Geo-technical study

Geo-technical studies are aimed at studying the physical properties of the sea bed, using direct methods. They study aspects such as the stratigraphic profile of the terrain, geo-technical parameters of the layers penetrated or the bearing capacity of the terrain and footings to external forces. This study supplements the geo-physical study done using indirect methods (boomer, TOPAS, etc.).

In the case of the test bed, a geo-technical study has been done using non-extractive piezocones (CPT - PiezoCone Penetration Test). A study of this kind consists of driving a conical tip into the sea bed under pressure at a constant velocity and directly measure the effort required for the cone to penetrate, together with the geo-technical parameters of interest. In this case, sediments are not extracted, as happens using other methodologies. The cone perforates down until it reaches the edge of the unconsolidated sediment layer and arrives at the rocky substrate (rejection depth). The direct measurement of certain geo-technical parameters in turn, allows other parameters of interest to be calculated indirectly, such as a description of the kind of sediment for example.

11 CPTs have been run in the test bed area at different depths until they reached the rejection depth. The positions and results of these tests are shown in the next graph and table.

Parameter Unit Relation
Cone Resistance -   (qc) MPa Direct measurement
Sleeve Friction Resistance -   ( fs) (kPa) Direct measurement
Porewater Pressure - (u2) (kPa) Direct measurement
Slope Indicator  -(°) (º) Direct measurement
Corrected Cone Resistance -  (qt) MPa Deduced fromqc
In Situ Pore Pressure -  (u0) (kPa)

Si z < zw ;u0=0

Si z ≥  zw, uo=(z-zw)xƔw

Friction Ratio -   (Rf) (%) (fs/qc×)x100 ó(fT/qT×)x100
Pore Pressure Ratio -(Bq) -- Bq= ∆U /(qT-sv0)
Material description(SBT) -- Represented according to Robertson1990

Relative Density  - (Dr)

(%)

1. Baldi et al. (1986); Al-Homoud& Wehr(2006)

2. Jamiolkowski et al.(2001)

3. Kulhawy & Mayne(1990)

Undrained Shear Strength – (su)

(kPa)

1.s= (qt - σvo)/Nkt; or (qc -σvo)/Nk

2. su = qt/Nkt; orqc/Nk

 

 

3. Wroth(1984)

4. Trak et al. (1980), Terzaghi etal.

5. Robertson (2009), su =∆u/N

The next graph shows the vertical profile of the parameters measured at sampling point CPT A by way of example. As we can see from the profile on the right of the graph, the kind of sediment can be inferred from the relative density.