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Published By Lankelma

Lankelma is the foremost contractor for onshore in-situ soil testing in the UK. An acknowledged specialist in CPT, Lankelma also offers a worldwide consultancy and training service.

A.P. van den Berg develops, designs and manufactures geotechnical and environmental soil investigation equipment for onshore and offshore applications. Specialists in CPT systems and equipment.


Gardline Geosciences offers worldwide marine geotechnics, in-house consutancy and services with marine investigations ranging from nearshore to full ocean depth (down to 3000m).

About the Author

Hans Brouwer studied civil engineering at Delft University in The Netherlands. He has worked as a part-time lecturer at Amsterdam Polytechnic and was senior partner in a structural engineering consultancy. He has written a standard textbook in Dutch about the design of building foundations. He now lives in England where he writes technical textbooks in English, hopefully to reach a bigger readership.

Chapter 10

Offshore testing

Seabed soil samplers


 Figure 96   

Grab sampler
The grab sampler (Figure 96) is one of the simplest forms of seabed sampling. It is a grab bucket very similar to that used on land.
The grab units tend to be either hydraulically or manually operated. The advantage of the manually operated version is that they are very simple and not really restricted to any water depth other than length of wire and winch capacity. These units can work in up to 4000 m of water. They give a good idea of the index properties of the seabed material.
The unit is typically deployed from a vessel’s crane or A-frame to recover the samples back to deck. These units are also useful in obtaining bulk samples that can be used back in laboratories for model testing such as soil pipeline interaction. 
     Figure 97 
Box corer
The box corer has become a standard sampling tool for surveys in soft or deep sediments. The enlarged surface area of the box (0.25 m2) allows for relatively large sample sizes to be recovered in deep water where the time required to deploy and recover the instrument is significant.
The box corer is built within a gimballed hexagonal frame. The
instrument is triggered by a trip as the main coring stem passes through its frame. The depth of penetration (maximum 50 cm) can be controlled to prevent over-penetration in softer sediments (Figure 97).
The recovered sample is completely enclosed after retraction, reducing the loss of finer materials during recovery. Stainless steel doors, kept open during the deployment to reduce any ‘bow-wave effect’ during sampling, are triggered on sampling and remain tightly closed, sealing the sampled water from the water column.
On recovery, the sample can be processed directly through the large access doors or via the removal of the box completely, together with its cutting blade. A spare stainless steel box and galvanised cutting blade can then be added, ready for an immediate re-deployment. Box corers are available in different sizes. The largest is the 0.25 m2 type, while smaller 0.06 m2 mini box corers can also be provided.
Box corers provide a very high quality bulk sample which again, like the grab sampler, has no electronics fitted, so is only limited in depth of water for operation by the length of wire and winch.


Gravity and piston corers
The length of a gravity corer ranges from 3–8 m. These tools are capable of obtaining continuous core samples in any water depth, subject to the availability of a suitable vessel and installed deployment system. The gravity corer, which drops in free-fall from a limited height, penetrates the seabed merely under gravity.
The stationary piston corer is a gravity corer which also drops in freefall from a limited height but has the lower end enclosed by a piston, until penetration into the soil commences (Figure 98). The piston, connected to the main lift cable by wire which becomes taut when the coring tube comes into contact with the seabed, remains approximately
stationary as the tube penetrates. The presence of the piston creates a negative pressure in the coring tube, enabling the frictional forces of the core on the walls of the tube to be overcome. This generally results in recovery rates which are better than those obtained with a standard gravity corer. A piston is particularly suited to soft cohesive soils.
The piston corer comprises basically of:

  • main lift cable
  • release mechanism
  • main weight
  • counterweight suspended from release system
  • core barrel and PVC liner (�� 85 mm)
  • tulip core catcher and cutting shoe
  • piston connected to main lift cable
  • special launch and recovery chute
  • special trolley for easy core barrel management.

The geotechnical quality of the recovered core depends on the shape of the corer, the coefficient of penetration, and how it penetrates – a constant push rate is best. The larger the core diameter the better the likely core quality. Samples are suitable for geological logging and all types of index and classification tests. Basic strength and consolidation tests can also be undertaken although it must be accepted that the
results may not be representative.
Gravity and piston corers can be operated from a large variety of nonspecialised survey vessels, having adequate handling capabilities(crane, derrick, boom, or portal or A-frame). Each type of system should be operated in strict accordance with the Safety Procedures forDeployment of Corers. When using a standard gravity corer, the system is used with a special launch and recovery chute which ensures that the corer is operated safely and at no time is it suspended above the deck.

Figure 98

Corers have now been developed into jumbo piston corers (JPCs) which
are able to take cores of up to 30 m length with a stationary piston. The
main obstacles with this are deployment and handling.
Typical applications for these samples are:

  • pipeline route surveys
  • cable route surveys
  • site assessment surveys
  • pre-dredging geotechnical surveys
  • shallow penetration sampling
  • calibration of near-surface geophysical data
  • geochemical sampling surveys
  • reclamation quality control
  • near-shore site investigations
  • microbial hydrocarbon exploration. 
Figure 99 

A variation of the gravity core is the vibro-corer (Figure 99). This corer uses motors to generate a centrifugal force to vibrate the sample barrel into the ground. It enables samples to be taken in granular material and in stiff clays where free-fall devices, such as a gravity corer, would meet refusal.
The high-powered vibro-corer is powered by an electric twinlinear vibrator motor delivering over 9000 kg of centrifugal force.
Standard size vibro-coring equipment will produce 86 mm diameter core samples to a maximum depth of 6 m. In coarse aggregates larger diameters up to 150 mm can be obtained.
To minimise sample disturbance and coring time, the units can be fitted with an integral penetrometer and data recorder supplying online information on penetration against time and penetration rate (m/sec). Data is subsequently used to assist in the evaluation of actual layer thickness compared with recovered length. Typical sea bottom vibrating time is up to ten times less than with a standard vibro-corer, improving performance rates and minimising core disturbance.

The corer is capable of coring up to 6 m into the seabed in water depths
of up to 120 m. Recovered cores can be retained within their semitransparent liners until formal examination onshore. Alternatively, the samples can be split and logged onboard to enable rapid assessment. 
Typical applications involve:

  • mineral exploration
  • aggregate resource evaluation
  • pre-dredging geotechnical surveys
  • sand search
  • reclamation quality control
  • near-shore site investigation
  • cable route surveys
  • pipeline route surveys.

 Deep water sampler
With the need for higher quality samples in the deepwater environment,
a new deep water sampler has been developed. This is a variation on
the jumbo piston corer but intended to provide higher quality samples.
The deep water sampler was designed in cooperation with the
Norwegian Geotechnical Institute to sample soft soils in deep water.
The aim is that the sample length will be at least 10 m and with a sample
diameter of 110 mm. One of the most important goals is the recovery
ratio of 95% or higher of the soil. This means that the sampler has to
penetrate the soil with minimal disturbance and handle the sample with
care during retraction. 

The system uses a ROSON seabed frame. The drive wheels of the ROSON push the sampler into the soil at a speed of 20 mm/sec. A piston is linked to a fixed point and the penetration depth is logged by a
depth encoder. During penetration, data is logged every 10 mm. On completion of the test, an ASCII file is generated containing test details in the header followed by the test data.
The ROSON 100 kN seabed frame is modified to take a 15 m mast to support the deep water sampler tube in the vertical position during sampling (Figure 100).
The deep water sampler is made up of several component parts.

  • Cutting shoe
  • Core retainer
  • Sample tube
  • Piston
   Figure 100
 Cutting shoe
The cutting shoe (Figure 101) protrudes into the soil and guides a soil
sample into the sample tubes behind the cutting shoe. The shape of the
cutting shoe is designed to obtain the least possible deformation of the
soil. Another function of the cutting shoe is to lock the spring loaded
core retainer during penetration. As soon as the whole sampler is pulled
back, the friction force on the cutting shoe triggers the core retainer. 

Figure 101
Figure 102     
Core retainer
The core retainer (Figure 102) consists
of several fingers made out of spring
steel. The function of the core retainer
is to cut the soil sample and support it,
while lifting the whole sampler. The
core retainer is spring loaded, which
gives it its strength. The retainer is
triggered by the retraction of the
Sample tube
The sample tube (Figure 103) consists of an outer tube, a liner and a set of rings and seals to connect the liners. The outer tube protects the
liner and guides the pushing and retracting forces which are being
imposed upon it. The liner guides the sample and acts as a container
for the retrieved sample. 

Figure 103  
The piston (Figure 104) fits closely into the liners. A seal on the piston
can withstand possible pressure differences between the soil and the
inside of the empty liners. This seal is also used as a one-way brake so
that the piston can only move upwards in the liners.

Figure 104
The piston stays stationary to the soil during penetration. The reaction
force is lead through a load cell and chain to a fixed point above the
sampler. The piston is used to seal the sample and to keep it steady
inside the liner. Pressures are monitored by sensors inside the piston.
If the pressure is too low or too high, a safety valve will open to avoid
damage to the sample and sampler.
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