Empirical correlations of overconsolidation ratio, coefficient of earth pressure at rest and undrained strength

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Michael Norman Stanley
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1 Second Conference of Jnior Reearcher in Civil Engineering 88 Empirical correlation of overconolidation ratio, coefficient of earth prere at ret and ndrained trength Vendel Józa BME Department of Geotechnic, Abtract Generally, parameter of the in it oil tate are determined from laboratory and in-it tet. The tree and the deformation of the oil depend on the tre hitory, among the other. The degree of the preloading i claically expreed with the overconolidation ratio (OCR. The goal of thi paper i to compare the vale of OCR which are determined from onedimenional oedometer tet and from cone penetration tet (CPT a well. Determination of OCR from CPT i baed on meared vale of net and effective cone tip reitance. Three mathematical fnction are decribed from laboratory and CPT tet. The correlation coefficient (R 2 of the fnction are very high, ranged between 0.82 and Relationhip for OCR, coefficient of earth prere at ret and ndrained trength were calclated. Keyword: cone penetration tet, overconolidation ratio, tre hitory, coefficient of earth prere at ret, ndrained trength Introdction Claical CPT probe, meared cone reitance (q c, leeve friction (f and pore prere mearement ( 2 were ed in Dnaújváro, middle of Hngary, right ide of the Danbe. The depth of the borehole and the cone penetration in the analyed ection were abot 30 m and the grond-water level wa detected 10 m below rface. OCR vale were determined from 22 oil ample of 7 borehole and from 13 CPT diagram with 53 different depth. Three calclation method were compared and preented. The regreion between OCR and depth wa calclated with Software Wolfram Mathematica. Stre hitory and overconolidation ratio A. Stre hitory from laboratory oedometer tet The tre hitory of clay oil i claically determined from one-dimenional oedometer tet on nditrbing ample. The yield point in one-dimenional loading denote the preconolidation tre ( p '. In normalized form, the degree of preconolidation i termed the overconolidation ratio (eq.1. OCR = p (1 where ' i the effective vertical geotatic tre. p ' i normally interpreted from oedometer tet from void ratio (e - log ' relationhip ing the Caagrande graphical techniqe. Fig. 1 how the Caagrande method for determining p '. The following tep decribe thi contrction. Fig. 1. Caagrande Method for Determining Preconolidation Stre

The tree and the deformation of the oil depend on the tre hitory, among the other. The degree of the preloading i claically expreed with the overconolidation ratio (OCR.

2 Second Conference of Jnior Reearcher in Civil Engineering Chooe by eye the point of minimm radi (or maximm crvatre on the conolidation crve (point A in Fig Draw a line tangent to the crve at point A. 3. Draw a horizontal line from point A. 4. Biect the angle made by tep 2 and Extend the traight-line portion of the virgin compreion crve p to where it meet the biector line obtained in tep 4. The point of interection of thee two line i the preconolidation tre. The preconolidation tre wa determined from oedometer tet ing for or five conolidation tre point with available maximm vale of 400 kpa. B. Stre hitory from CPT For depoit OCR can be expreed by the following imple formla (ee [1]. q t OCR = k ( (2 where k i the overconolidation factor with a range of 0.3 to 0.5 and i the vertical geotatic tre. For intact clay, a firt-order etimate of the preconolidation tre can be obtained from net cone tip reitance ed k = 0.33 (ee [2]. p = 0.33 ( qt (3 Thi expreion nderetimate vale for fired clay. Thi i becae the reaon i the macrofabric of crack and fractre affect the field mearement of the CPT (ee [2]. From a theoretical perpective, the vale of preconolidation tre can alo be acertained from the effective cone tip reitance in the cae of holder filter element (ee [2]. Relt of analyi p = 0.60 ( q t 2 OCR can be evalated from the following relationhip depending depth (z. The following expreion baed on eq.1 (Method A, eq.3 (Method B and eq.4 (Method C ed Software Wolfram Mathematica. where z = depth OCR A = ( OCR B = ( OCR C = (7 The calclated mathematical fnction baed on the relt of previo reearch and frther analyi of oil and method claification can be relted more accrate approximation for Hngarian oil. The maximm vale of loading tre of oedometer tet wa ed only 400 kpa relted lower preconolidaton tre then the derived vale from in-it tet. Higher loading tre cold be relted more accrate yield point to define preconolidation tre. The obtained fnction and calclated correlation coefficient (R 2 are hown in Fig.2. In general low effective vertical tre and high cone tip reitance are relted nrealitically high OCR in the pper 3 m thick layer. Calclated vale can be able below 3 m depth for geotechnical oftware ing finite element method. Another in-it tet - for example elf-boring preiometer hold be ed to compare the etimated vale and harmonie the calclation method. (4

Extend the traight-line portion of the virgin compreion crve p to where it meet the biector line obtained in tep 4. The point of interection of thee two line i the preconolidation tre.

3 Second Conference of Jnior Reearcher in Civil Engineering 90 Fig. 2. Image of variable OCR with Depth Coefficient of earth prere at ret for overconolidated oil (K0.OC i given with eq. 8. (ee [1]. K 0.OC = 0.1 ( qt v 0 v 0 (8 Relationhip for OCR and K0.OC i hown in Fig. 3. Fig. 3. Relationhip for OCR and coefficient of earth prere at ret OCR vale between 3 m and 30 m depth are ed to analye the fnction. The following expreion are connected to Fig. 3 with method B and method C. K 0.OC (OCR A = (OCR A (9 K 0.OC (OCR C = (OCR C (10 The coefficient of earth prere at ret from CPT and OCR with method B are not independent of each other and the relationhip are not analyed. From conideration of critical tate oil mechanic (CSSM, the ndrained trenght ( from direct hear

Fig. 3. Relationhip for OCR and coefficient of earth prere at ret OCR vale between 3 m and 30 m depth are ed to analye the fnction. The following expreion are connected to Fig.

4 Second Conference of Jnior Reearcher in Civil Engineering 91 imple tet (DSS can be expreed in normalized form (ee [2] (eq. 11. ' 1 = in( φ' OCR 2 λ (11 where λ = platic volmetric train potential, and φ = internal friction angle. For clay of low to medim enitivity, 0.7 < λ < 0.8, frthermore for enitive and trctred clay, 0.9 < λ < 1.0 can be oberved (ee [2]. If the parameter λ and φ are not known with confidence, eq. 12 can be ed (ee [2]. ' = 0.22 OCR The ndrained trenght can be etimated from CPT relt with the following form 0.80 (12 qt = (13 14 th the ndrained trength and OCR with method B are not independent of each other and the relationhip are not analyed. The relationhip for ndrained trenght and OCR with method A and method C are preented with eq. 14. and eq. 15. (ee Fig. 4. Fig. 4. Relationhip for Undrained Strength and OCR OCR vale between 3 m and 30 m depth are ed in the following fnction (eq. 14 and eq. 15. = OCR ' A (14 Conclion ' = OCR C Claical CPT probe with cone reitance (q c, leeve friction (f and pore prere mearement ( 2 were ed in Dnaújváro. The depth of the borehole wa ranged between m. 22 oil ample from 7 borehole and 53 characteritic point from 13 CPT diagram wa ed in the calclation. Moreover, different determination method were compared baed on laboratory and in-it tet. The following (15

9 < λ < 1.0 can be oberved (ee [2]. If the parameter λ and φ are not known with confidence, eq. 12 can be ed (ee [2]. ' = 0.

5 Second Conference of Jnior Reearcher in Civil Engineering 92 behavior were oberved. 1 Overconolidation ratio decreaed de to depth, depended on bai of determination method. The pper 3 m thick layer i relted in nrealitically high vale of OCR becae of the low effective vertical tre and high cone tip reitance. Method A provide the lowet vale, followed in trn by method B and C. The calclated correlation coefficient of the mathematical fnction are ranged between 0.82 and The relationhip for coefficient of earth prere at ret for overconolidated oil and OCR i etimated with R 2 = 0.61 and The expreion are independent of the internal friction angle and the power of OCR are oberved for method A and for method C. Method C cold be ed to etimate K 0.OC if vale of OCR are acceptable. 3 The relationhip for ndrained trength and OCR i etimated with R 2 = 0.54 and The relt of method A are hown large variability, therefore approximation of preconolidation tre hold be ed by in-it tet. Undrained trength increaed de to OCR. Undrained trength, coefficient of earth prere at ret, tree and deformation of retaining and other geotechnical trctre can be inflenced by overconolidation ratio, therefore frther analye may be needed. Acknowledgement The work reported in the paper ha been developed in the framework of the project Talent care and cltivation in the cientific workhop of BME" project. Thi project i pported by the grant TÁMOP B-10/ The athor wold like to thank to aociated profeor Zoltán Czap (Bdapet Univerity of Technology and Economic, Department of Geotechnic, Baláz Váárhelyi (Univerity of Péc, Department of Strctral Engineering for helpfl advice. The ample and CPT graph were preented by Roland Tóth (Geo- Engineering Kft.. The athor deeply appreciate him and other peron who contribted. Reference [1] Lnne T., Roberton P.K. and Powell J.J.M.: Cone Penetration Teting in Geotechnical Practice, Taylor & Franci, London, (2002 [2] National Cooperative Highway Reearch Program (Synthei 368: Cone Penetration Teting, Wahington, D.C. (2007

Method A provide the lowet vale, followed in trn by method B and C. The calclated correlation coefficient of the mathematical fnction are ranged between 0.82 and 0.93.

EXPERIMENT 11 CONSOLIDATION TEST

119 EXPERIMENT 11 CONSOLIDATION TEST Purpoe: Thi tet i performed to determine the magnitude and rate of volume decreae that a laterally confined oil pecimen undergoe when ubjected to different vertical