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Dr. C.N.V. Satyanarayana Reddy |
High rise buildings are defined as the buildings that have 13 floors or above. Skyscrapers, the buildings with over 40 floors also come under the high rise category. In India, construction of structures with more than 35-40 storeys has gained momentum in metropolitan cities over the last two decades. High rise structures experience predominantly large unbalanced horizontal loads due to wind/ seismic forces. Particularly in coastal areas, wind load becomes predominant with prevailing wind speeds up to 220 kmph with gusts of 260 kmph experienced during recent Hudhud and Phailin Cyclones. These wind speeds are much higher compared to the maximum basic wind speed of 50 m/s prescribed in IS 875 part 3 (2015) for coastal areas. The stability of high rise structures requires foundations laid in competent soil strata to sustain large lateral loads and moments arising from such cyclonic winds. High rise structures are constructed with basements to facilitate parking and storage facilities. The constructions of basements involve deep excavation and necessitate adoption of suitable soil retention systems to prevent soil slides and to ensure stability of foundations of neighbouring structures. For the design of foundations and soil retention systems, complete information on subsoil strata is required based on extensive site investigation. Understanding the code specifications on bearing capacity estimation for shallow foundations and load capacity of piles is essential for the design engineers to make stable and economic design of foundations for the structures. Also, the importance of performing field tests for characterization of soils and interpretation of supporting strength of soil and load tests on Piles shall be realized as they play significant role in foundation design. The useful information compiled from the relevant IS codes of practice and field practice with respect to foundations of high rise constructions are given below.
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” High rise structures experience predominantly large unbalanced horizontal loads due to wind/seismic forces.” |
Planning Of Subsurface Exploration
Detailed subsurface investigation is essential to adopt the suitable type of foundation for the high rise constructions. Subsoil exploration shall be done in accordance with IS 1892-1979. The spacing and number of boreholes shall be decided based on extent of site, uniformity of subsoil strata and the height of structure. For a compact building in a site of 0.4 hectare, investigation with one bore hole in the centre and one bore hole at each corner is suggested. For larger areas, sounding/penetration tests are to be performed at a spacing of 50m to 100m by dividing the area in a grid pattern. The depth of exploration shall be decided based on the significant depth (i.e., the depth at which vertical stress due to proposed construction becomes equal to 10 percent of initial effective overburden pressure). Generally, soil exploration to the depth to which pressure bulb extends below a foundation is considered adequate. Depth of exploration for foundations of structures suggested by IS 1892 is given below.
Increase in depth of exploration for adjacent footings and adjacent rows of footings with decrease in clear spacing is due to the fact that the higher stress in overlapped zone increases the significant depth.
The investigation shall be preferably carried out up to rock strata or hard strata (where refusal is recorded in Standard Penetration Test). In hard strata, the investigations shall extend at least 3m. In case of SDR/soft rock, the investigation shall be extended at least 6m to keep scope for estimation of load capacity of Piles in side socket shear resistance for allowable length of 6d for piles of diameter of 1000mm. Preliminary investigation with Geophysical exploration methods help in getting information on position of ground water table, subsoil layer boundaries and bed rock position. It also helps in proper planning of exploratory boreholes for detailed analysis of subsoil strata and useful in reducing the number of exploratory boreholes if soil strata are observed to be homogenous.
The borehole investigation shall have the scope for conducting Standard Penetration Tests (IS 2131) if sub soil layers are granular in character and Static Cone Penetration tests (SCPT) in soft to medium clays, silts and fine to medium sand (IS 4968 part 3). The strength and stiffness parameters of granular and cohesive soils shall be determined from Standard penetration tests and cone penetration tests respectively for reliability and better acceptability. Supplementing SPT with SCPT will enable correlation of N values to soil properties in a better way. Also, cone penetration tests help in identifying presence of any thin layer of weak soil as the test continuously measures penetration resistance of soil. In SPT, there is scope for missing such weak strata as test is performed in boreholes at certain intervals. Field vane shear tests shall be performed in clay strata of very soft to soft consistency for determination of in-situ shear strength (undrained cohesion). For laboratory analysis, undisturbed samples are to be collected using thin walled sampler/piston sampler/rotary sampler from different subsoil strata. The area ratio of samplers used for collection of undisturbed samples shall be less than 20% and in soft clays, preferably less than 10%.
Bearing Capacity/Supporting Strength Of Ground
Adoption of appropriate bearing capacity for design of foundations with respect to design loads and prevailing subsoil conditions is key factor governing the stability of structures. The field tests used for determination of bearing capacity shall be appropriate for subsoil conditions and shall be conducted following the standard procedures. Static cone penetration tests shall be preferred to Standard Penetration tests in saturated clays, silts, fine to medium sand as they measure static resistance of soils and provide continuous information on strength and stiffness of ground. Plate load tests shall be conducted for precise estimation of allowable bearing capacity in case the site is comprised of gravelly soil with refusal recorded in SPT results. However, it is essential to have supporting bore log data indicating uniformity of soil for validity of test results to foundation design. The Plate load tests are to be carried out with test plates of size as large as possible. Also, footing load tests (Gupta et al., 2016) shall be conducted in important structures to eliminate the uncertainties associated with plate load tests in terms of size effect and for judging the response of foundation soil with respect to actual footings.
As high rise structures are designed for seismic loading, the permissible increase in allowable bearing capacity of shallow foundations and pile foundations suggested by IS 1893 part 1 (2016) shall be followed while designing foundations. For Type A soils (rock and hard soils with N>30), 50% increase in bearing capacity is allowed while for Type B soils (Medium soils with N = 10-30), 25% increase in bearing capacity is allowed. No increase in bearing pressure is allowable in soft soils (N<10). The standard penetration resistance (N) values shall not be less than 15 for depths less than 5m and 25 for depths greater than 10m to keep structures safe against risk of liquefaction in seismic zones III and above.
Liquefaction potential evaluation of subsoil strata (loose saturated fine sand and silty sand) shall be done as per IS 1893 part 1 (2016) following the procedure suggested in Annex F. The Cyclic Stress Ratio (CSR) shall be determined based on Peak Ground Acceleration (PGA)/using amax/g value as seismic zone factor (z) for the site and Cyclic Resistance Ratio (CRR) shall be determined for anticipated/design earthquake magnitude. Factor of safety (FS) with respect to initial liquefaction determined as the ratio of CRR to CSR shall be more than 1.2. A sub soil stratum liquefies if FS is less than 1. In such a case, deep foundations terminated in hard stratum, preferably rock are to be adopted for the high rise structures and measures are to be taken to compact or stabilize the subsoil layers vulnerable to liquefaction to support other infrastructure facilities. The contribution from liquefaction susceptible strata shall not be considered in evaluation of lateral load capacity of piles.
Choice Of Foundation
Choice of foundation for high rise structures shall be carefully done based on the prevailing subsoil conditions and the structural loads, ensuring safety against the risks of foundation soil failures (shear/bearing failure and settlement failure) and without affecting the stability of neighbouring existing structures (Teng, 1962; Bowles, 1996). Unless rock is encountered at proposed foundation level, isolated foundations shall be avoided in high rise structures. Usage of raft foundation eliminates the chances of differential settlements arising from huge column loads and moments due to super structure load and large lateral load from wind/seismic conditions. Also, it will be viable option as it can be advantageously used as base slab for basement and basement walls can be also constructed over it treating it as base slab.
This eliminates excavation required around the perimeter of the site for basement wall construction. Beam and slab type rafts, Cellular type Rafts/Box structured Raft and Piled Rafts are generally used to support high rise structures. Cellular type rafts (Fig. 1) with RC basement walls and slabs has capacity to withstand heavy column loads and large bending moments. The walls increase stiffness of raft foundation and serve as deep beams or ribs for supporting heavy column loads. Deep beams of the raft resist the large bending moments arising of large column loads and large spans.
High rise structures at sites comprising of incompetent subsoil layers extending to greater depth, Pile foundations are to be adopted and Pile groups are to be provided to support the column loads. Deep foundations terminated preferably in rock are to be adopted for the high rise structures as any unprotected excavations for adjacent structures do not affect the stability of the structure. Piled Raft foundation (Fig. 2) is preferred and promising choice for high rise structures as it offers more stability under lateral loads and moments. Also, if there are any ineffective piles, the raft facilitates some degree of load redistribution to other competent piles and reduces the influence of the weak piles (if any) on the overall performance of the foundation. Piled raft helps in reducing the settlement of structure and controlling the differential settlements.
 Fig. 1: Box Structured Raft |
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While adopting Pile foundations, estimation of load capacities shall be done properly following the relevant code specifications (IS 2911 part 1 – sections 1 and 2 (2010); IS 14593 (1998); IRC 78-2014) and are to be finalized based on the safe load capacities arrived at by performing stipulated number of pile load tests as per IS 2911 part 4 (2013). Load capacity of piles with termination in sound rock or rock masses shall be estimated preferably as per IS 2911 and IS 14593 compared to IRC 78 unless structures are built nearer to water bodies, as the values estimated by the latter method are conservative. Socketing length in rock strata influence not only vertical capacity of piles but also, significantly influences the lateral load capacity of the piles. The socketing lengths of 1 to 2D, 2 to 3D and 3 to 4D for piles in sound homogenous rocks, moderately weathered rock and soft rock respectively, suggested by IS 14593 (1998) are generally adequate to meet the requirements. Karthigeyan and Rajagopal (2012) reported that socketing lengths of 0.5-1.0D (D is diameter of Pile) are adequate in sound rocks to cater to lateral loads and control of deflections. However, the safe load capacity of piles for design shall be finalized after making necessary revision to the estimated values based on initial pile load test results and need verification through routine tests to rule out any deficiencies.
The estimated load capacities from static analysis shall be taken as the reference values for preliminary design and shall be revised based on the initial pile load tests conducted on test piles by loading up to 2.5 times the anticipated safe load or until a settlement of 10% of pile dia. occurs, whichever is earlier. The minimum number of test piles shall be 2 if the number of piles proposed in a project is up to 1000 and one additional test for every 1000 piles and part thereafter. The minimum test piles shall be 2 in any project. The safe load capacity of piles evaluated from initial tests shall be verified by performing routine test on working piles by loading up to 1.5 times estimated safe load. However, the test load in compression should not cause settlement in excess of 12mm for piles of diameter up to and including 600mm and settlement in excess of 18mm or 2% of pile diameter, whichever is less, for piles of diameter more than 600mm. The number of routine tests on working piles shall be half the number of test piles, preferably not less than 2 for high rise structures. The requirement of initial and routine tests for evaluation of compression, uplift and lateral load capacities shall be decided for each category of piles, i.e., diameter wise for different lengths and varied capacity piles in different location of the site for given diameter and length. The reaction loading for pile testing is taken from a kentledge/ reaction piles/cement grouted anchor piles. For load tests on large diameter piles or at sites, where kentledge cannot be supported by weak soil prevailing near ground surface, Bi-directional static load tests (ASTM D 8169) with pressurized load cells shall be conducted.
The quality of constructed piles shall be monitored by performing pile integrity tests (IS 14893 -2001). The tests provide information about continuity/structural integrity of piles, pile length and help in detecting the defects such as necking, soil incursions, and changes in cross section. The tests shall be performed on all the piles and any identified defects shall be rectified. It shall be noted that pile integrity tests do not provide any information on load capacity of piles. Hence, it is mandatory to check the load capacity of working piles by performing routine tests.
In case shallow foundations are to be supported on rock and rock masses, the safe bearing capacity shall be determined as per IS 12070 (1987). The estimation of safe bearing capacity of sound rocks with discontinuities spacing more than 1m shall be done based on Rock Mass Rating (RMR). Rock core strength shall be used to determine safe bearing capacity if spacing of discontinuities is between 0.3-1.0m. For rocks of low strength (< 500 kg/cm2) and rock mass with discontinuities at closer spacing (0.05-0.3m), limit pressure determined from pressure meter shall be used for evaluation of safe bearing capacity. In rocks of very low strength (< 250 kg/cm2), fragmented rock and weathered rock, plate load tests are to be conducted to determine safe bearing capacity for foundation design. The foundations shall be anchored to rock by providing dowel bars if minimum embedment of foundation (0.5m) is not done into rock. The dowel bars (ribbed tor steel bars) are to be designed to resist the base shear of structure under wind/seismic loads and provided in cement grouted holes made into rock. The dowel bars shall extend into rock (in cement grout hole) as well as foundation to a length at least equal to development length of bars in shear.
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Selection Of Soil Retention Techniques In Deep Excavations For Basements
While making deep excavations for basements and foundations, proper soil retention techniques are to be adopted suiting to the subsoil conditions (Teng, 1962). Excavations without supporting systems result in soil slides into excavations and affect the stability of neighbouring structures. Also, the construction workers life will be in danger as they do not find time to escape if soil sliding starts from excavated faces. The problem of instability of soil in excavations is aggravated with subsurface seepage of water/excavation extends below water table. The soil movements from unprotected deep excavations/inadequate supporting systems may even lead to collapse of neighbouring structures, particularly those founded on shallow foundations (Finno et. al. 2005; Sayin et al. 2016).
If site conditions permit, shallow excavations shall be made with slopes not exceeding 1V to 2H in granular soils and 450 in cohesive soils. If the safe height of excavation in nearly vertical excavation determined from stability analysis is less than the proposed depth of excavation, suitable soil retention measures are to be adopted to protect soil from sliding. If subsoil has cohesion more than 7.5 kPa, soil nailing system (Fig. 3) can be adopted and designed with either driven/grouted nails with shotcrete facing to retain soil in excavation (Gassler and Gudehus, 1981). If cohesionless soils are present, diaphragm walls, sheet pile walls, secant piles, contiguous piles (tangent piles) shall be adopted. If hard stratum prevents extending the walls to designed embedment depth below the excavation level, anchorages are to be provided for achieving required lateral stability. A diaphragm wall (Fig. 4) is a structural reinforced concrete wall constructed in a deep trench excavation (supported by bentonite slurry), either in cast in situ method or using precast concrete components. Diaphragm walls are used for making deep excavations for basements in congested sites, sites with close surrounding structures and where ground water table is at shallow depth and soil is collapsible in nature.
Sheet pile walls are used for temporary retention purpose and once the construction of basement perimeter retaining wall is complete, they are removed while backfilling the space between protected soil and constructed retaining wall. A contiguous/tangent pile wall (Fig. 5) is an earth retention system formed by installing closely spaced bored cast-in-situ piles with a small gap between adjacent piles while a secant pile wall (Fig. 6 and Fig. 7) is an earth retention system formed by installing overlapping bored cast-in-situ piles.
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Deep excavations in clayey silty sands are challenging as they appear to stand vertical in dry state to depths of about 5-7m due to binding of the particles provided by clay binder. The soil disintegrates upon saturation due to reduced bonding and results in sudden sliding of large mass of soil from excavation. One such failure occurred in Visakhapatnam (Fig. 8) on a rainy day from a deep vertical excavation made up to 7.5m depth in low clay binding sandy soil (Satyanarayana Reddy, 2021) and claimed the lives of four construction workers. The subsurface seepage water through excavated slopes further aggravates the instability problem. Such soils can be retained in deep excavations using soil nailing technique as they possess undrained cohesion generally more than 7.5 KPa.
 Deep Excavation |
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Conclusions
Construction of High rise structures require detailed soil investigations with appropriate in-situ tests to adopt suitable type of foundations, proper selection of design loads (particularly Seismic and wind loads), Identifying the risk of liquefaction in subsoil layers during anticipated earthquakes, load tests on foundations, NDT tests to check construction quality of foundations, adoption of proper soil retaining techniques in deep excavations required for basements construction and foundation laying. Hence, geotechnical engineers play a vital role in designing appropriate substructure elements for the high rise constructions in varied subsoil conditions. High rise structures in coastal areas shall be designed for maximum wind speeds recorded in the region if wind speeds during recent past cyclones are higher than those specified in codes for better long term stability.
References
- ASTM D 8169 (2018): Standard test methods for deep foundations under Bi-directional static axial compressive load, ASTM International, West Conshohocken,
- Bowles, E (1996). Foundation analysis and design, 5th edition, Mc. Graw Hill publishing co., New York.
- Finno, J, Voss, F.T, Rossow, E. and Blackburn, J.T (2005). Evaluating damage potential in buildings affected by excavations, Journal of Geotech. Geoenviron. Eng., Vol. 131, No.10, pp.1199-1210.
- Gassler, G and Gudehus, G (1981). Soil nailing – Some aspects of a new technique, Proceedings of 10th International Conference Soil Mechanics and Foundation Engineering, Stockholm, 1981, 3, pp. 665-670.
- Gupta, S, Sundaram, R and Gupta, S (2016). Footing load tests on sand: Validating theoretical predictions, proceedings of Indian Geotechnical Conference, e- proceedings, December 15-17,
- IRC 78 (2014): Standard specifications and code of practice for road bridges; Section: VII Foundations and Sub Structure, Bureau of Indian Standards, New
- IS 12070 (1987): Code of practice for design and construction of shallow foundations on rocks, Bureau of Indian Standards, New
- IS 14593 (1998): Design and construction of bored cast in-situ piles founded on rock-guidelines, Bureau of Indian Standards, New
- IS 14893 (2001): Non destructive integrity testing of piles (NDT) – Guidelines, Bureau of Indian Standards, New
- IS 1892 (1979): Code of practice for subsoil investigation, Bureau of Indian Standards, New
- IS 1893 Part 1 (2016): Criteria for earthquake design of foundations- General provisions and buildings, Sixth Revision, Bureau of Indian Standards, New
- IS 2131 (1981): Code of practice for standard penetration test, Bureau of Indian Standards, New
- IS 2911 Part -1 Sections 1 & 2 (2010): Codes of practice-Design and construction of pile foundations – Driven Precast and Bored cast in-situ concrete piles, Bureau of Indian Standards, New
- IS 2911 Part 4 (2013): Code of practice – Design and construction of pile foundations: Load test on piles, Bureau of Indian Standards, New
- IS 4968 part 3 (1976): Method for subsurface sounding for soils – Static cone penetration test, Bureau of Indian Standards, New
- IS 875 part 3 (2015): Design Loads (other than Earthquakes) for Buildings and structures – wind loads, Bureau of Indian Standards, New
- Karthigeyan, S and Rajagopal, K (2012). Influence of rock socketing on the lateral load response of single pile, Indian Geotechnical Journal, 42, No.1, pp. 49-55.
- Satyanarayana Reddy, N.V (2021). Issues related to erosion and its effects on structures in Visakhapatnam, Proceedings of ISSMGE TC 213 workshop on “Scour and Erosion”, Visakhapatnam.
- Sayin, B, Yildizlar, B, Akcay, C And Cosgun, T (2016). Damages in adjacent structures due to foundation excavation, Fourth International Conference on Advances in Civil, Structural and Environmental Engineering – ACSEE 2016, Rome, Italy, 77-81. Teng, W.C (1962). Foundation design, 5th edition. Prentice Hall Inc