30

20- e a Z

the appropriate settlement parameters for 10- use in foundation design on Dublin Black Boulder clay. It gives the relevant results of a ground investigation which included in situ plate tests and a 1.5mby 1.5mtest footing which was loaded to over 1000kN/m .The measured settlement 0 10 20 40 50 60 70 80 90100oefusal under the test footing is compared with 30 N value that estimated from the findings ofthe in situ and laboratory tests. >g 2 Eistorgram ofSPTrosldts none were noticed in the trial pit Site and geology excavations on the Tallaght site. The study was camed out on a site in An electron microscope and Xray Tallaght on the south western boundary of diffraction study camed out by Ellis Dublin. Settlement parameters (1987) onthe particles passing the 425/Im of Dublin Black Glacial till deposits more than 17m thick sieve showed that there is little if any clay Boulder clay overlie limestone bedrock. The till on this mineral present. The soil particles site comprises lm to 3m of a generally firm comprise predominantly quartz and calcite with small dolomite by Dr ERFarrell BA, BAI, MS, CEng FIEI, Brown Boulder clay over a hard Black a amount of and and MICE Trinity College, Dublin Boulder clay which extends to bedrock. plagioclase felspars a trace of These are terms used locally to illite or mica. Those particles which are dimerentiate the two glacial deposits 'clay size'ere in fact very cubical and KKTFHT'IIFl rnalenee n au llllll'» which underlie much ofthe city. angular in shape unlike the normal plate

vari lloTSSIshear strength of o WZ !STiZ el IIrora =el' I P'IITSOT In The Black Boulder clay has atypical the Black Boulder clay on the Tallaght site saei lee oi"feei leaieei n i la re'iTee ~ lTI I ~ ii almost straight line grading when plotted because ofthe difficulties in obtaining s TTZ ilb% Tlua lmlPPi IIII oaoeaanuellreliae i, on a particle size distribution chart Fig. I. undisturbed samples. On the basis ofN fi:l ~ e IME ST ~ a ~ ou eIIsonn~ Siai ~ ooaiae ~ a The fines content varies from 24% to 46%, values, together with laboratory results IIm; serss mflrmsraa:sr which means that portion ofthe deposit and those ofthe in situ plate tests, it is should more correctly be called a gravel considered that the undrained shear when described in accordance with strength is about 250kN/m .The effective Introduction BS5930.The soil has a low liquid limit of stress parameters are c' 0 and Is' 37. Much of Dublin and its environs is between 19%and 31%and a plasticity underlain by a hard lodgement till which is index which is generally about 10.These Laboratory consolidated tests colloquially called Dublin Black Boulder plot on the plasticity chart as a clay oflow Consolidation tests were canied out on clay. It is difficult to obtain undisturbed plasticity index. The natural water content samples ofthe soil, with particles greater samples ofthis soil because of its stone is generally between 8% and 11%. than Smm removed, compacted to various content, and where such samples have Hanrahan (1977)'eported fissures in the densities in the standard oedometer. The been obtained, laboratory derived Black Boulder clay previously, however samples tested had specified volumes of parameters generally result in settlement between 1.286and 1.34.The coefficient of predictions which conflict with volume compressibility m„on the initial engineering experience. ~O TEST ~IETES load curve for the applied pressure In practice it is normal to design increments of 825kPa and 550kPa (after an foundations using an allowable bearing al initial bedding pressure of 20kPa) was I pressure of 400kN/m', subject to between 0.0293m'/MN and 0.058m'/MN confirmation ofthe in situ strength from The value on the unload stage varied from SPT results. However this approach has 0.002 /MN at the initial load stage to serious limitations for large footings or for I I I I 0.02m'/MN over the complete unload structures very sensitive to differential increment to the seating pressure. I I I settlement where an estimate ofthe The coefficient ofconsolidation amount of settlement is required. interpreted from the consolidation tests Ee Furthermore the introduction ofthe J II. was very high at over 70m'/year. A pore I Eurocode EC7 in 1992will put a greater pressure dissipation test on a 38mm emphasis on the estimation of foundation i. ~ 1.0 ~oi sample gave a c„ofabout 76m'/year. The SISE I I settlement in order to satisfy the rate of consolidation ofthese samples was serviceability requirements ofthe code. considerably faster than that interpreted This paper presents the results of a study from the on site tests, as will be discussed 3Q camed out into methods of determining Fig.l.Particle size chart later. GROUND ENGINEERING JULY 1989 Foundationsise 1 5m x I 5m Initial dead load of 88.9kPa

o ~L ..7./,. Cl „..t 500- CL

alCl a LC between the immediate settlement and In situ field tests )0 2 4 6 8 10 12 14 I the total settlement is given by SPT results Settlement(mm) A total of over 70 SPTs was carried out on s; 1 the site. There was no noticeable variation Fig.4.Searing pressure-settlement curve during 1.5m square foundation ofthe N value with depth. Results are s, 2(l-v') plotted in the form of a histogram on load test. Fig.2. These were performed using a It was sufficiently greater in area than where v' effective stress Poisson's ratio 'olid cone to prevent damage to the point. plate tests to show up the effect of size on the immediate settlement. Plate tests The results therefore indicate that the For logistic reasons it was necessary to Two 305mm diameter plate tests gave Black Boulder clay has a Poisson's ratio of load the footing in stages. The values ofthe undrained elastic modulus of about 0.22which is reasonable for a soil of settlement readings indicated that the 98MN/m and 87MN/m2 under a maximum this type which has a low plasticity index. primary settlement was complete prior to applied pressure of 1200kPa. There was Using this value and the measured total the application ofthe last load stage. no indication ofpotential failure under this settlement, the estimated drained elastic load. The load —settlement curves are The settlement versus average bearing modulus (E') is about 82MN/m . shown on Fig. 3. pressure is shown on Fig. 4 and time- The estimated coefficient ofvolume settlement under the last loading stage on compressibility mv assuming that the total Fig. S. settlement is approximately equal to that Plate dia. 305 mm 1500- Time (hours) determined from the oedometer (Burland et al 1977),is 0.012m /MN. The 500 1500 2000 m„, 1000 estimated from the consolidation EO E settlements only is ofthe order of jZ a 0.004m /MN, which is very low. 1000- j10 Interpretation ofthe coefficient of a m consolidation was problematic due to CL the small sl deQections which were involved, and because vandals destroyed dial 20'ime-settlement curve at 223t (1013kPa) gauges at a critical stage. This did not CL 500- affect the overall settlement readings as a Fig.S.rhyme-settlement curve for last backup system of level readings onto Test 2 loading of223t, 10l3irpa bearing graduated scales was sufficiently prtsssuze. accurate for that purpose. However it is considered that the in situ value of cv is Interpretation about 4m /year or 30m/year depending on I I I I I whether the vertical drains are 05 1 15 2 25 oftest footing results considered to have functioned efficiently Plate settlement mm Inspection ofthe settlement load curve or not at all. The determination ofthe Fig.3.Load-settlement curve during Fig. 4, indicates an almost linear response parameter was not a crucial part ofthe 30Smm diameter plate bearing test. between the immediate settlement and study. the applied load. This is illustrated by the Foundation test near constant slope ofthe loading portion ofthe curve. The maximum pressure The foundation test involved loading a Settlement parameters applied was over 1000kPa. Back analysis Immediate settlement 1.5mby 1.5m square reinforced concrete ofthis test using the slope ofthis line, and footing with 228t of dead load. This gave a assuming that the soil is a linear elastic Both the in situ plate tests and the maximum bearing pressure of 1013kPa. semi-infinite half-space with an undrained foundation test indicate that the The load was applied by concrete blocks Poisson's ratio of 0.5,gave an undrained settlement-load relationship can be and a steel support frame (see pic. l).The elastic modules of 1000MN/m'. treated as linear for practical purposes up concrete footing was placed on a 50mm (~ to a loading which corresponded to a high thick sand bed which was kept flooded. The ratio of immediate settlement (Q to proportion ofthe ultimate shear strength. Four 50mm diameter holes were drilled at total settlement (Sc)using this value of E„ The value ofthe undrained modulus 700mm centres to a depth of 1.5m beneath determined from the in situ plate tests and the footing. Full details ofthe test from the foundation test was reasonably arrangement and the settlement similar at between 87MN/mz to 100MN/ma. measuring system are given in Farrell et al —= 0.64 It must be appreciated however, that in (1989).It is considered that the total both cases some drainage would settlement readings are accurate to 3.5%. inevitably have occurred. An important The test footing was the largest size which Burland and Wroth (1974)'howed that for factor to be noted from these results is that could reasonably be loaded by kentledge. a linear elastic isotropic soil the ratio Eu determined from the in situ test gave a 31 GROUND ENGINEERING . JULY 1989 reasonable prediction ofthat estimated would be appropriate in view ofthe centre, and consulting engineer TJ from the settlement of the test footing. limited in situ results available to date. O'onnor &Associates for their kind Marsland et al (1985)'reported the ratio to permission to publish the results ofthe Total settlements be 0.37for the Cowden till ground investigation and ofthe test loaded to give a origin It has long been recognised that for factor of safety of 3 against undrained footing. overconsolidated clays the settlement shear failure. However the Cowden till estimated by summing the immediate contains a considerable proportion of clay References settlement and the consolidation minerals which are essentially absent in 1.Hanrahm E H 'bish Glacial tsls: an settlement determined from the the Dublin Black Boulder clay. chuacteristics'n Fores Forbartlm repor RC 184, 19TI. laboratory value for mv grossly The above 8.Hhs 'Analysis ofDubhn Black Boulder clay'AI overestimates the settlement that actually comments apply to a 4 monotonically increasing load. VeQing degree proiect, Trinity CoRege, Dublin 1987. occurs in practice. This was shown by the 3.FaneS E R, BunniNG, Mulhgan The Bearmg and others J, settlement ofthe test footing which was (1974) have shown that Capacity of Dublin Black Boulder clay'. Transactions of continuing settlments can occur under the ofEngineers ofheland, 1988. about 30% ofthat estimated using the mv conditions where the live load is a 4.Burland~JBand Wmth C P, 'Settlement ofbugdin{m determined from the loading sequence of significant portion ofthe dead load. and associated damage'roc Conf on settlement of the standard oedometer test and the suructurte, Cambridtfe, 1974. immediate settlement estimated using S.Burland JB,Brome B,and De Meso VFS behaviour of Eu = 100MN/ms. The total settlement was foundations and ructures'Proc 9thICSMFE Tokyo, Conclusion Voia, 1877. about 50% ofthat determined from the 1.Dublin Black Boulder clay is a hard 8.Vesing G, Settlement ofthe sugar silos on moraine unload sequence ofthe oedometer tests. days in Godev, Denmark'roc Conf on the senlement lodgement till which is difficult to sample A Skempton-Bjerrum correction factor of tructurtm, Cambridge, 1874. and test because ofits high stone content. applied to the latter gave a reasonable 7. A and Powell JJ, Tield and laboratory It is particularly difficult to determine the investigations~ ofthe day tsls at the buMng research prediction, however it was difficult to appropziate settlement parameters for estabhshed test site at CowdenHoMness.'roc Int justify the precise cozrection factor. Conf on construction in glacial tsts and boulder

design offoundations. The reasonable days'dinburgh, Burland et al (1977)have shown that, for a 1988. estimate ofthese parameters will soil which essentially behaves elastically, be required for major structures designed in the settlement estimated using mv detezmined form the oedometer is accordance with Eurocode EC7 which will introduced in 1992. nozmally within 10% ofthe exact total be 2.A comparison ofthe settlement of test settlement. The estimated settlement of a footing with that estimated from the test footing using this approach and parameters obtained from in situ and the mv determined from the unload laboratory tests on samples from a site in sequence ofthe test are generally within Tallaght, Co Dublin has indicated that the 30% ofthat recorded for the foundation settlement can reasonably be estimated test. The settlement estimated using the using the relationship. loading curve is about three times that measured. It must be appreciated that there are —= 0.5 serious limitations to the tste ofthe laboratory determined parameters as the tests were carried out on recompacted The immediate settlement Sican be samples with particles greater than Smm estimated from the undrained elastic removed. It was not possible to obtain modulus determined from in situ plate undisturbed samples suitable for testing. tests. Making the normal assumption that a 3.Settlements estimated using the heavily overconsolidated soil such as the unloading cuzve of standard oedometer Dublin Black Boulder clay behaves as an tests on recompacted samples with elastic matezial within normal working particles greater than Smm removed were stresses, the ratio 5/St can be related to reasonably in line with those measured on the elastic parameters. Burland et al (1977) the test footing. In this type ofanalyses the repozted measured values ofthis ratio on settlements computed using oedometer existing structures ofbetween 0.32and 0.8 parameters are taken as approximately with average values of O.S8 and 0.63on equal to the total settlement. individual sites. The ratio of 0.64recorded The use ofthe initial load consolidation on test footing on the Tallaght site is in line curve grossly overestimates the with these results. This relationship, settlement. together with the E,determined from the in situ plate tests, offers a simple and practical method ofestimating the total Ecknowledgements The author would like to thank Monarch 32 settlement of a foundation on this materiaL It is considered that a ratio of~of0.5 Properties, the developer ofTallaght town GROUND ENGINEERING JULY 1989