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Free fall penetrometer: a performance evaluation
Free fall penetrometer: a performance evaluation UMESH DAYAL Civil Engineering Department, Indian Institute of Technology, (Received 10 May 1979; revised 10 August 1979)
Kanpur-208016,
U.P., India
The application of a free fall penetrometer for determining the in situ shear strength properties of marine sediments is discussed. The penetrometer is instrumented with three sensors for measuring the acceleration/deceleration, cone thrust and local side friction simultaneously and continuously. The instrument is simple and fast in operation (the entire operation of deploying, testing and retrieving the instrument takes only 20 minutes) and may prove to be a useful ground truth equipment for deep toe acoustic surveys. Recent trials offshore Newfoundland are discussed. Preliminary field test results indicate that with this system the dynamic strength profile, the soil type and the location and depth of different stratigraphic layers can be directly evaluated up to penetrated depths. A procedure is outlined for estimating the static strength profile from the dynamic strength profile by applying the correction for penetration rate effects. The penetrometer developed to date is capable of penetrating to a maximum depth of about 4 m but, in the future, it may be possible to obtain soil profiles to a depth of approximately 15 m (50 ft) with some modifications in the instrument design.
INTRODUCTION Engineering knowledge of seafloor materials is required for the design and construction of offshore platforms (gravity and jacket type), artificial islands, (fill and embankment materials), airports, break waters, excavation and burial for cables and pipelines, slope stability analysis, anchor design, breakout resistance analysis, and problems related to sediment erosion, liquefaction, traflicability and penetration resistance. The maximum water depth of offshore construction has increased rapidly since 1947 and predominant structures at the present time are oil production platforms and pipelines. In this context, geotechnical studies are mainly concerned with delineating soil strata, shear strength, compressibility and stressstrain relationships under static and dynamic loads. Properties such as the grain size, water content, density, specific gravity, Atterberg limits and mineralogy are used mostly to determine the general physical characteristics of different strata. Many techniques used for subsurface investigations of seafloor are extensions or modifications derived from terrestrial soil mechanics. Sampling and in situ testing procedures, currently in practice for seafloor investigations, are summarized by Noorany’, Dayal and Nayak3. The present sampling techniques are known to cause soil disturbance which is especially undesirable in shear strength testing’v4-‘. However, the practice of collecting soil samples will continue in seafloor investigations because of the need to observe and identify geological features and conduct standard laboratory tests. Obviously, the in situ strength measurements provide data of the highest quality. A variety of techniques used for terrestrial in situ strength measurements can be adapted to seafloor work le3 . Nevertheless, most of the techniques require modification to overcome the environmental constraints. New techniques are being considered for cost effectiveness and to satisfy the en-
0141-l 187/80/010039-05/S02.00 @ 1980 CML Publications
vironmental constraints and industry’s needs. In situ test devices, such as the wire line vane shear apparatus5*8’9 and the static penetration test”- l3 have found common acceptance among the offshore engineers. However, in addition to enormous development cost, the complexity involved in deploying and operating these devices have restricted their use to only very sophisticated projects. To overcome the problems generally associated with the use of conventional methods for the in situ testing of marine sediments, a free fall penetrometer was proposed by Dayal and Allen14 and later a few pilot test results were obtained in oceanic environments’ 5. This paper demonstrates the potential value of the free fall penetrometer and discusses the results obtained during successful offshore trials. This third generation instrument is being currently developed at the author’s institute for greater penetration depths.
FREE
FALL
PENETROMETER
Originally the free fall penetrometer was developed at the Memorial University as an instrument for quick and economical evaluation of the surfacial sediments16. Depending upon the strength of the sea floor sediments, 10 to 15 m of penetration is possible with such an instrument. This can be used for geotechnical surveys, feasibility and site selection studies for offshore structures and investigation of large tracts for pipeline routings. The operating principle of the free fall penetrometer is similar to that used in the triggered corer for collecting cores”. Figures l(a), (b) and (c) show conceptual views of the penetrometer and the three stages of its operation, i.e. lowering, release and penetration. The release mechanism contains a lever arrangement in which the weight of the trigger line attached to the long arm of the lever counterbalances the weight of the penetrometer on the short arm of the lever. The release mechanism is connected to a
Appl. Ocean Res. 1980, Vol. 2, No. 1
39
Free fall penetrometer: a performance evaluation: U. Dayal INTERPRETATION
a
Figure 1.
b
c
Free fall penetrometer
Obtaining meaningful soil strength data from the impact penetration test results requires very careful interpretation because of the dynamic nature of the events. Implicitly, the penetrometer is subjected to a resistance derived from the dynamic soil-bearing capacity from its entry until the penetration terminates. The major causes of the differences in dynamic bearing capacity over static bearing capacity are the modes of failure and penetration rate effects20*21. Many investigators have worked on high velocity penetration (velocity of 50 m/set and above) and numerous theoretical and empirical relationships have been proposed between the penetration depth, the soil properties, the impact velocity and the penetrometer characteristics. The available theoretical relationships have little value from a practical point of view. Thompson” has proposed a more comprehensive theoretical approach by considering the continuity equation, the equation of state and the constitutive soil relationships. Unfortunately, his theoretical relationships could not be solved because it required the laboratory determination of a number of soil parameters which could not be established. Dayal and Allen22 studied the penetration rate effect on remoulded clay and sand samples in the laboratory for low velocity penetration (velocity range of 0.1 cm/s to 600 cm/s) and concluded that: (1)
winch of the ship and is designed to allow the penetrometer to fall freely for a short distance before it strikes the seafloor. When the trigger weight hits the seafloor, the penetrometer slips off the short arm of the lever and falls freely towards the sea bottom. The penetrometer continues to travel into the seafloor sediment until it is brought to stop by the soil resistive forces. After being released by the lever arm, the penetrometer is directly connected to the winch by the loop wire. Upon completion of the test, the penetrometer is pulled out of the seafloor and recovered from winch aboard the ship. Using this system the impact velocity can be preselected (up to the terminal velocity) according to the requirements. Figure l(d) gives a cross-sectional view of the penetrometer system. The basic dimensions of the original penetrometer model had standard 35.6 mm diameter (area 10 cm’) penetrometer (Fig. 1) with 60” cone tip and 150 cm2 friction sleeve. The system was developed so that weights up to a maximum of 1200 N (270 lb) could be attached at the top. In the later model of this instrument, the outer diameter of the penetrometer and shaft has been increased’* to 75 mm to overcome the frequent buckling failure of the shaft”. The penetrometer has been instrumented with accelerometer, cone load cell and friction sleeve load cell to measure acceleration/deceleration, cone thrust and local side friction simultaneously and continuously. An umbilical cord transmits the signals to an on-board high speed instrumentation tape recorder. Recently, several sea trials were made with a 75 mm diameter penetrometer in water depths of 180-250 m in the North Atlantic. Figure 2 is a typical trace of the deceleration, cone thrust and sleeve friction resistance”. In this trial, a penetration of the entire 3.7 m length of the penetrometer was obtained in medium to stiff clay. In cases of layered soils such as clay overlain by sand, the velocity and friction profiles clearly identified the layers.
40
Appl. Ocean Res. 1980, Vol. 2, No. I
OF TEST RESULTS
(2)
(3)
there is no significant difference in failure patterns obtained within the velocity range of 0.1 to 600 cm/s; the effect of penetration velocity on the cone and sleeve friction resistances are insignificant for granular soils within the velocity range tested (-0.1 to 600 cm/s; an increase in penetration velocity for cohesive soils causes an increase in the cone and friction resistances and within the velocity range tested (0.13 to 550 cm/s), the ratio of the dynamic soil resistance (soil resistance measured at some instantaneous penetration velocity) to the static soil resistance (soil resistance calculated at some insignificant penetration velocity) can be represented by the following empirical relationship:
‘yc
= KL
c
lOOr
2
Y
log
$ s
(
I
I1 10
‘_I
,/.~_/‘\,~,___JSIeeve
;,:; ‘d-7
1 39
ADeceleratIon
/
\__ A i___
1g
me kc1
Figure 2. Typical output of deceleration, cone thrust and sleeve.friction’*
Free fall penetrometer: a performance evaluation: U. Dayal Soil viscosity coefficient (KL)
Table 1.
Shear strength (lb/ft 2)
K L from cone
64 200 960 1070 1350 1670
1.5 0.25 0.21 0.15 0.12 0.03
resistance
Note: 1 lb/ft 2 = 4 7 . 9 P a
"
:i . . . . . . . . . . . . . . . .
.............. ____..... ~--
Fi#ure 3.
"
"
~
Free fall penetration test results
where qcd is unit dynamic cone resistance at velocity V, qc is the unit static cone resistance at velocity V~(calculated at some insignificant penetration velocity; this author used 0.13 cm/s in his analysis), V is the penetration velocity, V~ is the lowest penetration velocity used for calculating the static strength, K L is the soil viscosity coefficient, the values of which are given in Table 1. t h e example given in Fig. 3 illustrates the procedure for obtaining the static strength profile from the trace of the dynamic strength profile obtained from the free fall penetrometer. Depth vs. acceleration, deceleration, cone resistance and the sleeve friction profile date are depicted (Fig. 3). These data were obtained during initial sea trials at the coast of St. John's, Newfoundland, Canada. Analysis of the results clearly shows the travel of the penetrometer into the water at 1 g until the weight barrel hits the water at around 7.32 m/sec. Drag force at this speed cuts down acceleration considerably in water. When the penetrometer hits the bottom, it is travelling at 8.48 m/s and penetrates about 1.83 m. Records from both the accelerometer and the cone pressure cell show a series of spikes indicating the presence of cobbles and gravel. This was confirmed both by the divers and from a core sample taken at the site. An in situ vane shear test, which was carried out by the divers at the test site, gave strength values of 150 lb/ft 2 (7.2 kPa) at the top 1 ft to 216 lb/ft 2 (10.3 kPa) at 3.5 ft (107 cm) depth. The divers were unable to drive the vane any further. Assuming an average vane shear strength of 183 lb/ft 2 (8.8 kPa) the average soil pressure for a 60 ° cone is obtained from Meyerhof23 bearing capacity formula as 1740 lb/ft / (83.3 kPa).
The dynamic cone resistance profile obtained from the Free Fall penetration test (Fig. 3) indicates the variation of dynamic cone resistance from 3000 lb/ft 2 (144 kPa) at a depth of 1 ft to 4000 lb/ft 2 (192 kPa) at a depth of 4 ft (122 cm) (neglecting the peaks which are due to the presence of cobbles). Assuming the average dynamic cone resistance of 3500 lb/ft 2 (168 kPa), the static cone resistance is obtained as 1810 lb/ft 2 (86.7 kPa) from relationship (1) for static velocity of 0.13 cm/s and KL=0.25 (obtained from Table 1 for average vane shear strength value of 183 lb/ft2; 8.8 kPa). The static cone resistance values computed from the free fall penetrometer test results are in good agreement with those estimated from Meyerhof's formula. As discussed above, the free fall penetrometer can be used for determining dynamic strength profile and the location and depth of different layers up to penetrated depths. Upon applying the correction the static strength profile can also be estimated. This result and ihose reported earlier by the author 16 have been obtained under highly idealized conditions. However, in practice it can not be readily applied unless a good correlation is established between static and dynamic strength profiles. This would be obtained by conducting free fall penetration tests in conjunction with the static penetration tests at various sites encompassing diversified soil conditions.
PENETROMETER EXPLORATION
FOR
GREATER
DEPTH
The penetrometers developed to date are capable of penetrating to a maximum depth of about 4 m in soils having average strengths of 20 to 25 kPa. Many soil mechanics applications require the knowledge of the sea floor strength to depths of l0 to 15 m. This section discusses the possibility of using this instrument and describes the specifications required for obtaining strength profiles to deeper depths. A penetration theory based on the momentum considerations was developed for a cone-tipped right circular cylinder impacting on a c-(p soil target. A relationship has been established 16 between the instantaneous velocity, various static and dynamic soil properties, penetrometer characteristics and the instantaneous depth of penetration. The relationship was obtained assuming that the impact causes shear failure and the resistance to the motion of penetrometer is provided by the inertial resistance of the accelerated soil mass plus the dynamic soil resistive force distributed over the base and shaft of the penetrometer. The soil resistive force is calculated on the basis of plastic theory modified for dynamic conditions and extending the previous analysis for the static condition 24. The relationship obtained provides a velocity profile and can also be used for calculating the maximum penetration depth. The theoretical model is in good agreement with experimental results conducted in laboratory under fully controlled conditions. The same model is used for designing the penetrometer for greater penetration depths. In general, the design of the free fall penetrometer is based on a number of variables, for example, impact velocity, weight, diameter and overall penetrometer shape, dynamic strength of target, target material and its unit weight, and the maximum penetration depth requirements. In addition, the stability of the penetrometer during free fall, the structural and mechanical require-
Appl. Ocean Res. 1980, Vol. 2, No. 1 41
Free fall penetrometer: a per]ormance evaluation: U. Dayal f
2ao<
2¢00C
o
20000
FULL LINE~RE~CED D~AMETER SHAFT TYPE pENETROMETER
//
240:
2OOC
/
35oc LB
#Z~;40
Y //z :3o'
////'?//.~Zm:'~O' / • aooo
4000
~
8oc
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DIAMETER OF PENETROMETER
(~N)
Figure 4. Design weioht of penetrometer jor different diameter and different penetration depth ments such as the buckling load on the penetrometer shaft and strength requirements of different components during impact loading are to be considered in the detailed design of the penetrometer. The basic design requirements of the complete system are weight, diameter and length of the penetrometer and data acquisition system. The necessary weight of the penetrometer has been calculated (Fig. 4) for penetrometer diameter ranging from 3.81 to 10.16 cm and for maximum penetration depths of 9.15, 12.2 and 15.25 m assuming the penetrometer impacts at a velocity of 10.75 m/sec in a cohesive material soil target having shear strengths of 19.16 kPa 25. As an example, the design weight of 10.1 cm diameter penetrometer is 10 t for maximum penetration of 15.25 m. This compares favourably with the Woods Hole Oceanographic Institution's piston corer results 26 in which a 30.3 m long and 12.96 cm diameter piston sampler weighing 15 t, penetrated 28.5 m and recovered a 22.7 m long silty clay core. Submarine sediments often consist of fine soils in which a major part of the resistance to the penetrometer is offered from adhesion/friction on the shaft. If this resistance can be eliminated or minimized, a considerable reduction in the penetrometer weight can be achieved for the same diameter and penetration depth requirements. For example, by making the diameter of the shaft 6 mm less than the cone head diameter, an all around clearance of 3 mm between the surface wall and cavity wall could be had, thereby eliminating most of the resistances offered to the penetrometer shaft during impact penetration. The calculations to this effect have been made (Fig. 4). In these calculations, a safety factor of two has be~n applied to take into account the bulging and collapsing effects, which may cause some resistance to the shaft. The results indicate that the reduced diameter shaft type penetrometers require only about 16% of the weight of those required for uniform diameter type penetrometers. However, before developing the reduced diameter type penetrometer for greater depth capabilities, some experimental laboratory studies should be made under controlled conditions to study the response of this type of
42
Appl. Ocean Res. 1980, Vol. 2, No. 1
penetrometer to different test conditions, and to observe its stability during the penetration event. The results provide a tentative guideline for optimizing the diameter of field penetrometers for weight, penetration depth and buckling load requirements (Fig. 4). Detailed design and development of mechanical and electrical systems is needed before the reduced diameter type penetrometer could become an effective tool. CONCLUSIONS Sea trials of the free fall penetrometer described above prove it to be a useful equipment for obtaining the in situ strength profiles, depth and location of different layers. Penetration up to 4 m is easily possible in its present stage of development and in the future, it may be possible to achieve the penetration up to 15 m with some modification in the design. The entire operation of deploying, testing and retrieving the instrument takes only about 20 min. The instrument could ultimately become an inexpensive tool for reconnaissance of vast areas, especially of large tracts such as those for offshore pipe lines. In order to attain a higher degree of confidence, additional field and laboratory tests are being planned in conjunction with static penetration tests under diversified soil conditions. REFERENCES 1
2 3 4
5 6 7 8
9
10
II 12 13
14 15
Noorany, I. 'Underwater soil sampling and testing a state-ofthe-art review', Syrup. Underwater Soil Sampling, Testing, and Construction Control, STP 501, American Society for Testing and Materials, 1971, pp. 3 41 Dayal, U. 'Recent trends in underwater in situ soil testing', J. Oceanic Eng. 1EEE, 1978, 3, 176 Nayak, B. U., 'Offshore Site Exploration', Proc. GEOCONINDIA, Conf Geotech. Eng. 1978, lI, 136 Anderson, V. C., Clinton, J., Gibson, D. K. and Kirstan, O. H. Instrumenting RUM for in situ sub-sea soil survey, Syrup. Underwater Soil Sampling, Testing and Construction Control, ASTM STP 501, American Society for Testing and Materials, 1971, pp. 212 228. Kraft, L. M., Ahmad, N. and Focht. J. A. 'Application of remote vane results of offshore geotechnical problems, Proc. 8th OJJshore Technol. Conj. 1976, OTC 2626, pp. 77 96. Richards, A. F. and Keller, G. H. A plastic barrel sediment corer, Deep Sea Res., 1961, 8, 306 Richards, A. F. and Parker, H. W. 'Surface coring for shear strength measurements', Proc. Conj. Civil Eng. Oceans I, ASCE, San Francisco, 1967, pp. 445~488 Doyle, E. H., McClellan& B. and Ferguson, G. H. Wire line vane probe for deep penetration measurements of ocean sediment strength, Proc. 3rd Offshore Technol. Conf. 1971,1, OTC 1327, pp. 21 27 Richards, A. F., McDonald, V. J., Olson, R. E. and Keller, G. H. In place measurement of deep sea soil shear strength, Symp. Underwater Soil Samplino, Testin 9 and Construction Control, STP 501, American Society for Testing and Materials, 1971, pp. 55 68 Ferguson, G. H., McClelland, B. and Bell, W. D. Seafloor cone penetrometer for deep penetration measurement of ocean sediment strength, Proc. 9th Offshore Technol. Conf. 1977, OTC 2787, pp. 471 480 McClellan& B. Trends in marine site investigations: a perspective, Proc. Conl~ OJ]shore Europe, Aberdeen 1975, 20 pp. Ruiter, J. D. and Fox, D. A. Site investigations for North Sea Forties Field, Proe. 7th Offshore Teehnol. Conj'. 1975, OTC 2246, pp. 21 36 Zuid berg, H. M. Seabed penetrometer tests, Proc. Fugro Syrup. on Penetrometer Testing, The Hague, 1972
Dayal,U. and Allen, J. H. Instrumented impact cone penetrometer, Can. Geotech. J. 1973, 10, 397 Dayal,U. and Allen, J. H. The effectof penetration rate on the
F r e e f a l l penetrometer: a performance evaluation: U. Dayal
16 17 18 19 20
strength of remoulded clay and sand samples, Can. Geotech. J. 1975, 12, 336 Dayal, U. Instrumented impact cone penetrometer, Ph.D. Dissertation, Memorial Univ. of Newfoundland, Canada 1974, 222 pp. Preslan, W. L. Accelerometer-monitored coring, Proc. Conf. Civil En9 Oceans II, ASCE, 1970, pp. 637-641 Chari, T. R., Smith, W. G. and Zielinski, A. Use of free fall penetrometer in seafloor engineering, Conf. Record, Ocean 78 IEEE-MTS Conf. 1978, p. 4 Jones, J. H. The use of an impact penetrometer in remote sensing study of the sea bed properties, 2nd CSCE Hydrotech. Conf. Amherst 1976 McNeill, R. L. Rapid penetration of terrestrial materials - - the state of the art, Proc. Conf. on Rapid Penetration of Terrestrial Materials, Texas A and M Univ., College Station, 1972, pp. 11126
21 22 23 24 25 26
Murff, J. D. and Coyle, H. M. A laboratory investigation of low velocity penetration, Proc. Conf. on Rapid Penetration of Terr. Mater., Texas A and M Univ., College Station, 1972, pp. 319-359 Thompson, L. J. Dynamic penetration of selected projectile into particulate media, Sandia Laboratory Report SC-R-R-66-376, Albuquerque, New Mexico, 1966, 194 pp Dayal, U., Allen, J. H. and Jones, J. M. Use of an impact penetrometer for the evaluation of the in situ strength of marine sediments, Marine Geotechnol. 1975, 1, 73 Meyerhof, G. G. The ultimate bearing capacity of wedge shaped foundations, Proc. Fifth Int. Conf. Soil Mech. Found. Eng., Paris, 1961, 2, 105 Keller, G. H. Engineering properties of some sea-floor deposits, J. Soil Mech. Div., ASCE 1969, 95, (SM6), 1379 Korites, B. J. A numerical technique for estimating the response of a gravity corer penetrating a marine sediment, J. Terramechanics, 1969, 6, (4), 35
Appl. Ocean Res. 1980, Vol. 2, N o . 1
43
Kanpur-208016,
U.P., India
The application of a free fall penetrometer for determining the in situ shear strength properties of marine sediments is discussed. The penetrometer is instrumented with three sensors for measuring the acceleration/deceleration, cone thrust and local side friction simultaneously and continuously. The instrument is simple and fast in operation (the entire operation of deploying, testing and retrieving the instrument takes only 20 minutes) and may prove to be a useful ground truth equipment for deep toe acoustic surveys. Recent trials offshore Newfoundland are discussed. Preliminary field test results indicate that with this system the dynamic strength profile, the soil type and the location and depth of different stratigraphic layers can be directly evaluated up to penetrated depths. A procedure is outlined for estimating the static strength profile from the dynamic strength profile by applying the correction for penetration rate effects. The penetrometer developed to date is capable of penetrating to a maximum depth of about 4 m but, in the future, it may be possible to obtain soil profiles to a depth of approximately 15 m (50 ft) with some modifications in the instrument design.
INTRODUCTION Engineering knowledge of seafloor materials is required for the design and construction of offshore platforms (gravity and jacket type), artificial islands, (fill and embankment materials), airports, break waters, excavation and burial for cables and pipelines, slope stability analysis, anchor design, breakout resistance analysis, and problems related to sediment erosion, liquefaction, traflicability and penetration resistance. The maximum water depth of offshore construction has increased rapidly since 1947 and predominant structures at the present time are oil production platforms and pipelines. In this context, geotechnical studies are mainly concerned with delineating soil strata, shear strength, compressibility and stressstrain relationships under static and dynamic loads. Properties such as the grain size, water content, density, specific gravity, Atterberg limits and mineralogy are used mostly to determine the general physical characteristics of different strata. Many techniques used for subsurface investigations of seafloor are extensions or modifications derived from terrestrial soil mechanics. Sampling and in situ testing procedures, currently in practice for seafloor investigations, are summarized by Noorany’, Dayal and Nayak3. The present sampling techniques are known to cause soil disturbance which is especially undesirable in shear strength testing’v4-‘. However, the practice of collecting soil samples will continue in seafloor investigations because of the need to observe and identify geological features and conduct standard laboratory tests. Obviously, the in situ strength measurements provide data of the highest quality. A variety of techniques used for terrestrial in situ strength measurements can be adapted to seafloor work le3 . Nevertheless, most of the techniques require modification to overcome the environmental constraints. New techniques are being considered for cost effectiveness and to satisfy the en-
0141-l 187/80/010039-05/S02.00 @ 1980 CML Publications
vironmental constraints and industry’s needs. In situ test devices, such as the wire line vane shear apparatus5*8’9 and the static penetration test”- l3 have found common acceptance among the offshore engineers. However, in addition to enormous development cost, the complexity involved in deploying and operating these devices have restricted their use to only very sophisticated projects. To overcome the problems generally associated with the use of conventional methods for the in situ testing of marine sediments, a free fall penetrometer was proposed by Dayal and Allen14 and later a few pilot test results were obtained in oceanic environments’ 5. This paper demonstrates the potential value of the free fall penetrometer and discusses the results obtained during successful offshore trials. This third generation instrument is being currently developed at the author’s institute for greater penetration depths.
FREE
FALL
PENETROMETER
Originally the free fall penetrometer was developed at the Memorial University as an instrument for quick and economical evaluation of the surfacial sediments16. Depending upon the strength of the sea floor sediments, 10 to 15 m of penetration is possible with such an instrument. This can be used for geotechnical surveys, feasibility and site selection studies for offshore structures and investigation of large tracts for pipeline routings. The operating principle of the free fall penetrometer is similar to that used in the triggered corer for collecting cores”. Figures l(a), (b) and (c) show conceptual views of the penetrometer and the three stages of its operation, i.e. lowering, release and penetration. The release mechanism contains a lever arrangement in which the weight of the trigger line attached to the long arm of the lever counterbalances the weight of the penetrometer on the short arm of the lever. The release mechanism is connected to a
Appl. Ocean Res. 1980, Vol. 2, No. 1
39
Free fall penetrometer: a performance evaluation: U. Dayal INTERPRETATION
a
Figure 1.
b
c
Free fall penetrometer
Obtaining meaningful soil strength data from the impact penetration test results requires very careful interpretation because of the dynamic nature of the events. Implicitly, the penetrometer is subjected to a resistance derived from the dynamic soil-bearing capacity from its entry until the penetration terminates. The major causes of the differences in dynamic bearing capacity over static bearing capacity are the modes of failure and penetration rate effects20*21. Many investigators have worked on high velocity penetration (velocity of 50 m/set and above) and numerous theoretical and empirical relationships have been proposed between the penetration depth, the soil properties, the impact velocity and the penetrometer characteristics. The available theoretical relationships have little value from a practical point of view. Thompson” has proposed a more comprehensive theoretical approach by considering the continuity equation, the equation of state and the constitutive soil relationships. Unfortunately, his theoretical relationships could not be solved because it required the laboratory determination of a number of soil parameters which could not be established. Dayal and Allen22 studied the penetration rate effect on remoulded clay and sand samples in the laboratory for low velocity penetration (velocity range of 0.1 cm/s to 600 cm/s) and concluded that: (1)
winch of the ship and is designed to allow the penetrometer to fall freely for a short distance before it strikes the seafloor. When the trigger weight hits the seafloor, the penetrometer slips off the short arm of the lever and falls freely towards the sea bottom. The penetrometer continues to travel into the seafloor sediment until it is brought to stop by the soil resistive forces. After being released by the lever arm, the penetrometer is directly connected to the winch by the loop wire. Upon completion of the test, the penetrometer is pulled out of the seafloor and recovered from winch aboard the ship. Using this system the impact velocity can be preselected (up to the terminal velocity) according to the requirements. Figure l(d) gives a cross-sectional view of the penetrometer system. The basic dimensions of the original penetrometer model had standard 35.6 mm diameter (area 10 cm’) penetrometer (Fig. 1) with 60” cone tip and 150 cm2 friction sleeve. The system was developed so that weights up to a maximum of 1200 N (270 lb) could be attached at the top. In the later model of this instrument, the outer diameter of the penetrometer and shaft has been increased’* to 75 mm to overcome the frequent buckling failure of the shaft”. The penetrometer has been instrumented with accelerometer, cone load cell and friction sleeve load cell to measure acceleration/deceleration, cone thrust and local side friction simultaneously and continuously. An umbilical cord transmits the signals to an on-board high speed instrumentation tape recorder. Recently, several sea trials were made with a 75 mm diameter penetrometer in water depths of 180-250 m in the North Atlantic. Figure 2 is a typical trace of the deceleration, cone thrust and sleeve friction resistance”. In this trial, a penetration of the entire 3.7 m length of the penetrometer was obtained in medium to stiff clay. In cases of layered soils such as clay overlain by sand, the velocity and friction profiles clearly identified the layers.
40
Appl. Ocean Res. 1980, Vol. 2, No. I
OF TEST RESULTS
(2)
(3)
there is no significant difference in failure patterns obtained within the velocity range of 0.1 to 600 cm/s; the effect of penetration velocity on the cone and sleeve friction resistances are insignificant for granular soils within the velocity range tested (-0.1 to 600 cm/s; an increase in penetration velocity for cohesive soils causes an increase in the cone and friction resistances and within the velocity range tested (0.13 to 550 cm/s), the ratio of the dynamic soil resistance (soil resistance measured at some instantaneous penetration velocity) to the static soil resistance (soil resistance calculated at some insignificant penetration velocity) can be represented by the following empirical relationship:
‘yc
= KL
c
lOOr
2
Y
log
$ s
(
I
I1 10
‘_I
,/.~_/‘\,~,___JSIeeve
;,:; ‘d-7
1 39
ADeceleratIon
/
\__ A i___
1g
me kc1
Figure 2. Typical output of deceleration, cone thrust and sleeve.friction’*
Free fall penetrometer: a performance evaluation: U. Dayal Soil viscosity coefficient (KL)
Table 1.
Shear strength (lb/ft 2)
K L from cone
64 200 960 1070 1350 1670
1.5 0.25 0.21 0.15 0.12 0.03
resistance
Note: 1 lb/ft 2 = 4 7 . 9 P a
"
:i . . . . . . . . . . . . . . . .
.............. ____..... ~--
Fi#ure 3.
"
"
~
Free fall penetration test results
where qcd is unit dynamic cone resistance at velocity V, qc is the unit static cone resistance at velocity V~(calculated at some insignificant penetration velocity; this author used 0.13 cm/s in his analysis), V is the penetration velocity, V~ is the lowest penetration velocity used for calculating the static strength, K L is the soil viscosity coefficient, the values of which are given in Table 1. t h e example given in Fig. 3 illustrates the procedure for obtaining the static strength profile from the trace of the dynamic strength profile obtained from the free fall penetrometer. Depth vs. acceleration, deceleration, cone resistance and the sleeve friction profile date are depicted (Fig. 3). These data were obtained during initial sea trials at the coast of St. John's, Newfoundland, Canada. Analysis of the results clearly shows the travel of the penetrometer into the water at 1 g until the weight barrel hits the water at around 7.32 m/sec. Drag force at this speed cuts down acceleration considerably in water. When the penetrometer hits the bottom, it is travelling at 8.48 m/s and penetrates about 1.83 m. Records from both the accelerometer and the cone pressure cell show a series of spikes indicating the presence of cobbles and gravel. This was confirmed both by the divers and from a core sample taken at the site. An in situ vane shear test, which was carried out by the divers at the test site, gave strength values of 150 lb/ft 2 (7.2 kPa) at the top 1 ft to 216 lb/ft 2 (10.3 kPa) at 3.5 ft (107 cm) depth. The divers were unable to drive the vane any further. Assuming an average vane shear strength of 183 lb/ft 2 (8.8 kPa) the average soil pressure for a 60 ° cone is obtained from Meyerhof23 bearing capacity formula as 1740 lb/ft / (83.3 kPa).
The dynamic cone resistance profile obtained from the Free Fall penetration test (Fig. 3) indicates the variation of dynamic cone resistance from 3000 lb/ft 2 (144 kPa) at a depth of 1 ft to 4000 lb/ft 2 (192 kPa) at a depth of 4 ft (122 cm) (neglecting the peaks which are due to the presence of cobbles). Assuming the average dynamic cone resistance of 3500 lb/ft 2 (168 kPa), the static cone resistance is obtained as 1810 lb/ft 2 (86.7 kPa) from relationship (1) for static velocity of 0.13 cm/s and KL=0.25 (obtained from Table 1 for average vane shear strength value of 183 lb/ft2; 8.8 kPa). The static cone resistance values computed from the free fall penetrometer test results are in good agreement with those estimated from Meyerhof's formula. As discussed above, the free fall penetrometer can be used for determining dynamic strength profile and the location and depth of different layers up to penetrated depths. Upon applying the correction the static strength profile can also be estimated. This result and ihose reported earlier by the author 16 have been obtained under highly idealized conditions. However, in practice it can not be readily applied unless a good correlation is established between static and dynamic strength profiles. This would be obtained by conducting free fall penetration tests in conjunction with the static penetration tests at various sites encompassing diversified soil conditions.
PENETROMETER EXPLORATION
FOR
GREATER
DEPTH
The penetrometers developed to date are capable of penetrating to a maximum depth of about 4 m in soils having average strengths of 20 to 25 kPa. Many soil mechanics applications require the knowledge of the sea floor strength to depths of l0 to 15 m. This section discusses the possibility of using this instrument and describes the specifications required for obtaining strength profiles to deeper depths. A penetration theory based on the momentum considerations was developed for a cone-tipped right circular cylinder impacting on a c-(p soil target. A relationship has been established 16 between the instantaneous velocity, various static and dynamic soil properties, penetrometer characteristics and the instantaneous depth of penetration. The relationship was obtained assuming that the impact causes shear failure and the resistance to the motion of penetrometer is provided by the inertial resistance of the accelerated soil mass plus the dynamic soil resistive force distributed over the base and shaft of the penetrometer. The soil resistive force is calculated on the basis of plastic theory modified for dynamic conditions and extending the previous analysis for the static condition 24. The relationship obtained provides a velocity profile and can also be used for calculating the maximum penetration depth. The theoretical model is in good agreement with experimental results conducted in laboratory under fully controlled conditions. The same model is used for designing the penetrometer for greater penetration depths. In general, the design of the free fall penetrometer is based on a number of variables, for example, impact velocity, weight, diameter and overall penetrometer shape, dynamic strength of target, target material and its unit weight, and the maximum penetration depth requirements. In addition, the stability of the penetrometer during free fall, the structural and mechanical require-
Appl. Ocean Res. 1980, Vol. 2, No. 1 41
Free fall penetrometer: a per]ormance evaluation: U. Dayal f
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Figure 4. Design weioht of penetrometer jor different diameter and different penetration depth ments such as the buckling load on the penetrometer shaft and strength requirements of different components during impact loading are to be considered in the detailed design of the penetrometer. The basic design requirements of the complete system are weight, diameter and length of the penetrometer and data acquisition system. The necessary weight of the penetrometer has been calculated (Fig. 4) for penetrometer diameter ranging from 3.81 to 10.16 cm and for maximum penetration depths of 9.15, 12.2 and 15.25 m assuming the penetrometer impacts at a velocity of 10.75 m/sec in a cohesive material soil target having shear strengths of 19.16 kPa 25. As an example, the design weight of 10.1 cm diameter penetrometer is 10 t for maximum penetration of 15.25 m. This compares favourably with the Woods Hole Oceanographic Institution's piston corer results 26 in which a 30.3 m long and 12.96 cm diameter piston sampler weighing 15 t, penetrated 28.5 m and recovered a 22.7 m long silty clay core. Submarine sediments often consist of fine soils in which a major part of the resistance to the penetrometer is offered from adhesion/friction on the shaft. If this resistance can be eliminated or minimized, a considerable reduction in the penetrometer weight can be achieved for the same diameter and penetration depth requirements. For example, by making the diameter of the shaft 6 mm less than the cone head diameter, an all around clearance of 3 mm between the surface wall and cavity wall could be had, thereby eliminating most of the resistances offered to the penetrometer shaft during impact penetration. The calculations to this effect have been made (Fig. 4). In these calculations, a safety factor of two has be~n applied to take into account the bulging and collapsing effects, which may cause some resistance to the shaft. The results indicate that the reduced diameter shaft type penetrometers require only about 16% of the weight of those required for uniform diameter type penetrometers. However, before developing the reduced diameter type penetrometer for greater depth capabilities, some experimental laboratory studies should be made under controlled conditions to study the response of this type of
42
Appl. Ocean Res. 1980, Vol. 2, No. 1
penetrometer to different test conditions, and to observe its stability during the penetration event. The results provide a tentative guideline for optimizing the diameter of field penetrometers for weight, penetration depth and buckling load requirements (Fig. 4). Detailed design and development of mechanical and electrical systems is needed before the reduced diameter type penetrometer could become an effective tool. CONCLUSIONS Sea trials of the free fall penetrometer described above prove it to be a useful equipment for obtaining the in situ strength profiles, depth and location of different layers. Penetration up to 4 m is easily possible in its present stage of development and in the future, it may be possible to achieve the penetration up to 15 m with some modification in the design. The entire operation of deploying, testing and retrieving the instrument takes only about 20 min. The instrument could ultimately become an inexpensive tool for reconnaissance of vast areas, especially of large tracts such as those for offshore pipe lines. In order to attain a higher degree of confidence, additional field and laboratory tests are being planned in conjunction with static penetration tests under diversified soil conditions. REFERENCES 1
2 3 4
5 6 7 8
9
10
II 12 13
14 15
Noorany, I. 'Underwater soil sampling and testing a state-ofthe-art review', Syrup. Underwater Soil Sampling, Testing, and Construction Control, STP 501, American Society for Testing and Materials, 1971, pp. 3 41 Dayal, U. 'Recent trends in underwater in situ soil testing', J. Oceanic Eng. 1EEE, 1978, 3, 176 Nayak, B. U., 'Offshore Site Exploration', Proc. GEOCONINDIA, Conf Geotech. Eng. 1978, lI, 136 Anderson, V. C., Clinton, J., Gibson, D. K. and Kirstan, O. H. Instrumenting RUM for in situ sub-sea soil survey, Syrup. Underwater Soil Sampling, Testing and Construction Control, ASTM STP 501, American Society for Testing and Materials, 1971, pp. 212 228. Kraft, L. M., Ahmad, N. and Focht. J. A. 'Application of remote vane results of offshore geotechnical problems, Proc. 8th OJJshore Technol. Conj. 1976, OTC 2626, pp. 77 96. Richards, A. F. and Keller, G. H. A plastic barrel sediment corer, Deep Sea Res., 1961, 8, 306 Richards, A. F. and Parker, H. W. 'Surface coring for shear strength measurements', Proc. Conj. Civil Eng. Oceans I, ASCE, San Francisco, 1967, pp. 445~488 Doyle, E. H., McClellan& B. and Ferguson, G. H. Wire line vane probe for deep penetration measurements of ocean sediment strength, Proc. 3rd Offshore Technol. Conf. 1971,1, OTC 1327, pp. 21 27 Richards, A. F., McDonald, V. J., Olson, R. E. and Keller, G. H. In place measurement of deep sea soil shear strength, Symp. Underwater Soil Samplino, Testin 9 and Construction Control, STP 501, American Society for Testing and Materials, 1971, pp. 55 68 Ferguson, G. H., McClelland, B. and Bell, W. D. Seafloor cone penetrometer for deep penetration measurement of ocean sediment strength, Proc. 9th Offshore Technol. Conf. 1977, OTC 2787, pp. 471 480 McClellan& B. Trends in marine site investigations: a perspective, Proc. Conl~ OJ]shore Europe, Aberdeen 1975, 20 pp. Ruiter, J. D. and Fox, D. A. Site investigations for North Sea Forties Field, Proe. 7th Offshore Teehnol. Conj'. 1975, OTC 2246, pp. 21 36 Zuid berg, H. M. Seabed penetrometer tests, Proc. Fugro Syrup. on Penetrometer Testing, The Hague, 1972
Dayal,U. and Allen, J. H. Instrumented impact cone penetrometer, Can. Geotech. J. 1973, 10, 397 Dayal,U. and Allen, J. H. The effectof penetration rate on the
F r e e f a l l penetrometer: a performance evaluation: U. Dayal
16 17 18 19 20
strength of remoulded clay and sand samples, Can. Geotech. J. 1975, 12, 336 Dayal, U. Instrumented impact cone penetrometer, Ph.D. Dissertation, Memorial Univ. of Newfoundland, Canada 1974, 222 pp. Preslan, W. L. Accelerometer-monitored coring, Proc. Conf. Civil En9 Oceans II, ASCE, 1970, pp. 637-641 Chari, T. R., Smith, W. G. and Zielinski, A. Use of free fall penetrometer in seafloor engineering, Conf. Record, Ocean 78 IEEE-MTS Conf. 1978, p. 4 Jones, J. H. The use of an impact penetrometer in remote sensing study of the sea bed properties, 2nd CSCE Hydrotech. Conf. Amherst 1976 McNeill, R. L. Rapid penetration of terrestrial materials - - the state of the art, Proc. Conf. on Rapid Penetration of Terrestrial Materials, Texas A and M Univ., College Station, 1972, pp. 11126
21 22 23 24 25 26
Murff, J. D. and Coyle, H. M. A laboratory investigation of low velocity penetration, Proc. Conf. on Rapid Penetration of Terr. Mater., Texas A and M Univ., College Station, 1972, pp. 319-359 Thompson, L. J. Dynamic penetration of selected projectile into particulate media, Sandia Laboratory Report SC-R-R-66-376, Albuquerque, New Mexico, 1966, 194 pp Dayal, U., Allen, J. H. and Jones, J. M. Use of an impact penetrometer for the evaluation of the in situ strength of marine sediments, Marine Geotechnol. 1975, 1, 73 Meyerhof, G. G. The ultimate bearing capacity of wedge shaped foundations, Proc. Fifth Int. Conf. Soil Mech. Found. Eng., Paris, 1961, 2, 105 Keller, G. H. Engineering properties of some sea-floor deposits, J. Soil Mech. Div., ASCE 1969, 95, (SM6), 1379 Korites, B. J. A numerical technique for estimating the response of a gravity corer penetrating a marine sediment, J. Terramechanics, 1969, 6, (4), 35
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