Compaction points are normally spaced at between 1.5 and 2.5 metres centre to centre. The actual spacing is best decided upon by carrying out test compaction patterns and monitoring the results by carrying out pre and post compaction soil strength measurements using the CPT and SPT methods as well as carrying out plate load tests on the compacted soil. If adequate compaction is achieved then a spread footing founded on the improved soil can be designed using a bearing pressure of up to 200 kPa. The zone of soil compacted should extend beyond the edges of the footing by about 10 percent of the depth treated. Normally the upper 1000 mm will not be compacted using this technique and this must either be removed or compacted in-situ using an impact roller or dynamic compaction.
INSTALLATION TECHNIQUE
There are two basic types of vibrator used for deep compaction: one which has horizontal amplitude and one with vertical amplitude. The former type has a built in motor with eccentric weights which rotate about a vertical axis thus providing horizontal amplitude. The latter type consists of a separate vibrator unit with eccentric weights which rotate about the horizontal axis thus imparting a vertical amplitude. The vibrator clamps to a long slender metal section which is called a probe.
Horizontal Amplitude
The vibrator used with this system is a large immersion type with the motor and eccentric weights located at the lower end of the unit. The eccentric weights rotate about a vertical axis so the amplitude of the vibration is in the horizontal plane. The vibrating section is coupled to a follower section by means of a flexible coupling. A suspension point for handling the vibrator is located at the upper end of the follower section. The whole vibrator unit and part of the follower section becomes immersed in the ground during the compaction process. The vibrator section has two or more water jets. Two high volume jets are located at the tip of the vibrator and are used for aiding penetration of the vibrator into the ground. On some vibrators there are another two low volume jets located above the vibrator and these are used to feed water into the cavity to keep the sidewalls stable. The hydraulic fluid for the motor and the water for the jets are fed down from the head of the vibrator through pipes located in the hollow core of the unit. Large quantities of water are used in this process and this can present a site control problem.
A crane is normally used to lower and raise the vibrator. With the vibrator set up on the compaction position the motor is started and the high volume water jets are activated. The crane lowers the vibrator into the ground. When the vibrator has penetrated to the full depth the water flow through the high volume jets is shut off leaving the upper low volume jets to feed water into the cavity. The vibrator is raised slowly and then lowered again into the soil which flows into the cavity under the tip of the vibrator. The raising and lowering of the vibrator continues in a repetitive cycle as it is gradually withdrawn. The water flow must be controlled and at some stage it must be shut off completely.
The effectiveness of the compaction can be monitored at all times using an oil pressure gauge mounted in the cab of the crane. The compaction process must continue up to ground level or a minimum of one metre above the footing soffit level.
Vertical Amplitude
The probe is a long steel section to which the vibrator is clamped at the head. The cross sectional shape of the probe can vary considerably and there are some patented types on the
The vibrator, which is either electrically or hydraulically powered, is clamped onto the head of the probe using a hydraulically activated mechanism. The whole unit is suspended from a crane which lowers and raises the unit as required. With this system only the probe enters the ground.
The probe with the vibrator attached is set up over a compaction position. The vibrator is activated and the probe is lowered slowly into the ground. There are no water jets or any other means for assisting penetration so the vibrator has to be powerful enough to drive the probe to the full depth. Once full penetration is achieved, the probe is lifted about a metre and then lowered again. This lifting and lowering is continued in a cyclic manner as the probe is gradually withdrawn. An indication of the degree of compaction can be obtained by monitoring the electric current in the case of electrically powered vibrators and the hydraulic pressure in the case of hydraulically powered vibrators.
The probe is not capable of compacting the upper one metre of soil due to limited containment so this must be compacted using an impact roller or dynamic compaction or alternatively, the level of the footing soffit must be below this depth.
Plate 13.1.1 shows a typical horizontal amplitude vibrator and Plate 13.1.2 a typical vertical amplitude unit with a y -probe.
VARIATIONS IN INSTALLATION TECHNIQUE
Variable Frequency Vibration
All soil profiles have a natural resonant frequency which varies depending on the profile. It has been found that the best compaction results are achieved when the vibrator is operating at this resonant frequency. In Europe variable frequency vibrators have been developed and some contracts have been completed. This is the latest in deep compaction technology but it has not been introduced to Southern Africa as yet.
POTENTIAL PROBLEM AREAS
Variable soil profile
Soft layers of silt, peat and clay cannot be compacted using the probe or for that matter any form of mechanical compaction. Even silty and clayey sands can prove difficult to compact. Soil profiles in Southern Africa often have strata of these materials present with the result that only a certain percentage of the profile can be compacted. This situation is not normally acceptable and the vibratory compaction solution has to be rejected for this reason. When