Mitigation options for sewer in landslide

[civilman72]

*Not sure if this belongs in the geotechnical forum, but I'll start here.*

I'm working on a proposed sanitary sewer design for new 15"-18" gravity-fed main, three total miles.

I just received a phone call from the geotechnical engineer and about 1000'-1500' of the proposed main crosses through a landslide material, some of which is active.

The geotechnical report has not been finalized, so I do not have details on the movement, depth, etc. of the material, but I'm wondering what options I might be able to explore.

I do not have the option of completely relocating the main to avoid the slide area, nor can my client afford the costs of stabilizing the slide material.

One option, that seems reasonable, would be to bring the main to the surface with pre-insulated pipe and install the pipe hangars into bedrock.

The other (cheaper) option might be to use buried C900 pipe with pipe restraints and have the District monitor the pipe for movement and possible repair. If this option is feasible, is there any advantage to casing the pipe?

Your thoughts would be appreciated.

 

[bimr]

Why would you put sewers through an area disturbed by a landslide? It would seem that the houses sould be abandoned.

Have you considered using a fused pvc or hdepe product. These produccts would probably stay together better than jointed pipe.

Not sure that there is much merit in installing pipe supports in bedrock. If the slide moves, one would think that the slide would shear off the suopports.

It would seem that a pumped system using flexible pipe products such as HDPE would have the longest lift expectancy. However, pumped systems are more practical at smaller diameters.

 

[civilman72]

Thanks bimr.

There are no homes (or services) proposed in the landslide area.

Installing a pump system would be counter-intuitive - the main reason this gravity-fed system is being proposed is to eliminate two existing lift stations, and I agree that the pipe size would make pumping undesireable.

I will look into fused pvc - I've used it for water transmission lines, not for sewer.

 

[rconner]

While many types of piping material could probably be furnished with devices that accommodate some magnitude of deformation and movement, it is a little hard for me to get my mind around the concept of a gravity sewer pipeline (normally installed to line and grade, and intended to remain reasonably so?) designed actually through a known landslide formation, and without somehow stabilizing either the slope or pipeline. Without knowing more, it is therefore kind of hard for me to imagine how this might help either the risks presented by the landslide or the pipeline itself. While it may not be the case in your situation, risks associated with various types of landslides would appear to be in and of themselves a quite complicated business, and sometimes with quite high stakes, as discussed at e.g.

http://www.oas.org/dsd/publications/unit/oea66e/ch10.htm

.

 

[cvg]

construct with as flexible pipe material as you can. use flexible connections at joints and manholes. construct with as steep of slopes as you can. then monitor the sewer so that when the eventual movement occurs, you can catch it before the line breaks. good luck with this last part. It might be useful to install fiber optic or other type of monitoring system to get real time data on the slide movement.

 

[civilman72]

Both posts from rconner and cvg mimic my thoughts/concerns.

On one hand, I appreciate the uncertainty and potential destructive power of a landslide, particularly considering a pipe rupture in this location could discharge waste into a river, which is upstream of the water intake for Town treatment facility. But I also realize (based on the geotechnical report evaluation) that the potential for a quick, total slope failure is almost nil, and that there are inexpensive, remote monitoring techniques that can give sufficient warning before the pipe integrity becomes suspect.

I received the geotechnical report yesterday, and they provided the following information:

- Landslide material exists along 3,500'-4,000' of proposed sewer main. Approximately 300'-400' is active. They observed signs of recent movement and 'toe bulging.'

- Average estimated thickness of landslide material is 20'.

- The mapped landslides generally appear to be 'stable', with safety factors just above 1 (except for toe bulge areas).

- The groundwater levels this year are much higher than normal, and this may be causing some of the movement observed this year.

The geotechnical mitigation recommendations are to use joint restraints, install an underdrain system with discharge pipes and monitor the system.

My next step is to discuss with my client (the Town) and the sewer district engineer and get their thoughts.

 

[dicksewerrat]

Just how are you going to construct this pipe? Dig a trench across the face of a slope? If there is movement now, watch it speed up once you start disturbing the slope with equipment.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[rconner]

While I guess their conditions could be more severe than what you are potentially dealing with, I noticed that at

http://www.bjservices.com/website/p...GEOPIGGeotechnicalInstability.pdf?OpenElement

it is reported that in just a relatively recent 7 year span a section of a several decades old 30 inch welded steel gas pipeline I suspect originally trenched subsided 0.3 m(nearly 12 inches) with transverse movement of 1.5 m(nearly 5 feet) in a now identified movement area on a 20 degree slope. While certainly not professing much expertise in such operations, it just seems like movements less than this could conceivably cause operational or other problems in a conventional gravity sewer design (normally kept about on grade).
While I realize cost and operation are not optimum like undisturbed gravity in an ideal world, any thought of e.g. say a double-barrel (some redundancy?) siphon design or forcemain perhaps installed (drilled/HDD?) comfortably below the soil strata your Geotech says is subject to "slide"?