Application of piezometer probes to determine engineering properties and geological processes in marine sediments

Applied Clay Science, 4 (1989) 337-355 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

337

Application of Piezometer Probes to Determine Engineering Properties and Geological Processes in Marine Sediments R.H. BENNETT', H. LI 1, J.T. BURNS*, C.M. PERCIVAL2 and J. LIPKIN3

'Naval Ocean Research and Developrnent Activity, John C. Stennis Space Center, MS 39529-5004 (U.S.A.) 2Sandia National Laboratories, Albuquerque, N M 87185 (U.S.A.) 3Sandia National Laboratories at Livermore, Livermore, CA 94550 (U.S.A.) (Received July 18, 1988; accepted after revision January 9, 1989)

ABSTRACT Bennett, R.H., Li, H., Bums, J.T., Percival, C.M. and Lipkin, J., 1989. Application of piezometer probes to determine engineering properties and geologicalprocesses in marine sediments. Appl. Clay Sci., 4: 337-355. Singie-sensor piezometer probes, 8 mm in diameter were developed and tested for deep-ocean geotechnical investigations in support of the Subseabed Disposal Program. Two probes were tested in a hyperbaric chamber pressurized to 55 MPa (8000 psi) during a scaled (0.28:1 ) simulation experiment conducted at the David Taylor Naval Ship Research and Development Center (DTNSRDC) in Annapolis, Md. Testing was performed for 30 days with the probes inserted in reconstituted illitic marine sediment. Small differential pore-water pressures were generated in response to both mechanically and thermally generated forcing functions. The piezometers sensed very small (approximately 1.7 kPa [0.25 psi] ) pore-water pressure events during the process of carrying out other experimental objectives. The pressure sensors exhibited excellent sensitivity and stability during other deep-ocean simulated laboratory pressure tests for periods of up to 750 h. In addition to the measurements of ambient and dynamic pore-pressure response to environmental forces, the piezometer test data can be used to derive the in-situ undrained shear strengths and permeabilities of seabed sediments. The piezometer probe technology is providing a quantitative means of assessing important geotechnical parameters of fine-grained seabed deposits.

INTRODUCTION A d e e p - o c e a n p i e z o m e t e r p r o b e was d e v e l o p e d for m a k i n g in-situ p o r e - w a t e r p r e s s u r e m e a s u r e m e n t s a t n o m i n a l o c e a n d e p t h s of 6000 m a n d at seafloor s u b b o t t o m d e p t h s of 1.0 m. T h e p i e z o m e t e r t e c h n o l o g y a n d m e a s u r e m e n t s were a n integral p a r t of t h e I n Situ H e a t T r a n s f e r E x p e r i m e n t ( I S H T E ) (Percival *This paper is dedicated to the memory of the late John T. Bums.

0169-1317/89/$03.50

© 1989 Elsevier Science Publishers B.V.

338 et al., 1980), developed in support of the U.S. Subseabed Disposal Program ( SDP ). The SDP objectives were to examine the feasibility of emplacing highlevel nuclear waste in fine-grained clay formations located in deep, tectonically stable regions of the world ocean basins (Hollister et al., 1981 ). ISHTE was planned for a one-year period in the central-north Pacific Ocean on the sea floor using a recoverable platform and a 400-W isotropic heat source which was to be implanted to approximately a one-meter subbottom depth in the illitic sediments. The response of the sediment to the thermal field produced by the heater during deep-ocean and laboratory simulation tests was evaluated using various in-situ measurement devices including piezometer probes, thermal sensors, a vane shear device and a pore-water sampler. Measurement and careful assessment of the pore pressures developed during ISHTE would be critical because excess pore pressure reduces the strength of the sediment and effective stress, and water flow will occur away from the heat source as the thermal gradients develop around the heat probe. Pore pressure increases can cause time-dependent changes in the physical and mechanical properties of the sediment and thus affect the integrity of the system's "isolation" capacity. A comparison of the in-situ measured responses, with the predictions of numerical models for thermal, mechanical, and chemical behavior of the sediment, was planned to evaluate the applicability of the modeling tools being developed to assess the feasibility of subseabed disposal (Percival et al., 1980). A major step in developing the technology needed to field ISHTE was the completion of a scaled laboratory experiment (ISIMU:ISHTE SIMULATION) designed to simulate environmental conditions that are expected for the field experiment (Percival, 1982). ISIMU was conducted at the David Taylor Naval Ship Research and Development Center (DTNSRDC) in Annapolis, Md, during November and December 1981. This laboratory experiment was carried out using a cylindrical container filled with approximately 1 m 3 of remolded, reconsolidated, illitic sediment and placed inside a large pressure vessel capable of maintaining 55 MPa at 4°C for approximately one month. A calculated scale of approximately 0.28:1 allowed adequate thermal equilibrium to be reached within one month. The simulation experiment was used to test essentially all of the components of ISHTE. The sediment was recovered by dredge hauls from the central-north Pacific. The ISHTE test site (MPG-1) conformed to SDP specifications (Percival, 1982). The dredged sediment was reconstituted in a rotary mixer until a homogeneous slurry was formed. The pore-water salinity was approximately 34.2 %0. The test sample was formed by building the sample in layers, first adding a slurry layer approximately 20 cm thick, and then applying a vacuum to each layer to remove trapped air. Reconsolidation, over a 10-week period, was achieved to simulate porosities of the deep-ocean sediments (Silva et al., 1983). The tank, sediments, and emplaced instrumentation were pressurized to 55 MPa at 4 ° C and experiments were run for one month. Additional details of the

339

simulation test can be found elsewhere (Percival, 1982; Silva et al., 1983; Bennett et al., 1985). This paper reviews the performance of the piezometer probes under high pressure (55 MPa) for a period of 30 days and presents selected significant pore-water pressure events that occurred during ISIMU with emphasis on response of the pore pressure to the induced thermal gradients. Procedures and methods used to derive in-situ undrained shear strengths and permeabilities are discussed. These methods and technologies can now be readily applied to study geological processes and sediment geotechnical properties in the marine environment. ISIMU DEEP-OCEAN PIEZOMETER PROBE: INSTRUMENTATION The piezometer probe design specifications and the types of materials required were established on the basis of I S H T E experimental objectives and expected environmental conditions. Limitations were placed on the probe size (that portion of the probe inserted in the sediment) in order to minimize perturbations in the thermal field induced by the heat source. The required precision of the pressure sensors was 0.34-0.69 kPa in order to achieve experimental goals. Materials and electronic components were selected on the basis of their ability to withstand rather severe environmental conditions such as: high ambient hydrostatic pressure (68.9 MPa), high-temperature differentials (approximately 296°C with a range of 4-300°C) along the 196 cm length of the probe, strong electrolyte in contact with materials (approximately 35 parts per thousand seawater salinity), potential for thermogalvanic corrosion, and the predicted small changes in sediment pore-water pressures both spatially and temporally. These factors were considered the most important in the design of the probe, although other environmental factors were studied, such as soil strength, biofouling, oxidation/reduction potential, and probe proximity to other instruments. The piezometer probe consists of an 8 m m diameter titanium tube that attaches to a tip having a cone angle of approximately 5.3 ° (Bennett and Faris, 1979; Bennett et al., 1982a). A porous stone, which allows pore-water pressure to be transmitted to the pressure sensor, is fastened between the titanium tube and the probe tip (Fig. 1). Pore pressure is transmitted through the porous stone to an internal tube fastened to the positive side of the pressure sensor. The differential pressure sensor is pressure balanced by a similar internal tube that runs from the negative side of the pressure sensor to the top of the porousstone retainer but is isolated from the porous stone. The pressure sensor is enclosed in a stainless-steel housing which is pressure-compensated to in-situ hydrostatic pressure (Fig. 1). The stainless-steel pressure-sensor housing is physically separated from the titanium by high-dielectric polycarbonate material. The total lengths of the piezometer probes can be changed depending upon

340 POROUS STONE

~

TUBING

TO PRESSURE

TRANSDUCER (POREWATER)

DI F F ERENTIAL PRESSURE

TRANSDUCER

CONE A N G L E = 5.3 ° TI

IUM I TUBING TO PRESSURE TRANSDUCER (.HYDROSTAT IC)

j~

"~'-"~-~POLYC A RBO N AT E HYDROSTATIC J P R E S S U R E PORT

SIGNAL j CONDUCTING ~ CABLE

CAVITY FOR ELECTRONICSIGNAL CONDITIONER

O

i

196c m

~"STAIN LE S S STEEL 5cm

!

F

"l

Fig. 1. General mechanical configuration of the deep-ocean piezometer probe developed for the In Situ Heat Transfer Experiment (ISHTE).

the experimental design objectives for ISHTE. Because of limitations in the probe diameter, only one pore-pressure measurement at a preselected depth below the sediment-water interface (mudline) is possible with each piezometer probe. This limitation is imposed on the probe design because of the requirement to restrict the maximum probe diameter to 8 mm. The piezometer system electronics consist of three main components: pressure sensor, signal conditioner, and interface. Solid-state signal-conditioning electronics are enclosed (at atmospheric pressure) in a stainless-steel capsule and located directly above the pressure-sensor capsule (Fig. 1 ). A variable reluctance differential pressure transducer measures excess pore-water pressure directly (differential above hydrostatic). Variable reluctance pressure transducers were laboratory-tested at high hydrostatic pressure {68.9 MPa) over a period of 750 h to determine sensor characteristics and long-term stability (Bennett et al., 1982b). The pressure sensors exhibited a zero shift during pressurization but displayed excellent long-term stability and linearity under high pressure. The zero shift may be eliminated in future transducers by "matching" transducer components at the factory. Pressure-sensor integrity and stability are tested in three phases: (1) at atmospheric pressure prior to high-pressure tests; (2) at 68.9 MPa for both long and short periods of time; and (3) at atmospheric pressure following high-pressure tests. Additional details of the piezometer instrumentation can be found elsewhere (Bennett et al., 1982b, 1985).

341 G E O T E C H N I C A L T E S T S ON T W O I S I M U C O R E S

Two cores (PC-5 and PC-12) were recovered from the ISIMU tank and shipped to NORDA for testing and evaluation of geotechnical properties. The results from grain-size analyses I). The particle-size subsamples.

indicate that the sediment

distribution

of the material

is a silty clay (Table

is very consistent

between

TABLE I G r a i n size analysis (in % ) Interval (cm)

Gravel (>2 mm)

Sand

Silt

Clay (<2/lm)

A. I S I M U 1-3 36-38 58-60

PC-5 1.1.1 0 0

0.04 0.05 0.01

32.4 32.0 31.8

66.4 68.0 68.2

B. I S I M U 3-5 35-37 60-62

PC-12 0 0 0

0.03 0.06 0.03

31.5 31.6 29.5

68.5 68.5 70.5

*1Manganese nodule. T A B L E II Consolidation analysis, core PC- 12 Load (TSF)*I

e

av (cm2/g)

my (cm2/g)

Cv (cm2/s)

k (cm/s)

1/64 1/32 1/16 1/8 1/4 1/2 1 2 4 8 16 32

2.56 2.48 2.41 2.37 2.17 1.93 1.70 1.48 1.28 1.07 0.88 0.67

6.29 X 10 -3 1.70× 10 -3 1.34 × 10 -3 1.06 × 10-3 1.68 X 10-3 9 . 6 × 10 -4 4.8X 10 -4 2.3 X 10 -4 1.0× 10 -4 5.5 × 10 -5 2.4 × 10 -5 1.4 X 10 -5

1.81X 10 -3 4.94X 10 -4 3.93 X 10 -4 3.18 X 10 - 4 5.37 × 10 -4 3.3 X 10 -4 1.8X 10 -4 9.4 × 10 -5 4.5X 10 -5 2.7 X 10 -5 1.3 X 10 -5 8.6 × 10 - s

2.8X 10 -8 5 . 4 × 10 -3 4.4 × 10 -3 3.3 X 10 -3 2.0 × 10-3 1.9 X 10 -3 2.2 X 10 -3 2.4 X 10 -3 2 . 4 × 10 -3 2.1X 10 -3 2.0X 10 -3 1.5 X 10 -3

5.2 × 10 -6 2.7X 10 -6 1.8 X 10 -6 1.1 × 10 -e 1.1 X 10 -6 6.4 X 10 -7 4.1X 10 -7 2.4 X 10-v 1.1 × 10 -7 5.9 X 10 - s 2.7 X 10 -8 1.3 X 10 - s

Section 15-18 cm; water c o n t e n t w = 94.3 %; specific gravity G = 2.75; initial void ratio eo = 2.60; initial porosity n = 72.2%; Cv based on x / T method; u n i t weight of sea water 7w= 1.024; wet u n i t weight of sample 7t = 1.48 g/cm3; initial degree of saturation S = 99.6%. *I(TSF X 95.8=kPa).

342 TABLE III Consolidation analysis, core PC-5 Load (TSF)

e

a~ (cm2/g)

my (cm2/g)

Cv (cm2/s)

k (cm/s)

1/32 1/16 1/8 1/4 1/2 1

2.46 2.44 2.41 2.31 2.08 1.84

8.19 × 4.26X 4.92X 8.52 × 9.42X 4.79X

2.37 X 10 -4 1.24 X 10 -4 1.44× 10 -4 2.58X 10 -4 3.06X 10 -4 1.68X 10 -4

8.5 X 10 -3 8.2 X 10 -3 6.6X 10 -3 4.8X 10 -3 2.1 X 10 -~ 2.6X 10 -:~

2.1× 10 -6 1.1 × 10 -6 9.7X 10 -7 1.3X 10 -6 6.7X 10 -7 4.5X 10 -7

10 -4 10 -4 10 -4 10 -4 10 -4 10 -4

Section 15-18 cm; water content w=89.7%; specific gravity G=2.76; initial void ratio eo=2.48; initial porosity n = 71.2%; Cv based on x / T method; unit weight of sea water 7w= 1.024; wet unit weight of sample 7t = 1.50 g/cm3; initial degree of saturation S = 99.9%.

Two whole core subsamples (one from each core) were tested in back-pressure consolidometers. The sample from PC-12 was consolidated to 30.6-102 kPa (32 TSF) and sample PC-5 was loaded only to 95.8 kPa (1 TSF). Data on soil properties and consolidation parameters derived from the test are presented in Tables II and III. The test data appear to be reasonable; permeability (k), coefficient of consolidation (Cv) etc., are presented for each consolidation load and void ratio (Tables II and III ). The URI geotechnical data bases (Silva et al., 1983) compare favorably with our test results and were used in our analysis. P O R E - W A T E R P R E S S U R E R E S P O N S E TO M E C H A N I C A L AND T H E R M A L E V E N T S

Piezometer insertion data During the ISHTE simulation test, pore-water pressures were monitored with two piezometer probes placed at specific positions with respect to the heater. One near-field piezometer (designated Probe 1 ) monitored pore pressures 1.5 cm from the heater (skin-to-skin) and 16.9 cm below the mudline, which placed the center of the porous stone 10.1 cm above the center of the heater. Far-field piezometer (designated Probe 2) measurements were monitored 26.4 cm below the mudline and 34.2 cm from the heater (skin-to-skin) (Fig. 2). Each probe was inserted independently at atmospheric pressure following heater insertion. During probe insertion, sediment deformation occurs and excess pore-water pressures (Ui) are generated and reach a maximum pressure along the probesoil interface (Bennett and Faris, 1979). Both near- and far-field insertion pressures and their dissipation were determined. Saturation was ensured prior to pressurization of the sediment in the hyperbaric chamber.

343 NE~'I ;ar field Piezometer

Far field Piezometer

Seawater (4.6°C) •J, I

~/

:~APL Suooort Structure

:

! ~:

~'

1.8cm

Reconstituted Sediment ( lllite )

E E E

o

Porous Stone

5

o Center of Heater Heater

Element

34.2cm "----" '._.j

Porous Stone

Fig. 2. Deep-ocean piezometer probes (8 mm dia.) in reconstituted iUitic "Red Clay" soil during a high-pressure (55 MPa) simulation test. Diagram depicts position of piezometers in relation to heater probe.

During insertion of the probes, induced pore pressures were determined and pore-pressure decay was monitored. The induced excess pore-pressure (Ui) decay was normalized with respect to Um~ expressed as percent dissipation; plots of Ui as a function of the log of time, reveal significant differences in the decay curves (Fig. 3) when the log-fitting method from consolidation theory is utilized (Lambe and Whitman, 1969). Approximately 100 percent dissipation of induced pore pressure (tloo) is approximately 20.3 rain for the nearfield probe and 61.5 rain for the far-field probe. In addition, the induced pore pressure decay at Probe 2 lags probe insertion pressure by 0.6 min, whereas Probe 1 (near field) displays nearly instantaneous decay of pressure following probe insertion. In concert with the major differences observed in the induced pore-pressure characteristics between Probes 1 and 2, the maximum induced pore pressures generated by Probes 1 and 2 are 6.6 kPa (0.96 psi) and 12.9 kPa (1.88 psi ). The differences observed in the induced pore pressures and their respective decay characteristics suggest significant differences in the geotechnical prop-

344

0

---

Oz 20

~ . ~ e z o m e t e r ~ 2

~

<

~.

ISHTE

SIMULATION

6O

~ 8o

~ I00

t

1

i

i

i i i L ,ll ]0

. . . . . . . . TIME

.

.

.

.

.

.

~

~

5 m ~n

1 0 0 : 2 0 3rain

I 10.0

,

i

i i i ,ill

] O0

i i i iiii 1000

(MINUTES)

Fig. 3. Time-dependent dissipation of induced pore pressures ( Ui..... ) as a function of the logarithm of time for the 8-mm dia. probes. Note different decay times for the near-field (#1) and far-field (#2) probes.

erties of the sediment in proximity to the two probes. The piezometer insertion data including Um,x and the decay curves are used to derive the in-situ undrained shear strengths and permeabilities, and are discussed in the following section. Undrained shear strengths and permeabilities derived from pore-pressure measurements Existing models describing stress changes and consolidation-time effects around a pile driven into fine-grained cohesive soils and limited laboratory measurements are used to derive in-situ undrained shear strengths and permeabilities. Theory and models of interest in this study were developed earlier (Soderberg, 1962; Esrig et al., 1977; Randolph et al., 1979; Wroth et al., 1979) and will be discussed later in limited context as applied to the derivation of selected geotechnical parameters. A summary of the geotechnical data from ISIMU relevant to these analyses is found in Table IV, most of which has been abstracted from available data (Silva et al., 1982) and agrees well with data presented earlier in this report.

345 T A B L E IV Oedometer test data (abstracted from Silva et al., 1982) Core

Depth (cm)

Initial water c o n t e n t *~

Saturation (S; % )

(Wi; ~ ) HC-1 GC-05 HC-1 GC-05 HC-1 GC-05

24-28 31-35 84-88 95-99 145-149 155-159

101 101 104 108 118 115

94 98 97 94 97 100

Initial void ratio

Permeability at eo

Average coeff, of consolidation

(eo)

(k; crn/s)

(Cv; cm/s)

3.01 2.88 2.97 3.16 3.44 3.16

5.82 X 10 -6 2.39 × 10 -6 1.03 X 10-5 2.14 X 10 -6 4.05X 10 -6 1.55 X 10 -6

1.49 X 10 -3 9.96X 10 -4 5.39 X 10 -4 2.42 X 10 -4 8.90X 10 -4 6.60 X 10 -4

*1Corrected for 35 p p t salt.

General theoretical considerations During the insertion of cylindrical pipes (piles and probes) in normally and underconsolidated cohesive fine-grained soil, significant induced excess pore pressures (Ui) are generated. The time required for these excess pressures to dissipate to ambient pressure is, to a first approximation, a function of the probe (pipe) radius and the soil coefficient of consolidation, which depends upon the permeability of the material. The governing equation for the excess pore pressure for radial consolidation (cylindrical cavity expansion) is expressed as:

Ou/cgt= Cv{ ( l / r ) (O/Or) [r(Ou/Or) ]}

(1)

where u is the excess pore-water pressure, Cv is the coefficient of consolidation, t is the time, and r is the radial coordinate. The following assumptions are made in deriving eq. 1. (1) The medium (here, marine soil) is assumed to be a porous elastic matrix containing a viscous fluid (seawater). (2) Pore-fluid flow can be related to the pore pressure by Darcy's law. (3) The consolidation process assumes, primarily, radial flow of the pore fluid and radial expansion of the elastic matrix. (4) Thermal effects are not considered in the formulation. The solution of eq. 1 for a piezometer probe (or pipe) of radius ro can be obtained if the initial pore fluid pressure distribution and the coefficient of consolidation are known. For radial expansion, Ch (horizontal coefficient of consolidation) is considered approximately equal to Cv and is a reasonable assumption for surficial submarine soils having a random soil fabric (Bennett et al., 1981, 1986; Bennett and Hulbert, 1986). Two solutions were obtained (Soderberg, 1962) for the initial pore fluid pressure distributions from the pile skin ro to radial distances r, assuming the

346

soil is (1) an elastic-plastic material, and (2) a viscous material. The most important assumption in deriving the pore-pressure distribution, analytically, is that the excess pore pressure generated during probe insertion is proportional to the imposed radial stresses. Given the initial pore-fluid pressure distributions for the two assumed plain stress soil systems (elastic-plastic and viscous), Soderberg obtained solutions for the pore-pressure dissipation at a pile (probe) skin (at ro) and derived a general solution (curve) that lies approximately between the two initial solutions. Solutions for the decay of pore pressure at a probe (pile) surface were obtained (Wroth et al., 1979). U/Ur~axis plotted as a function of time T= Cvt/r2o and with um,x/Su(o) as a parameter where Su(o) is the initial value of the plane strain undrained shear strength. In this solution, the coefficient of consolidation is expressed as Cv= [k2G(1

-

v' ) ]/[Tw(1

-

2v' ) ]

(2)

where: k-- the coefficient of permeability; Yw--the unit weight of water; G = the elastic shear modulus of the soil; and v' --Poisson's ratio (drained) of the elastic soil. With solutions obtained from Soderberg (1962) and Wroth et al. (1979) and the expression of Cv in eq. 2, the coefficient of permeability can be estimated from the pore-pressure measurement obtained with piezometer probes. Based on a soil modeled as an elastic perfectly plastic material (Randolph et al., 1979), the maximum excess pore pressure at the probe surface is given by: Ui.... =Suln(G/Su)

(3)

since Ui.... is obtained from piezometer probe measurements and Su (o) can be determined in the laboratory, or from in-situ vane probe tests, the shear modulus (G) is easily calculated from eq. 3. The relationship: Ui--6Su

(4)

has been suggested for lean inorganic soils of moderate to high sensitivity (Esrig et al., 1977). The value of six (6) is considered reasonable since studies indicate that the predicted changes in radial total stress and pore pressure are relatively insensitive to the soil modulus ratio (EJSu), where Eu is the soil modulus and S, is the undrained shear strength (Esrig et al., 1977; Bennett et al., 1982a). Given a value of Ui.~x (insertion pressure) obtained from piezometer measurements, the undrained shear strength can be calculated using either eq. 3 (if the shear modulus is known) or eq. 4. The nondimensional time unit Tso can be determined for 50% consolidation, and Tso can be obtained from the piezometer probe measurements from normalized plots depicting the time-related dissipation of induced pore pressures (Bennett et al., 1979). The coefficient of consolidation can be determined by:

347

(5)

Ch ~- Cv= Tsoro2/ tso

Thus the coefficient of permeability can be estimated assuming a reasonable value for Poisson's ratio (drained). It should be noted that the assumption of a soil modeled as an elastic perfectly plastic material can be modified (Randolph et al., 1979), and if the shear modulus G is known for the material, eq. 3 is not required for the permeability estimate. Pore pressure results

Laboratory piezometer data were analyzed to obtain estimates of the in-situ undrained shear strength Su, permeability k, and shear modulus G using the above relationships for the prediction of pore fluid pressure generation, dissipation, and soil strength for piles driven into cohesive soil. Piezometer observations for the ISHTE simulation test are presented in Table V. The timedependent dissipation of pore pressure for the 0.4-cm radius probes is depicted in Fig. 3. Using the relationship Ui = 6Su for the determination of undrained shear strength suggested earlier (Esrig et al., 1977), values were calculated for the ISIMU soil. These values are compared with both in-situ vane probe measurements and laboratory miniature vane tests (Table VI). Vane shear probe measurements for the ISIMU soil have been discussed by others (Babb and Silva, 1983; Silva et al., 1983). Reasonably good agreement is observed between the calculated and measured undrained shear strengths (Table VI). The differences observed, however, in the induced pore pressures and their respective decay characteristics for the ISHTE piezometers suggest significant differences in the geotechnical properties of the soil in proximity to the two probes. The significantly lower induced pore pressure of Probe 1 and its rapid decay compared with observaTABLE V Piezometer observations: I S H T E simulation Probe

Piez. #1 (near-field) Piez. # 2 (far-field)

Water depth (m)

Length of data recorded (rain)

Sensor *l depth below mudline (cm)

Porous filter

Induced insertion pressure (kPa)

(psi)

~ 1

75

16.9

Corundum

6.6

0.96

~ 1

300

26.4

Corundum

12.9

1.88

* 1Differential pressure transducers.

348 TABLE VI Calculated versus measured shear strengths: ISHTE simulation Probe

Piez. #1 Piez. #2

Depth of porous stone below m.1. (cm)

U~ (psi)

16.9 26.4

0.96 1.88

Su (calc.) Ui/6

Su (meas.) *~ (kPa; psi)

Depth below mudline (cm)

Test type

2.2; 0.33 2.6; 0.38

17 26.5

in-situ vane in-situ vane

(psi) 0.16 0.31

Su = undrained shear strength; Ui/6 = calculated undrained shear strength. *Wane measurements determined 20 h after heater insertion and 19-21 cm from heater {pretest unheated; Silva et al., 1982).

tions from Probe 2 (Fig. 3) indicates (1) a reduction of soil strength of approximately 51% at the near-field probe compared with the far-field (Table VI), and (2) a significantly shorter drainage path at the near-field probe for the induced pore pressures to dissipate, compared with the far-field piezometer. The time delay of induced pressure prior to dissipation at the far-field probe also supports these conclusions. The severe cracking and disturbance of the soil observed at the surface during heater inserticn was probably a major factor responsible for the observed differences in the induced pore-pressure characteristics (Sandia, 1983 ). Vane shear measurements were determined at distances of 19 to 21 cm from the heater, and the values reflected in Table VI were selected from the strength profile (Silva et al., 1982), corresponding to depths approximately equal to depths ofpiezometer measurements. Vane tests were performed about 20 h following heater insertion, thus the influence of pore pressures generated from heater insertion were not significant. Also, vane measurements were sufficiently distant from the heater to eliminate the influence of pore pressures caused by the heater. Using eq. 3 for the estimation of Ui..... for the ISHTE simulation soils and comparison with pore-pressure measurements indicated agreement to within + 1% of the observed data (David McTigue, pers. commun., 1983). These data indicate reasonably good agreement between the predictive capabilities of the models and observed measurements. Knowing U i . . . . and Su from piezometer and vane shear measurements, the shear modulus G was calculated (Tables VI and VII) for use as an input parameter to eq. 2. Assuming a drained Poisson ratio of u' = 0.3, coefficients of consolidation Cv ~ Ch were calculated from eq. 5 using piezometer data (induced pore-pressure decay, normalized with respect to Ui..... as a function of the logarithm of time t) (Fig. 3). The derived values for the coefficient of consolidation and permeability for the 0.4 cm radius probes are given in Table VII (compare with Table VIII) using both models (Soderberg, 1962; Wroth et al., 1979). Derived values for undrained shear strength using piezometer insertion pressures and the models are in reasonable agreement with in-situ and

349 TABLE VII Estimate of the coefficientof permeability (k) and coefficientof consolidation (Ch) for the 0.4cm radius piezometerprobe Site units

Pressuresensor depth below m.1. (cm)

Shear modulus Estimated values undrained shear strength (Wrothet al., 1979) G/S,(O) Ch.1 h (cm2/s)

Piez.#1 16.9 (near-field) Piez.#2 26.4 (far-field)

18.5

(cm/s)

(Soderberg,1962) Ch

k

(cm2/s)

(cm/s)

2.19×10 -3 1.50×10 -7 2.44×10 -3 1.68×10 -7

132

2.05×10 -3 1.69×10 -v 5.13X10 -4 4.24×10 -s

*ICh~- C v.

TABLE VIII Measuredvaluesofthe coefficientofpermeability (k), coefficientof consolidation(Ch),and shear modulus-strengthratio (G/S,) Coefficientof consolidation C~ (cm2/s)

Permeability k (cm/s)

Shear modulus G/S.

2.8 to 17.2x 10-4 7.5X10 -4 average

2.6 to ll.6X 10-7 5.5X10 -7

15.3.1 --

"1G=72.0 kPa, S,=4.7 kPa (Silva et al., 1982). laboratory measurements (Table VI). Additional tests on various soil types would enable refinement of the solutions and models (Bennett et al., 1986). The estimated values of the coefficients of consolidation and permeability are in the same range as laboratory results (Tables VII and VIII) for tests performed on I S H T E simulation soil samples (Silva et al., 1983 ). It should be noted t h a t the estimated values for k and Ch depend on the solutions of the consolidation equation. To a first approximation, reasonable agreement is indicated between the models and the observed piezometer measurements. Additional research of the material properties (various soil types) and more piezometer measurements are required in order to refine the analytical solution of the consolidation models ( B e n n e t t et al., 1986). Eq. 1 would undoubtedly require modification for consolidation problems involving significant heat flow or a partially saturated soil (gas-liquid mixture). Additional models and equations would need to be developed to solve

350 similar problems regarding estimates of undrained shear strength, permeabilities, and related geotechnical parameters. Pore pressure response to thermal field

Following pressurization of the ISIMU sediment tank including piezometers and other instrumentation (November 11, 1981), heater power was initiated to induce thermal gradients in the illitic sediments (Percival, 1982). The positions of the piezometers with respect to the heater probe are depicted in Fig. 2. The power selected was 115 W which generalled a maximum temperature change of approximately 206 ° C. Excess pore pressures in the near-field (piezometer #1 ) responded reaching a maximum of approximately 4.8 kPa (0.7 psi). The far-field piezometer detected excess pore pressures after approximately 240 min {240-300 min) (Fig. 4). Following stabilization of the thermal field (steady state approximately 211 ° C ) the near-field pore pressures diminished and approached "stable" excess pressure while the far-field pore pressure increased and also "stabilized" with a slight excess pore pressure (Fig. 4). The steady state temperature of 211 ° C fell short of the predicted maximum temperature of 280 ° C at the heatersediment interface. In order to reach the required temperature level, a power of 160 W was generated which produced a peak temperature of 291 °C. This increase in heater power generated a smaller change in temperature ( A T = 80 ° C, power @ 160 W, steady state temperature 291 ° C). Excess pore pressures followed the same general trends (Fig. 5 ) with m a x i m u m excess pressures reaching approximately 1.4 kPa (0.2 psi ) above the steady state excess pore pressure established during the first stage of heater power which induced thermal gradients of 211 °C. The temperature isotherms established following stabilization of the thermal field at a power level of 160 W are depicted in Fig. 6. POWER 115 WATTS ~,T --- 206"C STEADY STATE - 211"C PIEZ. #2

~:

if

I

~= 10 - :

FAR FIELD

HEATER ON *" ~-

"-~_~.... --~---~-----------"EAH nE'O

~[

a'o

UO = 0.7 psi

6;

9;

,~o

1;o 1~o

)I

.......... """'"~/.

l\

2~o 24o4~a .........

ml 11 NOV 1981

24;0 ..........

:~92o

TIME (minutes)

Fig. 4. Pore-pressure response to induced thermal field as measured by ISIMU-I (power of 115 W).

351

,, "f

\\

PIEZ.#I

T ".÷

1/

ixe ~ 0.17 psi

0, ÷~- . . . .

"~"

AT = 80"C STEADY STATETEMP.291"C

11 NOV 1981

. . . .

I . . . . 50

0

I . . . . 100

I . . . . 150 TIME (minutes)

I . . . . . . . . . . 200 260

I 2503

Fig. 5. Pore-pressure response to induced thermal field as measured by piezometers #1 and #2 during ISIMU-I (power of 160 W). HEATER

MUDLINE

o~...

t

14" \ --

\

I~\.

N~/

..

I

l~'~c 501 ~ 0

"\ \ "<.\ ~ \

\.

\

,

/./. /

/

\

\

,

/

"

R A D I A L D I S T A N C E FROM HEATER CENTER

50

(cm)

Fig.6. Steady stateisothermsgeneratedby 160 W of heaterpower. Maximum temperatureof heater190°C.Dots areactualmeasuredvalues. The thermal fields (temperature history) for piezometers #1 and #2 locations (center of porous filters)were determined for the heater power of 115 W and 160 W . These thermal fieldswere estimated by C. Hickox (personal communication and letterto R. Bennett, Sept. 7, 1982) by interpolation from measured temperatures at specificlocations around each piezometer in the I S I M U test tank. The data reveal that the pore pressure and thermal response at the near-fieldpiezometer is very rapid (Figs. 4 and 5) in response to heater power of 115 and 160 W . The pore pressures (u) began to decay a few hours after the

352

heater power had been turned on despite the continued gradual increase in the temperature field. The pore pressure and thermal response at the far-field (piezometer #2) location was considerably slower than that at the near-field (piezometer #1 ) location (Figs. 4 and 5). A theoretical analysis of the pore pressure response to the induced thermal field based on linear theory for porous, thermoelastic materials was carried out using the ISIMU data (McTigue and Gartling, 1986 ). Comparison of the data with model predictions of the pore pressure rise versus time indicates reasonably good agreement; however, over a longer time interval of> 100 min, the model (ibid.) underestimates the pore pressure (Ue). Pore pressure response to distal events

The near-field piezometer measured small pore pressure perturbations in the near field during the final stages of ISIMU. A m o m e n t a r y loss of heater power (on the order of i min ) followed by "power up" caused a slight surge in pore pressure (approx. 1.4 kPa [0.2 psi] ) near the heater and piezometer #1 (Fig. 7 ). A very slight positive pressure (increase in ue) was observed (Fig. 7) between reference time 30 and 45 min. The pore pressure again stabilized to its initial value. No effect was felt by the far-field piezometers. At about 1 h and 27 min (Fig. 7 ), the pore-water sampler extracted interstitial water which created a negative pore pressure ( - 2 . 8 kPa [ - 0 . 4 psi] ) detected by piezometer #1 (near field). A slight residual negative pressure was maintained and at 2 h and 25 rain the vane shear probe was retracted from the sediment which again produced a negative pore pressure which reached a peak at 2 h and 30 min. When the heater power was turned off after approximately 3 h (for absolute times see A P L - U W Report, 1982), the sediment temperature began to decrease with a corresponding decrease in pore pressure as measured by piezometer #1 (Fig. 7). These data show the high sensitivity of the piezometer probes as a function of small rather subtle changes in the pressure field. The sensitivity of these piezometer pressure sensors is estimated to be approximately _+0.069 kPa ( _+0.01 psi) based on experimental data. POREWATERSAMPLER (POREWATER EXTRACTION) LOST HEATER VANE SHEAR REFRACTION POWER -0.8

-0.4

I

°,

,

,

0~- L + 4 ,~

i

1T

~_~

' /,

,

"\ ,

~

,,

,/ ,/,

TO = TIMezeno ":~

L

7. P i e z o m e t e r

,-1

/"~-

TO I

Fig.

/

TO

HEATER OFF

I

I

~¢1 r e s p o n s e

I

I

I

I

to dist~L] pore-pressure

l

I events.

I

I

I

I

I

I

353 CONCLUSION

The high pressure (55 MPa) ISHTE Simulation ( I S I M U ) t e s t provided a unique opportunity to test the deep-ocean piezometer probes for a period of one month. The data indicated excellent stability and sensitivity of the system's components, and measured capability for the duration of the experiment. The test clearly emphasized the significance of pore pressure measurements to the applications of subseabed disposal of nuclear waste. Determination of the in-situ pore pressures provides a quantitative means of assessing the consolidation state of seabed deposits, time-dependent changes in the effective stress, the permeability and undrained shear strength, and other in-situ soil properties. Pore-pressure response to induced thermal gradients at 115 and 160 W of power showed a rapid rise in excess pressure (ue) followed by a fall in Ue, compared to a very stable longer-term rise in the surrounding temperature of the sediments. The piezometers displayed excellent sensitivity (_+ 0.069 kPa) to not only positive pore-pressure perturbations but also to negative pressures caused by pore-water extraction and retraction of surrounding probes from the sediment. Theoretical modeling of the pore-pressure response to induced thermal gradients is being conducted at Sandia National Laboratories in order to improve existing theory and predictability of the behavior of seabed deposits. These investigations have demonstrated the feasibility of making precision deep-ocean piezometer (pore pressure) measurements. The technique has application and importance to (1) engineering problems and the derivation of important soil properties, and to (2) the study of geological processes in deep-sea sediments. ACKNOWLEDGEMENTS

This project was funded by Sandia National Laboratories, Subseabed Disposal Program, Albuquerque, NM, in support of the In Situ Heat Transfer Experiment. Partial funding for the first and second authors was provided by the Naval Ocean Research and Development Activity. The authors appreciate the assistance of F.L. Nastav and P.J. Burkett in preparing figures, tables and proofing the manuscript. D. Lambert and G. Romero completed the geotechnical tests on cores PC-5 and PC-12. The authors acknowledge the effort of C. Hickox in providing the thermal data and analysis and the data provided by D. McTigue during the modeling of the pore-pressure response to the thermal fields. Critical review of an early version of this manuscript by P.J. Valent is appreciated. The late John T. Burns made significant contributions to the development of both shallow-water and deep-ocean probes from 1974 through 1988. John played a major role in fielding of the instrumentation and working cooperatively with engineers and scientists from various highly respected industrial firms, government agencies, and universities.

354 REFERENCES APL-UW Engineering Report, 1982. ISHTE Simulation. Applied Physics Laboratory, University of Washington, Seattle, WA, 60 pp. Babb, J.D. and Silva, A.J., 1983. An in situ vane system for measuring deep sea sediment shear strength. OCEANS '83, Marine Technology Society/IEEE Conference Records, San Francisco, CA. Bennett, R.H. and Faris, R.J., 1979. Ambient and dynamic pore pressures in fine-grained submarine sediments: Mississippi Delta. Appl. Ocean Res., 1 (3): 115 - 123. Bennett, R.H. and Hulbert, M.H., 1986. Clay Microstructure. International Human Resources Development Corporation (IHRDC) Boston, Houston, London, 161 pp. Bennett, R.H., Bryant, W.R. and Keller, G.H., 1981. Clay fabric of selected submarine sediments: fundamental properties and models. J. Sediment. Petrol., 51 (1): 0217-0232. Bennett, R.H., Burns, J.T., Clarke, T.L., Faris, J.R., Forde, E.B. and Richards, A.F., 1982a. Piezometer probes for assessing effective stress and stability in submarine sediments. In: Marine Slides and other Mass Movements. Plenum, New York, N.Y., pp. 129-161. Bennett, R.H., Burns, J.T. and Lambert, D.N., 1982b. Fabrication and testing of deep-ocean piezometer system and components for the Sandia Subseabed Disposal Program (SDP). In: Subseabed Disposal Program Annual Report, January-September 1981, Vol. II. Appendices, Pt. 1. Sandia National Laboratories, Albuquerque, NM, SAND 82-0664111, pp. 643-646. Bennett, R.H., Burns, J.T., Nastav, F.L., Lipkin, J. and Percival, C.M., 1985. Deep-ocean piezometer probe technology for geotechnical investigations. IEEE J. Ocean. Eng. OE-10(1): 1722. Bennett, R.H., Li, H., Valent, P.J., Lipkin, J. and Esrig, M.I., 1986. In situ undrained shear strength and permeabilities derived from piezometer measurements. In: R.C. Chaney and K.R. Demars (Editors), Proc. Symp. Strength Testing of Marine Sediments - Laboratory and In Situ Measurements. ASTM Spec. Publ. STP, 883: 83-100. Esrig, M.I., Kirby, R.C. and Bea, R.G., 1977. Initial development of a general effective stress method for the prediction of axial capacity for driven piles in clay. 9th Annual Offshore Technology Conference, OTC 2943, Houston, TX, 2-5 May, pp. 495-501. Hollister, C.D., Anderson, D.R. and Heath, G.R., 1981. Subseabed disposal of" nuclear wastes. Science, 213: 1321-1326. Lambe, T.W. and Whitman, R.V., 1969. Soil Mechanics. Wiley, New York, N.Y., 553 pp. McTigue, D.F. and Gartling, D.K., 1986. Numerical Simulation of Thermally-Driven Pore Pressure Rise in the ISHTE Simulations. (Informal letter report, March 31, 1986, Sandia National Laboratories), 14 pp. Percival, C.M., 1982. Laboratory simulation of deep-ocean in situ heat transfer experiment. In: OCEANS-82. Proc. Annu. Conf. Marine Technol. Soc., Sep., pp. 679-684. Percival, C.M., McVey, D.F., Olson, L.O. and Silva, A.J., 1980. In situ heat transfer experiment (ISHTE): The Decade of the Oceans. In: Proc. Annu. Conf. Marine Technol. Soc., Oct., MTS/ IEEE Conf. Rec., pp. 567-573. Randolph, M.F., Carter, J.P. and Wroth, C.P., 1979. Driven piles in clay - the effects of installation and subsequent consolidation. G~otechnique, 29 (4): 361-393. Sandia, 1983. The Subseabed Disposal Program: Status Report. SAND83-1387, Sandia National Laboratories, Albuquerque, NM, 179 pp. Silva, A., Criscenzo, S.J., Jordan, S.A. and Babb, J.A., 1983. ISHTE Annu. Rept. No. 2, 19811982. URI Geotechnical Program of the In Situ Heat Transfer Experiment URI Rept., Feb. 1983, Narragansett, RI. Silva, A.J., Criscenzo, S.B., Jordan, S.A., Babb, J.D. and Levy, W.P., 1982. URI Geotechnical Program of the In Situ Heat Transfer Experiment. Sandia Annual Report, SAND 82-0664/ 11, pp. 649-687. Silva. A.J., Jordan, S.A. and Criscenzo, S.J., 1983. University of Rhode Island Technical Report

355 of Simulation Experiment for In Situ Heat Transfer Experiment Project. Subseabed Disposal Program, University of Rhode Island, Kingston, RI, 51 pp. Soderberg, L.O., 1962. Consolidation theory applied to foundation pile time effects. G~otechnique, 12: 217-225. Wroth, C.P., Carter, J.P. and Randolph, M.F., 1979. Stress changes around a pile driven into cohesive soil. In: Recent Developments in the Design and Construction of Piles. Institution of Electrical Engineers, Savoy Place, London, pp. 255-264.