Advantages and equations for pile design in Brazil via DPL tests Thomas Nilsson B.Sc. Civil Engineering, MSc., Thomas Nilsson Geoconsultores Ltda., Brazil
Renato Cunha B.Sc. Civil Engineering, MSc., Ph.D., Dept. Civil & Env. Engineering, University of Brasília, Brazil
Keywords: Modified DPL system, penetrometer, tropical soil, geotechnics, torque, pile design ABSTRACT: Difficult access to sites usually complicates and sometimes restrains a geotechnical field-testing campaign. Tight deadlines, stress, lack of planning and absence of good will from client and constructors are factors that also compromise the campaign. Hence, in order to overcome some of these problems, a modified version of the Light Dynamic Penetrometer Penetrometer DPL, designed, developed developed and patented by Thomas Nilsson under the name DPL NILSSON, herein denominated as modified DPL system, is presented and discussed. Compared with the conventional DPL, it can be easily disassembled in smaller parts, it is equipped with an alignment platform, and it contains rebound dampers plus a separate torque testing unit. The modified DPL system is appropriate for foundation design, giving in any soil conditions the cone´s tip resistance and torque measurements (hence producing an additional value for the cone´s lateral friction resistance, which can be used in classical pile design formulae once scaled and empirically corrected). Although the equipment is still under scrutiny by researchers of the University of Brasília, preliminary results and interpretative methodology for Brazilian soils are presented on a pioneer basis.
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GEOTECHNICAL CHALLENGES IN BRAZIL
Steep inclined slopes, remote investigation sites, primitive roads, r oads, uncultivated open fields, floods and swamps, jungles and many other kinds of difficult to access areas are encountered in Brazil. As in any development, they also have to be surveyed and geotechnically assessed, which poses a major problem to geotechnical consultants and companies in this country. Sometimes, given the level of difficulty, the t he geotechnical site campaign can be delayed or even cancelled (or changed to less intrusive types of tests, as photo interpretation or geophysics). Besides, tight deadlines, stress, lack of vision from both client client and constructors are present factors to be considered by any geotechnical company. Heavy and voluminous machines and equipments, imported high-tech units, delays to obtain spare-parts, the lack of specialized professionals in remote areas, and, of course, the eternal loan payment to international banks, are factors which increase the price of the geotechnical geotechnical information in Brazil. This is specially true for regions far from the large developed centers, where much of the aforementioned problems are somehow lessened. Most constructions in such remote areas don’t have any kind of geotechnical information. Others
have some information, but lack in technical quality or do not offer quantity enough to guarantee reliable statistics or quality design assurance for the client. Besides, the few projects designed with satisfactory information most frequently cost an above to average market price – given the high costs to obtain the required geotechnical data.
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MODIFIED DPL SYSTEM
The modified DPL system changes the bias on the geotechnical market, in social, logistical and technical terms. It is assembled in parts smaller than 1 meter, and packed in two volumes which occupies less than 0,17 m³, with the longest dimension of 1,06 m, i.e., easy stored in a common car. None of the com ponents weight more than than 10 kg. It can also be easily be operated by unqualified operators and technical assistants, if properly supervised by a trained engineer. It does not need energy or water on site s ite . This equipment was forecasted, designed and introduced in Brazil in 2001 by the first author (Nilsson 2003) and it was idealized with several modifications when compared to the conventional motorized DPL. The principal developments were the separation of the whole unit in smaller parts, the
design of a vertical alignment platform, the usage of stress dampers and ergonomic parts, and the development of a separate torque testing unit, as previously commented. Figure 1 presents a general view of the whole equipment and its several parts, while Figure 2 shows it assembled and ready for use in the field in a normal testing operation.
The major equipment parts are listed as follows: a) 90º cone of 100 mm height and diameter of 35,7 mm; b) 1000 mm rods of diameter 22 mm coupled with M16 bolts; c) 10 kg hammer and wood cushion; d) Centralizer rod and alignment platform; e) Torque measurement unit. To pull out the driven cone, an easy manageable composition is assembled by using a lever, metal connectors and a steel chain.
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Figure 1. Modified DPL system – DPL NILSSON
Figure 2. Ready for use DPL system
USAGE CONCEPT & FIELD OPERATION
The field usage of this DPL simply requires a vehicle with capacity to carry 3 operators and the 2 equipment boxes, respectively with volumes of 40 and 130 liters. The total weight of the equipment is 82 kg, being possible to be manually carried within the site in areas that are not accessed by car. The assemblage procedures take in overall 5 minutes, leaving an occupied ground area of less than 1 m² in the site and demanding a maximum of 2,0 m of height space. Under bad weather conditions it is suggested the employment of a canvas to cover the equipment, enabling the continuation of the insertion procedure in the usual manner – as it will be presented next. Thus, the major steps in the field are: a) Definition of the testing point, location and cleaning of the area; b) Assemblage and vertical field alignment; c) Elevation of the 10 kg hammer to a height of 50 cm, letting it fall free and drop onto the damper cushion (wood); d) Registration and counting of blows at every 10 cm of cone penetration. This record is defined as N10 by the international ISSMGE standard; e) Torque assessment, at each 1 m interval of rod change – using the appropriate torque measurement system, as presented in Figure 3 below.
Figure 3. Torque measurement with an special screw connector to the rod
In general the test continues up to its operational limit, which, as a practical guideline, is considered to occur whenever N 10 reaches the value 100, or whenever 3 consecutive N 10 respectively record 80 blows, or whenever 5 consecutive N 10 reach the value of 60. To get an idea of such procedure in relation to other tests, it can be said that these limits are comparable to qc´s (CPT) in the order of 8-10 MPa and NSPT´s between 15 to 30 blows. The system and site procedures of this DPL make it easily handled in the field, which contrasts with advanced high level technology apparatuses that sometimes are difficult to be used in remote areas. For instance, in countries with rich spectra of different soils, for example, in Scandinavia or Brazil, the sounding can be suddenly obstructed by boulders (in the first case caused by the glacial era, whereas in the second by soil laterization). With such equipment the reinstallation of a new hole is also easily handled around the original testing location. A lot of different types of edifications can be pro jected by modified DPL system data, as it provides design data for all kinds of residential units of one to two floors, industry plants, commercial centers and even tall buildings (as statistical complement to deep sounding/standard penetration tests). It can also be used for projects of new roads as it can be employed to detect soil stabilization locations. It is also used in engineering problems related to reinforcement of existing constructions with structural pathologies, since it fits small space areas and disturbs, to a minimum level, the foundation soil. This system is also bringing improvements to the geotechnical investigations in these remote zones. In the more than 200 services it has attended up to today (three years), this special DPL has provided a fast manner to obtain surveys in difficult regions, with soundings up to 12 meters in depth. In Brazil, in particular, it is under operation mainly in its Southern region. It is also under study on a research academic basis by the Geotechnical Group of the University of Brasília. This group is trying to understand its working principle under distinct soil conditions, and to verify its existing (practical) correlations in regard to laboratory reference values and other in situ tests from their recognized research site (with results of CPT, DMT, and SPT with torque measurements). This on going research, however, is still in its initial stage, yet lacking to provide definitive parameters for an overall engineering design in Brazil. Nevertheless, some guidelines will be drawn herein on a tentative basis. Details on the standard testing procedures for DPL systems are given by ISSMFE (1989) and were simply summarized herein.
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DESIGNING WITH MODIFIED DPL SYSTEM
This equipment supplies several important parameters. The stratigraphy is presented by graphs including small details and discontinuities and the natural water level is detected without any interference on the operation. When it is combined with the torque measurement, it is possible to provide the lateral friction in addition to the tip resistance. These values can be used in practice for pile design once they are cali brated by empirical coefficients that are acquired via large scale field loading tests. These loading tests are directly compared to the DPL reduced values to calibrate the design equations with the empirical coefficients. For instance, this can be exemplified by physically comparing the DPL penetration with the real pile penetration into the soil. By using an energy formula for pile driving, as the Hiley equation, and a simple moment equation for the measured torque, the pile´s (uncorrected) base and side resistances are obtained. As explained, such resistances can be further calibrated, via field loading tests, to consider pile scale differences and construction methodology into the equations. Therefore, using above concepts and taking on account the experience gathered so far, it is possible to propose on a preliminary basis a design DPL rule for usage in piles founded in Brazilian soils – em pirically corrected to account for field effects. The following formulae can then be adopted:
z ⎡ ⎤ P = R ⋅ ⎢q pf ⋅ A p + ∫ f ⋅ dAs ⎥ 0 ⎣ ⎦
R =
Pf ( s )
(1)
( 2)
Pu
q pf = α 1 ⋅ α 2 ⋅ qd
(3)
f = β 1 ⋅ β 2 ⋅ f s
(4)
or z ⎡ ⎤ ⎢q pf ⋅ A p + ∫ f ⋅ dAs ⎥ ⎣ 0 ⎦ P =⋅
SF
(5)
Where P = Pile load capacity – working load (kN); R = Settlement factor from field load test (whenever available);
Pf(s) = load for accepted settlement (load test) (kN); Pu = failure load (load test) (kN); SF = Safety factor ≥ 2 as standard in Brazil; q d = DPL tip resistance (kPa);
friction values. This particular preliminary set of results indicate that by keeping precision apart, it is possible to roughly infer CPT static penetration results with modified DPL system values, assuming close to one relationships between the variables of both testing equipments.
ƒs = DPL side resistance via torque (kPa);
qc (MPa)
q pf = Pile failure tip resistance (kPa);
0
ƒ = Pile failure side resistance (kPa); 2
A p = Pile tip cross sectional area (m ); dAs = Pile side area, per length dz (m 2); α´s and β´s = empirical factors as given by Ta bles 1 and 2. Table 1. Empirical factors for soil type Soil
α1
β1
Curitiba (General)
0.2
0.6
Curitiba (Guabirotuba formation)
1.8
1.3
Curitiba (Course alluvionar material)
1.5
1.8
Campinas (Porous collapsible clay)
0.5
1.0
Brasília (Porous collapsible clay)
0.5
1.1
Londrina (Porous collapsible clay)
0.2
0.3
1
2
3
4
5
0 1 2 3 4 5 6 ) 7 m 8 ( h t 9 p e10 D 11 12 13 14 15 16 17 18
Figure 4. Comparison between tip resistances of DPL system (dots) and CPT (Mota 2003)
modified
fs (MPa)
Table 2. Empirical factors for pile type Soil
α2
β2
Bored pile
0.2
1.2
Drilled caisson
0.4
0.1
Continuous flight auger
0.2
2.0
Omega pile
0.6
2.8
Precast driven pile in porous clay
0.5
0.7
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COMPARISON WITH CPT
Comparisons between the tip resistance and the calculated lateral side friction of this equipment and those from CPT tests have been carried out in the University of Brasília research site by Mota (2003). This particular site is a well known reference study site for the collapsible porous clay of the Brazilian Central Plateau, and has been depicted in papers elsewhere (Cunha et al. 2001). Figures 4 and 5 present one of such comparisons for close apart tests, in this same site. Although very few points of the site are available, the comparisons, as those of these figures, show promising results and a close match between tip resistances and lateral
0,00
0,02
0,04
0,06
0,08
0 1 2 3 4 5 6 ) 7 m 8 ( h t 9 p e10 D 11 12 13 14 15 16 17 18
DP3
CP12
Figure 5. Comparison between side resistances of modified DPL system (dots) and CPT (Mota 2003)
The practical applications for that are unquestionable, in particular in the Central Plateau of Brazil, where only one CPT rig system is available for an
area of the size of the whole Western Europe. By carring out simple and robust DPL tests in remote zones of this region, and by converting from one to another testing variable, it is possible to infer design parameters with the well known correlations for CPT. For instance, this approach has already been used with local modified DPL and CPT correlations via transformed variables for a large transmission tower project in the central area of Brazil (F. Villa 2002, personal communication). The potential for use and interpretation of this tool in this region is therefore high, as demonstrated in this particular paper. Although some guidelines have been presented and discussed, the authors also realize that much still have to be done to turn this equipment into a daily tool of the Brazilian practice. Better and improved correlations, and more dissemination of ideas, as carried out in this paper, are undoubtly necessary for achieving such goal.
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CONCLUSIONS
The modified DPL system is a must in a huge, diversified country as Brazil, composed by different environments, from slowly accessed mega-cities like São Paulo, or slope surrounded Rio de Janeiro, to untouched areas with extensive native forests, crossed by lot of rivers, and with the lack of roads and bridges, as the states of Mato Grosso and Amazonas. In several occasions the modified Brazilian version of the DPL is an essential geotechnical investigation equipment, able to enter remote areas in a fast and robust manner, and able to obtain site variables of competitive cost x benefit relationship for most civil engineering projects. This kind of DPL is under careful and methodic tests for some years now, being already used as ma jor site investigation tool in several projects which were mainly aimed to the design of piles, stratigra phy and water table assessment. It has also been used for land reclamation and incorporation projects, road designs, investigation of contaminated areas, slope stability analyses, building reinforcement and determination of geotechnical resistance parameters. This paper shows a glimpse of the possible uses and advantages of this tool, and presents, in a preliminary pioneer basis, a possible interpretation methodology for pile design in Brazilian soils. Although preliminary, this methodology has already been used successfully in many sites. Some preliminary comparison results between CPT and this modified DPL are also presented, showing a good match between the compared variables. Indeed, this indicates that the modified DPL can rather be compared with the CPT than to any other in situ tool, perhaps given the quasi-static penetration manner of this former equipment and the
geometric similarity of both apparatuses. Besides, by measuring the torque, the modified DPL is capable of acquiring one of the main advantages of the procedure which is currently employed in major cities with the most popular in situ testing in Brazil, the SPT. This procedure is exactly the adoption of a torque measurement adapted at each rod change interval, turning the SPT into a SPT-T. It shall be finally pointed out that this paper presents results from an on going research and collaborative work between the University of Brasília and Thomas Nilsson Geoconsultores Ltda. Much of the obtained data are still under scrutiny and, therefore, the results expressed herein can not be universally applicable. ACKNOWLEDGEMENTS Special thanks are given to FINEP via the CTINFRA funding provided to the University of Brasília, allowing this institution to acquire a modified DPL tool. REFERENCES Cunha, R.P., Pereira, J.H.F., Soares, J.M., Mota, N.M.B. and Poulos, H.G. 2001. Backanalyses of field loading tests on deep foundations in a tropical clay. XV International Con ference on Soil Mechanics and Geotechnical Eng ineering – XV ICSMGE, Istanbul, Vol. 2, pp.869-872 ISSMFE 1989. International reference test procedures for dynamic probing (DP). Report of the ISSMFE Technical Committee on Penetration Testing of Soils –TC 16 with Reference Test Procedures. Swedish Geotechnical Society, 49p. Mota, N.M.B. 2003. Advanced in situ tests in the unsaturated porous clay of Brasília: Interpretation and application to foundation design. D.Sc. Thesis, University of Brasília, 336 p. Nilsson. T. 2003. Initial Experiences of DPL NILSSON , I Central Brazilian Plateau Seminar . CD Rom
