Vibroflotation Control of Sandy Soils using DMT and CPTU

ABSTRACT: Vibroflotation is a typical improvement method for the cohesionless deposits of high thickness. The compaction method was applied to densify sandy deposits in Gdynia Port. Compaction control was verified with CPTU and DMT tests. Some examples of interpretation of soundings in pre-treated and compacted sands are shown. The classification diagrams are given for pre-treated and compacted sand. The stress history of the deposits is analysed. For a given relative density a considerable increase of lateral stress index was recorded after compaction. Some acceptance criteria for compaction control are discussed. The sensibility of CPTU and DMT methods to compaction control is analysed.

1 INTRODUCTION

A set of buildings was designed near the President Harbour in Gdynia Port. Heterogeneous soil conditions – with Holocene sand containing some mud inclusions and recent loose to medium dense sand fills of variable thickness – needed some improvement works to establish more uniform and less deformable subsoil. The vibroflotation method was applied to densify sandy soils by means of electric vibrating unit.

2 VIBROFLOTATION

2.1 Soil conditions
The thickness of Holocene sandy deposits in the considered area varied from 4 to 11 m. Below, there is a very dense Pleistocene sand layer. The water table is about 1 m below ground level. Hydraulic fills deposited underwater covers partially the considered area. The preliminary CPTU tests shown that the sandy deposits fulfill the suitability conditions for the use of vibroflotation according to diagrams proposed by Massarsch&Fellenius 2002 and Lunne et al. 1997. The silt fraction in sandy deposits was less than 7% and the uniformity coefficient was in the range from 2.2 to 6.8. The granulometric curves are presented on the diagram (Fig. 1), with the suitability zones for vibroflotation, as defined by Brown 1997. Here, the soil granulometry signifies that the sandy deposit is ideally compactable (B) or compactable (C) using vibroflotation method.

2.2 Compaction works
Deep soil vibratory compaction with granular material supply from the surface was used. Vibrator with power of 120 kW, frequency of 30 Hz and vibration amplitude about 20 mm was used. Under the influence of vibration in fully saturated conditions the loose sand particles are rearrangedinto a denser state with simultaneous increase of lateral stress in the soil mass. Additionally, some infill gravelly sand material was supplied from the surface level to reduce the soil settlements. Such action induces an additional increase of the lateral stress within the subsoil.
Some preliminary tests were performed on the trial field to determine the appropriate grid size and vibration time sequences. Finally, the compaction was performed in regular square grid 3×3 m.

2.3 Compaction criteria
Compaction effectiveness can be monitored during the works and with penetration testing afterwards. During vibroflotation the input power consumption and surface settlements were measured. The maximum recorded settlement for 8 m thickness of deposits was 49 cm. The consumption of the infill material was also monitored at each point of vibratory compaction. Some CPTU and DMT tests were performed before and after the works midway the vibroflotation points. The tests were executed more than three weeks after the works completion in order to take into account an increase of soil parameters due to aging. The minimum average constrained modulus over the soil profile equal 80 MPa was fixed as an acceptance criterion for the post-treated subsoil.

3. COMPACTION CONTROL

3.1 CPTU/DMT profiles
The results of CPTU tests before and after vibroflotation are given on Fig. 2. The control tests were realised from the working platform, so they are shifted about 1.4 m regarding initial ground level. An important, about 3.2 in average, increase of cone resistance is registered within the compacted layers. Only slight sleeve friction augmentation is observed compared to cone resistance. The corresponding normalized friction ratio decreases after compaction, which is consistent with findings of Slocombe et al. 2000 and Debats&Scharff 2009. One should notice that sandy deposits with only very limited fine content can be densified using vibroflotation. As an example, marginally compactable silty interbedding at about 4 m depth are found in the soil profile (see Fig. 2).
Typical DMT tests were performed and standard parameters were derived according to Marchetti 1980 interpretation. The DMT profiles are given for pre-treated and compacted subsoil (Fig. 3). An important growth of the lateral stress index KDMT in post-treated soil was recorded due to soil density increase, soil prestraining under repeatable load and lateral stress increase. The material index IDMT, however, decreases after vibroflotation. Significant increase of internal friction angle determined with Marchetti et al. 2001 correlation was obtained within compacted strata. This uniform distribution of internal friction angle within compacted layer confirms the quality of the compaction works.

Considerable augmentation of dilatometer and constrained moduli from DMT is registered. One should notice that the constrained modulus MDMT over the compacted soil strata exceeds by far the acceptance criterion. The post-treatment constrained modulus MDMT is in average 7.6 times higher than before compaction (Fig. 3). It means that dilatometer is more sensitive tool of compaction control that CPTU. The mean increase of the constrained modulus MDMT within compacted sandy layer is about 2.3 times higher than corresponding qc increase. This result confirms the previous observations of Schmertmann et al. 1986 concerning dynamic compaction and of Jendeby 1992 on monitored deep compaction of sandy fills using vibrowing. They found that after compaction works the constrained modulus MDMT increases more than twice the cone resistance qc. The acceptance criterion defined in terms of constrained modulus was achieved by far in the compacted strata.