Centrifuge penetration tests in saturated layered sands

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1 Proceedings ISC-2 on Geotechnical and Geophysical Site Characterization, Viana da Fonseca & Mayne (eds.) 2004 Millpress, Rotterdam, ISBN Centrifuge penetration tests in saturated layered sands M.F. Silva & M.D. Bolton Engineering Dept., University of Cambridge, Cambridge, UK Keywords: centrifuge, piezocone, layered soil, sands, viscous fluid ABSTRACT: In principle, the penetration resistance of saturated sands should be a function of relative density, relative crushability, and relative speed of penetration. Excess pore pressures due to high penetration velocities should be measurable, and related to soil permeability, and therefore grain size. Piezocone tests (PCPT) have been carried out in centrifuge models to investigate the sensitivity to layering and grain size. Models consisted of fine silica sand sandwiched between two layers of coarse silica sand. A 12-mm diameter model piezocone was pushed into the models at different penetration rates to observe the effect of layering under different loading conditions and permeabilities of the sand layers. Additionally, a more viscous pore fluid was used in a second series of penetration tests to correct the centrifuge modelling of penetration process and flow of fluid within soil. The data are used to discuss the methodology of characteristic grain size interpretation and the use of viscous fluid in centrifuge tests. 1 INTRODUCTION Since its introduction as an in situ investigation device the cone penetration test (CPT) has been providing information which geotechnical practioners have used to identify soil stratigraphy. With the addition of pore pressure measurement the PCPT (or piezocone test) can lead, by careful interpretation, to good judgments about the characteristic grain size of soils. However, the charts to identify the characteristic size of the soil grains are based on the uniformity of soil conditions. In reality, soils are the most heterogeneous of materials and the occurrence of layers of different grain size in a soil deposit can complicate the interpretation of PCPT data. As the cone tip advances into different soil layers, the piezocone senses the effects of an approaching layer as well as being influenced by the soil properties behind its tip. This is due to the disturbance in both soil and pore fluid conditions surrounding a piezocone, which are associated with quantities q c (cone resistance), f s (sleeve friction) and u (pore pressure) measured on the probe. Figure 1 shows schematically the zones affected by the insertion of a cone. The disturbance zones interact with any inherent layering, changing the PCPT output. Cone penetration data are used most directly in pile design. Following Kerisel s (1961) observations of embedment and size effects on full scale piles, De Beer (1963) used a classical bearing capacity solution to explain that the penetration resistance depends on the development of a static failure mechanism. However, De Beer s (1963) theoretical explanation does not correctly reflect the steady penetration of a PCPT through different layer conditions. Investigations on the influence of layers on the penetration resistances were hence conducted mostly on model pile tests (e.g. Meyerhof and Valsangkar, 1977), calibration chamber tests (see Lunne et al., 1997) and other special cases (e.g. Treadwell, 1976). Figure 1- Soil disturbance during cone penetration (adapted from Muromachi, 1981) 377

2 Table 1- Investigations on the effects of penetration rate in centrifuge models References Rate range (mm/s) Probe diameter (mm) g-level * Fine-grained soils Almeida and Parry (1985) Remarks g Penetration rates between 2 and 20 mm/s have little effect on q c in kaolin clay Renzi et al. (1994) g Increasing the penetration rate in silty clay, both cone resistance and pore pressure increased (q c increased by 15%) Coarse-grained soils Ferguson and Ko (1981) g Moderate variations in the penetration rate would not influence q c adversely. However, paper shows great scatter of data, q c showing lower values at lower penetration speed Corte et al. (1991) g Saturated sand showed lower q c than dry sand. Though neglected, the lowest speeds showed greater q c values than the greater speeds Van der Poel and Schenkeveld (1998) Sharp et al. (1998) * g is the gravity acceleration g The cone resistance of the fastest CPT falls in between the slowest and medium CPT 9g 4.5g 3g Excellent data agreement between the dry and saturated soil models Recently, the layering effect has also been studied by numerical simulation (Ahmadi et al., 1999) and by centrifuge testing (Morley, 1997; Wright, 1998). The observed variability of soil behaviour type is inferred due to the occurrence of peaks and troughs on the cone penetration-graphs. The troughs are generally believed to be entirely due to more compressible soil layers, but the question arises whether they can alternatively be indicating layers of low permeability. In this paper, centrifuge penetration tests into a three-layer silica sand system are described with particle size differences between successive layers as the prime variable. The intermediate layer consisted of fine sand to reduce the model permeability. To make the penetration resistance independent of changes on soil strength, the system consisted of sands of the same origin and the same relative density. To identify the fine sand layer the penetration tests were performed at different rates in the saturated soil models. A special 12-mm diameter model piezocone was designed to provide information as an in situ electric piezocone. 2 THE PENETRATION RATE EFFECT IN LAYERED SOILS Soils create positive excess pore water pressures ahead of an advancing CPT probe. For the full range of possible penetration rates, undrained to partially or fully drained penetration can take place depending on the soil permeability and loading conditions. However, the excess pore water pressure in the vicinity of the piezocone filter element results from a combination of the physical displacement of soil and fluid during the penetration of piezocone, as well as from shear stresses generated in Zone 1, Fig. 1 (Burns and Mayne, 2002). The common trend is that increasing the penetration rate, the dynamic pore pressure and cone resistance also increase (Danziger and Lunne, 1997, apud Lunne et al., 1997). On the other hand, measured negative excess pore water pressure in stiff clays, silts or dense fine sands indicates that soil relaxation or dilatancy is occurring, even though large positive pore pressures may exist just a few cone diameters beneath. In the case of soil with layers of different permeabilities, the piezocone insertion modifies not only the properties of the surrounding soil from its initial condition but also the ambient total and effective stresses of the approaching layer due to pore pressure effects. The presence of a fine layer may hence restrict the water flow in the compressive zone ahead of the cone tip (Zone 4, Fig. 1). This could strongly influence the penetration resistance and it is more pronounced in fine-grained layers. Santoyo (1982), for example, observed consistent differences in in-situ penetration resistances at different speeds in soft clays interbedded with sand and hard silty clay lenses. 3 CENTRIFUGE MODELLING CPTs in centrifuge models are useful to delineate layering, to demonstrate inhomogeneity within layers, and to validate comparisons with required prototype conditions (Bolton et al., 1999), and to investigate soil density changes provoked during testing, such as following earthquakes (Teymur, 2002) Millpress, Rotterdam, ISBN

3 Strain gauges Strain gauges Friction load cell Friction sleeve Tip load cell Pressure transducer i.e. under identical stresses. However, if the fluid viscosity is increased by a factor of 50, for example, then even the smaller, slower centrifuge CPT should create 7 times larger excess pore pressures than would a CPT in the same soil in the field. In order to begin an assessment of this situation, two soil models were prepared for CPT sounding at different speeds with an identical fine sand layer, and saturated either with water or with a fluid whose viscosity was 50 times greater than water. 4 EQUIPMENTS AND INSTRUMENTATIONS Filter element Cone tip Figure 2- Model piezocone (not to scale) The degree of drainage during a penetration process can best be inferred by changing the penetration rate, or by measuring ambient pore pressure within the soil, as well as on the piezocone. To date, most attempts to find the penetration rate effect were restricted to dynamic pore pressure measurements in fine-grained soils in the field. Few penetration testings to study the penetration rate effect using centrifuge facilities are found in the literature, and some are summarized in Table 1. In quasi-static modelling at Ng, centrifuge tests are pleased to accept the N 2 factor on consolidation rate in accordance to Terzaghi s theory for a model with its drainage distances reduced by a factor N. In earthquake testing, centrifuge modellers must increase the frequency of cyclic loading by factor N to keep earthquake accelerations in proportion. They must therefore reduce the fluid viscosity by the model scale factor, so that the rate of creation of excess pore pressures is correctly balanced by the rate of dissipation to the model drainage boundaries. This strategy of reducing soil permeability makes it more important to understand when the penetration speed of a penetrometer will itself induce excess pore pressures u during probing. Dimensional analysis of the penetration process reveals that u D vv K (1) where is the kinematic viscosity (e.g Nm -2 s for water at 20 0 C), D is the diameter of the penetrometer, v is the mean volumetric compressive strain of the soil influenced by the passage of the penetrometer, v is the rate of penetration, and K is the intrinsic permeability of the soil. It is clear from Eq. 1 that a model CPT probe of 12 mm diameter and 10 mm/s velocity, for example, should create smaller excess pore pressure by a factor 7.4 compared with a standard 35.7 mm diameter probe travelling at 25 mm/s in identical soil saturated with the same fluid and compressing similarly, 4.1 Model piezocone A conical tip piezocone with 12-mm outer diameter was designed as shown in Figure 2. The length dimensions were proportionally scaled down by a factor of three from specifications in the International Reference Procedure (IRPT, 2001). The conical tip and friction sleeve consisted of stainless steel so that consecutive penetration tests could be performed without replacement due to excessive abrasion. Two load cells were designed to measure the cone resistance and sleeve friction. The adopted design also allowed the strain gauges to be glued to the load cell inner walls. The dynamic pore pressure u 2 was measured by a commercial pressure transducer located inside the tip load cell. The pressure transducer was held in place using silicone rubber, which was also used as a sealing between the load cells and the piezocone shafts. The cone tip is removable so that the piezocone pressure system can be saturated. Sintered bronze was used as the filter element located at the cone shoulder. The pressure transducer was calibrated inside a vessel filled with water. Compression was applied in the vessel, and both pressure and output readings were recorded. The process also allowed to verify the sealing against water leakage inside the piezocone, and to measure the cone area ratio a due to the effect of cone unequal area. A value of a equal to 0.75 was hence determined. 4.2 Driving mechanism The mechanism for driving the model piezocone was designed to be used in soil models with high shearing resistance. The equipment design criterion consisted of a high load capacity up to 10 kn at any linear vertical speed (max. 10 mm/s). The actuator allows a maximum displacement of 300 mm, monitored continuously by a laser displacement transducer. Figure 3 illustrates the driving mechanism. The model piezocone is attached to a carriage plate by a Proceedings ISCʼ2 on Geotechnical and Geophysical Site Characterization, Viana da Fonseca & Mayne (eds.) 379

4 480 Pillow block Bracket Carriage plate Bearing rail Ball screw Model probe Motor Pulley belt series of aluminium connections. The carriage plate is driven by a ball screw and is guided by two slide rails. Four pillow blocks connect the guide rails to the carriage plate, providing linear vertical movement and thus not allowing rotation. The ball screw is connected to a 35 V DC servomotor by a timing belt on the bottom of the equipment. On the top of the screw system a spherical bearing supports the entire axial load which is generated during both upward and downward movement of the carriage plate. The load is then redistributed on the equipment frame. Also, the moments generated by the pillow blocks on the slide rails are supported by a vertical 20-mm thick aluminium plate. 180 (a) Rear view 280 (b) Section view Figure 3- Schematic design of the driving mechanism (dimensions in mm, not to scale) Piezocone PPT Actuator Frame WT B-Sand E-Sand B-Sand Figure 4- Section of the centrifuge model (units in cm, not to scale) PPT-1 Standard 850-mm tub PPT PPT PPT-2 Steel plate Figure 5- Location of the penetration tests and instrumentations in the 850 mm tub (units in mm, not to scale) Table 2- Properties of the silica sands Property Fraction B Fraction E Grain size d 10 (mm) Grain size d 50 (mm) Grain size d 90 (mm) Specific gravity (G s ) Maximum void ratio (e max ) Minimum void ratio (e min ) Friction angle at cons. volume ( cv ) Hydraulic conductivity (m/s) 0.7x x Soil model Commercially available silica sands of E and B British Standard fractions were used for the tests. The mechanical behaviour of the sands is well documented by high quality experimental data (e.g. Lee 1989, Tan 1990). Relevant geotechnical parameters are listed in Table 2. The soil models consisted of three layers as shown in Figure 4. The sands were pluviated dry inside an 850-mm diameter steel tub to a given relative density (75%), instrumented with pore pressure transducers (PPTs), and saturated layer by layer. The first sand poured was the fraction B with 150 mm thickness. The 50-mm thick intermediate layer consisted of fraction E silica sand. The upper layer was again fraction B sand. The sand layers total is 300 mm in height. 4.4 Pore pressure transducers (PPTs) The pore pressure transducers used for the centrifuge penetration tests were the Druck PDCR (see Konig et al., 1994). A sintered bronze metal with high air entry was used as a porous element to separate the soil from the PPT pressure diaphragm for measurement. The pore pressure transducers were positioned in the fine layers during the sand pouring process. They were inserted halfway through the dry fine sand layers, at a horizontal distance of 11 mm from the location of the piezocone penetration (or 5 mm from the piezocone shaft, Fig. 5). 5 TEST PROCEDURES 5.1 Saturation techniques The saturation processes took place before the models were transported to the centrifuge. The first soil model was saturated with de-ionised water. The water was inserted from four entrances located at the bottom of the tub at very low hydraulic gradient (i~ 0.1). The process of saturation of the first model Millpress, Rotterdam, ISBN

5 did not take more than 15 minutes. The model contained a total volume of 68 litres of water. The second model was saturated with a 50-cP viscous fluid. The viscous fluid consisted of a solution of hydroxypropyl methylcellulose at a mass concentration of 2.2%. The concentration was selected according to the calibration curve shown in Figure 6. The curve was obtained following similar procedures adopted by Stewart et al. (1998). The equation is valid for the range of concentrations tested, between 0.8 and 3.6%. Additionally, a quantity of phenol equal to 0.5% of the mass of methylcellulose in the solution was added to minimize biological contamination. In order to saturate the second model with the viscous fluid, a pressure lid was sealed over the top of tub and vacuum was then set up, creating a pressure gradient. The fluid feeding system was connected up to the four holes located at the bottom of the tub. Due to the high pressure gradient expected, the fluid rate of flow was then set at 0.7 l/h to avoid hydraulic failure by liquefaction. The saturation process took four full days to be completed. A total of 68 l of fluid was used to saturate the soil model. 5.2 Penetration tests In each of the two soil models, four penetration tests were performed at different penetration rates. To avoid the boundary influence on the penetration resistances the piezocone was located according to the recommendation by Bolton et al. (1993): the penetration of a 12-mm diameter probe would not affect the properties of the soil outside a 480-mm diameter cylinder. The best suited locations and sequences of the penetration testings are shown in Figure 5, numbers 01 to 04. The cone tip was also stopped at 10 cm above the tub bottom to avoid the base boundary effect (Lee, 1989). Kinematic viscosity (cp) at 20 0 C Eq. including the viscosity range of most centrifuge use: 20 = 3.76 C(%) Hydroxypropyl methycellulose concentration (%) Figure 6- Kinematic viscosity at a temperature of 20 0 C ( 20 ) versus solution concentration of hydroxypropyl methycellulose The centrifuge tests were performed at a centripetal acceleration of 50g. For each centrifuge penetration test, time was allowed after the centrifuge spinning-up for stabilization of the instrumentation readings. In each soil model, the first penetration test was performed at 0.4 mm/s. The second, third and fourth penetration test rates were respectively 0.8, 4.0 and 8.0 mm/s. A fifth penetration test (control test) was also performed at the centre of each soil model. The objective of this test was to indicate any disturbance due to the four previous tests in the soil model as well as of the centrifuge stoppages to change the position of the driving mechanism. The penetration rate of the control test was the same as the first test: 0.4 mm/s. No significant changes of the cone resistance were observed when compared with the first penetration tests at 0.4 mm/s. Settlements were also measured during stoppage, but no changes were found. 6 RESULTS The results of the first test series using the first soil model are shown in Figure 7. Only the four penetration tests at different rates on the locations specified in Fig. 5 are plotted. It can be observed that up to 60 mm model depth, the corrected cone resistances (q t ) at any rate increase almost linearly. The rate of increase then reduces over the next 40 mm (~ 3 cone diameters) in anticipation of the lower penetration resistance of the fine sand layer. The q t value in the fine layer returns to the higher trend-line about 3 diameters above the lower layer of coarse sand. Ultimately, at a depth of 180 mm, i.e. at an effective overburden pressure corresponding to a depth of about 9 m in the field, the corrected penetration resistance is approximately 15 MPa. Though noisier, the sleeve friction (f s ) curves of Fig. 7 show a similar trend with depth, including a dip as the cone tip approaches the intermediate layer. A value of about 300 kpa is measured at a depth of 180 mm. The friction ratio curves (R f ) show a steady average value of about 1.8% from 60 mm depth. Figure 8 shows the penetration results in the second soil model. As in Fig. 7, the corrected cone resistances seem to increase linearly with depth at first. Differences are observed when the cone tip approaches the intermediate fine layer. As before, the corrected cone resistances approach the original trend curve when the cone has entered the lower coarse layer. The q t values at 180 mm depth in the second model are seen to be about 20 MPa. The sleeve friction curves in Fig. 8 also show increasing f s with depth, but with much smaller frictional resistance compared to the first soil model results. The f s value at 180 mm depth is only about 60 kpa. Consequently, the average friction ratio values are also reduced (R f ~ 0.8%). The dynamic pore pressures (u 2 ) measured behind the conical tip can be seen in Figs. 7 and 8, respectively for the first and second models. The piezocone Proceedings ISCʼ2 on Geotechnical and Geophysical Site Characterization, Viana da Fonseca & Mayne (eds.) 381

6 recorded only the hydrostatic pressure u 0 at any penetration rate. Figure 7 only shows the dynamic pore pressure measurement at 8 mm/s penetration rate. The lower values of u 2 compared to u 0 above 30 mm are due to the process of re-saturation of the porous element during the penetration test. It is believed that the piezocone pressure system desaturated during the centrifuge spinning-up at position 4 (Fig. 5) while the cone tip was held above the model water table. No perceptible changes of pore water pressure were read when the cone tip passed close to the pressure transducers located in the intermediate fine layer. Figure 9 shows the pore water pressure readings with cone tip displacement during penetration tests of the viscous fluid model. An average value of 64 kpa was read by all PPTs throughout the penetration processes. Figure 10 shows the normalized cone resistance Q t = (q t v0 )/ v0 with depth for the two soil models. In the first soil model an average Q t -value of 110 was obtained. A greater scatter of Q t at different penetration rates was however obtained in the second soil model, having an average value of 160. q t (MPa) f s (kpa) R f (%) Pore pressure (kpa) Model depth (mm) mm/s 0.8 mm/s 4.0 mm/s 8.0 mm/s Coarse sand = 20.6 kn/m 3 e 0= 0.53 Fine sand = 19.5 kn/m 3 e 0 = Coarse sand = 20.3 kn/m 3 e 0 = 0.54 u0 u2 Figure 7- Results of the penetration tests in the first soil model saturated with de-ionized water 0 q t (MPa) f s (kpa) R f (%) Pore pressure (kpa) mm/s 0.8 mm/s 4.0 mm/s 8.0 mm/s Model depth (mm) Coarse sand = 20.8 kn/m 3 e 0= 0.53 Fine sand = 19.6 kn/m 3 e 0= Coarse sand = 20.9 kn/m 3 e 0= 0.52 u0 Figure 8- Results of the penetration tests in the second soil model saturated with 50-cP viscous fluid Millpress, Rotterdam, ISBN

7 Pore pressure (kpa) mm/s 0.8 mm/s 4.0 mm/s 8.0 mm/s PPTs position: 5 mm from piezocone shaft 125 mm deep Cone displacement (mm) Figure 9- Pore pressure readings in the middle of the intermediate fine layer Model depth (mm) Q t mm/s 0.8 mm/s 4.0 mm/s 8.0 mm/s 1st soil model Q t nd soil model 200 Figure 10- Normalized cone resistance with depth The normalized pore pressure B q = (u 2 u 0 )/(q t v0 ) is zero with depth, because u 2 corresponds to the hydrostatic pressure u 0 in all tests. On the other hand, the normalized friction ratio F r = f s /(q t v0 ) shows the same disparity of values as does the friction ratio R f, because of the small values of total vertical stress v0 when compared to the corrected cone resistance q t. From the data shown above it is possible to use Robertson s (1990) soil behaviour type chart to verify its validity for centrifuge penetration tests. Analyzing both Q t vs. F r and Q t vs. B q charts, the soil models can be classified as silty to clean sands. The intermediate fine layer could not be identified by the interpretation of the piezocone parameters. 7 DISCUSSIONS For the piezocone size and soil model geometry utilized, Figures 7 through 10 yield the following general discussions: The change in penetration resistance three cone diameters ahead of the layer boundaries can be due to the perceived difference in soil stiffness. After the cone tip crosses the interface of two layers the cone resistance is also dependent on the soil layer above the cone tip. However, the influence of the upper coarse layer is not so easily observed when penetrating the intermediate fine layer if the layer was of insufficient thickness, because the cone tip may be already sensing the lower coarse layer. Penetration rates up to 8 mm/s appear to have little influence on the cone resistance, even in fine sand with a pore fluid 50 times more viscous than water. The dynamic pore pressures (u 2 ) are also unaffected. The viscous fluid had the effect of reducing the sleeve friction by a factor of about 2.5 compared with the water-saturated model. Apparently, methylcellulose solution acts as a lubricant for sand sliding against stainless steel, though this needs further confirmation from direct shear tests. Although each of the soil layers in the second soil model is slightly denser than the corresponding layer in the first model, the 35% average increase in q t seems surprisingly large. There is certainly no evidence that the methylcellulose solution reduced the angle of friction of sand on sand, therefore. In Fig. 8, the transitions between the intermediate fine and the coarser layers are more pronounced in the fast penetration tests than in the slow tests, though this could be coincidental. Further tests are required before cone resistance can properly be correlated with the relative density and crushability of various soil types. 8 CONCLUSIONS Results of centrifuge penetration tests within saturated layered sands were presented. The penetration tests were performed at 50g with a 12-mm diameter piezocone and at different rates of penetration. It was expected that the identification of the intermediate fine layer would occur due to change of soil permeability. However, no sharp changes of cone resistance or generation of excess pore water pressure were measured by the piezocone. Only smooth changes of gradient on the penetration-graph curves were observed, which may be not easily associated with the penetration rate effect. The effect of a different soil layer could be identified when the cone approached within a distance of three cone diameters. This distance of influence is observed only in soft soils in CPT tests (Treadwell, 1976; Lunne et al., 1997), but is within the range adopted by empirical methods for the prediction of pile end bearing capacity based on CPT results, which assume that the failure mechanism in sands extend to a depth around 0.7 to 4.0 pile diameters (e.g. Begemann, 1961; Te Kamp, 1977). The difference in magnitude of the sleeve friction in the first and second soil models seems to be at- Proceedings ISCʼ2 on Geotechnical and Geophysical Site Characterization, Viana da Fonseca & Mayne (eds.) 383

8 tributable to the lubricating properties of the hydroxypropyl methylcellulose. Further research on the subject is ongoing. The differences in the penetration resistance seem not to be related to the change in pore fluid, but rather to variations in relative density. There is therefore no need to suppose that methylcellulose reduces the effective angle of friction of sand. Its use in dynamic centrifuge modeling to slow down pore fluid migration may therefore be acceptable if the apparent lubrication of metal interfaces can be overcome. ACKNOWLEGDMENTS The authors gratefully acknowledge the work of the staff members involved in the research, in particular Chris Collison, Steve Chandler, Jason Waters and Chris McGinnie. We would also like to thank Keith Wilkinson from Wilkinson Associates for the design of the centrifuge actuator. The first author is very grateful to the Brazilian agency CNPq for the financial support given during the research study at Cambridge University. 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(1990), Centrifuge and theoretical modelling of conical footings on sand, PhD Thesis, Cambridge University. Te Kamp, W.C. (1977), Static cone penetration testing and foundations on piles in sand. In: Fugro Sounding Symposium, Utrecht, October. Teymur, B. (2002), The significance of boundary conditions in dynamic centrifuge modelling, PhD Thesis, Cambridge University. Treadwell, D.D. (1976), The influence of gravity, prestress, compressibility, and layering on soil resistance to static penetration, PhD Thesis, University of California, Berkeley. Van der Poel, J.T. and Schenkeveld, F.M. (1998), A preparation technique for very homogeneous sand models and CPT research. In: Centrifuge 98, A.A. Balkema Ed., Tokyo: Wright, L. (1998), Centrifuge modelling of piles and anchors in calcareous sands, Fourth Year Project, Cambridge University Millpress, Rotterdam, ISBN