U. S. DEPARTMENT OF AGRICULTURE
FOREST SERVICE
WASATCH NATIONAL FOREST

ALTA AVALANCHE STUDY CENTER

TRANSLATION NO. 6

AVALANCHE FORCES
AND
THE PROTECTION OF OBJECTS

by
E. Sommerhalder
Lawinenkraefte und Objectschutz
Winterbericht des Eidg. Institut fuer Schnee- und Lawinenforschung, Nr. 29, 1964/65


Translated by E. LaChapelle
November 1967

1. Introduction

This contribution is addressed primarily to the practical man who comes into contact with the problems of avalanche effects on structures in the course of his professional work in the mountains. An attempt is made to estimate avalanche forces by assembling a number of relatively simple formulas. But a certain experience and familiarity with snow and avalanches are still assumed for an intelligent treatment of the subject.

The following exposition is based on the latest available theoretical studies, on current experience from test installations of the Institute for Snow and Avalanche Research (SLF), and on the study of specific problems in the Swiss Alps.

B. Salm: Contribution to avalanche dynamics. Mitteilung Nr. 24 des Eidg. Instituts fur Schnee- und Lawinenforschung vom. Februar 1966

B. Salm: Lawinenwirkung und bauliche Schutzmassnahmen. (Avalanche effects and construction measures for defense.) Referat anlasslich eines Lawinenzonenkurses vom 8./9. Nov. 1962 (Unpublished)

A. Voellmy: Ueber die Zerstorungskraft von Lawinen. (On the destructive force of avalanches.) Sonderdruck aus der Schweiz. Bauzeitung, 73rd year, Vol. 12, 15,
17 and 37 (1955) Available in English as Transl. No. 2, U. S. Forest Service Alta Avalanche Study Center.


As a rule, these cited works deal with simple avalanche models in which are introduced materials constants for static and moving snow whose numerical values are only partly known.

For this reason a high degree of accuracy cannot be assigned to the results obtained up to this time. The practical problems are often complex and from case to case have to be adapted to the formulas.

A thorough treatment of the whole subject would lead far afield and, moreover, is not the purpose of this manual. Those more deeply engaged in avalanche problems who want to delve further into the subject will refer to the pertinent literature.


2. Problems

Structures often have to be protected from damaging avalanche effects by constructive measures directly at the protected object or in its immediate vicinity.
- reinforced walls
- backfills
- wedges
- deflection walls
- roofs
- snow sheds, etc.

The protective structures conform to the expected loading from static and moving snow. In calculating these forces, exceptional avalanche conditions have to be considered on one hand, but on the other it often is not possible on financial grounds to allow for the maximum conceivable conditions when calculating the normal permissible loads. For such cases it is necessary to turn to the normally employed safety factors. There will be a certain residual., unprotected risk which those in charge of the construction have to take into account and allow for.


3. Preparatory Work

3.1 Field Check

- Inspection of the locality. (All the important features cannot be seen on a map or plan.)
- Investigate terrain forms and exposure clear up to the avalanche fracture zone.
- Listen to statements of local residents, but accept these critically.

3.2 Study of Records

- Snow depths, weather influences (wind, temperature, etc,) from existing records of nearby stations.
- Reports of earlier avalanche activity (possibly old records or chronicles).

3.3 Determinations from Maps or Overall Plan

- formation zone ---                                          
- fall path ----------    altitudes, height differences, sizes, area, slopes
- deposition zone---

Figure 1 gives an example in 100-meter steps of height differences.



Figure 1



3.4 Judging the Profile Forms


Figure 2

Establishment of a longitudinal profile along the entire avalanche path (overview!)
1 - even fall path
2 - Outrun zone bounded above by a steep section:
    - the snow cover at "b" is stabilized by "all, thus as a rule exceptionally heavy snowfalls are not cause for alarm.
3 - steep section in the fall path:
    - large snow deposits are possible at "all, dry loose snow avalanches develop into powder avalanches at "b".

Many loose snow avalanches occur on slopes steeper than 40 degrees to 45 degrees. As a rule, the slopes unload themselves in succession, especially when they are interrupted by a steep cliff band. Fall path cross-sections:


 
Figure 3
1- slope
2 & 3 - gulley forms (augmented friction) - gulley avalanches with a large depth of flow


4. Assumptions

4.1 Possible or Noteworthy Avalanche Characteristics

Avalanche situations:
- persistent snowfall
- poor snow cover development
- snow cover wet through
- local snow accumulation by wind transport

Avalanche types:
- powder avalanches mid-winter - dry loose snow avalanches
- wet loose snow avalanches., ground avalanches - spring

4.2 Standard Avalanche Situations: A given thickness of the sliding snow layer over the entire release zone is assumed for the various situations.

- Normal: h_o = 50 cm for the fundamental planning (e.g., snow removal from mountain highways,, avalanche zoning plans).
- Unusual: h_o = 120 cm for force calculations
- Extreme: h_o 120 cm is seldom allowed for (catastrophes)

4.3 Location of the Protected Objects.
- directly in the fall zone of the avalanche path: the object is exposed to all types of avalanches.
- In the run-out zone (deposition area): velocities are reduced, but the deposited masses of avalanche snow have to be considered.
- in the edge zones, on elevated sites: object outside the sliding and deposition zones, possibly exposed only to air blast.

4.4 Bases for the Calculations



Assumptions about the individual quantities:
h_o  Unusual avalanche conditions
      h_o = 1.2 m for normal slope profile and normal deposition (possibly higher assumed value for wind drift areas and short slopes).
      h_o = 0.5 m for slopes broken by cliff bands





   







5. Calculations

For slope breaks:


Figure 4


h₁       Gulleys:
           h depends on the form of the gulley cross-section and the quantity of flowing snow., Q




Figure 5

Assume different values of h (h₁ .... h_n). Use these to calculate different flow quantities Q₁....Q_n, using the hydraulic radius. The value of Q_i which corresponds to Q_o gives the sought value of h₁ as well as v₁.
F = cross-section area
U =  circumference of ground surface affected by avalanche
R =  hydraulic radius
Q =  snow quantity (discharge quantity)
b_m =  mean avalanche width

Slope break in gulleys:
The desired flow height, h_u, is obtained by trial in analogous manner with the table for calculating flow height in gulleys.





s           Outrun path (after Voellmy-Salm gives more uncertain values, but the formula does not lend itself to approximations).
             - does the object lie within the avalanche path?
             - forces reduced?
             - deposition of avalanche snow? etc.




6. Force Calculations

For exposed structural parts, an upward vertical force in addition to the pressure in the direction of avalanche impact has to be considered:



- The protective structures are stressed to the height of the avalanche.
- The deposited (natural or by avalanche) snow masses have to be considered as well as the moving ones.
- Ramming effects of transported foreign bodies (stones,, timber, etc.)
- The damaging effect of wind blast is light, maximum 0.5 t/m2.
- 0.1 - 0.2 t/m2 is sufficient to break in windows or weak doors.


The Important Types of Loading