The DMT as tool for the monitoring of the effect of pile installation on the stress state in the soil
5 PROCEDURE FOR THE EXPERIMENTAL DETERMINATION OF THE INFLUENCE
∂
∂u= ⋅ ∇
t c 2u (7)
4 OVERVIEW OF PILES EXAMINED
The research was originally mainly focused on the experiments determination of the stress state close to the pile shaft of srew piles of the displacement type.
Besides of the torque available, also the design and the shape of the drilling tip is of big importance, not only to asily reach the bearing layer but also for the assurance of a good pile shape.
The auger tip of a screw pile of the soil displace-ment type has a specific shape, designed to penetrate with displacement as quickly as possible, depending on the mechanical soil properties (fig. 5).
Later on, more recently, measurements were done close to driven piles during and after installation:
− close ended tubular piles (site Aarschot)
− driven precast concrete piles (site Limelette)
− driven Franki—piles with enlarged basement The same test procedure was followed, with meas-urements close to the pile during pile installation.
Later on, the measurements during installation were omitted, because of the high risk of damage of the DMT-blade and-equipment.
Following piles were examined (tests executed indicated in the table 1.
5 PROCEDURE FOR THE EXPERIMENTAL DETERMINATION OF THE INFLUENCE OF PILE INSTALLATION ON THE STATE IN THE SOIL
5.1 General considerations
Generally, foundation piles are installed to transfer the load from surface to a deep bearing layer. The pile shaft is mainly surrounded by weak layers (weak clays, loose sandy layers). In order to obtain reliable data of the influence of pile installation on the stress state in the soil, one has to make a judicious choice for the interdistance pileshaft to DMT-blade.
This distance has to be as small as possible in order to measure important variations in horizontal stress.
Preferably onde measures in the zone where plastic soil deformations have occurred. This zone is deter-mined by:
− the diameter of the pile
− the soil characteristics (especially for cohesive soils the deformation and shear resistance charac-teristics and for non cohesive soils also the relative density)
Taking into account:
− the distance pile shaft to DMT-blade has to be as small as possible in order to perform reliable meas-urements in the plastic zone around the pile tip
− the distance to be sufficiently large on the other hand in order to avoid damage of the membrane and the blade on one hand, and to reduce the influ-ence of non-verticality on the other hand
− the extent of the plastic zone varies with depth
− preferring only one procedure for all tests, the DMT-blade is placed at a distance smaller than the Figure 5. Different types of auger tip.
Table 1. Piles examined paper.
DMT DMT DMT
Type of pile before during after
Atlas x x x
Omega x x x
CFA x x x
PCS x x x
Franki x x x
(enlarged basement).
Tubelar (screw) x x x
Tubelar (driven) x x
Gravel column x x x
Conc. precast (driven) x x x
theoretically predicted plastic radius Rp (Cavity Expansion Theory).
5.2 Theoretical prediction for the distance DMT-blade—pile shaft
The expression for Rp is given by (1). The Ypresian clay is a tertiary overconsolidated fissured clay. For the relation between cone resistance and undrained shear resistance, the extensive research of the Ghent University lab in this type of soil, allows for the eval-uation of the Ypresian clay parameters as:
qc = Nk cu + ⋅ σv,0 (8)
where σv,0 = vertical total stress at the depth con-sidered and Nk= 27 for stiff fissured clay having a marine origin.
For the test site in Koekelare where the Ypresian clay was situated beneath a depth of five meters and taking in account a pile length of 13 m one can esti-mate cu as follows.
sv,0= ≈ 180 kPa and qc ≈ 2.7 Mpa. This results in cu= 93 kPa.
This is in good agreement with De Beer (1979):
cu= 64 + 16.5 ⋅ z (z = depth in meter, cu in kPa).
For slightly overconsolidated soils, M ≈ (1.3 to 3.3) qc of M ≈ 6.25 MPa for the test in Koekelare.
Out of the theory of elasticity one can derive G0= 1.04 MPa (ν = 0.40). Eq. (1) results finally in Rp≈ 3,3 ⋅ r0. Based on this result the interdistance between pile shaft and dilatometerblade is taken equal to the pile diameter. The same interdistance is taken for non-cohesive soils because of the lack of a reliable theoretical model on the one hand and the required uniformity for the test precedure on the other hand.
6 CHANGES IN TOTAL HORIZONTAL STRESSES DURING PILE INSTALLATION—
EXPERIMENTAL RESULTS—SCREWED PILE
6.1 Omegapile at Vorst test site
In a research program on Omega piles (Socofonda) the test site of Vorst was selected. The pile diam-eter was 51 cm. The dilatomdiam-eterblade was placed at a distance 77 cm from the pile axis and a depth of 10.60 m. The total pile length was 25 m. Out of the available geotechnical maps the Ypresian approved to be situates between 9.60 m and 23.60 m.
On figure 6 one can see that the pressure Po reaches a first peak value the moment the auger
tip passes the installation depth of the DMT-blade (downward movement). Immediately after reach-ing these peak values there is a considerable drop down of the total horizontale pressure. This can be explained by exceeding the tensile strenght in the soil leading to the formation of micro fissures. This results in temporary faster pre-pressure dissipation until the fissures again are closed by the inward soil deformation. This theoretical explanation is discussed under 8. Besides this, the decrease of Po is explained by decompression of the soil after the passing by of the auger, the dissipation of the excessive pore water pressure and the change in total stress state around the dilatometerblade.
6.2 Franki-Atlaspile at Koekelare test site
For this program the test setup is presented in figure 7. Two dilatometerblades and 1 pore water pressure transducer (CPTU-cone) were installed close to the pile, all at the same distance from the pile axis. The results are presented in figure 8 for the piles examined. One can detect only a small change is more pronounced. This probably can be explained by an inappropriate installation of the dilatometerblade (outside the plastic zone around the pile).
Figure 6. DMT-test installation Omegapile at Vorst test site.
Figure 7.
6.3 PCS-piles in sand
Two PSC-piles of Socofonda company were exam-ined. The details gatherd on the folowing table.
Pile Pile Installation Site length diameter depth DMT ID
Doel 19.07 m 0.40 m 8.5 m 2.9
Dendermonde 16.52 m 0.45 m 7.5 m 2.0 Both test programs were performed this time in a sandy soil. The DMT test measure there for predominantly during most of the pile installation period changes in effective stress around the pile.
At Doel test site, the dilatometerblade was directed tangentially (deformation of the membrane perpen-dicular to the radius from pile axis).
6.3.1 Doel ( fig. 9)
When the installation depth of the DMT—blade is reached, the total stress shows a sudden peak valeu, what can be explained by a sudden pore water pres-sure peak followed by an immediately dissipation.
Afterwards, due to arching, the tangential stress will increase because of soil decompression. Dur-ing castDur-ing the concrete a similair phenomenon can be seen. At the moment the augertip passes. Pore water pressure are induced but they immediately are dissipated. At the end, when the concrete overpressure is taken away, some decrease in tangential stress is induced leading to an increase in tangential stress.
One can notice that the difference in tangential stress for the upward movement is much less than daring the downward movement. This can be explained by higher decompression during the boring stage of pile execution what could be expected, since the PCS screw pile is basically only a CFA type, with some precaution on high enough vertical downward speed and pressurized concrete casting.
6.3.2 Dendermonde (fig.10)
Here one can see a quite similar picture as at the Doel test site. The dissipation of pore water pressure goes on however more slowly. This can be explained by the more silty sand character of the soil. A second remarkable difference comparing with the Doel test site is only a partial reduction of this radial horizontal stress after taking away the concrete over-pressure.
Also this can be explained by a somewhat delayed dissipation in this soil type.
Figure 8. Results at Koekelare.
Figure 10. DMT test during pile installation of PCS-pile at Dendermonde test site.
Figure 9. DMT test during pile installation of PCS-piles at Doel test site.
Figure 12. Changes in normalized total horizontal stress (ID ≥ 1.8).
7 COMPARISON DMT MEASUREMENTS