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International Conference on Case Histories in (1988) - Second International Conference on Geotechnical Engineering Case Histories in Geotechnical Engineering

01 Jun 1988, 1:00 pm - 5:30 pm

Geotechnical Services for a Bridge in a Seismic Area

Filippo Ciuffi I.S.P.I.S. Geotechnical Division, ,

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Recommended Citation Ciuffi, Filippo, "Geotechnical Services for a Bridge in a Seismic Area" (1988). International Conference on Case Histories in Geotechnical Engineering. 7. https://scholarsmine.mst.edu/icchge/2icchge/2icchge-session4/7

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This Article - Conference proceedings is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in International Conference on Case Histories in Geotechnical Engineering by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact [email protected]. Proceedings: Second International Conference on Case Histories In Geotechnlcal Engineering, June 1-5, 1988, St. Louis, Mo., Paper No. 4.40 Geotechnical Services for a Bridge in a Seismic Area Fiiippo Cluffl Director of l.S.P.l.S. Geotechnlcal Division, Potenza, Italy

SYNOPSIS: The purpose of this paper is to describe the different stages and the final results of the geotechnical investigations and foundation design for a bridge in a seismic area, in the south of Italy. Particular attention is given to the procedure followed to study dynamic soil­ pile interaction. Also described in summary fashion are a number of "borehole shear tests" carried out and their results.

INTRODUCTION

The soil-pile interaction analysis for a vember 1980. bridge in a seismic area plays a very impor­ tant role . ·in the procedure- for the foundations The morfology is characterized by slope aver­ to withstand earthquake. In particular, the aging 33% (right slope: 41%; left slope: correct prediction of the bridge's piers 25%). displacements during earthquake ground motion has significant effects upon the stability of the whole bridge. The construction of a road between Forenza and , two small towns in southern Italy, has required the construction of a bridge on the Macchiaro­ tonda gorge (Fig. 1).

Decribed in this paper are the different stages and the final results of the geotech­ nical investigations and foundations design for the bridge above mentioned. Particular stress is given to the procedure followed to design piles against earthquake.

The geotechnical consultancy services and the foundation design have been ordered by National Office for Extraordinary Works in Southern Italy.

LOCATION OF BRIDGE AND GROUND CONDITIONS

The area under study is located approximately 2. 5 km north of Forenza and 50 km north of Potenza, the regional chief city. The area, at the present time, is classified by italian law as a "seismic area". It is opportune to recall that Region has been greatly damaged by the earthquake of 23rd No-

Fig.1. Location of the Site

877 As regards the geology, the soils are mostly Borehole Shear Tests. In fact, for belonging to the following formations: the the first time, Borehole Shear Tests so-called "Daunia Formation" (Miocene) and have be.en pe rfo rme d in Italy. "Varicoloured Clays" ( Oligo-Miocene). The first one is characterized by marly lime­ (iv) To summarize the results of the above stones. The second one is characterized mentioned analysis, a tridimensional by alternations of varicoloured clays (preva­ soil profile map has been drawn to lent clolours: red and green) and marly a scale of 1:200. The site conditions clays. At the bottom of the Macchiarotonda have suggested that bored cast in gorge, there are recent alluvional deposits. situ piles ( i = 100 cm) would be the appropriate choice.

( v) The foundations design has been car­ PROCEDURE ried out by calculating both the ultimate capacity of the pile group The following procedure has been used. and the soil-pile interaction under dynamic loads. (i) First of all, the site has been studied on the basis of the analysis of aerial photographs. BOREHOLE SHEAR TESTS (ii) A detailed investigation has been carried out, in particular on the Inasmuch as space is limited and we are unable sites on which the bridge's piers to give due attention to all investigations will be located: on the right slope made, we deem it opportune to limit ourselves and on the left slope of Macchia- to illustrating, at least with regard to rotonda gorge: basic essentials, a number of "borehole shear tests" carried out on representative points - Cone Penetration Tests of the area under study. The "borehole shear - Boring Tests test" measures shearing strength quickly Borehole Shear Tests and directly in situ in the sides of a 75 - Pore Pressure Measures mm diameter borehole. - Laboratory Tests The tests described have all been carried (iii) All data collected have been studied. out by the I.S.P.I.S. Geotechnical Division Particular emphasis has been given of Potenza (Italy). to the interesting results of Five ''boref.lole shear tests" have been conducted on the sides on which the bridge's piers will be located (Fig. 2).

. ,., . .-. :· ·

Fig. 2. Schematic Bridge'• Section

878 For each point a series of tests with increas­ It can be seen from this table that the values ing applied normal stress ( er) was performed, of friction angles of the left slope are while the corresponding shear stress ( T ) higher than that of the right slope. It may was measured. The consolidation time, before be of interest to note that, to evaluate increasing normal stress, was evaluated by the anisotropy, the B.S.T. 1' was performed, monitoring the dissipation of excess pore by rotating the apparatus 90°, in the same pressures generated after application of hole, at the same depth of B.S.T. 1. (Fig. the normal stress. Shearing strength was 3). then plotted versus normal stress to give a Mohr-Couloumb type failure envelope.

Test results are summarized in Table I. SOIL-PILE INTERACTION ANALYSIS

The structural consultants indicated that l.S.T. c the maximum load at the base of bridge's (•.) (P.s.i.) ~ piers is expected to be about 1600 ton. The 1 4,2 171,7' left •. safe capacity of the pile group, calculated on the basis of "cone penetration resistance" 1' 2,2 18',2' " (Meyerhof, 1976,-1983), was estimated as 1756 ton. 10 m long pile on left slope and 14 2 3,9 32',2' " m long pile on right slope were required.

3 4,3 31',1' It is important to emphasize that the geo­ " technical conditions of the left slope are 4 4,4 13',8' right a. decidedly better than those of the right slope. Table I. B.S.T. Results

BOUBOLI SllEil TEST a• I X BOREllOLE SllEil TEST a• I• •

ltnss 11,0 19,6 22.2 24,7 ..... 24.7 27.3 32 ..f (J•Q 14.5 (J s ii C.S.Tiat . Tia• 15 12 10 8 8 15 11 10 - ,. (Ml --~(J. i) 8.5 9,7 10.5 11.3 12.4 ---"'' 10.3 11.0 12.9 C=4.25 p 1 i fl= 17• 7· C= 2.2 • •i fl= 11·.2·

.,

'

• •• ., .. ' llanDll a..-(pa I) "

Flg.3. B.S.T. 'a 1 and 1'

8'79 Mostly to estimate the maximum displacement The soil profile has been separated for both of the pile heads, the soil-pile interaction the slopes in two discrete parts A and B under dynamic loads has been studied, follow­ characterized by the following parameters ing the procedure proposed by Prakash and (Table II). Chandrasekaran (Prakash and Chandrasekaran, 1980). This analysis is based on the following assumptions (Prakash, 1981): A B

(i) The pile is divided into a convenient 2 2 number of segments and mass of each Young's modulus : E 150 Kgcm 475 K1cm segment is concentrated at its center Constant of hor.izontal 3 point. subgrade reaction "h 1~03 Kgcm 3.61 Kgcrii3

(ii) The soil is assumed to act as a linear Winkler's spring. The soil reaction is separated into discrete parts at the center points of the masses. The soil modulus variation is consid­ Table II Soi I Parameters for Dynamic Analysis ered both constant with depth and linearly varying with depth. Note that the parts A and B are the same for both slopes, but while on left side the (iii) The mass of the superstructure is part A is 4 .10 m long, on the right side concentrated at the pile top. its lenght is 9.10 m (Fig. 4). The dynamic analysis has beerl performed in the following (iv) The system is one-dimensional in steps: its behavior. ( i) The piles on the left slope have ( v) The pile end conditions are either been studied. The maximum displacement completely free to undergo translation of the pile head ( sd left ) ' the maximum and rotation or completely restrained bending moment and the soil reaction against rotation but free to undergo have been computed. translation. (ii) The lengths of the corresponding piles on right slope have been deter­ mined by imposing a value of maximum displacement of the pile head sd right ) as follows:

5 d right E 1 ~ 15 5d left ' 1 >

0~~~10i:iiii-~20m.

right slo,e left slope

B

Fig.4. Soil Discrete Representation

880 The above described steps are illustrated in the Table III.

KNOWN DATA UNKNOWN DATA KNOWN DATA UNKNOWN DATA

Young's Stiffness Young· s Stiffness Factor Cll> Factor ""u Modulus !:! Modulus ;:: ~ en E en T en E en T ..... ii u: ..... - - &M 0 &M -u &M- o~ Constant -~ - Consfan t s~ en u 2 &M Mu. Depth en -~ &M Max. Depth u c c of Horizontal cs c of Horizontal .2 •c Factor >- c Factor c c 0- :z: Subgrade Cl c :z: Subgrade c u >- - -u ~ IL - IL l11ctio11 Zm11 Reaction Zmu .h Time lh Time Young · s Period en Young's Period !:! Tn Modulus Tn ~ Modulus en Cll> ~ Mu. &M - &M a.: Length !:! ....IC ~ E .....- &M E en Displacement ~ - :z: .. -u A. u L &Mai: c ..... &M Diameter of the Pile Head Diameter ..- ~u -=c c s z: I CD en u D c Max Bending :z:- - &M u Muimum Length --CD-~ &M Moment en -en,.2 en Vertical L &M -= M c Cl c 2 IL Load Mui mum Soi I Q.... , Vertica I Reaction Max ""c 2 Load Displacement pl o..... of the Pi le Head en s CD CK Max Bending - &M~ Moment PILES ON THE LEFT SLOPE ~2 en- &Mc 0 c M &M IL- Soil Reaction P,

PILES ON THE RIGHT SLOPE

Table Ill Piles Designing Procedure Representation

CONCLUSIONS

The geotechnical consultancy services just to obtain, in different soils, the same maxi­ illustrated have furnished,as was our inten­ mum displacements (tollerance 15%) during tion, a number or indications extremely useful earthquRke ground motion. to structural consultants, in that they served as exact documentation for correctly designing the birdge's structures against earthquake.

In particular, the soil-pile interaction under dynamic loads has been studied, mostly to predict the pile seismic performance. The .lengths of piles have been calculated

881 REFERENCES

Meyerhof., G.G., (1976), "Bearing capacity and settlement of pile foundations", .J .G.E. D., ASCE, GT3, 197-228.

Meyerhof, G.G., ( 1983), "Scale Effects of Ultimate Pile Capacity"., J .G .E., ASCE, n° 6.

Prakash, S., V. Chandrasekaran, (1980), "Anal­ ysis of Piles in Clay Against Earthquakes", Preprint n~ 80-109, ASCE, Convention and Exposition, Portla.nd, Ore.

Prakash, S.. ( 1981), "Soil Dynamics", McGraw Hill, New York, N.Y •.

882