Free-fall Penetrometer Research Articles

Pore Pressure Dissipation Induced by High-Velocity Impacts of a Portable Free-Fall Penetrometer in Clays

AbstractIn situ pore pressure dissipation in soil primarily depends on the coefficient of consolidation and permeability of the soil. This study represents an initial effort to investigate pore pre...

Numerical simulation of a free fall penetrometer deployment using the material point method

Free Fall Penetrometer (FFP) testing consist of a torpedo-shaped body freefalling into a soil target. The use of this type of device is becoming popular for the characterization of shallow sediments in near-shore and off-shore environments because it is a fast, versatile, and non-expensive test capable of recording acceleration and pore pressures. In recent years, the data analysis advanced considerably, but the soil behavior during fast penetration is still uncertain. Hence, there is a need to develop numerical models capable of simulating this process to improve its understanding. This paper proposes a numerical framework to simulate the deployment of an FFP device in dry sands using the Material Point Method (MPM). A moving mesh technique is used to ensure the accurate geometry of the FFP device throughout the calculation, and the soil-FFP interaction is modelled with a frictional contact algorithm. Moreover, a rigid body algorithm is proposed to model the FFP device, which enhances the performance of the computation and reduces its computational cost. The sand is simulated by using two constitutive models, a non-associate Mohr-Coulomb (MC) and a Strain-Softening Mohr-Coulomb (SSMC) that reduces, exponentially, the strength parameters with the accumulated plastic deviatoric deformation (Yerro et al., 2016). Variable dilatancy, which reduces as a function of the plastic strain, is also taken into account, and the strain-rate effects have been evaluated in terms of peak friction angle. In general, the behavior predicted by the MPM simulations is consistent with the experimental test. The results indicate that the soil stiffness has a big impact on the deceleration time-history and the development of a failure mechanism, but less influence on the magnitude of the peak deceleration and the penetration depth; the soil dilatancy reduces the FFP rebound, and the FFP-soil contact friction angle and the peak friction angle are highly linked to the peak deceleration.

Evaluating the sealing potential of young and thin mass-transport deposits: Lake Villarrica, Chile

Abstract Subaqueous mass-transport deposits (MTDs) can be important elements in hydrocarbon systems, forming potential reservoirs or seals. Most research has targeted outcrops or moderately to deeply buried MTDs and, therefore, the petrophysical properties of near-seafloor MTDs, and their influence in the trapping and release of shallow fluids, is poorly studied. Here, we investigate shallow MTDs in Lake Villarrica (Chile) by combining sub-bottom profiles, free-fall penetrometer data, pore pressure dissipation tests and geotechnical properties of sediment cores. Low undrained shear strength under a surficial MTD indicates underconsolidation caused by sudden loading and rapid sealing. Larger, buried MTDs show acoustic signatures of free gas at their base, indicating effective sealing. This is supported by degassing core gaps just below MTDs and by excess pore pressure ratios c . 30–70% within MTDs. Acoustic windows below rafted blocks suggest local fluid escape. MTDs exhibit elevated undrained shear strength and reduced porosity compared to surrounding sediments, but are comparable to upslope source sequences. This suggests that MTD sealing capacity in Villarrica relates to the apparently overconsolidated nature of the slope sequence, leaving a minor role for shear densification. This study shows that shallow MTDs can form a relatively rapid seal for fluid migration, locally degraded by rafted blocks.

Coupled sedimentological and geotechnical data analysis of surficial sediment layer characteristics in a tidal estuary

In situ geotechnical testing of surficial sediment layers in areas of active sediment dynamics can provide essential information about physical and geotechnical variations of sediment properties, which have the potential to contribute to engineering activities in subaqueous environments. Portable free fall penetrometers have been used for the rapid geotechnical investigation of subaqueous sediment layers. However, data analysis methods and interpretation are still hampered by a lack of understanding of surficial sediment layer mechanics. Field measurements were conducted at four different locations along the York River Estuary in Virginia, USA. A portable free fall penetrometer was deployed 45 times, and complementary sediment samples were collected. The collected samples were tested and analyzed to obtain vertical profiles of bulk densities and grain size distribution through the upper seabed, X-ray images of sediment texture, and the erodibility of the sediment surfaces. The data were investigated regarding the impact of sedimentological characteristics on the in situ geotechnical behavior of surface sediments. With regard to processes and forces governing the resistance against the penetrating object at the surface of the seabed, the role of soil drag and soil buoyancy was explored. The results indicated that soil buoyancy has a limited influence on the sediment resistance, but soil drag seemed to have a significant impact, especially in the case of loose sand or soft mud top layers. The erodibility of surficial sediments appeared inversely proportional to the geotechnical sediment strength, providing empirical insight into a possible correlation of sediment strength and erodibility. In conclusion, a coupled analysis of geotechnical and sedimentological properties of surficial layers allowed insights into a potential transition from fluid-like to soil behavior of the uppermost seabed surface layers, and potential relations between sedimentological characteristics and in situ geotechnical properties.

Estimating in situ relative density and friction angle of nearshore sand from portable free-fall penetrometer tests

The friction angle of sand in the nearshore zone of Cannon Beach, Yakutat, Alaska, was estimated from the deceleration measured by a portable free-fall penetrometer (PFFP) at 72 test locations. A correlation between the relative density and PFFP’s maximum deceleration was developed from controlled PFFP deployments into sand of different relative densities. Two approaches were tested: (i) a correlation between relative density and friction angle and (ii) bearing capacity theory. For the former, laboratory vacuum triaxial tests were performed to adjust an existing correlation between relative density and friction angle for the tested nearshore sediments. In situ peak friction angles were then determined using this adjusted correlation and estimates of relative densities. The resulting in situ relative density and friction angle varied between 32%–88% and 44°–56°, respectively. Two bearing capacity–based methods suitable for shallow penetrations were tested. For this approach, equivalents of static cone resistance were determined from the measured decelerations considering the strain rate effect. A range of empirical strain rate coefficients K = 0.1–1.5 were tested. A K value between 0.2 and 0.4 yielded matching results between the two approaches. The estimated friction angles agreed well with expected values and may be applied to problems of sediment transport or early site assessment.

Investigation of spatial and short-term temporal nearshore sandy sediment strength using a portable free fall penetrometer

In order to investigate the effect of wave processes on the geotechnical characteristics of topmost sediments, a Portable Free Fall Penetrometer (PFFP) was deployed in the energetic nearshore zone of the U.S. Army Corps of Engineers' Field Research Facility (FRF) in Duck, NC. A total of 335 deployments were conducted from the FRF's 560 m long pier over six non-consecutive days in September and October of 2016. During the surveying period, significant wave heights varied between Hs = 0.8–2.4 m measured at a water depth of 17.4 m. The results showed that the sediment strength is affected by the wave climate. The derived average maximum quasi-static bearing capacities (qsbc), which reflects sediment strength, during low energy wave periods (September 22–24 and 26; Hs = 0.8–1.3 m) ranged between 67 and 73 kPa with maximum qsbc values of 120–136 kPa, while the average values were 47–59 kPa with a maximum qsbc of 82–98 kPa during the high energy wave periods (October 5–6; Hs = 2.4 m). Results also showed that the sediment strength decreased in the shallow water depth regions in spite of the increase in the particle size distribution, due to the increasing wave impact on the seabed within the shallow water region. The area where seabed strength variations were affected by the wave action became wider in the cross-shore direction as the wave height increased. Results further showed that the strength of the topmost sediment layers varied spatially along the profile and temporarily with variations in significant wave height. This likely reflects the impact of waves on sediment strength.

A Simple Effective Stress Approach for Modeling Rate-Dependent Strength of Soft Clay

This paper proposes a simple effective stress method for modeling the strain rate-dependent strength behavior that is experienced by many fine-grained soils in offshore events when subjected to rapid, large strain, undrained shearing. The approach is based on correlating the size of the modified Cam-Clay yield locus with strain rate, i.e., yield locus enlarging or diminishing dependent on the strain rate. A viscometer-based method for evaluating the needed parameters for this approach is provided. The viscometer measurements showed that strain rate parameters are largely independent of water content and agree closely with data from a previous study. Numerical analysis of the annular simple shear situation induced by the viscometer shows remarkable agreement with the experimental data provided the remolding-induced strength degradation effect is accounted for. The proposed method allows offshore foundation installation processes such as dynamically installed offshore anchors, free-falling penetrometer, and submarine landslides to be more realistically analyzed through effective stress calculations.

In situ undrained shear strength characterization using data from free fall ball penetrometer tests

Large-deformation finite element analyses (FEAs) employing coupled Eulerian-Lagrangian technique are performed to simulate penetration of a spherical free fall penetrometer (FFP) in clay. FEA results are analyzed following the principle of energy conservation. Although final depth of penetration dp varies with penetrometer mass, diameter and impact velocity, bearing capacity factor at depth dp is a unique function of dp normalized with penetrometer diameter. A simple methodology is proposed for predicting in situ undrained shear strength profile using FFP test data. Shear strength values predicted using the proposed methodology are in excellent agreement with instrumented test data reported in literature.

Rapid sediment mapping and in situ geotechnical characterization in challenging aquatic areas

AbstractYakutat Bay, Southeast Alaska, is characterized by significant spatial variations in sediment type and dynamics. The northwestern side is supplied by sediments from the nearby glaciers, and is affected by longshore sediment transport processes, while the southeastern side has no direct sediment input, and is affected by human activities. In situ seabed investigations can be difficult, and expensive, due to logistical challenges in such remote locations. A portable free fall penetrometer (PFFP) was deployed 149 times along 16 transects in water depths of 2–48 m. The deceleration and pore pressure records during the probe's penetration into the seabed were used to characterize the surficial sediments. Equivalents of quasi‐static bearing capacity were determined using the deceleration‐depth signatures, and yielded strong variabilities ranging from 5 to 107 kPa at sediment depths of 10.3–41.9 cm. Correlating the PFFP results to visual field observations and literature, a regional classification scheme, and an updated sediment distribution map were derived. The pore pressure response was correlated to the different sediment types, and was used to assess the sediment's consolidation state. At the northwestern side, an increasing pore pressure trend indicated underconsolidated cohesive sediments. At the southeastern side, clayey sediments appeared to be more consolidated except of sediments of high organic content near the populated areas. The use of the pore pressure measurements represents a novel way for rapid sediment characterization using PFFP. The presented approach to create rapidly a regional sediment classification scheme offers a time‐ and cost‐effective method to derive seabed sediment maps in areas of difficult access and logistics.

An extended interpretation of the free-fall piezocone test in clay

Seabed strength may be determined rapidly using free-fall penetrometers as the tool is simply released from above the sea-floor and penetrates under gravity into the seabed. The speed and ease of deployment relative to conventional ‘push-in’ penetrometers is attractive, with the trade-off of more complex interpretation. This paper considers two approaches for deducing the undrained shear strength from a slender conical tipped penetrometer. The first requires as input only the vertical acceleration of the penetrometer, where the soil strength is determined indirectly by considering the various forces acting on the penetrometer and solving the equation of motion. The second determines the undrained shear strength more directly by combining tip load cell and u2 pore pressure measurements. In both cases, adjustments for drag resistance and strain-rate effects are necessary to deduce a strength compatible with that determined from the equivalent push-in penetrometer test. Application of both methods to a serie...

Variability of in situ sediment strength and pore pressure behavior of tidal estuary surface sediments

Tidal estuaries feature spatially and temporally varying sediment dynamics and characteristics. Particularly, the variability of geotechnical sediment parameters is still poorly understood, limiting the prediction of long-term sediment stability and dynamics. This paper presents results from an in situ investigation of surficial sediments (≤50 cm) in a tidal estuary in New Hampshire (USA), using a portable free fall penetrometer. The aim is to investigate variations in sediment strength and pore pressure behavior with regard to sediment type and seabed morphology. The study also provides a detailed analysis of high velocity impact pore pressure data to derive information about sediment type and permeability. The penetrometer was deployed 227 times, and the findings are correlated to 78 sediment samples. Differences in sediment strength and type were found when transitioning from tidal flats to the deeper channels. Finer-grained sediments located predominantly on the tidal flats appeared well consolidated with noticeable and spatially consistent sediment strength (reflected in an estimate of quasi-static bearing capacity qsbcmax ~10 kPa). Sediments with higher sand content (>75%) showed more variations in strength relating to differences in gradation, and likely represent loose and poorly consolidated sands (qsbcmax ~10–55 kPa). The rate at which the recorded excess pore pressures approached equilibrium after penetration was classified and related to sediment type. The data indicate that the development of excess pore pressures upon impact and during penetration may provide additional insight into the nature and layering of bed material, such as identifying a desiccated or over-consolidated dilative surficial layer. In summary, with varying sediment grain size distributions, bulk densities and morphology, sediment strength and pore pressure behavior can vary significantly within a tidal estuary.

Application of portable free-fall penetrometer for geotechnical investigation of Arctic nearshore zone

The Arctic is currently undergoing rapid changes with regard to sea ice extent permafrost thaw, and coastal erosion. In addition to hydrodynamic processes, the sediments in the Arctic nearshore zone are affected by freeze–thaw cycles, as well as an increase of abundant suspended sediment introduced by permafrost-induced mass movements, such as retrogressive thaw slumps, and increased river discharge. During the YUKON14 expedition to Herschel Island, Yukon, in situ geotechnical testing of nearshore zone sediments was conducted using a portable free-fall penetrometer. Approximately 200 sites were tested, and four different geotechnical signatures identified and grouped. Most locations were characterized by a soft sediment top layer that exhibited a noticeably lower sediment strength than the underlying sediment. In some cases, multiple layers of different sediment strength were detected in the upper meter of the seabed surface. The results were correlated to existing sediment grain size records and backscatter information from a phase measuring bathymetric sonar. Strong spatial variations in sediment type and stiffness were observed, as well as in abundance and thickness of a top layer of very soft and loose sediment. The geotechnical signatures were correlated to site-specific hydrodynamic conditions, morphology, and vicinity to thaw slumps.

Free-Falling Penetrometers: A Laboratory Investigation in Clay

This paper presents the results of a series of laboratory model free-falling penetrometer (FFP) tests in kaolin clay. A thin-shafted FFP has been allowed to free fall and has also been pushed at a relatively slow constant rate into clay beds. Different configurations are used to examine the influences of various properties (FFP mass, tip diameter, tip shape, impact velocity, and clay-bed strength) on the rate effects in FFP tests. The rate effects are determined by comparing the dynamic and static penetration resistances measured from the free-fall and constant rate of penetration tests. The rate effects are found to be independent of the FFP mass, tip shape, and tip diameter but are influenced by the clay-bed strength and FFP velocity.

Erratum to: Sediment identification using free fall penetrometer acceleration-time histories

The previous acknowledgement of Dr. Robert A. McConnaughey failed to fully represent his contributions to this research. The unpublished FFCPT data and ground truth sediment samples discussed in this paper were acquired as part of his own research. The scientific oversight and technical planning provided by Dr. McConnaughey in his role as Chief Scientist on the 2006 Bering Sea Cruise of the NOAA ship Fairweather were also significant factors in successfully executing and concluding the field validation of the model described in this paper. This was a regrettable mistake that is hopefully resolved with this erratum.

Coupled penetrometer, MBES and ADCP assessments of tidal variations in surface sediment layer characteristics along active subaqueous dunes, Danish Wadden Sea

In-situ geotechnical measurements of surface sediments were carried out along large subaqueous dunes in the Knudedyb tidal inlet channel in the Danish Wadden Sea using a small free-falling penetrometer. Vertical profiles showed a typical stratification pattern with a resolution of ∼1 cm depicting a thin surface layer of low sediment strength and a stiffer substratum below (quasi-static bearing capacity equivalent: 1–3 kPa in the top layer, 20–140 kPa in the underlying sediment; thickness of the top layer ca. 5–8 cm). Observed variations in the thickness and strength of the surface layer during a tidal cycle were compared to mean current velocities (measured using an acoustic Doppler current profiler, ADCP), high-resolution bathymetry (based on multibeam echo sounding, MBES) and qualitative estimates of suspended sediment distributions in the water column (estimated from ADCP backscatter intensity). The results revealed an ebb dominance in sediment remobilization, and a general accretion of the bed towards low water. A loose top layer occurred throughout the tidal cycle, likely influenced by bedload transport and small events of suspended sediment resettlement (thickness: 6 ± 2 cm). Furthermore, this layer showed a significant increase in thickness (e.g. from 8 cm to 16 cm) related to periods of overall deposition. These findings imply that dynamic penetrometers can conveniently serve to (1) quantify potentially mobile sediments by determining the thickness of a loose sediment surface layer, (2) unravel sediment strength development in potentially mobile sediments and (3) identify sediment accumulation. Such data are an important complement and add a new geotechnical perspective during investigations of sediment remobilization processes in highly dynamic coastal environments.

Sediment identification using free fall penetrometer acceleration-time histories

Knowledge of physical properties of near-surface sediments is an important requirement for many studies of the seafloor. Dynamic or Free Fall Penetrometers (FFP), instrumented with accelerometers, are widely used to assess the mechanical properties of the sediment by deriving penetration resistance from the deceleration response of the probe as it impacts and embeds the seabed. Other field investigations, a priori knowledge or a very basic description of the type of sediment (such as a description of the sediment as soft, medium or hard) derived from studying the deceleration response (accelerometer-time histories) are used for sediment identification prior to the application of an appropriate strength determination model. In many cases this information is site-specific and in others the penetration resistance is overestimated due to the dilatory effects observed in sediment with an undetected grain fraction. In this study variables affecting a dynamic penetrometer-sediment interaction system are identified. Using data from field investigations and literature we found a relationship among five variables: peak acceleration, embedment depth, total embedment time, velocity of impact and grain size. This is used to formulate a sediment identification model. The model accounts for variables that may vary widely within one deployment and it can be applied to other FFPs with different physical characteristics (such as a different mass or size). This may lead to the increased use of FFP as a deployment tool for rapid in situ characterization of the seafloor.

Construction and use of an inexpensive, lightweight free-fall penetrometer: applications to paleolimnological research

This paper describes the construction of an inexpensive, lightweight, free-fall lake sediment penetrometer with application to paleolimnological research. This penetrometer is suitable for use in a variety of fresh water lake settings. Data are collected on a laptop PC, Palm OS handheld, or a Texas Instruments handheld data collection unit and are analysed using commercially available software. The quality of these data are adequate for determining changes in substrate type based on hardness. The unit can also be used to locate sites for optimum penetration of gravity cores and to calibrate sonar records. Addition of up to three optional transducers could expand the capabilities of the unit. Other potential uses include the determination of specific geotechnical properties of the lake sediment including the undrained shear strength.

Indentation of a Free-Falling Sharp Penetrometer into a Poroelastic Seabed

A solution is developed for the buildup, steady, and postarrest dissipative pore fluid pressure fields that develop around a conical penetrometer that self-embeds from free-fall into the seabed. Arrest from free-fall considers deceleration under undrained conditions in a purely cohesive soil, with constant shear strength with depth. The resulting decelerating velocity field is controlled by soil strength, bearing capacity factors, and inertial components. At low impact velocities the embedment process is controlled by soil strength, and at high velocities by inertia. With the deceleration defined, the solution for a point normal dislocation migrating in a poroelastic medium is extended to incorporate the influence of a tapered tip. Dynamic steady pressures, \IP\dD\N, develop relative to the penetrating tip geometry with their distribution conditioned by the nondimensional penetration rate, \IU\dD\N, incorporating impacting penetration rate, consolidation coefficient, and penetrometer radius, and the nondimensional strength, \IN\dD\N, additionally incorporating undrained shear strength of the sediment. Pore pressures may develop to a steady peak magnitude at the penetrometer tip, and drop as \IP\dD\N = 1/\Ix\dD\N with distance \Ix\dD\N behind the tip and along the shaft. Induced pore pressures are singular in the zone of tip taper for the assumed zero radius of the penetrometer, negating the direct evaluation of permeability magnitudes from pressures recorded on the cone face. However, peak induced pressure magnitudes may be correlated with sediment permeabilities, postarrest dissipation rates may be correlated with consolidation coefficients, and depths of penetration may be correlated with shear strengths. The magnitudes of fluid pressures evaluated on the shaft may be correlated with sharp penetrometer data (reported by Urgeles et al. in 2000) to independently evaluate magnitudes of strength and transport parameters.

Indentation of a free-falling lance penetrometer into a poroelastic seabed

A solution is developed for the buildup, steady, and postarrest dissipative pore fluid pressure fields that develop around a conical penetrometer that self-embeds from free-fall into the seabed. Arrest from free-fall considers deceleration under undrained condi- tions in a purely cohesive soil, with constant shear strength with depth. The resulting decelerating velocity field is controlled by soil strength, bearing capacity factors, and inertial components. At low impact velocities the embedment process is controlled by soil strength, and at high velocities by inertia. With the deceleration defined, the solution for a point normal dislocation migrating in a poroelastic medium is extended to incorporate the influence of a tapered tip. Dynamic steady pressures, PD , develop relative to the penetrating tip geometry with their distribution conditioned by the nondimensional penetration rate, UD , incorporating impacting penetration rate, consolidation coefficient, and penetrometer radius, and the nondimensional strength, ND , additionally incorporating undrained shear strength of the sediment. Pore pressures may develop to a steady peak magnitude at the penetrometer tip, and drop as PD51/x D with distance x D behind the tip and along the shaft. Induced pore pressures are singular in the zone of tip taper for the assumed zero radius of the penetrometer, negating the direct evaluation of permeability magnitudes from pressures recorded on the cone face. However, peak induced pressure magnitudes may be correlated with sediment permeabilities, postarrest dissipation rates may be correlated with consoli- dation coefficients, and depths of penetration may be correlated with shear strengths. The magnitudes of fluid pressures evaluated on the shaft may be correlated with sharp penetrometer data ~reported by Urgeles et al. in 2000! to independently evaluate magnitudes of strength and transport parameters.

Free-fall penetrometer: Impact and penetration in reconstituted clays (in French) : Levacher, D Can Geotech JV22, N1, Feb 1985, P129–135

Free-fall penetrometer: Impact and penetration in reconstituted clays (in French) : Levacher, D Can Geotech JV22, N1, Feb 1985, P129–135