Benefits of Ko Step Blade Test:
- Immediate results save the owner and engineer time and money
- Measures the horizontal stress of soils in place
Ko Step Blade (KSB): While engineers can estimate the vertical stress of soil relatively well, they cannot estimate the horizontal stress. The coefficient of horizontal stress, K, ranges from 0.2 to 6 times the vertical stress (Schmertmann, 1985). When a vertical force is applied to the soil, it is resisted by the soil in three dimensions, two of which are horizontal, emphasizing the importance of the horizontal stress.
Figure 1: Ko Step Blade and Control Unit
Unfortunately, horizontal stresses are difficult to measure. When we drill a hole, we remove them. When we push a device into the soil, we tend to increase them in looser soils and may decrease them in denser soils. Soil sampling causes too much disturbance for the engineer to measure horizontal stresses with laboratory tests.
The Ko step blade was invented to measure this difficult to obtain soil parameter. The blade contains four steps going from thin to thick from its bottom to top (Figure 1). At each step there is a circular membrane that is exerted outward, measuring the soil’s horizontal stress. It is recognized that even the thinnest step causes disturbance to the horizontal stresses when it is pushed into the soil. At the desired test depth, the engineer measures the horizontal stress of the soil for each blade step. By plotting the blade thickness versus the log horizontal stress, engineer can extrapolate the horizontal stress at a zero blade thickness (Figure 2). The documented accuracy of this method is +10% (Handy, 2008). (Note that the maximum 7.5‑mm‑thickness of the Ko step blade is half that of the 15‑mm‑thick DMT blade.)
Figure 2: Extrapolation of Ko value (Handy, 2004)