Observed and predicted settlement of shallow foundation

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1 n d International Conference on New Deelopments in Soil Mechanics and Geotechnical Engineering, 8-3 May 9, Near East Uniersity, Nicosia, North Cyprus Obsered and predicted settlement of shallow foundation Rasin Düzceer Kasktaş A.Ş.-İstanbul, Turkey KEYWORDS: Settlement, Shallow foundations, Load tests, CPT, SPT, Finite Element Method. ABSTRACT: The objectie of this paper is to compare the predictie capabilities of different methods of estimating settlements of shallow foundations on sands. For this purpose. x.. m square concrete footing was statically load tested. Prior to load test, standard penetration test (SPT), cone penetration test (CPT) and laboratory tests were performed to determine the engineering properties of soil layers. Predictions of footing settlement were performed by conentional (semi-empirical) and finite element method (FEM). The results of static load test reealed that the settlements were oer predicted by Finite element method. Finite element analysis using either SPT or CPT deried input parameters proided conseratie settlement estimates. Howeer, most of the empirical methods employed in this study proide reasonable estimates using CPT deried parameters as input. INTRODUCTION The design of shallow foundations on cohesionless soils is often controlled by settlement, rather than bearing capacity limitations. Seeral methods hae been proposed for predicting settlement of shallow foundations on cohesionless soils. Settlement prediction methods can be diided into two categories, conentional or semi-empirical methods and the finite element based methods. Semi-empirical methods are the predominant techniques used to estimate settlements of shallow foundations on cohesionless soils. These methods hae been correlated to large databases of tests such as the SPT and CPT (Kimmerling ). In this paper a.x.. m precast concrete footing was statically load tested to. times the proposed design load of kpa to examine the settlement behaiour of the footing. Prior to load test, SPT, CPT and laboratory tests were performed to determine the engineering properties of soil layers. Settlement of the footing resting on cohesionless soil was estimated by seeral methods based on semi-empirical correlation and FEM. Measured settlement of the footing was compared with the settlements estimated by conentional methods and FEM. REVIEW OF THE SETTLEMENT PREDICTION METHODS Allowable bearing pressure for footings on sand is generally limited by the consideration of settlement rather than safety against bearing capacity failure. Due to the difficulties of obtaining relatiely undisturbed samples of cohesionless soils, semi-empirical approaches rely on correlations between the obsered foundation settlements and some parameters from in situ tests. Many methods hae been proposed to predict the settlement of foundations on cohesionless soils based on SPT N alues and CPT point resistance, q c. Some of the methods used to estimate settlement are summarized in Table. There are seeral other methods used to estimate settlement of foundations based on dilatometer (DMT) and pressuremeter (PMT) deried parameters. 9

2 n d International Conference on New Deelopments in Soil Mechanics and Geotechnical Engineering, 8-3 May 9, Near East Uniersity, Nicosia, North Cyprus In addition to the conentional approaches in estimating settlement of the shallow foundations, new methods and techniques are becoming aailable as more sophisticated electronic and computational tools are being deeloped. These include centrifuge modeling (Sargand et al. 997), nondestructie test methods such as the wae-actiated stiffness (WAK) test (Briaud and Gibbens 997), and neural networks (Shahin, et al. ). Seeral studies were performed to compare the predictie capabilities of different methods of estimating settlements of shallow foundations on sands. Gifford et al. (987) concluded that the methods proposed by D Appolonia et al. (967) and by Burland-Burbidge (984) were more accurate than the other methods. The Peck-Bazaraa method had a tendency to underpredict the field settlement, while the methods by Hough and Schmertmann (97) often oerpredicted the field settlement. Briaud and Gibbens (997) conducted a surey among bridge and foundation engineers for research work for FHWA. Briaud and Gibbens concluded that the best predictions resulted from the methods by Briaud (99), Burland-Burbidge (984), Peck-Bazaraa and Schmertmann (986). Briaud (99) and Burland-Burbidge (984) were somewhat conseratie for their methods, while the other two were slightly unconseratie. Berardi and Lancellotta (Lancellotta 99) hae compared the reliability and accuracy of different methods. They concluded that the most accurate empirical methods appear to be methods suggested by Burland and Burbridge and D Appolania et al. 3 SOIL CONDITIONS Standard penetration test (SPT), cone penetration test (CPT) and laboratory tests were performed to determine the engineering properties of soil layers. The soil inestigation has reealed that ery loose to loose silty sand up to 4 m depth, followed by dense to ery dense silty sand down to m exist at the site. The ground water table was encountered at a depth of m below ground leel. The results of SPT and CPT are presented in Figure. Cone Tip Resistance qc (MPa) 3 4 Sleee Friction f s(kpa) 3 4 Friction Ratio. f R(%) SPT N Blows / 3cm 34 DESCRIPTION ( SPT ) Light brown to dark gray, ery loose to medium dense, fine to medium SAND with traces of silt (SP - SM) Depth ( m ) 4 Depth ( m ) 4 Depth ( m ) 4 Depth ( m ) Figure : SPT and CPT Results 9

3 Obsered and Predicted Settlement of Shallow Foundation Düzceer, R. Table Summary of Settlement Prediction Methods METHOD EXPRESSION FOR SETTLEMENT D'Appolonia qb S (967) M Department of the Nay (98) 4q B S K B DEFINATIONS S = settlement (inches);µ = embedment influence factor; µ = compressible strata influence factor; q = applied pressure (tsf); M = modulus of compressibility (tsf). B = footing width (ft) EXPLANATIONS K V = modulus of subgrade reaction (tons/ft3); Valid for B feet Peck and Bazaraa (Anderson et al. 7) q B S CDCW N B C D = embedment correction factor; Cw = water table correction factor; N = corrected SPT-N alue; C.4 D σ Cw σ ' γc f q Peck-Hanson- Thornburn (D'Appolonia 967) q S.C w N Cw = water table correction factor; N = aerage corrected SPT-N alue within depth of B below the base of footing. D w C.. w D B f N.77 log '. N for σ tsf σ ' N N for σ ' N.4N for σ '.tsf Anagnostropoulos (99) Bowles (996) Burland and Burbidge (984) q B S. N ( ) qb' S IsI E S s.( L/ B) Bq'. ( L/ B) 3 Meyerhof 8q (Anderson et al. S N ' 7) q B S N' B f S = settlement (mm); q = applied bearing pressure (kpa) B = footing width (m); N = aerage uncorrected SPT-N alue υ = Poisson s ratio; q = applied bearing pressure (ksf);b = B/ for footing center and = B for footing corner (ft); E s = modulus of elasticity of bearing soil (ksf); I s and I f influence factor. S= settlement (ft); α = a constant (.4 for normally consolidated sands;.47 for oerconsolidated sands); α = compressibility index; and α 3 = correction for the depth of influence; q' = applied stress at the leel of foundation (tsf); q = applied bearing pressure (ksf); B = footing width (ft); N ' Corrected SPT- N alue B 4 feet B 4 feet N'.N Schmertmann Z z I z (978) S CC q z Es Schultze and fq B Sherif (Anderson S et al. 7) 4. D 87. N B f S = (in); C = foundation depth correction factor; C = soil creep factor; q = applied pressure; Iz = strain influence factor; and Es = modulus of elasticity. f = influence factor; q = applied bearing pressure (tsf); B = footing width (ft); N = aerage SPT-N alue within B from the base of footing; and D f = footing embedment depth (ft). γd f C. q t C.log. Terzaghi and Peck (Anderson et al. 7) Buisman- De Beer (96) Hough (Kimmerling ) Janbu (CFEM 99) 3q B S CDCw N B C D = embedment correction factor; Cw = water table correction factor; N = aerage uncorrected SPT-N alue for depth B below the base of footing; q = applied pressure (tsf); and B = footing width (ft). H =thickness of layer; C=Compressibility of sand H ' S log C σ ' =Effectie stress; ' o σ =Change in Effectie stress due to applied load; q c = Cone resistance n σ' log oσ H =thickness of layer ; C =Bearing Capacity Index c S Hc i C ' σ ' o σ ' =Effectie stress; o σ =Change in Effectie stress due to applied load j j H n σ ' σ ' o S H =thickness of layer σ ' o =Initial Effectie stress; ' ' σ ' i mj σ r σ r = final effectie stress; σ ' r =Reference stress kpa; m =modulus number; j=stress exponent=. for sandy and silty soils C W =. for Dw B; C W =. for Dw B D D C D =.- D 4 B 4B qc C=. ' 9

4 n d International Conference on New Deelopments in Soil Mechanics and Geotechnical Engineering, 8-3 May 9, Near East Uniersity, Nicosia, North Cyprus 4 LOAD TEST RESULTS Full scale footing load test was conducted at site. A. x. m square concrete footing was used for the test. The test was conducted at the proposed foundation embedment depth. Load test set up is presented in Figure. Figure Load Test Set up The full scale load test was performed on a precast footing up to. times the proposed safe bearing capacity of footing. The settlement of the footing and the applied loads recorded during the test are presented in Table. Table Measured Settlements s. applied test loads Loading Sequence (kpa) Measured Settlement (mm) INPUT PARAMETERS FOR EMPRICAL AND FINITE ELEMENT METHODS Numerical calculations were performed with finite element method. Plaxis 9. (Brinkgree and Broere 8) was used for this purpose. The square footing is represented by an equialent area circular footing using axisymetric conditions. The Mohr-Coulomb model was used to conduct the analysis. The geometry and the generated mesh are gien in Figure 3. The angles of shearing resistance were correlated from SPT N alues from the following equation: = x (-.47 N ) () The angles of shearing resistance were correlated from CPT using the correlation based on the effectie oerburden pressure, q c alues (Robertson and Campanella 988) The following correlations were used to obtain the Modulus of Elasticity with SPT N alues and CPT q c alues for normally consolidated sands (Bowles 996): E (kpa) =(N+) () E (kpa) =-4 q c (3) The following semi emprical relationship between the soil modulus and the adjusted cone tip resistance, q c was used to obtain modulus number, m in Janbu method (Canadian foundation Engineering Manual 99), as proposed by Massarsch (994): 93

resistance; = Mean effectie stress ' m ' m = K ' 3 (6) K =Coefficient of horizontal earth pressure Input parameters used in finite element analysis and empirical methods are gien in Table 3.

5 Obsered and Predicted Settlement of Shallow Foundation Düzceer, R. q m a cm (4) r m = Modulus number ; a =Emprical modulus modifier, which depends on soil type q =Stress adjusted cone stress; =Reference stress= kpa cm r c q cm r q () c ' m q = Unadjusted cone resistance; = Mean effectie stress ' m ' m = K ' 3 (6) K =Coefficient of horizontal earth pressure Input parameters used in finite element analysis and empirical methods are gien in Table 3. Description of Soil Layers (CPT) Table 3 Soil properties from in situ tests Depth (m) N ( bl/3 cm) (kn/m3) SPT (Deg) E (MPa) q c (MPa) CPT (Deg) SAND-GRAVELLY SAND -. to SAND-SILTY SAND -. to SANDY SILT- CLAYEY SILT -3. to SAND -4. to E (MPa) Figure 3 Finite element mesh for analysis 94

6 n d International Conference on New Deelopments in Soil Mechanics and Geotechnical Engineering, 8-3 May 9, Near East Uniersity, Nicosia, North Cyprus 6 COMPARISON OF SETTLEMENT PREDICTION METHODS In this study, ten conentional methods among the aailable methods hae been selected to be incorporated in settlement predictions. The comparison of settlement predicted with conentional methods and FEM are summarized in Table 4 and Table respectiely. Comparison of predicted ersus measured settlement is presented in Figure 4. Table 4 Measured ersus predicted settlements by conentional methods Settlements (mm) Loading Sequence (kpa) Measured Buisman- De Beer (3) Burland and Burbidge () Elastic Theory () Anagnostropoulos () D'Appolonia () Hough () Schmertmann (3) Schultz & Sherif () Department of the Nay (NAVFAC) (), ( 3) Janbu Tangent Modulus (), () () SPT based Method; () Elastic Theory base Method; (3) CPT based Method Table Measured Settlements ersus finite element analysis with Plaxis using insitu data Settlements (mm) Loading Sequence (kpa) Measured SPT CPT DISCUSSION OF RESULTS Examination of Table 4 indicates that the six of the conentional methods inestigated in this study, oerpredict the settlement of the footing. The most accurate settlement was estimated with the Buisman - De Beer method,.4 mm. The next most accurate method is the Janbu method, with the settlement of 4.7 mm at 3 kpa. On the other hand, the Hough method is the least accurate method with 4 mm of total settlement. This method is considered to be the most conseratie among conentional methods in predicting settlement in sands. Howeer, the preious studies hae shown that, Hough method oerpredict settlement by a factor of.8 -. (Gifford et al 987). It is interesting to note that, an oerprediction factor of.88 was obtained in this study for Hough method. Following the Hough method Nafac method is the second conseratie method with a total settlement of 34 mm. On the other hand, D Appolania method, underpredicted the settlement by a factor of.48, which is ery close to the factor of. determined in preious studies (Duncan and Tan 99). Elastic and Shultz - Sherif methods also 9

7 Obsered and Predicted Settlement of Shallow Foundation Düzceer, R. underpredict the settlement. The other methods; the Schmertmann, Burland - Burbridge and Anagnostrospoulos methods are situated in the middle. As for the finite element analysis, better accuracy of the estimation is obtained using the input data from CPT testing. The results of settlement estimate corroborate the conclusion from the Anderson et al (7) studies. The settlement predicted from the CPT deried input parameters was smaller than SPT as the CPT estimated modulus of elasticity and angle of shearing resistances are higher. The predicted settlements from the SPT and CPT input parameters are 4. mm and 8.9 mm respectiely. The settlement predicted from the SPT input parameters is less accurate then CPT. Applied Pressure (kpa) 3 3 Settlement (mm) Measured Nafac D'Appolania Shultz-Sherif Burland-Burbridge Anagnostropoulos Elastic Buisman De Beer Hough Plaxis (CPT) Plaxis (SPT) Janbu Schmertman-978 Figure 4: Comparison of the predicted and measured settlements. 8 CONCLUSIONS. A static load test was conducted to study the settlement behaiour of the footing. The CPT and SPT data were used to estimate the settlement of shallow foundation on sand.. Among the CPT based conentional methods, Buismann-De Beer, proide more accurate estimations of settlement. 3. Janbu method using CPT deried modulus number m, proide good settlement estimate. The correlations proposed by Massarsch proide accurate estimates of modulus number. 4. The correlated input parameters from the CPT data are more consistent than the SPT blow count in both conentional methods and finite element method.. Finite element analysis using CPT deried input parameters proided reasonable settlement estimates whereas the SPT deried input parameters proided poor settlement estimates. 6. The settlement estimations using FEM with CPT and SPT deried parameters corroborate the results obtained from the preious studies. 96

8 n d International Conference on New Deelopments in Soil Mechanics and Geotechnical Engineering, 8-3 May 9, Near East Uniersity, Nicosia, North Cyprus REFERENCES Anagnostopoulos, A.G., Papadopoulos, B.P.and Kaadas, M.J.(99). SPT and Compressibility of Cohesionless Soils. Proceedings of the nd European Symposium on Penetration Testing, Amsterdam. Anderson B.J., Townsend F. C. and Rahelison L. (7). Load testing and settlement prediction of shallow foundation. Journal of Geotechnical and Geoenironmental Engineering ASCE. Vol 33, No :494- Briaud, J-L (99) The pressuremeter. Balkema,Brookfield,Vt. Briaud, J-L., and Gibbens, R. (997). Large Scale Tests and Database of Spread Footings on Sand. Report No. FHWA-RD-97-68, Federal Highway Administration, McLean, VA, Brinkgree, R. B. J., and Broere, W. (8). Finite element code for soil and rock analysis. Plaxis Version 9. Delf Uniersity of Technology and Plaxis b.. Netherlands. Bowles J. E. (996). Foundation Analysis and Design th Ed. Mc Graw Hill New York Burland J.B.,and Burbidge, M.C.(984) Settlement of foundations on sand and grael. Institution of Ciil Engineers, Glaskow and West Scotland Association, Glaskow, Scotland. Canadian Foundation Engineering Manual 99. 3rd Ed. BiTech Publishers Ltd. Richmond, Canada D Appolonia D. J., D Appolonia D. and Brissette R.F. (967) Settlement of spread footings on sand. Journal of Soil mechanics and foundation Diision. ASCE., Vol 94, No SM 3 : De Beer. (96) Bearing capacity and settlement of shallow foundations on sands. Symposium on bearing capacity and settlement of foundations. Duke Uniersity. Durham N.C :3-33 Duncan J.M. and Tan C.K. (99). Settlement of footing on sands- accuracy and reliability. Geotechnical Engineering Congress, Boulder Colorado. ASCE Geotechnical Special Publication No: 7 Edited by Francis G., Campbell DW A. and Harris D.W Gifford, D. G., Kraemer, S. R., Wheeler, J. R., and McKown, A. F. (987). Spread Footings for Highway Bridges. Report No. FHWA/RD-86/8, Federal Highway Administration, U.S. Department of Transportation, Washington, D.C. Kimmerling Robert E. (). Shallow Foundations Report No. FHWA-SA--4, Federal Highway Administration, McLean, VA Lancellotta Renato. (99) Geotechnical engineering, Balkema, Rotterdam. Massarsch K.R. (994) Settlement analysis of compacted fills. Proceedings of the 3th International Conference on Soil Mechanics and Foundation Engineering. ICMSFE, New Delhi. Vol I: 3-38 Department of the Nay. (98). Soil Mechanics. Design Manual 7.. NAVFAC DM-7.. Naal Facilities Engineering Command, Alexandria, VA: 348 Robertson, P. K., Campanella, R. G. (988). Guidelines for using the CPT, CPTU and Marchetti DMT for Geotechnical Design Vol II. Report no: FHWA-PA U.S. Department of Transportation, Washington, D.C. Sargand S.M., Hazen G.A., Masada T. (997). Field and laboratory ealuations of spread footings for highway bridges. FHWA Report no: OH/98-7 Sargand S.M., Masada T. and Abdalla B. (3). Ealuation of cone penetration test based settlement prediction methods for shallow foundations on cohesionless soils at highway bridge construction sites. Journal of Geotechnical and Geoenironmental Engineering ASCE., Vol 9, No : 9-98 Shahin, M.A., Maier H.R., Jaksa M.B., (). Prediction settlement of shollow foundations using neural network. Journal of Geotechnical and Geoenironmental Engineering ASCE., Vol 8 No:9 : Schmertmann, J.H (97)Static cone to compute static settlement oer sand Journal of Soil mechanics and foundation Diision. ASCE, Vol 96., No SM3 : -43 Schmertmann, J.H., Brown P.R., and Harman J.P (978). Improed strain influence factor diagrams. Journal of Soil mechanics and foundation Diision. ASCE, Vol 4, No 8 :

Use of CPT for design, monitoring, and performance verification of compaction projects

Cone Stress, q t (MPa) 2 3 Sleeve Friction (KPa) 2 4 Pore Pressure (KPa) 2 7, Friction Ratio (%) 2 3 4 Profile Mixed Y 2 2 2 2 CLAY 3 3 3 3 4 4 4 4 SAN D Use of CPT for design, monitoring, and performance