NETA Shear is a technique that turns any drilling machine into a geo-mechanical measurement tool.

Throughout the history of CFA (Auger Cast) piles, there were attempts to use drilling machines as a tool for pile performance and a geo-mechanical measurement. In many cases, CFA (Auger Cast) piles are applied in soft, weak, problematic soils. Due to the high-risk level and extreme changes in the properties of soil within the limits of the same site, the need aroused for an objective measurement tool to supply information on the strength of soil surrounding the pile.

Today, the most common technique is to monitor the torque in the rotary system. The basic assumption is that there is a direct, monotonous, relation between that torque and soil strength. As the strength of soil increases - the torque in the rotary system increases.

In reality, the torque of the rotary system is equally dependant on the soil and driller activities. It is almost impossible to separate between the two influences. Furthermore – there are kinds of soils where the correlation between the torque and strength of soil is reverse. It is problematic to use the toque of the rotary system as a geo-mechanical criterion.

TOMER SYSTEMS BV. present a new concept in using a drilling rig as a geomechanical tool – NETA Shear.

The auger surface creates a compression force on the mass of soil inside the auger.
The direction of this force is perpendicular to the auger surface. We will name it the normal force. The magnitude of the normal force depends on the shear strength of the soil only.
The normal force will increase until the soil is destructed (through shear) then it will be completely canceled.

As customary in engineering, we will split the normal force into two equivalent vectors – one horizontal and one vertical. Clearly if the normal vector depends on the soil’s shear strength, its components also depend on the soil’s shear strength.

The point of applying the horizontal vector is far from the auger’s axis of rotation therefore a moment around this axis is created. This moment, along with all friction forces, are neutralized by the torque of the rotary system.

The vertical force together with the proper weight of the rotary system, the auger and the mass of soil, creates a vector on the axis of rotation (due to symmetry) of the auger pointing downwards. This vector is neutralized by the tension in the lifting cable.

By knowing the proper weight of the auger and rotary system and the weight of soil inside the auger, we can easily calculate the vertical component of the normal vector – the NETA Shear.

NETA Shear is very similar to Shear Vane Test, but with twisted rips.

As stated before, the vertical component of the normal vector depends solely on the shear strength of the soil. In other words – by a simple measurement of the tension in the lifting cable of the auger we can obtain a figure, which is equivalent to the soil’s shear strength – NETA Shear.

In practice, the measurement process is carried out in 10 stages (refer to drawing no. 18):

Note that:

1. Throughout the process, the auger is rotating. All friction forces are in the horizontal plane and distant from the axis of rotation therefore they turn into an horizontal torque. This torque is neutralized by the rotary system only. The friction forces, like the moments they create, have no influence on the vertical vector or tensile force in the lifting cable (on the NETA Shear). (refer to drawing no. 6).
2. Most of the sections described in the process of the NETA Shear measurement are identical to those carried out in the MALI Effect process (refer to chapter 2).
3. The required tension gauge in the NETA Shear process is identical to the one required in the MALI Effect process.
4. As the auger structure is closer to a theoretical screw – more accurate results will be obtained.
5. Due to the “screw” structure of the auger, a large mechanical amplification is obtained in the NETA Shear system (like the mechanical mplification obtained in a worm). This amplification increases the sensitivity of the system.
6. NETA Shear represents the shear strength of the soil but is not necessarily identical to this strength.
7. NETA Shear is a sensitive, available, reliable and simple tool. It supplies data at real time and enables more accurate control of the soil properties – yet this is not an absolute tool and certainly not a standard one.
8. Using the NETA Shear must be done after careful consideration of the accuracy of pitches in the auger, the shape of the bottom part of the auger and the tool’s percentage of compression (compaction, displacement).

Clearly, it is not intended that the operator will carry out a perfect NETA Shear procedure on each pile. This procedure should be carried out from time to time or once in several piles.
Carrying out the full NETA Shear procedure reduces the rate of work since the operator has to stop the penetration each about 1 meter (3 foot) and rotate the auger for several seconds in its place (increases the penetration time in few dozens of seconds – approximately 10%).

Even in cases where the operator does not carry out a full accurate NETA Shear procedure, due to the nature of the auger penetration, the obtained results are a better reflection of the shear strength of the soil than obtained by measuring the torque in the rotary system.