In order to evaluate the applicability of high-speed waterjets technology to slot cutting in rocks from economic point of view, it is necessary to predict the depth of cut of an objective rock for various cutting conditions. The depth of cut is influenced by the cutting parameters such as driving pressure, traverse rate, standoff distance and nozzle diameter in addition to the mechanical properties of the rock.
In this paper systematic experiments were carried out both in air and in water for five rocks including mortar to know the effects of both driving pressure up to 98.1 MPa and traverse rate up to 198mm/min.
Main results obtained in this study are summarized as follows:
1) There exists a critical-driving pressure below which no cutting occurrs for both in air and in water.
2) The depth of cut in water is often greater than that in air when driving pressure is sufficiently large and traverse rate is small enough except granite. This is due to the cavitation errosion in water.
3) The depth of cut h can be expressed by the following equation as a function of driving pressure p and traverse rate V for both in air and in water.
h=A₁V^-n (p-p_c)
where P_c is a critical pressure, A₁ and n are coefficients.
4) The critical pressure is less dependent on the traverse rate in the range of this study and increases almost linearly with tensile strength of rocks.
5) The coefficient n is almost independent of the mechanical properties of rocks but depends mainly on the environmental medium.

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It is necessary to establish the empirical formula on the depth of cut for slot cutting in rocks with high-speed waterjets both in air and in water in order to apply the high-speed waterjets technology to various engineering fields such as tunnelling, pre-cracking around a deep borehole and rock excavation at the sea bottom as well as quarrying rocks.
In this paper, to clarify the effects of standoff distance up to its limit where no cutting occurs and nozzle diameter from 1 to 2 mm on the depth of cut in rocks, systematic experiments were carried out both in air and in water for welded tuff and a comprehensive empirical formula on the depth of cut in rocks was proposed from the test results and those from a previous work.
Main results obtained in this study are summarized as follows:
1) There exists a limit of standoff distance beyond which no cutting occurrs for both in air and in water. The limit of standoff distance increases with both driving pressure and nozzle. diameter, and becomes much smaller in water in comparison with in air.
2) The depth of cut depends on the distance from the limit of standoff distance rather than standoff distance itself.
3) The critical pressure is almost independent of both standoff distance and nozzle diameter.
4) The following formula is proposed as a comprehensive empirical formula on the depth of cut h for slot cutting in rocks both in air and in water.
h=k(1-L/L₀)^γd^βV^-n (p-p_c).
where L is standoff distance, L₀ is the limit of standoff distance, d is nozzle diameter, V is traverse rate, p is driving pressure, p_c is critical pressure, and k, γ, β and n are coefficients which depend on environmental medium and rocks.

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