An overview on Existing Dynamic Cone Penetration Test Research Related to the Central Area of Brazil L.C. Figueiredo B.Sc. Civil Engineering, Me., Dept. Civil Construction, Federal Institute of Education, Science, and Technology of Mato Grosso, Brazil
R.P. Cunha B.Sc. Civil Engineering, M.Sc., Ph.D., Ph.D., Dept. Civil & Env. Engineering, Engineering, University of Brasília, Brazil
W. Conciani B.Sc. Civil Engineering, Me., Dr., Dept. Civil Construction, Federal Institute of Education, Science, and Technology of Mato Grosso, Brazil
ABSTRACT: This paper presents and discusses several experiments and research carried out in the Central Area of Brazil, in which dynamic light cone penetrometers (DPL´s) have started to be tested into the existing tropical soil conditions of this region. Although the Standard Penetration Test (SPT) prevails among the dynamic penetration testing devices, it cannot always be adopted in situ, given ground conditions and the magnitude of the expected stresses. On the other hand, several initiatives to use the light cone penetrometer have been done for the prediction prediction of the resilience modulus modulus of subgrade pavement pavement layers, as well well as bearing capacicapacity values from foundations of light structures. Without detailing two much into the comparisons of the distinct penetration probes, this paper summarizes the t he advances attained at tained so far from existing publications, which results have already provided a reliable reference procedure, or background, for practical usage in the Central Brazilian area by the local geotechnical engineering community. It is another effort from the GPFees Group of the University of Brasília (www.geotecnia.unb.br/gpfees) (www.geotecnia.unb.br/gpfees) to to popularize in Brazil an economical and costeffective in situ device oriented towards the daily practice in tropical soils. 1 INTRODUCTION 1.1 General characteristics of penetrometers The estimated load capacity of the soil is a commonplace of the actions of civil engineering. It's almost instinctive to imagine the resistance to penetration of a rod into the soil. It is therefore natural that penetrometers have been preferred for obtaining subsoil information, when it is not always possible to collect samples or when the financial cost of the campaign neglects the geotechnical investigation and allows a low budget for that. In essence, a penetrometer consists of a thin metal rod, which is stuck or pushed through the blows of a hammer or a hydraulic jack system. The penetration resistance of the rod is measured at any depth and the data are plotted on a diagram representing the resistance to penetration in the abscissa and the depth on the ordinate. 1.2 Standard Penetration Test In Brazil, the preferred test is the Standard Penetration Test, or SPT, although not always appropriate when examining soil characteristics. In fact, the SPT is a good device for granular soils, however this is not the case for predominantly clayey soil materials. In addition to the problems encountered for
the characteristics of soils there are other factors that interfere with the SPT results, some of them detailed by Belincanta & Cintra Cintr a (1998), such as the free-fall characteristics of the hammer (in Brazil), the type of hammer, the anvil mass, and conservation status of the composition. Actually the indiscriminate use of the SPT test has brought geotechnical problems and mismanagement of resources in the field of geotechnical geotechnical testing. When faced with the scenario of central Brazil where it is largely noticed the presence of lowdensity soils with high void ratios, and deeply leached yellow and red latossols, the use of the SPT is a real challenge. Besides, there are the normal difficulties of access, especially for civil constructions in rural settlements, or in small constructions in ur ban areas. It is evident that t hat other penetrometers can be more useful and practical than t han the SPT, and, perhaps, more cost-effective for the region. 1.3 Cone and Piezocone Penetration Tests According to Schnaid (2000) the first references on the cone penetration test are from 1930 relating to test experiments that took place in Holland. The test consists of the continuous push of a cone shaped probe at a constant speed into the soil of 2 cm/s. The cone section can vary from 10 to 15 cm². A variation of the test is the piezocone penetration test, which
consists of equipment with a porous element, load cells and transducers that provide the record of the dynamic pore pressures. Hence, the pore-water pressure in the soil adjacent to the cone is continuously provided during the penetration of the cone. Note that the position of the porous filter on the base of the piezocone is not yet standardized. However, the International Society for Soil Mechanics and Geotechnical Engineering - ISSMGE- suggests that the porous filter must be placed just behind the cone, or at U2 position (Hasmat, 2000). The piezocone test is attractive, and in addition to eliminating the influence of the test operator it provides detailed description of the soil stratigraphy with continuous record of the penetration resistance. On the other hand, the financial costs of the equipment use and burden of the constant maintenance – besides of tests that do require rigs mounted on small trucks - prevents it wide use to areas of trails and unpaved paths, or densely forested regions, and are surely the inhibitors for a widespread use of this test in the central portion of Brazil. 1.4 The context of the Brazilian Central Area & States
Figure 1 shows the incidence of Oxisols or Latossols in Brazil especially in the central region of the country, where it has been established its capital, the city of Brasilia, around half a century ago. Plateaus with soils of low densities, deeply leached, and low resistance to penetration, about two to five blows of SPT up to 15 m deep, predominates in this region. It is undoubtedly the new agricultural frontier of the country, with many small towns and new pioneering settlements under establishement. Access to geotechnical campaigns is difficult due to the state of the paving quality of the roads, besides of the fact that this region is remote from ma jor urban centers (or the most developed corner) from Brazil. Nilsson & Cunha (2004) report that these problems often come together with the lack of the awareness of the problem by geotechnical customers and building contractors. In addition to that, it is usual that small constructions neglect any geotechnical test during site reconnaissance stage.
Figure 1 - Map of Brazil- Central area of Brazil, with large incidence of “l atossols” (modified after Furtado, 2010)
In this regard, this paper explains the advantage of the use of lightweight penetrometers, especially the Dynamic Penetrometer Light or DPL in such small scale works, especially at early design stages. 2 THE LIGHTWEIGHT PENETROMETERS In this section we argue succinctly about the light dynamic penetrometers. Generally they are made up of three parts: tip, cone-shaped, threaded rods, and a hammer that moves in free fall up to striken on top of an anvil. In the geotechnical literature the light penetrometers can be divided into three kind: The French Panda; Dynamic Cone Penetration or simply DCP; and Dynamic Probe Light or just DPL. 2.1 Panda The Panda is a equipment made by a company called Sol Solution, and is a lightweight dynamic cone penetrometer designed for soil investigation and compaction control. The equipment has an anvil with strain gages bridges that receives blows from a hammer with a variable energy level. A device of reception of signals generates files that can be analyzed later on with a geotechnical insight. In Brazil the Panda has been used in academic terms by the University of Brasilia among other research institutions, besides of some few industry companies. It is not yet used on a widespread design basis in the country. 2.2 Dynamic Cone Penetration The Dynamic Cone Penetration or just DCP was developed in the 60s in South Africa to evaluate the strength or stiffness of the pavement. According to Wu (2010), Vuuren (1969) designed the original DCP with a 30° cone, and later on Kleyn (1975) described another DCP design, which used a 60° cone tip, hammer with 78.48 N mass, and 575 mm free fall. This design was then gradually adopted by countries around the globe. In 2003, this lightweight penetrometer was standardized by the ASTM D6951-03 (2005), for shallow pavement applications (Wu, 2010). The DCP can be made with accelerometers, however the cost of the equipment is high to be employed in small buildings. Figure 2 shows a sketch of a DCP equipment. In Brazil the research about DCP has been carried out by the Federal University of Santa Catarina. Several articles on the use of the DCP can be accessed at internet about the methodology for characterization of unpaved roads (Triches & Dal Pai, 2006). According to Delgado et al. (2007) the DCP is used to characterize the bearing capacity of the soil in its natural or compressed state, and it also presents a good correlation with the CBR test. The
test is a "little destructive" of the original soil´s structure, and somehow this feature makes it possi ble a detailed study of the pavement layers and the sub-grade. In addition, it can be employed on a very wide working range, i.e., from soft soils or inconsistent ones, even on gravel layers, from bases or sub bases.
Figure 2 – Schematic assembly of DCP made by Kessler Soils Engineering Products, Inc. User’s Manual.
2.3 Dynamic Probing Light The main focus of this paper is the current literature discussion about the possible uses of the Dynamic Probing Light or DPL, especially in determining the load capacity of small scale (or load magnitude) foundations, in a way to complement the SPT test, among others, during the geotechnical investigation. Hasmat (2000) explains that the test with the Dynamic Probing became known in Europe between the two world wars. After 1945 this test was widespread around the world. Efforts to standardize the DPL test were performed by the past International Society for Soil Mechanics and Foundation Engineering - ISSMFE - with the publication of the re port of the Technical Committee on Penetration Testing - TC16 (1989), and the European Standard EN ISO 22476-2:2005 (2005). Nevertheless, in Brazil, it continues to be a quite “unknown” testing d evice in most of the regions, particularly in the inner parts of the country.
2.4 The research of the DPL in Brazil The test with the DPL is summarized by the penetration of a cone with 10 cm² of area coupled to a set of threaded rods 1 m in length. In the top there is an anvil that receives the impact of a hammer in a free fall manner, which mass is corresponding to 98.1 N. Each 10 cm of penetration is marked by the number of blows known as N 10. Most of the background research on the DPL has been basically targeted on comparisons between the SPT and the CPT test. Nilsson & Cunha (2004) conducted studies with the DPL for estimating load capacity of piles for a tropical soil with the presence of a particular “ porous clay”, as it is colloquially known in Brasília city . The authors assumed that the possible measurement of torque (a new feature) obtained in DPL tests could provide lateral friction resistances by the advancing tip and body of the DPL, which obviously can be associated with the lateral friction of a driven pile. Accordingly, these authors adopted an energy derived formula for pile driving, as the Hiley equation, and a simple moment equation for the measured torque, in order to derive the lateral DPL lateral resistance. As explained by these authors, such resistance could be further calibrated, via field pile loading tests, to consider into the equations a possible pile scale effect and the construction methodology – hence to be further employed into real pile analyses. Although it is particularly not a Brazilian research, Martins & Miranda (2005) conducted several tests with the aim to obtain correlations between various penetrometers focusing their comparisons between DPL and CPT tests in granitic soils from several urban locations in the North of Portugal, i.e. essentially Porto, Braga and Guimarães. The authors found good correlations between the CPT resistance and the DPL. Silva et al. (2006) proposed a semi-empirical method for estimating the shearing resistance in excavations of small diameter in residual and laterized soils from basalt in the region of Londrina / PR. The method was correlated with loads tests, and in an attempt to predict the shearing load the authors used the Super Heavy Dynamic Probing – DPSH, that has an energy level higher than those from the DPL. The authors concluded that further studies are needed to establish parameters for the tip resistance and side friction of the DPL. Sousa & Fonseca (2006) made correlations of the DPL with the SPT and the CPT, based on results of plate load tests and the use of various methods of load forecasting for shallow foundations, including the methods of Burland & Burbidge (1985) and Anagnastopoulos et al. (1991). The soils chosen by these authors were a granite residual soil, a schist soil, a massive natural residual soil of granite, and a soil of volcanic origin. The authors compared the prediction results, and showed that there was a good
agreement of the DPL with other penetrometers, with the exception of the case of the granite residual soil. Ávila & Conciani (2005) proposed the first testing correlations between SPT and DPL in the Brazilian state of Mato Grosso, also backed up by pile load tests. It is worth mentioning that the tests were performed with a home made DPL apparatus built with basis on the TC-16 - ISSMFE (1989) reference drawing dimensions, and that such study aimed the design of low cost residential units (with a social local appeal). Figure 3 shows a schematic drawing of this device. In this particular paper, the authors used the method of Décourt & Quaresma (1978) to estimate the capacity of the piles. The tests were performed in the city of Primavera do Leste, where the soil is typically a yellow non plastic sandy silt, which can be classified as SM in the unified system, with voids of the order of 1.7 (porous). The groundwater at the tested site is located approximately 5.5 m below surface. The authors found that the data from the DPL test was poorly correlated with the respective one from the SPT. However it was indicated the possible use of the DPL data to design deep foundations of the region, as the DPL predictions depicted values 35% lower than those from the experimental load tests. This option has proved to be feasible in situations where the soil presents low bearing capacity.
based on the results of SPT. The soil was a sandy clay, red, very soft to moderately soft in the city of Jaciara, Mato Grosso. The authors compared the data of dynamic penetrometers with load tests at com pression state and concluded that the use of the DPL in saturated soils should be avoided due to energy dissipations. Nevertheless, there was good fit between the predicted load capacity of the DPL data obtained with the empirical models of Aoki and Velloso (1975), and Décourt & Quaresma (1978), which are originally suited for SPT tests, and the results of pile load tests. Figure 4 shows a comparison of the cited methods and results. In this one it is noticed that the SPT does not provide good agreement for the test load. This in fact proves the inadequacy of the SPT for soils with a very low soil´s shearing resistance. Azevedo & Guimarães (2010) made a direct correlation between the N 30 of the SPT test and the N 10 of the DPL. The field experiment was conducted in a stretch of an electrical transmission line in the state of Acre, in Brazil, and the authors warned that more studies on this particular correlation would be necessary.
Figure 4- Comparison between the load estimation methods for SPT test, and DPL (Ribeiro Junior et al., 2007).
Figure 3 - Schematic assembly of the DPL (Ávila & Conciani, 2005)
Ribeiro Junior et. al . (2007) conducted a comparison between the predictions of load capacity, made from the DPL with statistical methods developed by Aoki and Velloso (1975), and Décourt & Quaresma (1978) to predict the load capacity of foundations
It is noticed that the geotechnical literature often fails to show a good agreement between the DPL and the SPT testing results. Nilsson (2004), Ávila & Conciani (2005), and Ribeiro Junior et. al. (2007) made valuable observations about the inadequacies of these comparisons. Nilsson (2004) made several considerations about the differences between the DPL test and the SPT. This latter author stated that by using these two tests simultaneously for situations where each one is more effective, one could surely obtain better correlations and results - im proving the efficiency of the design. Ávila & Conciani (2005) concluded that for soils of low resistance in Mato Grosso, the SPT and the DPL do not correlate well. According to Ribeiro
Júnior et al. (2007), the SPT test is best suited for granular soils, with SPTs (N 30) above 5, where the deep foundation generally has a length greater than 5 m, whereas the DPL test is more useful in finegrained soils of low resistance, where it could be used for the prediction of deep foundation capacity loads in buildings up to 5 m high (2 stores). It is worthwhile to say that the DPL test is extremely sensitive to small variations to stratifications within the soil deposit, feature generally not found in SPT tests. In addition to that, some studies have been conducted to compare different DPL configurations. Ávila et al. (2006) in a comparative study between two types of DPLs (DPL Nilsson, and DPL CEFET) concluded that the configuration of the equipment do influence directly on the efficiency of the test. However, Souza et al. (2008) concluded that the density of the rods have little influence on the results. Thus, the choice between a thicker or thinner than normal rod should solely affect the durability of the device. 2.5 Future research possibilities on the DPL There is no doubt that, among the dynamic cone penetrometers, the DCP type has more advantages when comparing to lightweight penetrometers, see for instance the article from Kleyn & Zyl (1998) which deals with applications of the dynamic cone penetrometer (DCP) to pavement design. The article of Kayalar (1998) is another good example for the future research with dynamic penetrometers of the DPL family type. In this work the adopted penetrometer was of the German Heavy Type (German standards DIN 4094) with a hammer weight of 0.50 kN, height of fall of 0.5 m and penetrometer cone diameter of 4.27 cm. This author established a statistical evaluation of the data to be used in the design of shallow foundations in cohesionless soils. The CEN-UNE-EN ISO 22476 (2008) of the European Union is another example of the European deep interest in this topic. This standard explains on possible data / field abnormalities and the influence on the derived data from particular geotechnical characteristics of the soil, as for instance the relative density of the granular material, as shown in Figure 5. Although one can not overlook the efforts of the research on the DPL, it is a fact that the comparative analyses with different penetrometers, especially with the SPT, fulfilled its role in the dissemination, and technical explanation of the advantages of the DPL usage for a large (previously unaware) audience. The moment now calls for more applied research on the energy assessment and direct measurements from the DPL test, and its possible use in design, likewise it has been done so far in other practical
oriented research lines (as the use of statistical or empirical correlations for bearing capacity with this same data).
Figure 5 – Influence of relative density on N 10 values (UNEEN ISO 22476 (2008)
Although in the market there are lightweight penetrometers with built in transducers, as the French Panda device, the standard DPL still lacks such feature. Of course, the reader may argue that if the Panda provides an answer through its own transducers, why measure the energy of the standard DPL? The answer is precisely because the DPL is a simple and easy to purchase (and manufacture) device, while, (nowadays in Brazil) Panda penetrometers are “ black boxes” that represent a high cost to engineering companies that are actually more familiar with the SPT tests – and not prone to invest further on an expensive new (and rather unknown around this region) imported tool. In this regard, a properly calibrated and low cost manufactured DPL could provide practical formulas for daily use in such companies, especially in small towns when oriented to low cost new constructions. 3 CONCLUSIONS A brief review on the use of DPL as an aid for estimating the capacity loads from foundations in Brazil shows that past research has been focused in the correlations to use the DPL based on existing experience with the SPT test, as this latter one is extremely used in Brazil. Nevertheless, although the predictions of capacity load for foundations with the DPL can in principle be done with experience borrowed from the SPT, it has also been demonstrated that such approach may be, in some cases, misleading to reliable results. The reviewed research shows that the DPL is suitable for tropical soils of low density and porous matrix. In cases where the magnitude of load is not significant, e.g., small buildings, the N 10 of the DPL is a handy information for design purposes. Research
also denotes that there are significant differences between both DPL and SPT tests, which range from equipment geometric differences to the magnitude of the enthru energy of the free fall hammer on the anvil. Additionally, soil characteristics influence the results distinctly, for instance, porous soils of lowdensity can respond differently to the advances of each of such aforementioned penetrometers. The evaluated research has also shown that the correlations of the DPL, with the CPT test, have yielded good agreement. One possible answer to that would be the closer “physical” resemblance, or geometric similarity, between these two artifacts in regard to other probes. An undeniable fact is that lightweight penetrometers assembled with accelerometers do yield very good results in terms of soil stratigraphy, but a major concern still remains: the attenuation of the input energy at the anvil along the assembled conjunct of rods and probe. As a final remark, one can firmly state that the research on the DPL device surpasses the common ground of empirical correlations, as usually done with the SPT. In fact, such former device does deserve a better insight in terms of the real enthru energy or its dissipation, so that a more physically accurate and rational way of interpretation could devised for its “ popularization” in the Brazilian practice. It has large potential for development and deployment, especially in Brazil where 80% of the population lives close to the coast (up to 100 km), and there are still vast extensions of tropical soil yet to be occupied by (generally low cost or low load) pioneering constructions from interior cities. A typical case is the state of Mato Grosso, where agriculture needs to be enhanced and urgently enforced to feed the needs of the country and, why not say, the world as well (given projections of 8+ billion inhabitants soon in future decades). This paper is an initial and simple contribution in this direction, based on an ongoing research with such well-defined cited and somehow discussed ob jectives (Figueiredo, 2010). 4 ACKNOWLEDGMENTS Special thanks to CAPES by funding of this capacitation, the University of Brasilia (UnB), that provides the course of doctoral, and Institute Federal Education, Science and Technology of Mato Grosso (IFMT), where if gives this research.
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