The SRP

Apollo 16 Preliminary Science Report: Self Recording Penetrometer
Overview
The SRP (fig. 1), the main quantitative data source for the soil mechanics experiment, was used to obtain data on soil-penetration resistance as a function of depth below ground surface. Maximum possible penetration depth of the SRP is 76 cm, and the maximum recordable penetration force is 215 N. The record of each penetration is inscribed on a recording drum contained in the upper houseing assembly. The lunar-surface reference plane rests on the lunar surface during a measurement and serves as a datum for measurement of penetration depth. A 2.54 by 12.7 cm bearing plate and two penetrating cones, each 30 degrees apex angle and base areas of 1.29 and 3.22 cm², were available for attachment to the penetration shaft.

Figure 1. Self-recording penetrometer
Quantitative Determination of Soil Properties
The results of simulation studies (8-2 and 8-5-8) and soil mechanics theories (8-9) are used as a basis for the deduction of quantitative values of soil properties. Soil-strength parameters are deduced from the results of the penetration tests in the following was, as shown by Durgunoglu (8-10).

Figure 2. Failure mechanism associated with wedge penetration.
From the results of model tests, it has been found that a failure surface as shown in figure 2 represents closely the actual failure surface associated with wedge penetration into relatively dense, fine, sandy soils. Equilibrium analysis of the failure zone shown in figure 2 leads to the equation.

Equation 1
where

The value of δ/Φ has been taken as 0.5 based on the results of friction measurements between a ground-basalt lunar soil simulant and hard anodized aluminum similar to that used for the SRP cones. Equations for evaluation of the factors N_c and Nγq are given in the appendix, and charts for N_c and Nγq as a function of Φ for a range of values of D/B, α, and δ/Φ are in reference 8-10. The ultimate penetration resistance of cones is best estimated by using the bearing capacity factors for wedges modified by shape factors. The appropriate equations are

Equations 2 and 3
where L is the length of the loaded area and B/L=1.0 for the SRP cones and 2.0 for the SRP bearing plate. Infinite combinations of c and Φ could satisfy equation 1 for a given penetration resistance and depth. If penetration resistance values are available for two sizes of cone penetrating the same soil conditions or if the soil deposit is homogeneous and the penetration resistance is known at two depths, then the specific values of c and Φ may be determined by simultaneous solution of two equations of the form of equation 1, one for each combination of q_f and D/B values
Appendix
Bearing Capacity Factors For Cone-Penetration Resistance
From the results of model tests (ref 8-10), it has been found that a failure surface as shown in figure 2 represents closely the actual failure surface associated with wedge penetration. The angle γ, which defines the plane shear zone OAC depends on the penetrometer-to-soil friction angle δand soil friction angle Φ and can be determined from different

Equation A-1
Values of γ determined from equation (a-1) for different values of Φ and δ/Φ are presented in reference 8-10. A logarithmic spiral bounds a radial shear zone to a point of vertical tangency at point E, above which the failure surface rises vertically to the ground surface. For large depths of penetration, such as shown in figure 2, the angle β , which locates point E,equals Φ. For shallow penetration depths, the logarithmic spiral breaks out at ground surface before vertical tangency is reached, and the corresponding value of β can be determined iteratively. Equilibrium analysis of the failure zone shown in figure 8-2 leads to

For wedges, the bearing capacity factors N_c and Nγq are given in reference 8-10 as

The bearing capacity factors for cones can be determined by using the bearing capacity factors for wedges modified by the shape factors computed according to equations 2 and 3.
References