instrumented impact cone penetrometer

Instrumented Impact Cone Penetrometer

Ocetrtl Et~gitrt,eritlg Grolrp, Ftrc111t.y qf Etlgitleeritlg rrtltl Applirtl Sciet~ct,, Met,~orinl Urli~,er.sity of Ne~vjiolrt~tllot~d, S t . Johtl ' .~, Nen:fiorrtrtNtrtld

Received January 29, 1973

Accepted February 15, 1973

The present paper describes the development of an instrumented impact cone penetrometer for a direct measurement of in sitlr strength properties of a soil target. The developed penetrometer, in addition to providing acceleration signatures (as obtained by previous investigators), is capable of recording cone thrust and local side friction simultaneously and continuously. The procedures have been outlined for estimating it1 si~rr strength properties and soil type of the target materials throughout the penetrated depth from the output records of these sensors. Typical test results generated from an on-going experimental program aimed at providing the data for (I) understanding the penetration mechanism. (2) development of penetration theory, and (3) designing the penetrometer for field tests, i.e. it1 sitrr testing of ocean floor soils, are also presented.

L'article decrit le developpement d'un penetrometre i impact instrument&, destine a la mesure directe de la resistance it1 sit11 d'un sol-cible. Le penktrometre realis6 permet I'enregistrement, non seulement des profils d'accCICration (tels qu'obtenus par des etudes antirieures), mais aussi de la resistance en pointe et du frottement lateral de f a ~ o n simultanee et continue. On presente les methodes d'interpretation des don- nees fournies par I'appareil, pour determiner les caractCristiques de resistance it1 sit11 et le type de sols sur toute I'tpaisseur de sol-cible traversee. On presente tgalement les rCsultats typiques d'essais obtenus dans le cadre d ' ~ l n programme en cours dont le but est (1) la comprChension du mecanisme d e penetration, (2) le developpement d'une thtorie de la penetration, (3) le design d'un pknetrometre de chantier applicable a I'etude en place des sols marins. [Traduit par le journal]

Introduction The problem of impact penetration into ter-

rcstrial materials is a classical problcm of terminal ballistics. Historically, this problem was attempted as early as the 15th Ccntury and an immense amount of literature has becn pub- lished on both theoretical and experimental aspects. The existing knowledge on impact penetration phenomena and the various dif- ficulties cncountered in this field have been summarized by Dayal (1972).

The major objective of the earlier research was to find the depth of penetration of an impacting projectile to provide the passive pro- tection against bombing or shelling for person- nel and underground installations (Robcrtson 1941 ) . Reccnt work has highlighted many possible avenues of engineering applications (McNeill 1972); among these arc applications for obtaining in situ strength properties of inaccessible terrestrial soils (Sandia Labora- tories 1968) , lunar soils (Anon. 1966) , and ocean sedimcnts (Scott 1967) .

With the advancement of electronics the present day penetrometers are instrumented

with accelerometers and are a useful tool in tracing the velocity profile and depth of penetration of the Denetrometer. To estimate the soil

I

strc~lgth properties from thc acccleromctcr signatures various thcorctical mcthods, based on Ncwton's Second Law of Motion, have bcen proposed; for cxamplc, Wang ( 197 1 ) ; Schmid ( 1969) ; and Hakala ( 1965). Thompson ( 1966) has proposed a morc cornprehcnsive theoretical approach by considering thc con- tinuity cquation, cquation of state, and consti- tutivc soil relationships. In thcsc considerations, howevcr, the target material has becn assumcd to bc homogeneoi~s isotropic, consisting of cither comprcssiblc elastic, incomprcssible plastic, or visco-plastic. Unfortunately, soil dcposits generally do not satisfy most of thc above propcrties, consequently the availablc thcorctical rclationshius have little value from a practical point of view. To overcome this, attempts have been made to categorize the soil strength parameters bascd on the experimental results. But the limitations of such attempts are Icgion. In each program the penetrometer has been of various dimensions

Can. Geutceh. J . , 10, 397 (1973)

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C A N . GEOTECH. J . VOL. 10, 1973

GROOVE FOR HOLDING THE PROBE

TO RECORDING SYSTEM

- - - - -ACCELEROMETER

-.-. ... .. - - LEAD WEIGHT (JP TO 4 OF APPROXIMATELY lo:a EACH )

----- WEIGHT CARRIAGE

-- -- PENETRATION ROD ( 3 6 MM 0.D - AND I

MM 1.D )

-STRAIN GAGES

2 _---FRICTION SLEEVE (150 CM)

6 0 CONE WITH I 0 SO. CM. BASE

[A) CROSS - SECTIONAL VIEW OF PENETROMETER

I CONICAL POINT 2 LOAD CELL 3 STRAIN GAGES

- 4 FR.CT.Oh S-EEVE 5 ADJUSTMENT RING 6 WATERPROOF BJStilNG 7 CABLE 8 CONNECTION WITH RODS

(B) DETAILS OF LOAD CELL AND STRAIN GAGES

ARRANGEMENT (LFTEK DE RUITER 1971)

(NOT TO SCALE)

FIG. 1. D e t a i l s of laboratory impact p e n e t r o m e t e r .

and frequently of different shapes and thus making a comprehensive interpretation very difficult. To obtain, in the laboratory, the varia- bility and combinations generally obtained in nature is a formidable task. The results to date indicate that it is not possible to obtain in situ strength of soil from the acceleration signature alone.

To fill this gap the authors have put forward a new instrumentation system by which the characteristic soil strength properties can be measured directly from an impact penetrometer test. The main eniphasis in this work is to develop the capability of the proposed instrument to measure the b z situ shear strength properties of the top 10-20 ft (305-610 cm)

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IIAYAL. A N D A L L E N : C

of marine sediments. The present papcr deals with the development of such a device which has demonstrated its usefulness during preliminary tests conducted in the laboratory on several different types of soils. The instrument is capable of recording acceleration during the free or forced drop, and deceleration, cone thrust at its tip, and local side friction simultaneously and continuously during impact penetration.

Impact Penetrometer Design The criteria for selection of the penetrometer

used in this investigation are based on the considerations of:

1. The ease with which the penetrometer could be handled and adapted to different environments. In particular the ability to work from a relatively small ship in adverse sea conditions and without special handling gear.

2. The sensitivity of the penetrometer response to variation in in sitzc soil properties with depth.

3. The ability to analytically evaluate penetrometer output.

The resulting instrument in its laboratory form is shown in the Figs. 1 and 2. The con- ceptual view of the proposed penetrometcr to be used for in situ strength testing of marine sediment is shown in Fig. 3. The operating principle of this instrument is very similar to a free falling or triggered corer (Preslan 1969) with velocity on impact being 15-20 ft/s (457.5-610 cm/s) when triggered 30 f t (915 cm) above the sea bed.

Ii7str~ii77eritation The impact penetrometer has been instru-

mented with three sensors: accelerometer, cone load cell, and friction sleeve load cell.

Acceleronzeter The accelerometer is housed inside the pene-

trometer and mounted in the direction of the penetrometer axis at approximately weight carriage height to facilitate the load cells wiring connecting to the recorder. The accelerometer used in this investigation is of the following specifications: 'Endevco' Model 2262-25 Serial No. AA47; Type, piezoresistive accelerometer; Rated range, * 25 g; Frequency response, DC to 750 Hz; Sensitivity, 19.87 MV/g (at 10 VDC excitation).

I N E P E N E T R O M E T E R 399

FIG. 2. Photograph of laboratory impact penetrometer.

Load Cells The load cells used for the impact pene-

trometer are the ones developed by 'Fugro' for the static cone penetrometer and a detailed designing description of which has been given by de Ruiter (1971). The following are the specifications of the adopted model of the

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CAN. GEOTECH. J . VOL. 10, 1973

WlRE TO SHIP

;ih-- CAST BRONZE WlRE CLAMP

SAFETY PIN - fuk

DEPTH - OPTIONAL )

f n

PRESSURE POWERED PIN I ARMS TRIGGER AT PRESET

TO PILOT WEIGHT -

RELEASE

FOR FREE

MECHANISM

FALL

I 1 STABlLlZER 8 ACCELEROMETER AND RECORDING CASE

EIGHT LEAD CABLE CONNECTED TO RECORDER

LEAD WEIGHTS 4 5 L B EACH ( FOUR STANDARD 9 UP TO SIX

MAY BE USED)

STEEL COUPLING ( A QUICK MECHANISM FOR CHANGING

I !Bl CORER 8 PENTROMETER)

PENETRATION ROD (36 M M 0 . D . AND 1-27 M M I. D. FIRST STAGE = SFT LENGTH'

STRAIN GAGES

LOAD C E L L 2

FRICTION SLEEVE (150 CM ) CONE WITH '0 So.CM BASE

FIG. 3 . Marine impact penetrometer.

'Fugro' penetrometer: Cone-Diameter of cone, Outer diameter, 1.4 in. (36 mm) ; Area of 1.4 in. (36 mm); Basc area of cone, 1.55 in."leeve, 23.25 i n . V l 5 0 cm2); Rated range, (10 cm2) ; Cone angle, 60"; Rated range, 2204 1653 lb (750 kg) ; Sensitivity, 305 lb (1 38.3 lb (1000 kg); Sensitivity, 194.4 lb (88 kg)/ kg)/MV at 10 VDC excitation. (Both load MV (at 10 VDC excitation). Friction sleeve- cells were recalibrated in the laboratory.)

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DAYAL A N D A L L E N : C O N E P E N E T R O M E T E R

SWITCH T O ENERGIZE RELAY

28 VDC (TO DROP PENETROMETER) 5 A

POWER SUPPLY 28V RELAY SOLENOID

ACCELEROMETER / PENETROMETER

SIDE FRICTION STRAIN GAGES

i

Recording System The output signals of accelerometer, cone

load cell, and friction sleeve load cell were initially recorded by means of a two channel 'Textronic' oscilloscope and a single channel storage scope equipped with polaroid camera. However, the existing recording arrangement has been modified by using a high speed mag- netic tape recorder to provide a better flexibility and reproduction facility; in addition the recorded tape can be fed directly to the computer for processing of the data.

DROP HEIGHT

POWER SUPPLY

STORAGE SCOPE

OSCILLOSCOPE

TEST CONTAINER

DIFFERENTIAL AMPLIFIER

FIG. 4. Experimental set-up for laboratory testing.

taneous free fall and simultaneously starting the recording system.

Laboratory Test Facility The testing facility provides a system by

which the various independent and dependent variables can be measured under fully con- troIled conditions. The present program is aimed at providing a rational approach for understanding the penetration phenomena and for studying the 'influence of various soil parameters on the penetration mechanism. Figure 4 shows the schematic diagram of the experimental set up being used in laboratory impact penetration test.

Triggerirlg Systenz Thc pcnctromctcr is triggered with the help

of a rcmotcly controllcd rclay system. The system includcs a 28 V rclay solenoid which works at 5 A currcnt. A switch has been pro- viclcd to cncrgizc thc rclay for giving instan-

Independent Variables ( I ) lnzpact Velocity As mentioned earlier, the final form of the

instrument would be used for measuring the in situ strength properties of marine sediment wherc the impact velocity will range from 15 to 20 ft/s (457.5 to 610 cm/s) for normal free fall height, the scope of the present laboratory investigations is limited to low velocity impact penetration. The impact velocity is controlled by the free-fall height of the penetrometer.

(2 ) Weight The laboratory penetrometer is being de-

signed to provide a varying weight system ranging from 15 to 45 1b (6.8 to 20.4 kg) so that a preselected weight can be used in particular tests.

Dependent Variables In the laboratory investigations a wide

variety of dependent variables can be measured; for example the deflection of the soil surface, the amount and distribution of ejecta, deformation pattern produced within the soil target, shear strength, unit weight, and moisture content of the target material.

Target Materials The preliminary tests have been performed

on different typcs of soils ranging from gravelly

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402 CAN. GEOTECH. J . VOL. 10, 1973

sand through to clay. A few tests have also Interpretation of Test Results been performed on artificially prepared layered Typical penetrometer output records for soil targets. The study includes the application gravelly sand and layered soils are shown in of impact penetration test results for estimat- ~ i ~ ~ . 5 and 6, respectively. ~~~~~i~~~~~ were ing the soil type on lines similar to the friction carried out in soils ranging from gravelly sand ratio concept developed for static cone penetra- to artificially prepared mud and layered soils tion tests. The friction ratio for the 'static' case for impact velocities between 17.1 alld 18.5 has been defined Schmertmann as ft/s (522 and 564 cm/s). For these soils, the dimensionless ratio of unit friction/ad- sleeve friction, cone thrust, deceleration, and hesion along the smooth steel friction jacket to velocity profiles are plotted in ~ i ~ ~ . 7, 8, 9, and unit bearing capacity of the standard cone 10, respectively. point and a definite relationship has been shown (Begemann 1965) to exist between cone (1) Penetration Resistance resistance, local side friction, and soil type. During the impact penetration, the pene-

trometer is subject to resistance due to 'dy- Processing of Penetrometer Output Records namic' soil bearing capacity all the way from The penetrometer provides the following entry until the conc~usion of penetration. Well

time dependent parameters of target material established formulae are available for estimat- at the moment of impact right down to full ing the bearing capacity for 'static' loading, penetration depth. (1 ) Acceleration/decelera- however, no formula is known to the authors to tion, ( 2 ) Cone thrust, and ( 3 ) Sleeve friction. be adequate for 'dynamic' loading. Further-

From the recorded acceleration/deceleration more, the behavior of soil under dynamic time history, penetration velocities and depth loading is still not well understood. It is be- are by numerical integration at lieved that the 'dynamic' case at low velocity is suitable time intervals (a time interval of 6.25 similar to the 'static' case. It is therefore de- ms has been adopted in the present analysis sirable to examine al] possible factors which which will further be improved in subsequent C O L ~ ~ cause differences between the 'static' and analysis by computerized calculations). ~t the 'dynamic' bearing capacities of soils at these instant of impact the time and penetration OW penetration velocities. The present dis- depth are assigned zero values whereas the cussion is confined, however, to the two major velocity assigned is that of the impact velocity factors: ( 1 ) Modes of failure; and ( 2 ) strain calculated from the lneasured free-fall height rate effect On characteristic strength parameters. neglecting air resistance, This can be verified These are believed to be directly associated from the accelerometer record. Using the cal- with the interpretation of impact penetrometer culated time vs. penetration depth relationship test results. the depth vs. cone thrust and depth vs. sleeve (I) Modes o f Failure friction profiles are obtained from recorded Thompson (1966) has performed both two- time vs. cone thrust and time vs. sleeve friction and three-dimensional model impact penetra- relationships by cross calculation. Similarly tion tests and full scale impact penetration tests depth vs. deceleration, and depth vs. velocity on sand and gelatine target materials to study profiles are also obtained from the time vs. the failure pattern. The motion of soil particles deceleration and time vs. velocity relationships, and penetrometer has been studied from high respectively. The obtained depth vs. sleeve speed photographs with penetrometer velocities friction profile, is modified by subtracting 0.22 ranging from 200 to 800 ft/s (6096 to 24384 ft ( 7 cm) for each calculated depth because cm/s). The motion of the target in the two- of the geometry separating the cone and sleeve. dimensional tests as well as the length of the The present penetrometer therefore enables the surface cracks and surface deformation in the following target characteristics to be computed. full scale tests indicate that the phenomena of 1. Depth of penetration. 2. Provides depth vs. impact penetration are primarily one of shear de- sleeve friction profile, depth vs. cone thrust formation. There appears to be a shear front, de- profile, depth vs. penetration velocity profile, fined as the line bounding the zone in which no and depth vs. acceleration profile. shearing of medium has occurred, traveling with

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DAYAL AND ALLEN: CONE PENETROMETER

FIG. 5. Impact penetration tost results for gravelly sand

the penetrometer. The shape of the leading edge of the front seems to be a log spiral in the case of a blunt-nosed projectile penetrating a half space. Similar observations have been reported by Colp (1965) and Chou (1972) from steady penetration tests performed on simulated cohesionless and cohesive soils, respectively. Fur- ther, on the basis of two-dimensional test photographs, Thompson ( 1966) has indicated that at any instant of time, the displacement of the particle is very similar to the displacement initially approximated by Prandtl for the in- dentation of an infinitely long footing or a punch into a half space of rigid-plastic weight- less materials. Though their study indicates that the shear front is similar to the log spiral surface observed in static penetration, it is not known if the shape is related to target materials and penetrometer shape and velocity.

Colp ( 1965 ) , Thompson ( 1966), Caudle et al. (1967), and McNeill (1972) believe that mounds or craters are formed during high velocity impact penetration and consequently no side wall resistance is offered to the pene-

trometer, whereas, based on a simple math- ematical model Murff and Coyle (1972) indicate that side wall resistance is a significant portion of the total resistance in clay. For sand and sand clay mixture, this effect is present but is less significant. It is believed that a critical velocity exists for all soils and for impact velocity above this critical value, cavitation occurs. Where cavitation, defined as encapsula- tion of the penetrometer by a cavity, does occur, resistance is developed only by the penetrometer nose. The critical velocity is a function of both soil and penetrometer properties. The only information available in previous investigations was acceleration signatures and as such no definite conclusion could be drawn whether or not the side resistance is developed along the side of the penetrometer. However, in the present investigation it is possible to record for the first time both cone thrust and local side friction during impact penetration, this will give an indication of the modes of failure and also leads to an assessment of the magnitudes of the cavitation effects.

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CAN. GEOTECH. J . VOL. 10, 1973

TARGET - TWO LAYERS 10 5 1267crn) THICK CLAY

LAYER OVERLYING COMPACTEDGRAVELLY SAND I 1 I I 1

FIG. 7. Plot of penetration depth vs. unit local side friction.

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DAYAL AND ALLEN: CONE PENETROMETER

FIG. 9. Plot of penetration depth \IS. acceleration.

The friction ratios as defined earlier are calculated by taking the ratio of unit side friction to unit bearing capacity of cone point for four different types of target materials and the average values are shown in Table 1. These ratios have been obtained beyond the penetration depth of four times the diameter of the penetrometer to take into account the geometry

of the penetrometer and the free surface effect on punching (Meyerhof 195 1 ) . Though these ratios have been obtained by impact penetration tests, they are in remarkably good agreement with the information reported by Begemann ( 1965) and Schmertmann ( 1969) from static cone penetration tests for the similar range of soils. Using some simplifying assump-

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CAN. GEOTECH. J. VOL. 10, 1973

VELOCITY ( f t / s I -

FIG. 10. Plot of penetration depth vs. V.

TABLE 1. Friction ratio

Friction ratio Type of soil (%)

Mud 10 Clay powder 5 Sand 2.5 Gravelly sand 1 . 5

tions one can arrive at a range of friction ratios similar to those reported herein by using any of the classical bearing capacity theories available for 'static' loading (for example see Begemann 1965). Thus, from the friction ratios obtained for different types of target materials from impact penetration tests which fall within the range of 'static' penetration values, it can be presumed as a first approximation that the failure modes during impact penetration are basically of a 'static' nature. It may be noted that at this stage of discussion the strain rate effect on the friction ratio has not been considered but it will be shown later that this will not alter the result significantly under the low impact velocity.

The friction sleeve records indicate the presence of adhesion/friction along the surface of the penetrometer. This clearly demonstrates that cavitation does not occur for the velocity

range used in the present investigation. Further- more, the side wall resistance is a significant portion of total resistance in both cohesive and granular soils.

( 2 ) Strain Rate Eflect The question of strain rate sensitivity on the

shear strength of soil arises wherever thc carth structures are loaded or deformed suddcnly. Taylor ( 1948), Casagrande and Shannon (1949), and Whitman (1957) have found that the strength of clay increases somewhat under dynamic loading while only a slight increase in sand was reported. Hampton and Yoder (1958) have also demonstrated a significant increase in the strength of clay soil in a rapid test, this increase is as much as 160% in loosely compacted samples. The experiments on sand by Whitman and Healy (1962) have shown that the value of the internal friction angle ( 4 ) changes by 2-3% over a loading speed up to 8.3 ft/s (254 cm/s). Schimming et al. (1966) reached the same conclusion, stating that "dynamic effects are minimal" for cohesionless soils. Based on the experimental results of Casagrande and Wilson (1951), Crawford ( 1959), Osterberg and Perloff (1960), and from his own results Peck (1962) indicates that as a first approximation, the change in the strength of many clays and clay

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D A Y A L A N D A L L E N : ( ZONE P E N E T R O M E T E R 407

soils can be represented by the following em- pirical equation:

where Sd = 'dynamic' shear strength a t strain rate a d , and

Ss = shear strength a t standard o r 'static' strain rate 0 , .

Also from the theory of rate process it has becn found that the logarithm of the strain rate is directly proportional to the shear strength of clay (Mitchell 1964).

Based on the preceding discussion it is believed that the strain rate effect on the friction angle ( 4 ) is insignificant for the velocity range considered here. On the other hand in fine grained soils strength is influenced by the rate of straining. As such, the soil strength properties obtained by the impact penetration test for clay soil are modified for the condition of 'static' penetration by applying Eq. [I] . It should be born in mind however that Peck's (1962) relationship is the ratios of failure strengths at 'static' and 'dynamic' strain rates. In this case velocity fields are unknown and consequently strain rates are also unknown. If, however, the strains for 'static' and 'dynamic' cases are the same then the ratio of the strain rates is equal to the ratio of 'static' and 'dynamic' velocities and Eq. [ I ] can be rewritten as :

s* - ss -- v, - 0.10 log,, - Ss v s

where I/, = 'Dynamic' velocity, and T/, = Standard o r 'static' velocity.

This relationship reveals that for a 10-fold increase in penetration velocity the increase in recorded strength would be 10% whereas for a 100-fold increase in penetration velocity the strength will increase by 20%. The 'dynamic' strength estimated by this relationship is well within the values suggested by Whitman ( 1970) for cohesive soils.

Applying Eq. [2], the 'static' sleeve friction values for a mud target have been calculated from the recorded 'dynamic' sleeve friction value. In this calculation the value of V,, at any particular depth is taken from the depth vs.

velocity plot (Fig. 10) and the value of V , equal to 0.0575 ft/s (1.75 cm/s)-the generally accepted driving speed in static penetration test is 1.5 to 2 cm/s (de Ruiter 1971 ). The modified 'static' sleeve friction values, thus calculated at different depths, are plotted in Fig. 7. The unit sleeve friction value in static penetration tests is generally regarded as a measure of the shear strength of cohesive soils (Begeimann 1965).

As the effect of strain rate on friction angle is insignificant for the low velocity range, thc friction ratio obtained for granular soils by the impact penetration test should be of the same order as those obtained from the static penetration test. In the case of cohesive soils, the strain rate effect gives the following expression for the friction ratio.

Friction ratio (dynamic) =

v, Ss 1 + 0.10 log,, - v s

1 + 0.10 log,, ?) + ? Z

where Nc = Bearing capacity factor, 7 = Unit weight of soil, and Z = Depth of penetration.

For the laboratory situation and many field penetrations less than 15 ft (457.5 cm) -yZ can be ignored, allowing the expression to be simplified to:

Friction ratio 1 Friction ratio - - - - - (dynamic) Nc (static)

The experimental results obtained so far are in agreement with the arguments expressed above.

(2) Layered Soil McNeill (1972) in his paper entitled "Rapid

penetration of terrestrial materials-The-state- of-the-art", emphasized that "in the present time the analysis of a layered system is not understood . . . .". In this investigation, tests have been performed on laboratory prepared layered soils. The instrument is equally sensitive for layered soil and an example has been given in Fig. 6 ( a 10.5 in. (26.7 cm) thick moist clay layer overlying gravelly sand). A sudden change in deceleration, cone thrust, and sleeve friction records clearly demonstrates the

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408 CAN. GEOTECH. J. VOL. 10, 1973

presence of the layered soil. Analysis of the data gave the following results:

(1 ) Depth of top layer = 10.7 in. (27.2 cm); ( 2 ) Avcrage unit cone thrust for top layer

= 5 194 lb/ft"2.59 kg/cm" ; (3 ) Average unit sleeve friction for top layer

= 224 1b/ft2 (0.1094 kg/cm" ; (4) Average unit cone thrust for bottom

layer = 28 000 lb/ft2 (1 3.18 kg/cm" ; and (5) Average unit sleeve friction for bottom

layer = 401 Ib/ft"O.196 kg/cm2). The friction ratio for the first layer equals

4.66%; friction ratio for the second layer equals 1.43 % .

Also, from Table 1, the soil type gives estimated friction ratios for the top and bottom layers as 5 and 1.5, respectively.

(3) Sorne Additional Points Wortlzy of Note ( 1 ) The penetrometer continues to accelerate

for some depth where the soil penetration resistance is less than the penetrometer weight.

(2 ) The penetrometer then decelerates until it reaches a maximum depth of penetration; the increase in deceleration being sharper in coarse materials than in fine.

( 3 ) On completion of penetration the penetrometer moves or bounces until damping eventually brings it to a stop. This can be observed on accelerometer records as well as on thc friction sleeve records.

Summary and Conclusions The objective of this study is to develop an

impact penetrometer capable of providing shear strength characteristics of marine sediments. The present paper gives details of an instrument which laboratory tests indicate is capable of achieving this objective. More detailed laboratory and field tests are continuing.

However, the following conclusions can be drawn from the present investigation.

(1) The concept of an impact/projectile penetration test, for measuring i ~ z sitii strength properties of a soil target can be achieved from the present instrument.

(2 ) The response of the penetrometer can be directly related to the strength governing parameters of target material.

(3 ) Experimental results indicate that failure modes during impact penetration are basically of a 'static' nature and cavitation does not occur

for the tested velocity range. Furthermore, the friction ratio concept developed from static cone penetration tests for assessing soil type can be extended to impact penetration tests also.

(4 ) The penetrometer has been shown to be sensitive in estimating depth, strength, and soil type for different layers.

Acknowledgments The authors wish to acknowledge the support

of this work from a National Research Council of Canada Negotiated Grant to the Faculty of Engineering at Memorial University of New- foundland to initiate work in Ocean Engineer- ing. The first author is in receipt of a Memorial University Fellowship. Our thanks are also ex- ~ressed to Dr. M. Bruce-Lockhard and Dr. J. I

Jones for their assistance and advice in development of the instrumentation.

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