Более конкретно: The so-called "Hardening Soil model" is an elastic-plastic constitutive model which incorporates shear hardening and volumetric hardening with an associated flow rule for the cap yield surface and a non-associated flow rule for the deviatoric yield surface

Мне кажется, 10-4 должен этот термин знать, потому что он встречается и при изучении скважин

Каким-то макаром это связано с Cap plasticity model (встречается в документации к ABAQUS

Очень надеюсь на помощь!!!!

Hydrostatic and deviatoric stress components:
Let us consider the stress matrix representation [σ] at a point in the body:
[σ] = [σxx σxy σxz
σyx σyy σyz
σzx σzy σzz] (23)
It is convenient and useful to split the stress matrix into two parts, one called the spherical
or the hydrostatic part and the other one the deviatoric part.

Сферическое (в Вашем сучае полусферное)/гидростатическое давление - усреднённое давление направленное радиально по всем направлениям.
Деривиаторное (разностное/дифференциальное, имхо) давление показывает разность между реальным и усреднённым давлением в каждой конкретной точке.

А вы не порекомендуете, что в Инете можно по этому поводу почитать? И почему в англоязычном Инете так много ссылок на документы, где yield и plasticity встречаются со словом cap, а в русскоязычном про эту полусферическую поверхность очень мало документов?

В этом источнике http://www.engr.utk.edu/~drumm/ce538ce561/PlasticityModels-2.pdf
графически показана геометрия CAP поверхности, но искаженной, с элиптической частью.

В русском источнике в таком разделении использован термин "шаровая" вместо "сферическая":
Шаровая и девиаторная часть тензора и инварианты
http://www.gauss.ru/soft/mathemat/pinega/a12/a12.asp#3

Cap Plasticity Model - это математическая модель, основанная на таком разделении напряжений, имхо

Sorry, Если бы я видел перед глазами текст, может быть что-нибудь поумнее предложил.
Мне кажется, в некоторых местах Cap yield surface - это сферическая поверхность пластической деформации.

Shear yield surfaces as indicated in Fig. 4.2 do not explain the plastic volume strain that is measured in isotropic compression. A second type of yield surface must therefore be introduced to close the elastic region in the direction of the p-axis. Without such a cap type yield surface it would not be possible to formulate a model with independent input of both and . The triaxial modulus largely controls the shear yield surface and the oedometer modulus controls the cap yield surface. In fact, largely controls the magnitude of the plastic strains that are associated with the shear yield surface. Similarly, is used to control the magnitude of plastic strains that originate from the yield cap. In this section the yield cap will be described in full detail. To this end we consider the definition of the cap yield surface:
f c = (a = c cotj) (4.17)
where a is an auxiliary model parameter that relates to as will be discussed later. Further more we have p = - (s1+s2+s3) /3 and = s1+(d-1)s2-ds3 with d=(3+sinj) / (3-sinj). is a special stress measure for deviatoric stresses. In the special case of triaxial compression (-s1>-s2=-s3) it yields = -(s1-s3) and for triaxial extension (-s1=-s2>-s3) reduces to = -d(s1-s3). The magnitude of the yield cap is determined by the isotropic pre-consolidation stress pp. The hardening law relating pp to volumetric cap strain is:
= (4.18)
The volumetric cap strain is the plastic volumetric strain in isotropic compression. In addition to the well known constants m and pref there is another model constant b. Both a and b are cap parameters, but we will not use them as direct input parameters. Instead, we have relationships of the form:
a « (default: = 1-sinj)
b « (default: = )
such that and can be used as input parameters that determine the magnitude of a and b respectively. For understanding the shape of the yield cap, it should first of all be realised that it is an ellipse in p- -plane, as indicated in Fig. 4.8.
The ellipse has length pp on the p-axis and app on the -axis. Hence, pp determines its magnitude and a its aspect ratio. High values of a lead to steep caps underneath the Mohr-Coulomb line, whereas small a-values define caps that are much more pointed around the p-axis. The ellipse is used both as a yield surface and as a plastic potential. Hence:
= with: l = (4.19)
This expression for l derives from the yield condition f c = 0 and Eq. (4.18) for pp. Input data on initial pp-values is provided by means of the PLAXIS procedure for initial stresses. Here, pp is either computed from the inputted overconsolidation ratio (OCR) or the pre-overburden pressure (POP) (see Section Ошибка! Источник ссылки не найден.).

Figure 4.8 Yield surfaces of Hardening-Soil model in p- -plane. The elastic region can be further reduced by means of a tension cut-off
For understanding the yield surfaces in full detail, one should consider both Figs. 4.8 and 4.9. The first figure shows simple yield lines, whereas the second one depicts yield surfaces in principal stress space. Both the shear locus and the yield cap have the hexagonal shape of the classical Mohr-Coulomb failure criterion. In fact, the shear yield locus can expand up to the ultimate Mohr-Coulomb failure surface. The cap yield surface expands as a function of the pre-consolidation stress pp.
-σ1
-σ2 -σ 3
Figure 4.9 Representation of total yield contour of the Hardening-Soil model in principal stress space for cohesionless soil

Там два рисунка и несколько формул, но с ними мне справиться не удалось (в смысле, чтобы они отобразились)
Может это поможет?
Я пока пойду погуглю "шаровую поверхность"

С нетерпением жду ответа

Еще раз спасибо!!!!!