Abstract
The Boussinesq Problem (Joseph Valentin B., 1842-1929) consists in finding the elastic state in a linearly elastic isotropic half-space, subject to a concentrated load applied in a point of its boundary plane and perpendicular to it. This problem has wide geotechnical applications.
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Notes
- 1.
The information items needed for this calculation are:
$$\begin{aligned} {\varvec{S}}\varvec{g}^1=\sigma {\varvec{r}},\quad {\varvec{S}}\varvec{g}^2={\varvec{S}}\varvec{g}^3={\varvec{0}},\quad {\varvec{S}}{\varvec{h}}=(\sin \vartheta )\sigma {\varvec{r}};\quad {\varvec{r}},_{\vartheta \vartheta }=-{\varvec{r}},\quad {\varvec{r}},_{\varphi \varphi }=-(\sin \vartheta ){\varvec{h}}. \end{aligned}$$With this, one finds:
$$\begin{aligned} {\varvec{S}},_\rho \varvec{g}^1+{\varvec{S}},_\vartheta \varvec{g}^2+{\varvec{S}},_\varphi \varvec{g}^3&=({\varvec{S}}\varvec{g}^1),_\rho +\;({\varvec{S}}\varvec{g}^2),_\vartheta -\rho ^{-1}{\varvec{S}}{\varvec{r}},_{\vartheta \vartheta } +\;({\varvec{S}}\varvec{g}^3),_\varphi -(\rho \sin ^2\vartheta )^{-1}{\varvec{S}}{\varvec{r}},_{\varphi \varphi }\nonumber \\&=(\sigma {\varvec{r}}),_\rho +\;\rho ^{-1}\sigma {\varvec{r}}\;+ (\rho \sin ^2\vartheta )^{-1}(\sin \vartheta ){\varvec{S}}{\varvec{h}},\nonumber \end{aligned}$$whence (5.4) easily follows.
- 2.
On differentiating (5.3) with respect to \(\rho \), we quickly find that
$$\begin{aligned} \int _{-\pi /2}^{+\pi /2}\big ( 2\widehat{\sigma }(\rho ,\vartheta )+\rho \widehat{\sigma },_\rho (\rho ,\vartheta ) \big )\cos \vartheta \,d\vartheta =0. \end{aligned}$$We are then driven to choose a mapping \(\widehat{\sigma }\) that satisfies the partial differential equation (5.5).
- 3.
Point \(\vartheta =0\) is the only one in the interval \((-\pi /2,+\pi /2)\) where Eq. (5.7) is singular. The other fundamental solution of this equation being singular at that point is:
Here is a method to construct this solution. It is not difficult to show that (5.7) is equivalent to
$$\begin{aligned}(\sin \vartheta \,W(\vartheta ))^\prime =0, \end{aligned}$$where
is the wronskian of \(\widehat{\tau }\) and . Hence, modulo a constant,
$$\begin{aligned} W(\vartheta )=\frac{1}{\sin \vartheta }\,, \end{aligned}$$and the combination of the last two relations yields the following first order ODE for :
which can be re-written in the form
The last bit of information needed is:
$$\begin{aligned} \int \frac{1}{\sin \vartheta \cos ^2\vartheta }=\frac{1}{\cos \vartheta }+\log \tan \frac{\vartheta }{2}.\end{aligned}$$ - 4.
- 5.
To obtain the last two relations, it is useful to recall that
$$\begin{aligned} \nabla {\varvec{u}}={\varvec{u}},_z\otimes \ {{\varvec{e}}}_1+{\varvec{u}},_r\otimes \ {\varvec{h}}+r^{-1}{\varvec{u}},_\varphi \otimes \ {\varvec{h}}^\prime , \end{aligned}$$and that the physical components of \({\varvec{u}}\) are:
$$\begin{aligned} u_z:={\varvec{u}}\cdot {{\varvec{e}}}_1,\quad u_r:={\varvec{u}}\cdot {\varvec{h}},\quad u_\varphi :={\varvec{u}}\cdot {\varvec{h}}^\prime . \end{aligned}$$ - 6.
For an alternative way to deduce this condition, one writes (5.13) and (5.14) in the form:
$$\begin{aligned}(r\sigma _4),_r=-r\sigma _1,z,\quad (r\sigma _4),z=\sigma _3-(r\sigma _2),_r; \end{aligned}$$differentiates the first equation with respect to \(z\), the second with respect to \(r\): and finishes by eliminating \((r\sigma _4),_{zr}\).
- 7.
To take the last step in the calculation, use has been made of the following alternative version of (5.13):
$$\begin{aligned} r^{-1}\sigma _4,_r+\;r^{-2}\sigma _4=-r^{-1}\sigma _1,z. \end{aligned}$$ - 8.
- 9.
\(R\) is star-shaped if there is a point \(p_0\in R\) such that the line segment from \(p_0\) to any point \(p\in \partial R\) intersects \(\partial R\) only at \(p\) itself.
- 10.
Revert to the footnote in Sect. 5.2.2.
- 11.
- 12.
In [7], Flamant himself recognizes his debts to Boussinesq.
- 13.
This result follows from the fact that
$$\begin{aligned}\int \frac{1}{x_2^2+x_3^2}\,dx_3=|x_2|^{-1}\arctan \frac{x_3}{|x_2|}.\end{aligned}$$
References
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Podio-Guidugli, P., Favata, A. (2014). The Boussinesq Problem. In: Elasticity for Geotechnicians. Solid Mechanics and Its Applications, vol 204. Springer, Cham. https://doi.org/10.1007/978-3-319-01258-2_5
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