Prof. Y.Satya Prabhakar, HOD; Dr. M. Potha Raju, Professor, Dept of Civil Engineering, GIT., GITAM University, Vizag; Dr. K. Manjula Vani, Department of Civil Engineering, J.N.T.U., Hyderabad, and B. Satyeswara Rao, Senior Engineering Manager, BHPV Ltd, Visakhapatnam (A.P)
On observation of the extensive cracking developed and cracking pattern, the presence of expansive soils was suspected, but could not be found at foundation levels. To establish reasons for building failures, extensive soil investigation was carried out up to a depth of 20 m below EGL. ER-Survey was also conducted to supplement the data up to 50m depth to have correlation of soil layers with physical observations. Interestingly, it is observed that most of theses buildings are founded in river alluvium within 1.5 to 1.8 m depth and underlain by expansive clay. GWT fluctuates between 0.5 to 8.0 m due to the proximity of river Nagavli, thus causing cyclic swelling /shrinkage in these soils. Tests like XRD and SEM were also carried out to identify the clay mineral. To mitigate the damages, different types of foundations are explored for adoption to counteract swelling and some innovative, foundations proposed.
However, the structural designs are found ok. Hence the failures could not be attributed to structural designs and since the underlain soils are reported as sandy (non– swelling) soil, the failures could not be attributed to shrinkage/ swelling failures. Thus it is concluded to crack the mystery only by thorough field investigations. It is also seen that in some of the Government offices, severe cracking and extensive damages occurred leading to collapse of some load bearing structures in this area within five years after construction. One of the buildings collapsed completely. Some were abandoned because of severe cracking and became un-inhabitable. Some buildings like District Civil Supplies Office, SETSRI, A.P. Irrigation D e v e l o p m e n t Corporation., are in severe distressed condition and District Industries Centre had collapsed. This dreadful problem necessitated an in depth study. The District authorities requested investigations to identify the cause of failure of the existing buildings and to suggest solutions to remediation.
InvestigationsOn summarizing the information, the available data collected from the site indicated sandy strata at and below the foundation level. The structures built were well engineered framed light structures. But the buildings experienced severe cracking. The cracking pattern indicates doming type heaving of the soils. GWT is located at 6.0m below EGL at the time of investigations. Fluctuations in GWT due to seasonal changes can be from 0.5m to 8.0m below GL, as the site is hardly 500 m away from river.
Field TestingTo investigate the reasons for failure of buildings, soil investigations were carried out by drilling two 150/100 mm diameter bore holes, using rotary rigs up to 20 m depth, below EGL for collecting disturbed and un-disturbed soil samples, and conducting SPT at regular intervals.
Generalized soil profile (Figure 1 & Table 1) indicates the following sub-soils:
- The first layer from 0 to 1.8 m below G.L is filled up earth consists of medium dense silty sand and is classified as (SM) according to I.S. classification.
- The Second layer from 1.8 to 6.0 m comprises of stiff expansive clays of CH group with FSI ranging from 40 to 50.
- The third layer from 6.0 to 14.0 m comprises of soft to medium expansive clays with sand of CI and CH groups.
- The fourth layer comprises of medium dense sand from 14 to 19 m depth.
- The fifth layer is fine sand from 19m till the termination depth of 20 m.
Bore hole investigations were supplemented with “Electrical Resistivity Survey” to map the ground up to a depth of 50 m below GL. ERS sounding was conducted using Schlumberger configuration. Results of interpretation about expected litho*logy was presented in Table-1 Based on this survey results, the analysis suggests the site to be a flood plain with river alluvium as top-soil. Granitic-gneiss out crops were seen in the river bed around 1.0 km from the site. The same type of rock can be expected at the site.
Laboratory TestingA detailed Laboratory testing was conducted on all the disturbed & undisturbed samples collected. Results for a typical UD–sample are presented in Table 2A, below.
The summary of the Lab Tests conducted is given hereunder in Table below Table 2.
X–Ray Diffraction & SEM Studies
Review & AnalysisThe GWT table which was at 6.0 m depth, during investigation time fluctuates significantly rising to about 1.0 m from EGL. Extensive cracks were observed. It is reported that crack widths are also changing with change in seasons. Crack pattern studies indicate the presence of expansive clay below GL. Only the top 2.0 m is covered with medium dense sand (SM) below which the expansive clay is present. As the existing structures are light structures with only ground floor or ground + first floor, there is not much downward thrust exerted by the buildings to counter the uplift forces from soil expansion. As previous soil exploration was not conducted up to deeper depths, the presence of expansive clays was not identified, leading to the failures & problems in structures. On analysis of bore log data and laboratory test results, barring the top soil layer of 1 to 2 m, the stratification mainly consists of clayey sub-soil of CI-CH group with high expansive behavior. FSI ranges between 35 and 50. The value of cohesion is 30 kN/m2 and the friction angle only 40. The swell test on the soil by the constant volume method indicates that the swell pressures to be in the range from 20 to 80 kpa. The swell potential was also estimated with a newly deviced equipment, which
provides quick saturation around the soil specimen, than a regular laboratory testing equipment. The test results indicated reasonably high swell potential. The probable cause for the structural failure could be the swell pressure exerted by the sub-soil. The structures were pushed up because of the swelling of soils leading to structural failure of buildings as shown in photographs 1 to 4.
(i) Deep support systems and
(ii) Shallow support systems. Each of these systems has an associated level of risk of damage that can occur to the building superstructure and architectural components due to differential foundation movements as well as an associated relative cost of construction. When comparing the various foundation systems, the level of risk is generally found to be inversely proportional to the level of cost. Higher risks are often accepted due to economic considerations. For example, shallow support systems typically have a relatively higher level of risk than deep support systems, but are often selected due to economics and affordability.
Many alternatives are available to deal with this type of expansive soils which are generally adopted.
(I) Deep Support Systems
Because of the relatively small void space that is used with this system, the bottom portion of the grade beams are normally cast directly on the soil, even though they are designed to span between the deep foundations. The slabs typically range in thickness from four to eight inches. The reinforcement can consist of a single or double mat of rebar. The structural slab is designed in accordance with the IS 456.
Void forms serve as formwork for the placement of concrete by acting as a temporary platform that supports the weight of the wet concrete. Void forms typically are made of corrugated paper arranged in an open cell configuration. The raft footing.
In the event of swelling, because of the anchorage developed by the anchor bars, pressure on the surrounding soil of granular anchor pile increases. There by the pore- pressure in the soil underneath the foundation increases, but dissipates into adjacent granular soil, thus relieving pore/swell pressures. Especially for the bulk / medium housing colonies this method is cost effective.
Anchored Geo GridAnchored Geogrid is one of the promising methods, which is useful in expansive soils. In this method, a Geo Synthetic / geogrid is spread at the bottom of the foundation. The geogrid is anchored by means of anchor pins into the soil.
Where these pins are provided, it is preferable that the soil underneath them is treated with chemicals in order to minimise the swelling nature. Since the anchors do not move, when there is a swelling in the adjacent soil, the grid tries to counteract the swell pressure., thus restricting the transfer of swell exterior surface may be wax impregnated to temporarily resist moisture. The forms are specifically designed to gradually absorb ground moisture, lose strength, disintegrate over time, and leave a void between the expansive soils and the concrete slab. If the soil below the concrete heaves, it can expand into the space created by the void form without lifting the foundation
By the Usage of Under-reamed Piles5UR-pile foundations are one of the best possible solutions for providing foundations in swelling soils. But investigations revealed that the main reasons of failures are due to improper workmanship, poor quality, lack of expertise, due to absence of gap between the plinth beam and the soil surface etc. In the above case study UR-Pile foundations are recommended due to the availability of expertise and the pile capacities were estimated in accordance with IS-2911 (part-III) (Table -3).
Granular Anchor PilesOther important alternate type of foundation is by providing Granular Anchor piles. GAP is an innovative Solution Sreerama Rao .A,. et al3 Since a mere granular pile gets sheared off when swelling occurs as it cannot resist uplift forces.
To counteract the heave, granular anchor piles are used. Usually a bore is drilled similar to under-reamed pile bore, with expansion bulbs in the pile stem. The bore is filled with granular material/ Sand.
Based on the uplift expected, two or three rods are lowered prior to filling of granular material and they are anchored at the bottom of pile as shown in the Figure 3, either by geo-grid or by embedding the rods into PCC. The rods are tied and anchored into the foundation pressures to the foundation. On the top of the geo-grid, a layer of granular soil is laid for the dissipation of pore pressure caused by swelling,
One of the combination methods i.e use of geogrids with micropiles technique is expected to give better results and the configuration is as shown in Figure 4 (a) & (b).
The anchor consists of a plate or series of steel plates formed into the shape of a helix to create one pitch of a screw thread. The shape of the plate permits easy installation, which is accomplished by applying torque to the shaft of the anchor and screwing it into the ground using rotary motors. The anchors can be used to resist a tensile or compressive load, which is accomplished by means of bearing pressure resistance on the area of each helix, and not by skin friction along the shaft. The plate helices of helical pier foundations are attached to a central high strength steel shaft that can be segmented to facilitate construction and to allow various combinations of the number and diameter of helices used. The pier is screwed into the soils until the applied torque readings indicate that the necessary load capacity has been achieved or until the desired depth below the moisture active zone of the expansive soils is obtained. In new constructions, the pier shafts are typically anchored to the grade beams by using fabricated brackets that are tied to the grade beam reinforcement before placing the concrete, and bolted to the top of the pier shafts.
(ii) Shallow Support SystemsThis foundation system is similar to that discussed in Section (i) A above, except that the slab is placed, without a void, over the expansive soils and new fill and the grade beams are supported directly by the underlying soils instead of spanning to deep foundations. The key advantage of this system is that the grade beams need only to penetrate a minimum of six inches into the competent natural soils.
By Providing CNS Layer Cushion1,2,6By Providing CNS Layer cushion, Katti.R.K1, along with solid waste materials like rice husk ash, fly ash etc. The efficiency of this method depends on the thickness of cushion as well as the density to which it is compacted. However, the performance is observed to be decreasing with repetition of swelling-shrinking cycles of the expansive soils. To improve the efficiency of [CNS(NS)] soils, they can be again stabilized by lime (3-5%) or cement2 before using them.
By Chemically Stabilizing the Ground7This is a cost-effective solution and chemical injections with chemicals like CaO / CaCl2 / Ca(OH)2 / KCl / FeCl3 etc. are used to stabilize the soils. These chemicals can be injected or grouted into the ground. However, due to fluctuating water table, these chemicals get washed away.
Miscellaneous MethodsThese may consist of raft on three point support system, piled-raft foundations, steel helical piers, known as screw anchors or screw in piles, moisture control systems etc.,
To Adopt Combination of the Above MethodsCombinations of the above methods were tried on an experimental basis and they were found to yield good results. Especially when the granular piles are used along with chemical stabilization, the soil has shown very minimal swelling. This appears to be one of the suitable methods, where the chemical injection facilities are also available and the structures to be built are of important nature.
It has been once again proved that Conventional Foundation are not suitable for expansive sub-soils.
Hence, it is once again emphasize us, how important the soil exploration is and not only the extent but also to the required depth etc.. At this juncture, the authors feel that the stipulations for rightful exploration shall be made mandatory as per IS guide lines, before taking up any construction. The authors have also explored the different foundations types which can resist the swell pressures and discussed. For the present case, it was recommended to adopt combination method Grade- Supported Stiffened Structural Slab, which is described in the para “E” above along with the chemical treatment of sub soils with CaCl2, prior to construction of foundation. For existing buildings with lesser cracks, it was recommended to immediately adopt Lime slurry stabilization around the building by excavating trenches, which had yielded good results and found that further propagation of cracks were reduced.
Granular anchor piles with surrounding chemically stabilized ground shows 20–25% increase in the up-lift capacities it is hoped to have a promising future in the application of these techniques. Some more investigations are in progress. Various methods which can be used in such type of swelling soils are summarized and also other innovative methods which tackle such type of soils are listed to address the problems due to swelling of soils. Experiments and research are in progress, especially to assess the functioning of the GA Piles and also the anchored geogrid type foundations, which seems to yield promising. These methods if popularized, building failures can be avoided.
AcknowledgmentsWe convey our deep sense of gratitude to District Collector and Sri. K. Bhaskara Rao, E.E., A.P. Social Welfare Dept., Srikakulam for awarding this research problem to our Instt. We also extend our heartful thanks our Principal, Dr. V.V. Kutumbarao, and all our colleagues in the Department of Civil Engineering for constant support and encouragement through out this project.
Our heartfelt thanks are due to Dr. M.R. Madhav, Professor emeritus, JNTU Hyderabad for his valuable suggestions, Dr. A.K .Singh and Dr. T.R. Nandi, Scientists o DMRL, Hyderabad for extending help, to interpret XRD and SEM results, to the alumni of GITAM, Er. Dola Tirumala Rao, DEE , A.P. Irrigation Dept, for the valuable help and hospitality during the execution of this work.
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