WHAT ARE BIMROCKS?
- block-in-matrix rocks
- mixtures of rocks composed of geotechnically significant blocks within a bonded matrix of finer texture
- melanges, fault rocks, weathered rocks, etc.
- geologically, spatially, and mechanically heterogeneous
- troublesome to geotechnical engineers, geologists, contractors and owners
The words "geotechnically significant" in the above definition of bimrocks mean that there are criteria for scale, strength contrast, proportion and size of blocks. In bimrocks, blocks must have mechanical contrast with the matrix and, at the scales of engineering interest (from laboratory to site) there must be enough blocks of a size range to contribute to the overall strength of the rock mass.
ISN’T “BIMROCKS” JUST A SILLY WORD?.
“Bimrocks” is a word that I coined in 1992 for my PhD Oral Examination. “Bimrocks” is not a geological word, but simply describes the vast range of geological fabrics of strong blocks mixed with weaker matrix. Since there are no geological shadings to the word, engineers can focus on the characterization challenges rather than fuss about the geological words and the geological origins of bimrocks.
ARE BIMROCKS RARE? No, they are very common.
Bimrocks include many complex geological mixtures of relatively strong blocks surrounded by weaker matrix: fault rocks, weathered rocks, lahars and melanges. Chaotic melanges are the most difficult bimrocks to characterize. (Personal note: a Northern California melange had a major impact in my life, as described in this story. )
ARE BIMSOILS THE SAME AS BIMROCKS? No. They are not.
Bimsoils are analogous to bimrocks but are composed of blocks embedded in soil (unbonded or uncemented materials) with contacts between blocks and matrix that are “unwelded” or are weak or have no strength. Bimsoils include weathered rocks, saprolites, lahars, debris flows, and colluvium.
ARE BIMSOILS AND BIMROCKS THE SAME AS SRMs? Usually: No.
In the last decade hundreds of papers have been published about SRMs (Soil-Rock Mixtures) generally by researchers describing widespread colluvium/debris flow deposits in China, in which the largest block sizes are often limited to about a meter. Because the soils are generally uncemented with little strength between blocks and matrix, they are limited-scale bimsoils: bimsoils at laboratory scale but not at the scale of a 30 m high hillside or excavation (because the blocks at that scale have little geomechanical effect). Nevertheless: SRM researchers persistently claim, (wrongly) that SRMs include bimrocks (such as melanges or fault rocks), and then inappropriately incorporate the many findings on bimrocks (which are rocks with blocks sometimes 100s of meters in size) ) into their analyses of SRMs (soils, with blocks less than about 1 meter).
NOTE: I confess that very early in my research writings I too sometimes called bimrocks “Soil/Rock Mixtures” to help geotechnical engineers (who generally work with soil) and rock engineers identify with the titles of my earlier papers – “bimrocks” was not a common word then.
SO WHAT? WHO CARES?
Bimrocks and bimsoils (and yes, SRMs) universally frustrate the economic and accurate characterization, design and construction of civil engineering works. Bimrocks and bimsoils are troublesome to geopractitioners, who include geological engineers, geotechnical engineers and engineering geologists. Owners and contractors care a lot, too – they lose money because of the troubles, although their lawyers help resolve the troubles.
NEED BASIC INFORMATION ON BIMROCKS?
Try the resources listed below (NB: there is much redundancy/duplication):
- 25 Years of Progress in Characterizing Melanges, Bimrocks, and Similar Rock-Soil Mixtures (Lecture, 2019)
- Field Trip Guide provides snap shot views of San Francisco area bimrocks (2018)
- Geopractitioner Approaches to Working With Anti-Social Mélanges, detailed book chapter Review of melanges and bimrocks (2011)
- “Geotechnical and Geological Engineering with Melanges, Fault Rocks and Other Bimrocks” taught at the National University of Columbia at Medellín, Colombia on September 18-22, 2017
- Introduction to Bimrocks (2006) (25 MB) an old-fashioned style of PowerPoint presentation on bimrock basics, but with animated graphics
- Presentation Notes (2006) abundant, very detailed notes that are useful as a guides to the “Introduction to Bimrocks” Presentation as well as being a stand-alone resource
WHAT ELSE IS ON THIS WEBSITE?
The bimrock website provides papers, presentations, dissertations, posters, Short Courses and photographs by myself and others who have kindly given their permission for me to host their work. (The only other site like this one is my original bimrocks site hosted by Geoengineer.org – which remains in its original clunky format.)
Please contact me if you want to contribute content to this site. Note that I only post useful and well-presented material.
Use the resources freely. Please credit authors! All resources are provided on the condition that they be used for sole use, and for the non-commercial purpose of scientific and engineering research.
Visit my bimrocks blog for infrequent thoughts, ideas, information and comments by myself and the occasional visitor.
Questions, comments, contributions? Contact: firstname.lastname@example.org