FOUNDATION SETTLEMENT

Transcription

1 FOUNDATION ETTLEMENT Dr.. K. Prasad Prfessr f Civil Engineering. J. Cllege f Engineering, Mysre 8.0 yllabus 1. elemen Analysis, Daa fr selemen analysis. Cmpuain f selemen 3. Cncep, Immediae, Cnslidain and ecndary selemens (n derivain) 4. Tlerance, BI pecificains fr al and differenial selemens f fings and rafs. (5 Hurs) 8.1 Definiins elemen is he verically dwnward mvemen f srucure due he cmpressin f underlying sil because f increased lad. Unifrm elemen Differenial elemen Building max Original Level max min Final Level L max Fig. 8.1 : Cnceps f unifrm and differenial selemen Maximum elemen : I is he abslue maximum dwnward mvemen f any par f building elemen. Maximum elemen max

2 Differenial elemen : I is he maximum difference beween w pins in a building elemen. Differenial elemen max - min Angular Disrin : I is anher mehd f expressing differenial selemen. Angular Disrin Differenial elemen/lengh f elemen ( max - min )/L Fig. 8. represens sil mvemen under differen circumsances a he grund level. The flucuain in he elevain f grund level depends n seasnal changes in expansive (Indian Black Cn) sils and Fig.8. : il mvemen in differen siuains Table 8.1 presens he differen ypes f mvemens experienced by varius sils. I can be niced ha nly few sils such as clay can experience swell. Furher, cnslidain selemen and creep selemen are mre prnunced in clay. Immediae r elasic selemen is bserved in each and every sil. Firs hree, namely Immediae, Cnslidain and Creep selemen cause dwnward mvemen f grund while swell causes upward mvemen f grund.

3 Principal il Type Table 8.1: Types f Mvemen in differen sils Type f Mvemen Immediae Cnslidain Creep well Rck Yes N N me Gravel Yes N N N and Yes N N N il Yes Minr N Yes Clay Yes Yes Yes Yes Organic Yes Minr Yes Yes 8. Exrac frm I : General Requiremens fr Design & Cnsrucin f Fundain I presens Table 1 which gives deails abu he permissible selemen in seel srucures, reinfrced cncree srucures, muli-sreyed buildings and waer wers and sils in w differen ypes f sils, namely (1) and and hard clay and () Plasic clay. The selemens cnsidered are maximum selemen, differenial selemen and angular disrin r il. The deails in his able can be fllwed in he absence f mre precise selemen suggesed by he user. In case f muli sreyed buildings bh RC frames and lad bearing wall srucures are cnsidered. Lad bearing srucures wih L/H and 7 are deal wih. Tw ypes f fundains cnsidered are islaed fing and raf fundain. Table 8. gives he exrac f I cde and Table 8.3 presens he same able in differen frm fr seel and RC srucures. A maximum selemen f 75 mm, differenial selemen f L and angular disrin f 1 in 666 is permied fr islaed fings.

4 Table 8. : Permissible unifrm and differenial selemen and il fr shallw fundains Table 8.3 : Permissible unifrm and differenial selemen and il fr shallw fundains and & Hard Clay Plasic Clay Max. elemen Diff. elemen Angular disrin Max. elemen Diff. elemen Angular disrin Islaed fundain i) eel ii) RCC 50mm 50mm L L 1/300 1/666 50mm 75mm L L 1/300 1/666 Raf fundain i) eel ii) RCC 75mm 75mm L 0.00L 1/300 1/ mm 100mm L 0.00L 1/300 1/500

5 Table 8.4 : Limiing Values f mvemen fr Geechnical rucures Design Applicain Parameer Typical Mvemen hallw Fundain Allwable Bearing Pressure 5 mm fr buildings Deep Fundain kin Fricin 10 mm fr skin fricin mbilize Reaining Wall Acive & Passive earh pressure 0.1% H fr Ka & 1% H fr Kp mbilize in dense sand Reinfrced Earh wall Fricin & Dilancy lad ransfer in sil & reinfrcemen 5 50 mm fr gegrid mm fr geexile Pavemen Ru deph based n srain due n. f repeiins 0 mm ru deph in majr rads & 100 mm ru deph in minr rads Embankmen elf weigh selemen 0.1% heigh f embankmen Drainage Tal selemen mm 8.3 Tal elemen Tal fundain selemen can be divided in hree differen cmpnens, namely Immediae r elasic selemen, cnslidain selemen and secndary r creep selemen as given belw. I + C + Here, Tal elemen I Immediae / Elasic elemen C Cnslidain elemen ecndary elemen 8.4 Immediae elemen

6 Immediae selemen is als called elasic selemen. I is deermined frm elasic hery. I ccurs in all ypes f sil due elasic cmpressin. I ccurs immediaely afer he applicain f lad I depends n he elasic prperies f fundain sil, rigidiy, size and shape f fundain. Immediae selemen is calculaed by he equain menined belw. I Here, 1 µ qbi ρ E I Immediae selemen Pissn s Rai f fundain sil E Yung s mdulus f Fundain il q Cnac pressure a he base f fundain B Widh f fundain I Influence Facr Table 8.5 presens he ypical values f Pissn s rai in differen sils. Table 8.6 represens he ranges f sil mdulus in clayey sil f differen cnsisencies in undrained sae. In he absence f mre accurae daa, he values in ables can be used. The influence facr I depends n he shape and flexibiliy f fing. Furher, in flexible fing I is n cnsan. Table 8.7 presens he differen values f I. Table 8.5 : Typical Range f Pissn s Rai fr differen sils Type f il Pissn s Rai auraed Clay 0.5 andy Clay Unsauraed Clay Less 0.44 il and

7 Rck Table 8.6 : Typical Range f il Mdulus in undrained sae il Type il Mdulus (kpa) Very f Clay f Clay Medium Clay Hard Clay andy Clay Table 8.7 : Typical Values f Influence Facrs I hape f Fing Flexible Rigid Cener Crner Mean Circle Recangle L/B Recangle L/B Recangle L/B Recangle L/B Recangle L/B Recangle L/B Prblem 1 Deermine he elasic selemen f a fing 3 m X 3 m resing n sandy sil given Es kpa and 0.3. Fing carries a lad f 000 kn. Take I 0.8 (Feb 00)

8 q 000/3. kpa B 3 m I 0.8 E k Pa 0.3 I m 11 mm Prblem I 1 µ qbi ρ E Esimae he immediae selemen f a cncree fing 1 m X 1.5 m in size, if i is funded a a deph f 1 m in sily sil whse cmpressin mdulus is 9000 kpa. Fing is expeced ransmi uni pressure f 00 kpa. Assume I 1.06, 0.3 Daa E 9000 kpa 0.3 q 00 kpa B 1 m I 1.06 I 0.14 m I 1 µ qbi E ρ Prblem 3 A series f plae lad ess was cnduced n hree plaes 300 mm, 450 mm and 600 mm square plaes. The lads and crrespnding selemens in he linear prins f P curves are as fllws a a sie. Find he immediae selemen f a fing m X m subjeced a lad f 1000 kn. Table 8.8 : Deails f Lad selemen fr differen plae sizes Plae size (mm) Lad (kn) elemen (mm) 300 X X X

9 Table 8.9 : Variain f qb wih selemen fr differen plae sizes B (m) P (kn) q (kpa) (m) qb (kn/m) I 1 µ qbi E 1 I µ I qb E 1 µ I E ρ ρ ρ ( kpa) qb (kn/m) elemen (m) Fig. 8.3 : Variain f qb wih selemen fr differen plae sizes Daa B m q 1000/(*) 50 kpa

10 I 1 E I µ ρ qb I m Prblem 4 The fllwing are he resuls f plae lad es n granular sil. Find he allwable bearing pressure if B m, Bp 0.3 m, permissible selemen in 1 mm. Table 8.10 : Values f Lad elemen frm Plae Lad Tes P (kn) (mm) Lad (kn) elemen (mm) P f 50 kn; q f kpa -9 Fig. 8.4 : Lad elemen curve fr Plae Lad Tes daa s s p f Bp ( B f + 0.3) B f ( Bp + 0.3) 0.3( + 0.3) ( ) + s 1mm p

11 Based n selemen Permissible plae selemen ~ 1 mm ABP 3 kn/(0.3x0.3) kpa Prblem 5 The fllwing resuls were bained frm a plae lad es cnduced n dry sandy sraum using square plae f 0.3 m widh. Deermine he selemen f square fing 1.5 m wide when he inensiy f lading is 10 kpa. Table 8.11 : Values f Lad elemen frm Plae Lad Tes Pressure (kpa) elemen (mm) Daa andy sraum B F 1.5 m B P 0.3 m P 3. mm Plae Lad Tes Resul elemen (mm) il Pressure (kpa)

12 Fig. 8.5 : Lad elemen curve fr Plae Lad Tes daa 8.5 Cnslidain elemen 1. I ccurs due he prcess f cnslidain.. Clay and Organic sil are ms prne cnslidain selemen. 3. Cnslidain is he prcess f reducin in vlume due expulsin f waer under an increased lad. 4. I is a ime relaed prcess ccurring in sauraed sil by draining waer frm vid. 5. I is fen cnfused wih Cmpacin. 6. Cnslidain hery is required predic bh rae and magniude f selemen. 7. ince waer flws u in any direcin, he prcess is hree dimensinal. 8. Bu, sil is cnfined laerally. Hence, verical ne dimensinal cnslidain hery is accepable. 9. pring analgy explains cnslidain selemen. 10. Permeabiliy f sil influences cnslidain. P F BP ( BF + 0.3) BF ( BP + 0.3) Table 8.1 : Cmpacin Vs Cnslidain ( ) COMPACTION CONOLIDATION F 1.5( ) + 1. Man made 1. Naural F 8.89 mm. Vlume reducin due. Vlume reducin due expulsin f air expulsin f waer 3. udden (hr durain) 3. Gradual 4. Dry densiy increases waer 4. Dry densiy increases waer cnen des n change cnen decreases 5. Applicable fr unsauraed sils 5. Applicable fr sauraed sils

13 Cmpacin Cnslidain Fig. 8.6 : Cmparisn beween Cmpacin and Cnslidain Fig. 8.7 : Cncep f Cnslidain selemen : Oedmeer Tes 1. Oedmeer es is als called cnslidmeer es.

14 . I is a labrary es deermine ne dimensinal cnslidain prperies f sil. 3. Cmpressin index and Cefficien f cnslidain are he impran prperies deermined frm his es. 4. A a given susained pressure, rae f cmpressin f sauraed specimen wih ime is measured. 5. The pressure level is gradually increased wih lading and decreased wih unlading. Cnslidain elemen in nrmally cnslidaed clayey sil is given by he expressin, c Cc 1+ e + H lg 10 c C c e H Cnslidain elemen Cmpressin Index Iniial Vid Rai Thickness f clay layer Iniial verburden pressure a he middle f clay layer Exra pressure due he new cnsrucin 8.5. : Cmpuain f ime aken fr cnslidain selemen in A any degree f cnslidain, cmparisn in ime aken fr cnslidain beween labrary & wih respec drainage pah is Time aken d Drainage pah lab d d lab Empirical equain fr Cmpressin Index based n kempn s wrk (1944) is given by, CC 0.009( ωl 10%) Here, L is he Liquid Limi in %. Cmpressin index has n uni. Iniial vid rai in sauraed sil is bained frm he equain, ωg e e ωg

15 Here, Waer cnen G pecific Graviy f il lids (assume.7) Degree f aurain (1 fr sil experiencing cnslidain) : Lad dispersin in pariculae sil medium H γ sa P H + B 45 (Dispersin angle) H H/ B H/ Fig. 8.8 : Cncep f Lad dispersin : Cefficien f Cnslidain I is ne f he impran prperies f cnslidain hery ha helps in evaluaing cnslidain selemen. I is deermined frm edmeer es. The belw menined frmula is used deermine cefficien f cnslidain. TV d CV Here, Tv is he ime facr, d is he drainage pah and is ime aken fr cnsldain : Time Facr

16 Time facr is bained frm he frmulae shwn belw. I depends n he degree f cnslidain. U TV π 4100 Cmmnly ime facr a 50 % and 90 % degrees f cnslidain are used and are as menined belw. (T V ) (T V ) T V lg (100 U %) 10 Prblem 6 The al ime aken fr 50 % cnslidain f clay layer is 4 years. Wha will be he ime aken fr 90 % cnslidain? (Aug 001) (T V ) (T V ) C V 0.848d ( TV ) d ( TV ) 0.197d years d Prblem 7 A layer f clay 8 m hick underlies a prpsed new building. The exising verburden pressure a he cener f layer is 90 kpa and he lad due new building increases he pressure by 100 kpa. C c 0.45, 50 %, G.71. Esimae he cnslidain selemen. (Aug 00) Daa C c 0.45 e H 8 m 90 kpa 100 kpa

17 c e Cc 1+ e ωg H lg (Dispersin angle) H H/ B H/ Fig. 8.9 : Cncep f Lad dispersin c Cc 1+ e H lg m Prblem 8 A nrmally cnslidaed clay layer is 18 m hick. Naural waer cnen is 45 %, sauraed uni weigh is 18 kn/m 3, grain specific graviy is.7 and liquid limi is 63 %. The verical sress incremen a he cener f clay layer due fundain lad is 9 kpa. Grund waer able is a he surface. Deermine he selemen. (Aug 003) Daa C c e 1.15 H 18 m 16 kpa 9 kpa

18 CC e γ 0.009( ω 10%) L ωg 1.15 sa 9 kpa Z 18* 18 16kPa c Cc 1+ e H lg m Prblem 9 A square fing 1. m X 1. m ress n a sauraed clay layer 4 m deep. L 30 %, sa 17.8 kn/m 3, 8 % and G.68. Deermine he selemen if he fing carries a lad f 300 kn. γ sa Z 17.8* 300 ( ) C e C H 4 m L kPa kPa 0.009( ω 10%) 0.18 ωg 0.8* c Cc 1+ e H lg m Prblem 10 A es n undisurbed sample f clay shwed 90 % cnslidain in 10 minues. The hickness f sample was 5 mm wih drainage a bh p and bm. Find he ime required fr 90 % cnslidain f fing resing n 5

19 m hick cmpressible layer sandwiched beween w sand layers. (Aug 007) Daa D lab 5/ 1.5 mm D 5000/ 500 mm lab 10/(60*4*365) years lab d d lab lab dlab d years AND 5m CLAY AND Fig : il prfile in he and deails f edmeer es 8.6 ecndary Cmpressin 1. This selemen sars afer he primary cnslidain is cmpleely ver.. During his selemen, excess pre waer pressure is zer. 3. The reasns fr secndary selemen are n clear. 4. This is creep selemen ccurring due he readjusmen f paricles a sable equilibrium under susained lading ver a lng ime. 5. This selemen is cmmn in very sensiive clay, rganic sils and lse sand wih clay binders. C α sec H lg 10 prim prim

20 C Cefficien f secndary cmpressin H Thickness f clay layer sec Time aken fr secndary cmpressin (usually life span f srucure) prim Time aken fr primary cnslidain cmplee (EPWP becme zer) Cefficien f ecndary Cmpressin Table 8.13 : Typical values f Cefficien f Cnslidain in differen sils Type f il C N. C. Clay Organic Clay > 0.03 O. C. Clay (OCR > ) < elemen C End f Primary Cnslidain Lgarihm f Time Fig : Time Vs elemen curve represening cefficien f secndary selemen Prblem 11

21 Deermine he creep selemen in a sensiive clay f hickness 6 m given C 0.01 when he labrary sample 0 mm hick wih duble drainage experienced cmplee cnslidain in 10 minues. The life span f srucure is 100 years. Daa sec 100 yrs H 6 m C 0.01 lab d d 36X10 prim lab / 5 mns 6.85yrs sec prim C α H lg m prim Fig. 8.1 : Deails f edmeer es Prblem 1 A m X m fing carrying a lad f 1600 kn ress n a nrmally cnslidaed sauraed clay layer 10 m hick belw which hard rck exiss. The life span f he srucure is 150 years. Time aken fr he cmplein f primary cnslidain f 0 mm hick labrary specimen wih duble drainage faciliy is 0 minues. Find he al selemen, if he sil prperies are as fllws. il mdulus 0 MPa, Pissn s rai 0.45, Influence facr 0.9, Liquid Limi 50 %, Naural waer cnen 5 %, pecific Graviy f

22 grains.7, sauraed densiy 0 kn/m 3 cmpressin and cefficien f secndary Tal elemen, I + C + Daa fr Immediae elemen E 0000 kpa 0.45 q 1600/ 400 kpa B m I 0.9 I 1 µ qbi ρ E I m 8. 71mm Daa fr Cnslidain elemen L 50 % 5 % G.7 C c 0.36 e H 10 m 100 kpa kpa C e C 0.009( ω 10%) 0.36 L ωg 0.5* γ H sa 100kPa c P H + B Cc 1+ e 11.11kPa + H lg 10

23 Daa fr ecndary elemen sec 150 yrs H d 10 m C d lab 10 mm lab 0 mns lab prim d d lab 38.05yrs C α sec H lg 10 prim prim m 4. 7 mm Tal elemen, I + C mm 8.7 Facrs Influencing elemen Many facrs influence he selemen f fundain sil when a srucure is buil n i. The fllwing are a few impran facrs be cnsidered in he evaluain f selemen. 1. Elasic prperies f sil. hape f fing 3. Rigidiy f fing 4. Cnac pressure 5. Widh f fing 6. Cmpressibiliy characerisics f sil 7. Iniial cndiins f sil (Densiy, vid rai ec.) 8. Degree f saurain 9. Over Cnslidain Rai 10. Time available fr selemen 11. Thickness f sil layer

24 1. Lad dispersin angle 8.8 Mdulus f ubgrade Reacin 1. Mdulus f subgrade reacin is defined as he rai f cnac pressure he crrespnding selemen. Is unis are in kn/m 3 3. I depends n shape, rigidiy and size f fing, deph f embedmen and ype f sil 8.9 Prbable Quesins 1. Disinguish beween 1. Cnslidain elemen and Immediae selemen. Cnslidain selemen and ecndary selemen 3. Unifrm selemen and Differenial selemen. Explain he cncep f elasic selemen. 3. Wha is influence facr? Wha facrs influence i? 4. Wha is cnac pressure? Wha facrs influence he same? 5. Hw are sil mdulus and pissn s rai deermined in labrary? Explain. 6. Explain he cncep f cnslidain hery. 7. Wha is cnslidain selemen? Hw is i deermined? 8. Explain he use f labrary cnslidain selemen in finding selemen f sil. 9. Lis he facrs influencing selemen. 10. Explain he cncep f creep / secndary cnslidain selemen 11. Define he erms Cefficien f secndary cmpressin Cmpressin Index Cefficien f Cnslidain

25 Influence facr Cnac Pressure 1. Wha are he ill effecs f al and differenial selemen f sil n buildings? 13. Wha are he differen ypes f selemens f fings? 14. Menin he relains used cmpue elasic and cnslidain selemens. 15. Wrie a ne n BI specificains fr differen ypes f selemens. 16. Explain he use f plae lad es in esimaing he prbable selemen f fing. 17. Define mdulus f subgrade reacin and indicae he facrs affecing i : References 1. Bwles, J. E. (1977) Fundain Analysis and Design, Mc Graw Hill Publicains, New Yrk.. Craig, R. F. (1983) il Mechanics, 3 rd Ediin, English Language Bk ciey & Van nsrand Reinhld C. Ld., Lndn. 3. Das, B. M. (007) Principles f Fundain Engineering, Thmsn 4. I India Ediin, New Delhi. 5. I : General Requiremens fr Design & Cnsrucin f Fundain 6. Punmia, B. C. (005) il Mechanics and Fundains, Laxmi Publicains Pv. Ld., Bangalre 7. m, N. N. and Das,. C. (003) Thery and Pracice f Fundain Design, Prenice Hall f India, New Delhi