Dar Magazine I9 Page 35

On solid ground

Two different tests were conducted on the trial area
after it was subjected to the improvement: Standard
Penetration Test (SPT) and Cone Penetration Test (CPT).
The tests analyzed:
· Liquefaction
· Bearing capacity for the most critical footings
· Settlement analysis for the critical footings
considering the interaction effect of adjacent foundations.
All calculations were in accordance with the
international BS/ASTM standards.

Cranes lift 35ton pounders to a 35m height and then
release the pounders, delivering, in a single blow, the energy
equivalent of 1,200 ton/m. The massive impact of the falling
weight triggers a series of actions:
· Immediate densification of granular soils
· Generation of a compression wave that builds pore water

pressure causing total or partial liquefaction
· Generation of high energy shear and surface waves that result in

a denser soil through the rearrangement of the grains
· Speeding up of the consolidation process.

Testing the technique

To determine the ideal parameters for the technique, the
contractor created an onsite calibration area (Figure 3).
The quality control tools selected were the PreTreatment
and PostTreatment Cone Penetration Tests (CPT) and the
Pressuremeter Tests (PMT).
The tests evaluate subsurface conditions from the relative density
point of view, before and after the improvement process. Also,
two Heave and Penetration tests were carried out to determine
the ideal number of blows.

The assessment parameters were:
· Number of blows per print
· Drop height
· Type and weight of the pounder
· Number of phases and passes.

Dynamic Compaction

Finding the first technique inadequate, Dar's team
turned to a second solution, Dynamic Compaction.
It targets unsaturated and saturated granular
materials, dramatically increases the geotechnical
properties of soil, and has the advantage of
reaching great depths.

Dynamic Compaction is typically applied to soils
with 3035% fines content. It is well adapted
to large areas and results in uniform ground
conditions all over the site. The process of Dynamic
Compaction is illustrated below Figure 2.

Energy is provided to the soils in several blows
for each impact position and in several phases,
following a preset (usually a square grid)

Table 1

Pretreatment and posttreatment tests that were used to

evaluate Dammam's sand layers after Dynamic Compaction

Primary (phase 1) and secondary (phase 2) grid to compact all layers
from the surface to intended depth of treatment

Typical phasing of

dynamic compaction

Figure 3

Dynamic compaction

calibration area

Figure 2

Dynamic compaction process

Evaluating the technique

The crushed stone technique proved unsatisfactory:
tests showed it to be unsuitable for the prevailing
subsurface conditions. This technique was only
able to treat layers up to maximum 2 m in depth.