INTRODUCTION OF EUROCODE 7 A.F. VAN TOL

Delft University of Technology, Delft, The Netherlands Deltares (former GeoDelft), Delft, The Netherlands

W. Bilfinger

Vecttor Projetos Ltda. Sao Paolo, Brazil

Victor Li

Victor Li & Associates Ltd., Hong Kong

Y. El-Mossallamy

Ain Shams University, Cairo, Egypt ARCADIS Consultants, Darmstadt, Germany

A. Mandolini

Department of Civil Engineering, Second University of Napoli, Italy

ABSTRACT: This Report of Discussion Session 1 on pile design and development and codes presents the summary of the presentations by the discussion leader and the panelists. These presentations deal respectively with Eurocode 7, pile developments, effects of construction procedure of bored piles and bored piles in difficult soils and piled rafts.

1 INTRODUCTION

For the conference topic 1, Pile Design Development and Codes 20 abstracts were submitted. The themes of these abstracts were analyzed and although it was not always clear what the exact content of the related paper would be, it shows the important subjects in the field of this conference topic. The most important developments from the abstracts are shown in table 1 (some abstracts deal with more then one item).

It appears that the abstracts cover the most impor-tant themes of this discussion session. Perhaps it was

expected that the imminent introduction of Eurocode 7 would lead to more abstracts on this item.

The content of the presentations of the discussion leader and the panelists was carefully selected in view of important themes in the topic 1 of this conference. The following themes are addressed in these lectures: the introduction of Eurocode 7, pile developments, bored piles in weathered rock, the effect of construction proce-dure of bored piles and piled rafts. These presentations cover the most relevant aspects of this conference topic.

The summaries of these lectures are presented below in this session report.

2 INTRODUCTION OF EUROCODE 7 A.F. VAN TOL

2.1 General

Eurocode 7, Geotechnical Design consists of two parts, namely:

− part 1 (EC 7-1), General rules, that was ratified in 2004

− part 2, (EC 7-2), Ground investigation and testing, ratified in 2005.

Table 1. Content of submitted abstracts.

Items Number Remarks

Pile capacity 5 Axial and lateral Installation effects 2

Piles in difficult soil 5 Bored piles and others Load displacement 6 Mainly numerical analyses Piled rafts 2

Eurocode 7 2

Others 1 Ground improvement

In addition to these codes, each Member State will issue a National Annex with the selected Design Approach and the magnitude of the partial factors to be applied. Some Member States will also publish a national code with additional requirements. Most Member States finalized these documents in 2007.

The implementation of EC 7-1 is planned step-wise. From 2007 on a period of Coexistence starts, in which Eurocode 7-1 and the existing national codes may be used. From 2010 on these national codes will be withdrawn.

2.2 Pile design according to EC 7-1

The important requirements for the design of pile foundations according to the 3 Design Approaches (DA) are given in chapter 2. The corresponding par-tial factors are presented in Table A.3 for action or the effect of actions (γF or γE), Table A.4 for ground parameters (γΜ), Tables A.6, A.7 and A.8 for resist-ances for piles (γR) and Table A.9, A.10 and A.11 for the correlation factors ξ.

Section 7 of EC 7-1 is devoted to the design of pile foundations under axial loads. In this section, clauses 7.6.2.2 and 7.6.2.3 deal with the assessment of the Ultimate compressive resistance from respectively static load tests and from ground test results. In this section the correlation factors, to derive a character-istic resistance from a number of static load tests or ground tests, are introduced.

The subsequent requirements for ULS for piles in compression or tension lead to the basic condition that the design value of the Actions F_d does not exceed the design value of the Resistance R_d:

F_d < Rd (1)

F_d= γF . F_k (2)

R_d= Rk/γt or R_d= Rbk/γb+ Rsk/γs (3) where F_k is the characteristic value of the actions, γF the partial factor on actions, R_k, R_bk and R_sk are the characteristic values of respectively the total, the base and the shaft resistance and γt, γb and γs the partial factors on respectively the total, the base and the shaft resistance.

The characteristic values of the resistance R are obtained from shall be based on:

− results of static load test (SLT)

− empirical or analytical calculation methods vali-dated by static load test

− results of dynamic load tests, validated by (SLT)

− observed performance, supported by ground inves-tigation and ground testing.

The characteristic values of the resistance are obtained with:

Rk = R/ξ (4)

where the correlation factor ξ is a statistical factor depending on the number and type of tests.

Table 2 presents the partial factors, from A.6, A.7 and A.8 of Annex A of EC 7-1 for Ultimate Limit State for driven, bored and CFA piles in compres-sion. It is striking that EC 7-1 recommends different partial factors for the different pile types in Design Approach 1 and not in DA 2 and 3. The explanation for the higher partial factors for bored and CFA pile will be the higher uncertainty (installation procedure dependent) regarding the base capacity of these piles compared to driven piles. Although this is an under-standable ground, the reason for the absence of this diversion in DA 2 and 3 is not consistent but will probably be explained by the lack of space for diver-sion, as the safety in these approaches is (nearly) completely located at the action side.

It is interesting to see how the Member States have dealt with this in their National Annex. In some Mem-ber States the mentioned uncertainties are covered by installation factors, but the calculation methods, wherein installation factors might appear are not uni-fied in this code. Schuppener (2007) presented an overview of the DA’s and partial factors that were cho-sen by the Member States in their National Annex. The overview is based on a questionnaire and presents the status at January 2007. Table 3 in Schuppener (2007) presents the selected partial factors for bored and driven piles. From this table it appears that 3 Member States, that chose for DA1 follow the recommendation and take higher partial factors for bored then for driven piles (P, IRL, LT) and one vice versa: higher factor for driven then for bored piles (R). It also can be seen that in DA2 two Member States chose higher partial factor for bored then for driven piles (CH, D).

2.3 Harmonization of Direct CPT-method

The final goal of the European Standardization project is to harmonize the design of constructions. The ques-tions is of the development of EC 7-1 succeeded to (partly) reach this goal for geotechnical design. After

Table 2. ULS, recommended values in EC 7-1 for γb, γs, γt. Design approach 1

Type

of pile Combination 1 Combination 2 DA2 DA3 Driven 1.0 1.0 1.0 1.3 1.3 1.3 1.1 1.0 Bored 1.25 1.0 1.15 1.6 1.3 1.5 1.1 1.0 CFA 1.1 1.0 1.1 1.45 1.3 1.4 1.1 1.0

the publication of EC 7-1 different committees ini-tiated exercises to design trial examples of standard design situations by representatives of the Member States. Some of these exercises showed a very large scatter in the design results and EC 7 was blamed.

The effect of differences in Design Approaches, par-tial and correlation factors however cannot be respon-sible for very large deviations. The real reason is the fact that the calculation methods that are not unified in EC 7-1. Therefore, more results can be achieved by trying to harmonize the calculation methods.

As stated above clause 7.6.2.3 of EC 7-1 deals with the assessment of the ultimate compressive resistance from ground test results. In Member States were Cone Penetration Testing is a standard soil investigation technique the method to derive the ultimate pile resistance directly from the cone resist-ance is generally accepted. Several National codes in Europe apply this direct CPT method to determine the pile capacity (Cock & Legrand, 1997). Eurocode 7, part 2, Ground Investigation and Testing address this method in paragraph 4.3.4.2 and in Annex D.7 an example of the method illustrated. The calculation procedures in different countries, like in the Nether-lands, France and Belgium are similar but with some important differences, like e.g. the factors account-ing for the installation effects, the pile geometry and the averaging procedures to assess the representa-tive cone resistance for the pile base. The method described in Annex D.7 in EC 7-2 at present is the Dutch method.

The Netherlands, France and Belgium started a working group to harmonize the method. The first action of this group was to gather a reliable data-base with static pile load tests. At present, the group gathered about 25 well-documented pile load tests on driven prefabricated piles (mostly concrete and some closed ended steel piles) and compared the results of these load tests with the predicted ultimate base and shaft capacity according to the Dutch, French and Belgium method. The next step will be to extend the database with driven, cast in place piles, bored and screwed piles.

The first results show that the Dutch and Belgium methods overestimate the base capacity consider-ably. The French method is more conservative and fits therefore better. This confirms the findings pub-lished by Puppala et al. (2002) based on the database of the Federal Highway Authorities and Xu & Lehane (2005) based on the UWA database.

3 PILE DEVELOPMENTS W. BILFINGER

The increasingly competitive global market is driving industry towards two critical and opposite,

thresh-olds: to build faster and cheaper, on one side, and, on the other side, to avoid underperformance, failures and mistakes.

For this reason, a segment of this industry is con-stantly searching for ways to improve the foundations of structures, which, in essence, mean to increase the allowable loads and/or to reduce settlements of piles, leading to proportionally cheaper foundations, with-out affecting the necessary safety margin.

The development of piles is currently focused on a better understanding of bearing capacity and load-settlement prediction and, to some extend, to improve construction reliability.

To evaluate the real possibilities of improving reli-ability and the capacity to predict bearing capacity and load-settlement behavior accurately, one has to consider the different factors that affect these pile characteristics.

Normally safety and reliability are affected by:

• Load uncertainties—in this paper, this issue will not be discussed;

• Material uncertainties, including strength and deformability of the pile and the surrounding material. The material uncertainties are normally divided into their intrinsic variability and those associated to representativeness, testing reproduc-ibility, including accuracy and precision;

• Method uncertainties, associated to the design cal-culations and assumptions.

For practical purpose, pile behavior predictions that include all factors above, with exception of load varia-bility, are based normally on field and laboratory test-ing of the foundation soil, and a model that uses these inputs. The most common soil testing performed in North and South America for pile design, i.e., bearing capacity and load-settlement prediction, are SPT and CPT tests (Paikowsky, 2002). Less frequently, other types of tests are performed. Comparison between observed and calculated bearing capacities by several methods shows that in roughly 50 years generalized formulae to predict ultimate bearing capacity have not gained significant reliability. Figure 1 presents

Figure 1. COV associated to different databases.

0

1960 1965 1970 1975 1980 1985 1990 1995 2000

COV associated to the databases presented by the different authors in their original papers. The data base used by Norlund in 1963 to develop the corre-sponding bearing capacity calculation method show a proportionally low COV, when compared to other methods. Later tests presented by Paikowsky (2002) lead to COV’s between 0,22 and 0,58.

This conclusion is backed by a study by O’Neill’s (2001) about side resistance of piles, that concludes that “… much still remains to be learned ... and that site specific load testing should remain an integral part of the design process for driven and drilled shafts”.

A way to reduce this variability is to perform site specific load test and calibrate generalized pile behav-ior prediction methods. A theoretical sound approach is the use of Bayesian inference to combine load tests with predicted bearing capacities (Bacher and Rackwitz, 1982), (Hachich et al. 2008). But often the problem of these approaches is the necessity of per-forming previous load tests, preferable more than one and, in some cases, high variability of results, even on the safe side (showing, for example, significantly higher loads than predicted), can lead to a theoretical need for higher safety factors.

Another approach is to use information from the site: in the case of bored and auger piles, a direct measurement of installation parameters is compli-cated, but possible. Interesting and promising results were presented by NeSmith (2003) and Saeki and Ohki (2003), were the installation effort is compared to pile bearing capacity. A statistic analysis com-paring probability of failure using not only bearing capacity predictions, but adding the information of the installation effort certainly will show significantly lower probabilities of failure or, in other word, more reliable foundations.

All the efforts of reducing variability and, therefore, reliability, can be seen in a broader view, a part of the initial phases of geotechnical risk management.

Risk management is an effective management and decision tool and is widely used in several engineer-ing activities. The introduction of risk management in geotechnical engineering is recent, but becoming increasingly important. The ITIG (International Tun-neling Insurer Group) published recently, together with the ITA (International Tunneling Association), a “Code of Practice for Risk Management of Tunnel Works” (ITIG, 2006), which consists in a rational way to manage risk in tunneling works. Some re-insurers are including the necessity of use of this Code in their contracts, showing that, probably in the near future, risk management will be almost obligatory in tun-neling works. This trend, probably, will be expanded to other geotechnical works, including deep founda-tion engineering.

Considering that risk management will be prob-ably “unavoidable”, it is important to realize that risk

management does not involve complicated theories or formulation. The key tools for effective risk manage-ment are available and the only necessity is system-atic use of available information, using a relatively simple methodology.

4 INFLUENCE OF CONSTRUCTION PROCEDURES ON PILE CAPACITY—SOME HONG KONG EXPERIENCE

VICTOR LI 4.1 Introduction

Most textbooks on foundation engineering present a picture that the estimation of pile capacity of bored piles is a relatively straight forward exercise. In real-ity, it is far from being the true story. Figure 2 shows a summary of average shaft friction of bored piles measured in Hong Kong (GEO, 2006). The wide-spread of data gives a clear message that prediction of pile capacity, or at least the shaft friction, of bored piles is difficult even when the bored pile data in Figure 2 represent bored piles constructed in fairly similar soil conditions in Hong Kong.

The shaft friction τ of a bored pile can be described by a simple relationship.

τ = σh tan δ (1)

where σh is the horizontal stress acting on the pile shaft and δ is the friction angle between the soil and pile shaft. Both parameters are highly affected by the method of construction, making the prediction of pile capacity a difficult task. In this note, the author likes to make a point by discussing some case studies in Hong Kong.

4.2 Use of permanent liner

In Hong Kong, permanent tubular liners made from a corrugated steel sheet are sometimes used for con-struction of bored pile to prevent necking of pile shaft during extraction of temporary casing. They are also used routinely for bored piles constructed

Figure 2. Measured shaft friction of bored piles in Hong Kong (GEO, 2006).

in cavernous marble formations to prevent loss of concrete into marble cavities. The permanent liner is placed inside the steel casing before concreting inside the permanent liner. As the diameter of the permanent liner is smaller than the temporary steel casing, a gap will be formed between the soils and the permanent liner when the temporary steel casing is extracted dur-ing concretdur-ing (Fig. 3). Experiences indicate that the gap can remain stable after extraction of temporary casing due to arching effect (Lam & Li, 2003). In this case, shaft friction will be low because σh is zero. If collapse of pile bore occurs to fill up the gap, σh will drop due to release of stress caused by soil movement towards the permanent liner. Also, δ will be low as the soils filling up the gap are likely to be in a much looser state then their initial states. Lam & Li (2003) reported a case study for a 60 m long, 1.5 m diameter bored pile constructed using a permanent liner. The measured average shaft friction is 17 kPa, which is practically the lowest limit of shaft friction one can obtain according to the data in Figure 2.

4.3 Concreting rate and setting time of concrete As discussed convincingly by Kay & Kalinowksi (1997), the confining stress σh in Eq.1 tends to be con-trolled by contact pressure derived from placement of concrete rather than in-situ earth pressure of soils. The contact pressure of concrete is governed by the rate of pour, setting time and delay of concrete pour, so will be the shaft friction of bored pile. Kay & Kalinowksi (1997) reported a case study of two 1.5 m diameter bored piles constructed at 6 m apart on a Hong Kong site, and apparently using similar construction method.

Loading test results indicated that one bored pile gave significantly lower shaft friction than the other. A review of construction records revealed that the bored pile with unexpectedly lower shaft friction had a con-siderable number of delays in concreting, amounting to 155 minutes in the total 342 minutes placement time, while concreting for the other bored pile with

higher shaft friction was completed in 267 minutes with much reduced loss in delay time.

4.4 Soft interface

Bored piles and barrettes are often constructed using bentonite slurry for supporting the pile bore. It is commonly recognized that slow rate of excavation and delay in concreting can lead to formation of thick filter cake along the pile bore. Ng et al. (2000) reported a case study of a 39.7 m long test barrette constructed on a Hong Kong site. The soil profile comprised fill, marine deposit, alluvium and com-pletely decomposed granite (CDG). The excavation of the barrette took 62 hours to complete and con-creting was commenced 43 hours after completion of excavation. Shaft friction measured along the bar-rette was low, and particularly so for the CDG layer which gave shaft friction comparably to that of the marine clay layer. Li & Lam (2001) argued that the low shaft friction was likely to be due to the unu-sually long duration of excavation and long delay in concreting, causing significant relaxation of confin-ing stress (i.e. σh), and thick filter cake to be formed along the bored pile (hence low δ). Ng et al. (2000) has therefore made an unintended contribution of demonstrating how poor construction method can lead to low shaft friction.

In summary, use of appropriate method and good control of construction for ensuring high σh and δ are important in achieving higher shaft friction of bored piles. Textbook analyses are unfortunately of little help in this respect.

5 PERFORMANCE OF LARGE DIAMETER