantages could be overcome by adopting simpler and more fool-proof techniques evolved by this author, in association with his research students.

Granular Pile-AnchorsA highly promising foundation technique, which not only inhibits heave but also possesses high load carrying capacity in both compression and uplift (Srirama Rao et al., 2007; Phanikumar et al., 2008), granular pile-anchor (Fig.2) is simple to construct. The foundation consists of a granular pile, consisting of small stones and coarse sand in a proportion of 20:80, compacted to a relative density of 70%. A granular pile per se cannot reduce heave of expansive clay beds because the particulate granular medium is not a good tension-resistant material. By providing an anchor rod, it becomes tension-resistant and counteracts the upward tensile force caused on the footing by the expansive clay. Therefore, the foundation is anchored at the bottom of the granular pile to a mild steel anchor plate, embedded in cement concrete by a mild steel rod passing centrally through the granular pile. Resistance to uplift, caused by the heave of the black cotton soil surrounding the pile, is developed as a result of

(a) weight of the granular pile acting downwards

(b) frictional resistance mobilized along the pile-soil interface.

Fig.2 Granular Pile-Anchor

Fig.3 illustrates the forces acting on a granular pile-anchor. The uplift force, P u, caused by the swelling pressure ps is given by the expression

……… (1)

The resisting force, PR, generated by virtue of the frictional resistance, is given by

… (2)

Where K is the coefficient of earth pressure and

Ks is the coefficient of lateral swell pressure

Wgp is the self weight of the granular pile

is the effective cohesion of the pole-soil interface

is the effective angle of friction at the pile-soil interface

is the effective vertical overburden pressure at the mid-depth of the granular pile.

Fig. 3. Granular pile-anchor concept, and forces acting on a GPA

It can be seen from Eq.(2) that the lateral component of swell pressure also contributes to the frictional resistance, an otherwise negative factor being made use in a positive way.

For a suitable factor of safety, given by the ratio, PR / Pu, the length and the diameter of the granular pile-anchor can be designed.

Fig.4 shows the pull-out behaviour with the applied load in kN plotted as ordinance and the corresponding upward movement of the granular pile anchor (GPA) of diameter of 200mm) in mm as abscissa. The lengths of the GPA are varied as 0.5, 0.75 and 1.0m. All the piles were tested to failure. From the figure, it can be seen that the failure pull-out load increased with an increase length of the GPA. Fig.5 shows the pull-out behaviour of GPAs of 1.0m length, with diameters varied as 100, 150 and 200mm. It can be seen from the figure that the failure pull-out load increased with an increase in the diameter of GPA. Thus, the uplift resistance of a GPA depends upon the surface area of GPA, besides the interface shearing resistance.

Fig. 4 Pullout behaviour of granular pile anchors of diameter 200 mm

Fig.5 Pullout behaviour of granular pile anchors of 1000 mm length

Stabilized Ash CushionsIn a previous section, it was mentioned that Katti’s CNS cushion, though effective in arresting the heave initially, failed to serve its purpose during the subsequent wetting and drying cycles. While the development of cohesive bonds in the cohesive non-swelling soil layer was proved beyond doubt, it was difficult to explain why, during the swell-shrink cycles, it failed to serve the purpose of arresting the heave. This gave rise to the thinking of an alternative to the CNS, while still retaining its basic principle. Fly ash is a material which has high pozzolanic properties due to the presence of silica and alumina in large quantities in it. Further, disposal of fly ash has become a big problem these days and should be utilized as a construction material, if possible. When blended with lime or cement, in the presence of water, fly ash is capable of producing cementitious bonds serving the same purpose as CNS. It was, therefore, felt that a stabilized fly ash cushion would be efficacious in minimizing the heave of the expansive clay and, accordingly, research was carried out by this author (Rama Rao et al., 2008; Rao et al., 2008) to verify this assumption.

Matetials Used

Dial gauge

Heave stake

Hollow PVC pipe

Test tank

Fly ash layer

200 mm thick soil bed

Sand drain all around and at the bottom

The fly ash was collected from the electro-static precipitation hoppers of VTPS, Andhra Pradesh. The silica and alumina contents were 63% & 25% respectively. Ordinary Portland Cement of 43-grade and commercial hydrated lime were used as the stabilizing agents.

Experimental StudiesFig.6 shows the experimental set-up. Fig.7 shows the variation of the swelling potential (the ratio of the increase in the thickness of the expansive clay bed to its initial thickness, expressed as a percentage) with lime content, for different thickness ratios of the Fly Ash Cushion (FAC) with a dry density of 1.3 Mg/m3. The swelling potential of the clay bed without any cushion was 26.5%. With an increase in the lime content, the swelling potential has been found to decrease. It can also be seen from the figure that the swelling potential decreases significantly with an increase in the thickness of the fly ash cushion. Pozzolanic action in the presence of water between the silica and alumina present in the fly ash and the lime produces cementitious bonds in the fly ash cushion, which results in the reduction of the heave significantly.

Fig.6 Experimental Set-up for swelling studies

Fig. 7. Plot of swelling potential against lime-stabilized fly ash cushion thickness: dry density 1.3 Mg/m3

Leaching studies in 5 cycles showed that the amount of lime leached decreased with each cycle. While the lime leached after the first cycle was 0.266%, the lime leached after the fifth cycle was only 0.123%. The cumulative lime content leached was 0.942% at the end of the fifth cycle, which is quite insignificant (Rama Rao & Sreerama Rao, 2007).

The investigation has shown that a cushion of fly ash, stabilized with 10% lime, is effective in arresting heave when placed between the foundation footing and the underlying expansive clay bed. With an increase in the thickness of the cushion, the amount of heave of the underlying clay bed decreases. Similar studies were carried out using cement also as the stabilizing agent in fly ash. Fig.8 shows the plot of swelling potential as a function of the cement content for different ratio of the thickness of the cushion to that of the soil.

tc / ts

Fig. 8. Plot between swelling potential and cement content for fly ash cushion of dry density 1.0 Mg/cu.m

Depending on the amount of heave needed to be arrested, the thickness of the cushion can be designed. For an expansive clay bed of 3m, it may not be necessary to remove the entire 3m thickness to obviate heave of the foundation structure. It may be sufficient to remove 1m or 1.5m of the expansive clay bed and replace it with stabilized fly ash cushion. This may yield the desired results in arresting the heave (Rama Rao et al., 2008).

Studies were also carried out to compare the efficacy of the above cushions vis-à-vis the sand cushion and CNS cushion. The results are presented in Table 1.

Table 1. Reduction of heave with different cushions

Type of CushionHeave (mm)

% reduction in heave

Heave ratio

No Cushion 39.73 -- --Sand Cushion 17.36 56.3 0.44CNS Cushion 16.35 58.8 0.41Lime-stabilized FAC 12.47 68.6 0.31Cement-stabilized FAC 9.93 75 0.24

Heave ratio=Heave of cushioned soil bed/heave of un-cushioned soil bed

Swell-shrink cycle studies (Srirama Rao & Rama Rao, 2010) have also shown that stabilized fly ash cushion does not suffer from the disadvantage of a CNS cushion. With every cycle, the swelling potential decreased significantly (Fig.9). Similar results were obtained by using cement as the stabilizing agent for the fly ash. Fig.10 shows the results of the study.

Fig.9 Cyclic swell-shrink behaviour of expansive soil provided with lime-stabilized fly ash cushion

Fig.10 Cyclic swell-shrink behaviour of expansive soil provided with cement-stabilized fly ash cushion

CONCLUSIONS

The problems associated with foundations in expansive soils are discussed. The merits and the demerits of the different foundation techniques in vogue are highlighted. Two new innovative foundation techniques, the granular pile-anchor and stabilized fly ash cushion, have been proposed.

Granular pile-anchor not only reduces heave but also exhibits considerable uplift resistance. The lateral swell pressure also contributes to the uplift resistance.

Stabilized fly ash cushions, using either lime or cement as the stabilizing agent, have been found to be efficacious in arresting the heave of the expansive clay beds. Development of cementitious bonds due to pozzolanic action in the cushion has been found to be responsible for the mitigation of heave. The technique has been found to be more effective than either the sand cushion or the CNS cushion. It does not suffer from the drawbacks of either.

REFERENCES

I.S: 1498-1970, 1982 (revised). Code of Practice for Classification of Soils, Bureau of Indian Standards, New Delhi.

I.S.: 2720, Part XL, 1977. Code of Practice for Classification of Soils, Bureau to Indian Standards, New Delhi.

I.S: 2911, Part III, 1980. Code of Practice for Design and Construction of Pile Foundations, (1 st Revision), Bureau of Indian Standards, New Delhi.

Katti, R.K. (1979). Search for Solutions to Problems in Black Cotton Soils, Indian Geotech. Jrl., 9, 1-80.

Phani Kumar, B.R., Srirama Rao, A, Dayakar Babu, R and Suresh, K (2007). Response of Granular Pile-Anchors Under Compression. Ground Improvement, 161(6), G 13, 121-129.

Rama Rao, M, Sreerama Rao, A and Dayakar Babu, R (2008), Efficacy of Cement-stabilized Fly Ash Cushion in Arresting Heave of Expansive Soils, Geotechnical and Geological Engineering, Springer, Netherlands, 26, 189-197.

Rama Rao, M, and Sreerama Rao, A (2007). Leaching Studies on Lime-stabilized Fly Ash Cushion. Asian Jrl. of Microbiology, Biotechnology and Environmental Science, 9(3), 569-576.

Rao, M.R., Rao, A.S., and Babu, R.D. (2008). Efficacy of Lime-stabilized Fly Ash Cushion in Expansive Soils. Ground Improvement, 161(11), 23-29.

Srirama Rao, A, Phani Kumar, B.R., Dayakar Babu, R and Suresh, K (2007). Pullout Behaviour of Granular Pile-Anchors in Expansive Clay Beds in Situ. ASCE Jrl. of Geotechnical and Geoenvironmental Engineering, 133(5), 531-538.

Srirama Rao, A and Rama Rao, M (2010), Behaviour of Expansive Soils Under Stabilized Fly Ash Cushions During Cycle Wetting and Drying, Intl. Jrl. of Geotechnical Engineering, 4(1), 111-118.

Subba Rao, K.S. (2000). Swell-shrink Behaviour of Expansive Soils – Geotechnical Challenges, Indian Geotechnical, Jrl., 30(1), 1-69.