anvesh rfeeddy and dey, gsp238, 2014

Compensated Raft Foundation on a Preloaded Soil Improved by Vertical Drains

Arindam Dey1 and Mamidi Anvesh Reddy2

1Assistant Professor, Department of Civil Engineering, Indian Institute of Technology Guwahati, Assam 781039, India; [email protected] 2Business Analyst, Mu-Sigma, Bangalore, Karnataka 560066, India; [email protected] ABSTRACT: Compensated raft foundation is used to support heavily loaded structures resting on soft and low-permeable soils, and aids in the reduction in settlement due to lowering of the stress transferred to the underlying soil. Such soils often require preloading and pre-treating with vertical drains to arrest the majority of the final settlement under the actual construction in lesser time. This paper reports the attainable efficacy in adopting the above methods for the foundation of the 10th Boys Hostel at the Indian Institute of Technology, Guwahati. Soft clayey and marshy fill soil is prevalent in the hostel site. FEM modelling using PLAXIS 2D v2012 has been used to interpret the possible benefit of the adopted method. In comparison to the condition when no preload was used, it has been observed that the application of staged preloading resulted in a reduction of the total settlement in the range of 45-90%. Moreover, in comparison to the untreated soil, the application of vertical drains significantly accelerated the rate of consolidation and dissipation of pore-pressure (~30-50% and 15-25 times respectively). The comparative results reveal that the adoption of above methods will substantially help to improve the settlement characteristics of the soft soil site in discussion. INTRODUCTION

Foundation comprises of the substructure footing and the surrounding soil within a influence zone (Das 2009), and serves a direct load transferring medium to the underlying soil or bedrock. Based on the depth of embedment (D), foundations are commonly classified as shallow or deep foundations. Raft/Mat foundation is a preferred category of shallow foundation and is preferred when the individual isolated footings provided under the structural columns occupy more than 50% of entire foundation area (Tomlinson 2001). Providing a common platform, such foundations help in reducing differential settlement arising due to spatially variable soil profile,

515Ground Improvement and Geosynthetics GSP 238 ASCE 2014

and/or variation in loading between adjacent columns. Spreading of the external applied load over a large area also assists in the reduction of generated contact stress beneath the raft foundation.

Compensated raft foundation is a special category of mat foundation where the net pressure on the foundation soil is further reduced by higher embedment depth of the foundation and simultaneous construction of a basement wall (Fig. 1). In this case, the excavated soil is not used further as a backfill which aids in significant reduction of overburden pressure. Moreover, such adoption helps to bypass the weak overlying soil to let the footing rest on a comparatively stiffer stratum. The weight of structure is partially or fully compensated by the weight of excavated soil and hence, provides a reduction in the subsequent settlements. When the structural load is fully compensated, the foundation is specially termed as buoyant raft or a floating foundation (Fig. 1).

(a) (b)

Fig. 1. Types of raft foundations: (a) Compensated raft (b) Buoyant raft.

(a) (b)

Fig. 2. Preloading and surcharge (a) without and (b) with vertical drains

Preloading generally refers to the process of compressing the soil under applied vertical stress (certain percentage of the expected post-construction stress e.g. 90%) prior to the actual construction (Staplefeldt 2006, Indraratna et al. 2012). Generally, the preload is applied in stages (Fig. 2) and is to be ideally removed when the preload-induced settlement approaches the expected design settlement and/or the pore-pressure attains a minimal magnitude (~1-5 kPa) after dissipation. As depicted in Fig. 2, for low-permeable soils, the rate of pore-pressure dissipations is often


accelerated using vertical drains. After the preload is maintained for a stipulated time and then removed, a residual pore-pressure remains which is further eliminated after the actual construction load is applied. Hence, the actual post-construction settlement mostly has to tackle the settlements arising due to the dissipation of the residual pore-pressures. Vertical drains accelerate the rate of dissipation and hence, aid in counteracting the damaging long-term settlement of structures. In some cases, a surcharge load in excess of a preload is also applied to attain further pre-construction settlement.

The present article explores the potential of a compensated raft foundation for the hostel sites at IIT Guwahati. The area is mostly flanked by thick bed (~ 22-25 m) of soft and compressible marshy fills. Pile foundations have been the general practice for the foundation construction. In spite of using deep foundations, many hostel structures have revealed excessive differential settlements and hazardous rigid rotation and tilting. Fig. 3 depicts the tilted Dibang Hostel at IIT Guwahati, which hints at improper functioning of the deep foundations due to the gradual dissipation of pore-pressure and delayed consolidation effect of the soil. Unequal distribution of the external load might be another cause triggering the tilting of the structures. Owing to the advantages of compensated raft foundation mentioned earlier, the potential of the same is explored though numerical modeling and behavioral interpretation of the foundation system. The site being made of low-permeable soil, the suitability of preloading the site accompanied by vertical drains have also been investigated, the results of which have been found to be encouraging and has been described in the subsequent sections.

Fig. 3. The tilted Dibang Hostel of IIT Guwahati Campus

PROBLEM STATEMENT

The present investigation pertains about the upcoming 1000 seater 10th Boys Hostel in the IIT Guwahati Campus. The structural detailing and the layout of the hostel have been collected from the Engineering Cell, IIT Guwahati. The hostel consists of two blocks (A and B) that are geometrically placed as a mirror image of each other. Figure 4 depicts the floor plan of Block A depicting the plinth and the floor columns. The hostel have already been designed to be supported on pile


foundations; however, because of the reasons stated above, this article attempts to provide an alternative proposal of foundation design for similar upcoming hostels in the same site. The alternative foundation ventured is a compensated flat-plate mat foundation resting on a pre-treated soil, the improvement been attained by means of preloading and vertical drains.

Fig. 4. Layout of the Block-A of 10th Boys Hostel, IIT Guwahati Campus

The dimensions of the entire block as shown in Fig. 4 are 113 m x 67 m. The block

is so built that two rectangular annulus regions are surrounded by the residential rooms for the boarders, the annulus regions being of the dimensions 35 m x 45 m. A compensated raft foundation beneath the loaded area has been considered as a modified embedded flat plate mat wherein the soil is not backfilled over the footing. Rather, the embedment is provided with surrounding structural walls which leads to the formation of a basement floor. The ground surface can then formed as a grillage plinth or a floor slab which is connected to the floor of the basement by several prop columns. In this manner, the compensated raft would facilitate the distribution of peripheral load over a large area (superstructure load passed on from the columns to the plinth grillage or plinth slab, which is transferred subsequently to the basement raft through several of the prop columns), and would result in a basement raft footing resting on the underlying subsoil and acted upon by nearly uniformly distributed load. Moreover, in such a scenario, the upper weaker subsoil layers are bypassed and the compensated raft gets rested on comparatively stiffer layers having more bearing capacity. This also assures lesser settlement due to the lower compressibility characteristics of the underlying strata. The differential settlements are also subsequently reduced due to uniform distribution of stress on the compensated raft.


PLAXIS FE MODELING

Finite element modelling of the problem has been carried out using PLAXIS 2D v2012, a finite element package intended for the two dimensional analysis of deformation and stability problems in geotechnical engineering. A plane-strain analysis of the problem has been carried out under special considerations. It is understandable that only the portions of the mat foundation located on the sides of the rectangular annulus may behave as a plane-strain system (the mat foundation at the corners of the annulus definitely behaves as a biaxial, and hence, a 3D system). The primary objective of this study is to assess whether a compensated raft resting on a pre-treated soil can be a better foundation option at the specified site. Hence, keeping the primary purpose in view, a plane-strain analysis of the mat foundation has been carried out, considering the section of mat being 14m wide.

The stratified subsoil has been modelled based on the borehole stratigraphy data available from the site. Eight borehole surveys (BH1-BH8) were carried out up to a depth of 30m to identify the soil stratification and estimate their properties. Standard Penetration Test (SPT) was conducted to estimate the strength of soil in terms of N-values. Soil samples were collected using a split-spoon sampler and subsequently tested in the laboratory to identify the primary engineering properties. Water table was also identified during the borehole survey, and the natural moisture content of the soil determined from the laboratory tests. Fig. 5 shows the layout of the borehole survey carried out at the site.

Fig. 5. Layout of the boreholes at the 10th Boys Hostel Site, IIT Guwahati

The borehole surveys (BH1 - BH8) were carried out up to a depth of 25-30 m. The borehole investigation provided the information about the various parameters of the soil strata in an interval of 1.5 m. Fig. 6 depicts the variation of the various soil properties obtained for all the borehole surveys. It can be observed that there is a significant variation in the strength characteristics of the subsoil strata, although the


unit weight, void ratio and natural moisture content do not show substantial changes. Such a spatial variation in the subsoil strength definitely calls for a 3D analysis. However, in order to assess the efficacy of the proposed compensated raft foundation, a 2D modelling of the same has been adopted considering single borehole stratification at a time.

Fig. 6. Engineering properties of subsoil as obtained from eight borehole surveys

The analysed 2D mat has been considered to be resting on the subsoil profile obtained from each of the eight boreholes. The entire PLAXIS FE model dimension was chosen as 50 m x 50 m and the mat is placed at the centre of the model geometry to minimize any model boundary effect. The compensated raft foundation (M25 concrete) of length 14 m and thickness 1m has been modelled by an elastic plate. The base of the compensated raft is assumed to be located at a depth of 3 m from the ground surface, which would provide nearly one storey at the basement level. Standard fixities has been set to the model boundaries which allow for only vertical movement along the vertical far vertical boundaries, while the bottommost boundary is restricted from movement in any direction. The water table is set at a location as obtained from the borehole investigation (mostly located at a depth of 3 m from the ground surface, and hence, beneath the base of the compensated mat). Borehole investigations revealed the presence of both sandy and soft clayey soils at different


locations of the substrata. The highly permeable sandy soil is model as Drained Mohr-Coulomb (MC) model for both loading and unloading conditions. In order to accommodate the compression-relaxation-recompression behaviour of the clayey layers under loading-unloading-reloading phenomenon, the Soft Soil Model (SS) has been used.

Preload is provided on the ground surface in the form of sand embankment over the region where 14 m mat will be constructed. The embankment is trapezoidal in shape with 2:1 slope distribution and a height of 2.5 m. Unit weight of sand is taken as 22kN/m3 and slope stability of the embankment is assured by providing higher angle of friction for material used. Vertical drains are provided with a spacing of 2m c/c beneath the embankment and 3m c/c beyond it. Drains considered are Prefabricated Vertical Drains (PVDs) whose permeability is theoretically infinite. Compensated Raft is provided by excavating 3m of soil under the mat and a basement in form of walls is provided. Material properties of wall used are similar to that of Mat (M25 concrete) and thickness of wall is taken as 500 mm, as it is a load bearing shear wall. Based on the structural drawing, the load estimated on the compensated raft is estimated to be 46.6 MT. Due to the floor construction basement floor, load distribution on the compensated raft can be treated as uniform loading and the magnitude of the estimated stress is obtained as 54 kPa (considering only dead and live loads with factor of safety as 1.5). Fig. 7 depicts the PLAXIS 2D model for the problem as developed.

Fig. 7. PLAXIS 2D FE model as adopted in the present study

Phases of execution of the FE model Phase 1: Initial stress generation Under self-weight of soil without any external

load. Drains are not active in this phase. Phase 2: Construction of preload 20 days stage construction period executed as

a consolidation analysis phase for the same period, commencing immediately at the end of initial phase. PVDs are activated from this phase


Phase 3: Preloading Preload embankment maintained for 100 days resulting in further dissipation of pore-pressures and executed as a consolidation analysis phase.

Phase 4: Preload removal - Preload removed quickly within 2 days after the specified period to ensure lesser rebound of the overconsolidated soil, and executed as unloading swelling phase.

Phase 5: Raft construction The raft construction period of 100 days is executed as consolidation analysis commencing from the end of Phase 4.

Phase 6: Long-term settlement Executed as consolidation analysis with a time duration being equal to the life-span of building (For the present study, it is set to 20000 days which is approximately equal to 50 years).

In order to assess the benefit of preloading in reducing the post-construction

settlement, additional analysis has been carried out with the raft resting on PVD treated subsoil where no preloading has been used.

RESULTS AND DISCUSSIONS Estimation of ultimate bearing capacity of compensated raft foundation

The ultimate bearing capacity (qu) of the compensated raft is estimated from the results from the load-deformation analysis of the same. The raft is subjected to a progressive staged uniform loading, the maximum magnitude of which is set as 10 MPa/m, and stress-settlement characteristics at a point beneath the centre of the mat is recorded. Hence, in this manner, application of any empirical or semi-empirical expression to estimate the bearing capacity is avoided. Fig. 8 depicts the stress-settlement plots as obtained considering the subsoil profiles from various boreholes, and as expected, different bearing capacities are obtained owing to the variation in the subsoil profiles.

Fig. 8. Ultimate bearing capacity and settlement of compensated raft


The bearing capacity is estimated from the magnitude of Mstage (defined as the ratio of the applied stress to the maximum stress of 10 MPa) at the point when the settlement of the raft increases significantly due to the slightest increase in the applied load. This is indicated in Fig. 8 by a sharp drop in the settlement, with the applied stress being almost constant. Fig. 8 enlists the bearing capacities and the corresponding ultimate settlements of the compensated raft foundation. It can be observed that although the supporting subsoil has a good bearing capacity, the foundation settlement is significantly high (Permissible settlements 60 mm for mat foundations on clayey soils; Punmia, 1998). This ensures the necessity of pre-treatment of the ground with preloading and subsequent improvement using PVDs.

Efficacy of vertical drains

In order to reduce the post-construction settlements, preloading has been used accompanied by vertical drains, with a spacing of 2 m c/c, to accelerate the process of consolidation (Atkinson and Eldred 1982, Indraratna et al. 2000). It can be observed from Fig. 9a that there is a significant increase in settlement in a same time-frame when the vertical drains is used, thus suggesting that the pore-water had been dissipated in an accelerated manner. In this process, the final settlement of the preloaded soil can be obtained with significant saving in time. For example, a settlement of 6cm is attained in 50 days when no drains have been used, while the same is attained in 17 days when preloading is assisted by vertical drains. Similarly, in 120 days, the excess pore pressure dissipated without drains is in the range of 1kPa, while the same is obtained as 22 kPa when assisted by vertical drains.

(a) (b)

Fig. 9. Efficacy of (a) Vertical Drains and (b) Preloading

Efficacy of preloading

In order to assess the benefit of preloading, the compensated raft resting on PVD treated subsoil is analysed without preloading. Fig. 9b depicts a typical time-displacement curves for both types of analyses. Without the use of preload, it is observed that the raft undergoes 34 mm of consolidation settlement. When preloading is applied, 77 mm of consolidation settlement is recorded in 120 days followed by a


rebound of 23 mm as preload is removed. Upon construction of the raft, a further consolidation settlement of 9 mm is observed. Hence, a net post-construction settlement of 16 mm is observed when the soil is treated with preloading in presence of vertical drains. This ensures the efficacy of preloading in reducing the post-construction settlement. This exercise is repeated considering the subsoil profiles obtained from all the borehole surveys, and in each case, in comparison to the settlements obtained for a non-preloaded soil, a significant reduction in the post-construction settlement in the range of 20-135mm had been observed. Moreover, the post-construction settlement of the compensated raft foundation resting on preloaded soil assisted by vertical drains considering any subsoil stratigraphy ranges from 16 mm-34 mm. These values are well within the permissible limits of maximum settlement of raft foundation allowed in clayey stratum (~60 mm).

RESULTS AND DISCUSSIONS

Based on the present study, it has been recognised that the compensated raft foundation could substantially enhance the bearing capacity and resist the large settlement problems in the concerned area. It has been also noted that preloading, if used without vertical drains, is not as such beneficial since the time required to achieve the final settlement is not reduced. When used in combination with vertical drains, which help in accelerated consolidation, the efficacy of the preloading has been observed to be significant in limiting the post-construction within the permissible limits. For the subsoil profiles obtained from various boreholes in the site, adoption of such technique has resulted in the reduction of total post-construction settlement in the tune of 20-135 mm. The post-construction settlement of the preloaded soil has been reduced to magnitudes of 16-30 mm, which are well within the permissible limits of the mat foundations resting on clayey soil (~ 60 mm). Hence, the attempt made is designing an alternative foundation for the hostel site is found to be efficient and satisfactory.

REFERENCES Atkinson, M.S. and Eldred, P.J.L. (1982). Consolidation of soil using vertical

drains. Vertical Drains, 33-43. London: Thomas Telford Ltd. Das, B.M. (2009). Shallow foundations: Bearing capacity and settlement, CRC Press,

Boca Raton, USA. Gupta, S.C. (1997). Raft foundations design and analysis with a practical approach,

New Age International Limited, India. Indraratna, B., Salim, W. and Redana, I. W. (2000). Predicted and pbserved behavior

of soft clay foundations stabilized with vertical drains Proc. ICGGE, I, 1-7. Indraratna, B., Rujikiatkamjorn, C., Balasubramanium, A. S. and McIntosh, G. (2012)

Soft ground improvement via vertical drains and vacuum assisted preloading Geotextiles and Geomembranes, 30, 16-23.

Meyerhof, G.G. (1951). The ultimate bearing capacity of foundations. Geotechnique, 2, 301-322.


Punmia, B.C. (1998). Soil Mechanics and foundations, LPP Ltd, India. Stapelfeldt, T. (2006). Preloading and vertical drains, Helsinki University of

Technology, Norway. Tomlinson, M.J. (2001). Foundation design and construction, Pearson Education

Ltd., USA.


anvesh rfeeddy and dey, gsp238, 2014

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