Not for the first time, I recently received an enquiry from a researcher who is trying to sort out the abundant literature on Soil-Rock Mixtures and the relationship between SRMs and bimrocks/bimsoils. There seems to be a lot of money spent on research on SRMs in the extensive colluvial/debris flow deposits of interest for dams, power-generating facilities and the like. The SRM research is very worthwhile and some of the resulting papers are excellent. (I like to think that the bimrock/bimsoil/SRM literature is composed of a few really good papers floating in a matrix of not-so-good stuff…).
The growing jumble of bimrocks/bimsoils/SRM literature is now confused even more by the 2021/2022 publication of two books by Dr. Yu Wang purporting to be about bimrocks: The Geomechanical Behaviors of Bimrocks and Cyclic and Fatigue Behaviors of Bimrocks .
I am glad to see that somebody finally wrote some books on bimrocks. I always intended that person to be me but, I have not had the energy, time nor ambition to do so. So: kudos to Yu Wang for being the first!
Based on a cursory review of the Wang bimrocks books: they are not comprehensive treatments of bimrocks, being essentially collections of Wang’s papers focused on relatively narrow research of the mechanical behaviors of lab-scale Soil Rock Mixtures. A major deficiency (at least from my limited review) is a lack of any geological context to bimrocks/bimsoil/SRMs. Further: though it is confirmed that mechanical behaviors depend (mostly) on Volumetric Block Proportion, there no examination of how one goes about estimating VBP at site scales, and how to adjust those estimates to account for uncertainty, as described by Medley (1997) and Napoli et al (2022).
I shall be reviewing Wang’s books in more detail the future.
This post is inspired by how the the Wang books approach SRMs – he includes bimrocks, bimsoils etc. as SRMs, in much the same way that most writers approach SRMs in the now hundreds of papers devoted to these (mostly) colluvial/debris flow deposits in China. Then by inference, the findings on bimrocks and bimsoils are incorporated into those for SRMs by declarations that bimrocks are SRMs and/or bimsoils. (To be fair to Wang: he offers other definitions of SRMs in his “Cyclic and Fatigue” book.)
A typical declaration is the one on page 102 of Wang’s book Cyclic and Fatigue Behaviors of Bimrocks :
“The soil and rock mixtures (SRMs) are a complicated formation composed of stiff blocks of various lithologies and sizes, embedded in a fragmented matrix of finer grained material, such as soil matrix (Figure 1). The geomaterials of opencast dumps, cemented waste rock backfills, weathered rocks, coarse pyroclastic rocks, mélanges breccias, coarse grain colluviums and alluviums are examples of SRM. In some literatures, these special inhomogeneous and unconsolidated geomaterial are also commonly referred to as blocks-in-matrix rock (bimrock) (Medley and Lindquist, 1995; Coli et al., 2011; Afifipour and Moarefvand, 2014) or blocks-in-matrix soils (bimsoils) (Kalender et al., 2014; Wang et al., 2017).“
p. 102, “Cyclic and Fatigue Behaviors of Bimrocks” (Wang, Y., 2022)
The principal problem with this definition is that it masks the vast range in geological character between (say) open cast dumps (very coarse soils) through colluvium and alluvium to melanges (rocks) – although the geomechanical behaviors may well be similar for some of the materials.
During my and Eric Lindquist’s 1994 PhD research on melanges, we discovered the pioneer paper of Irfan and Tang (1992) which was focused on the slope stability behavior of coarse, granite boulder-freighted colluvium in Hong Kong. That research yielded results almost identical to those of Lindquist’s lab-scale physical model melanges. Very different materials – similar geomechanical results. Yet it would be silly to term Lindquist’s model melanges as “colluvium”!.
The mechanical contrasts/constructabilty differences between the many geologically/geotechnically distinct components of Wang’s definition of SRMS can be extreme. Melanges may require blasting in excavation and tunneling due to often extremely strong and large blocks. Colluvium and alluvium can be often excavated with a hand or mechanical shovel. Some of the differences are apparent in the photos below:
My many technical reviews of SRM papers over the years started in 2005 with Xu et al’s (2007) Geotechnique paper “Some geotechnical properties of soil-rock mixtures in the Houtiao geoge area of China“- which went through a two-year process, including three extensive Medley-driven edits. Both Geotechnique and Medley were tired of the process and we have never worked together again. This was a pioneer paper on SRMs, despite the editorial pain I inflicted on the authors and Geotechnique.
Since reviewing Xu et al (2007) I have tried my best to help SRM researchers better discriminate bimrocks/bimsoils from SRMs. I will continue to do that until I am otherwise convinced that SRMs – which are generally loose to unlithified/uncemented to weakly cemented colluvial/debris flow deposits – with blocks less than 1m – are the same as bimrocks like melange, lahar or fault rock bimrocks, which contain blocks ranging from cms to 100s of meters in size and fit the definition of bimrocks of Medley (1994):
“mixtures of rocks composed of geotechnically significant blocks within a bonded matrix of finer texture“
– where “geotechnically significant blocks” 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.
With melange bimrocks, for example, lab-scale characterizations can be extrapolated to site-scale because of the scale independence of blocks sizes. Lab-scale masses are actual scale models of site-scale rock masses, and the mechanical behaviors are thus reasonably assumed to be similar. The same cannot be said of SRMs – until at least more studies are performed on the scale independence of block-size distributions in SRMs.
Certainly there is likely some scale-independence at the mm to dm scales. But for an SRM at the scale of a 30 m high slope (say) ,1 m fragments are barely even blocks – (using the height of the slope as the characteristic dimension Lc of 1.5 m and greater, and the block/matrix threshold of 0.05Lc) – and so the small blocks provide little mechanical benefit to the the overall stability of the slope. (See photos above).
I have reviewed scores of reviews of SRM papers over the years, and my often very blunt Reviewer observations about the differences between SRMs and bimrocks seem to have been acknowledged in the subsequently published papers. But then I am continually surprised that similar distinctions are not made in later papers published even by the same author and I (as Reviewer) have to write the same editorial education as for previous papers. Bimrocks being SRMs seems to be a sticky concept…
I tend now to say NO! to editors sending me SRM papers to review. It is tedious to say the same thing time and time again while reviewing the ceaseless flow of SRM papers. Eventually – just by brute force and publishing of “bimrocks” books like Wang’s – will the SRM experts succeed in enfolding bimrocks melanges and fault rocks into SRMs?.
It is likely time to start calling bimrocks/bimsoils/SRMs/etc. by more generic, all-encompassing terms like “bim masses“, bimstuff”(!) or, “complex geological mixtures”- an mouthful of an expression I have occasionally used. The trouble with the latter term is that it could also encompass the universal complexity of geological mixtures such as ore within gangue; contaminated ground water lenses surrounding by pristine groundwater; and so on. So “complex geological mixtures” is too broad.
In the 2022 paper “Practical classification of geotechnically complex formations with block-in-matrix fabrics” Dr. Maria Lia Napoli, Prof. Andrea Festa and Prof. Monica Barbero attempted to make more inclusive the enormous range of bimrocks/bimsoils/SRMs/etc by using the term “complex formations with block-in-matrix fabrics” which has particular appeal since the term “Complex Formations” was first coined in Italy and historically used in the 1970’s by geotechnical engineers and academics researching the behaviors of Italian olistostromes and melanges. Unfortunately the geological stratigraphic word “formations” could suggest well-behaved regional stratigraphic coherence – which is rarely the case when dealing with bimrocks at site scales. For this reason the world-famous “Franciscan Formation” of Northern California went through several name changes before landing on it’s current descriptive appellation of “Franciscan Complex”. “Complex” does a better job of suggesting the geological jumble of the Franciscan than the word “Formation”.
Napoli et al (2022) also introduce the appealing word: “bimunits”, to embrace the vast range of geological conditions expressed by fragmented and mixed materials (bim materials):
“The high internal heterogeneity and compositional variability of block-in-matrix units (“bimunits” in the following), which is mainly due to the strong rheological contrast between blocks and the matrix, extends the geotechnical complexity over a wide spectrum of complex formations, ranging from rocks to soils, with a significant engineering and societal impact (Medley and Zekkos, 2011).” – Napoli et al, 2022.
The “unit” in bimunit” is a geological stratigraphy word which Wikipedia defines as: ” A stratigraphic unit is a volume of rock of identifiable origin and relative age range that is defined by the distinctive and dominant, easily mapped and recognizable petrographic, lithologic or paleontologic features (facies) that characterize it.”
One could object to the “unit” component of “bimunit” as being too bound to geological stratigraphy, but I still think it is a better word than “formation”.
I encourage efforts to find a broader term for bimstuff along the lines of Napoli et al’s (2022) “bimunits” and “geotechnically complex formations”. By the way: Napoli et al (2022) has excellent image collections of bimunit examples (see here, also).
A less geological, looser expression is the term “bim materials” that I introduced (Medley,1992) and “block-in-matrix materials” (as well as “bimsoils”) in my PhD dissertation (Medley, 1994). Sonmez et al (2021) also used the term “bim materials” in “An Empirical Method for Predicting the Strength of Bim Materials Using Modifications of Lindquist’s and Leps’ Approaches“. I cannot recall where, when and how often the terms have been used in publications over the years, but “bim materials” and “bim masses” also seem to be reasonable catchall terms for the very broad range of complex mixed rocks/soils/etc. ranging through a broad range in scales of engineering interest such as: coarse alluvium, mine waste, colluvium, debris flows, weathered rocks, fault rocks and melanges.
So for now I suggest “bim materials” or the Napoli et al (2022) term “bimunits”.
So: bimrocks are NOT SRMs! Do not use “SRM” if what you are actually referring to is bimrocks like melanges or fault rocks!.
NOTE: this post is periodically revised whenever I am inspired by a random thought.