Deep compaction control of sandy soils

The reclamation works in civil and maritime engineering together with the compaction of existing deposits of cohesionless soils increase the use of deep soil compaction. The complex mechanism of deep soil compaction is related to void ratio decrease with grain rearrangement, lateral stress increase, prestressing effect of certain number of load cycles, water pressure dissipation, aging and other effects. An important issue is the choice of the appropriate method of the compaction control, the test parameters and the compaction criterion to be fixed. CPTU is the most widely used method for deep compaction verification (Schmertmann [1], Mesri et al. [2], Slocombe et al. [3], Massarsch and Fellenius [4]). However, it is not so sensitive to the stress history, lateral stress increase and prestressing effect as DMT and CPTU cannot fully take into account the major phenomena related to the soil improvement due to deep compaction (Marchetti et al. [5], Lee et al. [6]). Additional advantage of DMT or pressuremeter test (Gambin [7]) in quality control is that they provide supplementary data on soil deformability and stress state. It is thus possible to get a comprehensive measure of compaction control including the soil strength and deformability characteristics. The application of CPTU and DMT in quality control of compaction works will be discussed in this paper.

The criteria for the compaction control can be fixed in terms of relative density, specific value of cone resistance, modulus of deformation and soil settlements on the trial field. CPTU is particularly useful when a certain relative density over the compacted strata should be obtained. Taking into account the soil mineralogy and compressibility at a given site, the suitable correlation – established from calibration chamber tests – between the cone resistance and relative density should be applied. The improper estimation of the sand compressibility can produce the uncertainty in the evaluated relative density up to 20%. One should also remember that these correlations are established from calibration chamber tests on freshly deposited sand, their use for in-situ conditions and in aged or cemented sands gives an equivalent relative density (Schmertmann et al. [1], Jamiolkowski et al. [8]).

The choice of a specific value of cone resistance as a compaction criterion over the strata considered will produce an excessive compaction effort in the upper layers and non uniform soil densification. The better solution is to express (Massarsch and Fellenius [4]) compaction specification as a cone resistance value adjusted to mean effective stress. If the compaction requirement is to obtain a minimum modulus of deformation in the strata considered, then the dilatometer or pressuremeter is a very useful tool to check the compaction effectiveness. The constrained modulus from dilatometer test MDMT can be considered simply as one-dimensional, compression modulus M, because the settlement observations of the real structures (Schmertmann [1], Marchetti et al. [5]) are very consistent with those calculated with the constrained moduli from DMT (i.e., M ≈ MDMT). Values of the constrained modulus from DMT can be thus compared directly to the fixed criterion. When a minimum value of the relative density should be obtained in the compacted strata, the DMT is useless because no method is available to derive the relative density from dilatometer test data alone (Marchetti et al. [5]).

A set of buildings was designed near the President Harbour in Gdynia Port. Heterogeneous soil conditions – with Holocene sand containing some mud inclusions and sand fills of variable thickness – imposed the soil improvement under the slab foundation. The vibrocompaction method was applied for densifying sandy soils by means of electric vibrating unit. Under the influence of vibration in fully saturated conditions
loose sand particles are rearranged into a denser state with simultaneous increase of lateral stress in the soil mass.

3.1. SOIL CONDITIONS
A simplified soil cross section is given in Fig. 1. Sand fills and aged Holocene sands with silt and mud inclusions are found. The water table is about 1 m below the ground level. Some parts of the superficial layers were hydraulically placed fills. Below dense sand found near the surface a medium dense to loose sand is found with local seems of silts or mud. Loose sand with silt and mud inclusions was detected near the harbour from 9 to 11 m. A very dense Pleistocene sand is found below. The roof of a very dense Pleistocene sand declines towards the harbour.

3.2. COMPACTION WORKS
Deep soil vibratory compaction with granular material supply from the surface was used. S-type vibrator with power 120 kW, frequency 30 Hz and vibration amplitude of about 20 mm was used. The application of
infill material assists in maintaining site levels. It provides an additional increase of the lateral stress within the subsoil, induces arching phenomena – extra reducing the expected settlements. Compaction was performed in regular square grid 3 × 3 m. Neither water nor air was introduced during the vibrator insertion and the consecutive compaction phases. The area to be compacted was about 7000 m2. The soil thickness subjected to vibro compaction increases towards the harbour. During the works typical parameters were recorded.  The settlements of the soil surface due to deep soil vibratory compaction were monitored. Up to 44 of the surface settlement was measured within the compacted area. The consumption of infill was monitored at each point of vibratory compaction. The power input was controlled and recorded in each profile of vibratory compaction. The minimum average constrained modulus over the soil profile equal 80 MPa was fixed as an acceptance criterion for the post-treated subsoil.cm