A COMPARISON OF THE DIGITAL RESISTOGRAPH WITH THE RAM PENETROMETER. R. L. Brown1 Karl Birkeland2 ABSTRACT

Transcription

1 A COMPARISON OF THE DIGITAL RESISTOGRAPH WITH THE RAM PENETROMETER R. L. Brwn1 Karl Birkeland2 ABSTRACT The digital resistgraph (DR) is a micrprcessr-based prbe which is used t rapidly btain strength index prfiles similar t the prfiles btained with the ram penetrmeter. The current versin f the DR can cllect as many as 500 strength prfiles in a wrk day. In this study the DR is cmpared t the ram penetrmeter fr repeatability, learnability and ease f use. Additinal studies were cnducted t determine just hw sensitive the DR readings are t penetratin rate. The test results did shw that the instrument has a repeatability at least as gd as the ram penetrmeter. The DR has a decided edge in terms f learnability. Tw nvices were fund t give rughly the same DR prfiles after a minute f instructin, whereas the ram penetrmeter is widely knwn t shw a wide range f results when used by nvices. One f the the primary prblems with the DR was durability, as the instrument frequently malfunctined and failed t wrk prperly in cld cnditins. The DR shws much prmise fr becming a substantial imprvement ver the ram penetrmeter, prvided a mre reliable and durable versin can be engineered. INTRODUCTION The ram penetrmeter has fr many years been the standard instrument fr btaining strength prfiles f alpine snw cver (Perla and Martenelli, 1975). Once mastered, the instrument can give repeatable strength prfiles in typical alpine snw. Hwever snw cver with thin weak layers r ice crusts always psed a prblem fr all but very experienced users. In additin an instant prfile plt culd nt be btained, since the strength index had t be calculated and then pltted by hand. Typically tw persns were necessary t btain the prfiles, as ne wuld perate the prbe and 1. C/AE Department, Mntana State University, Bzeman, MT Earth Science Dept., Mntana State University, Bzeman, MT

2 hammer while the ther recrded the data. In spite f these shrtcmings the ram penetrmeter prved t be a simple, durable and reliable instrument which in the hands f an experienced peratr prvided repeatable prfiles f snw cver strength. C. C. Bradley (1966) later develped the resistgraph in an attempt t reslve sme f the shrtcmings f the ram penetrmeter. This instrument culd prduce a graphical plt f the strength prfile which culd be instantly reviewed in the field. In this way it helped the field persn make an n-site stability assessment, smething that culd nt be dne with the ram penetrmeter. In additin the resistgraph culd btain prfiles cnsiderably mre quickly than with the ram penetrmeter. Sme disadvantages included user sensitivity, an appreciable learning curve, an a significant weight factr which affected its prtability. As such it did represent a significant imprvement ver the ram penetrmeter. Hwever, its weight and cst prevented it frm becming widely accepted as a field instrument. The digital resistgraph (DR) was recently develped in an attempt t btain an instrument which culd quickly prvide strength prfiles similar the ram penetrmeter (Dwd and Brwn, 1984). The primary mtivatin was the develpment f an instrument which culd digitally stre the prfiles and allw rapid and repeated prfiling and instant graphical display f strength prfiles in the field. In additin it was designed s that the data culd later be ff-laded t btain hard cpies f the prfiles. The riginal instrument develped by Dwd and Brwn (1984) satisfied all f these requirements. Hwever this riginal mdel culd nly stre abut 25 prfiles, and the data culd nt be cnveniently ff-laded nt a persnal cmputer. Rather it was designed t laded directly nt an X-Y pltter, but this was a very time cnsuming prcess. In a research study by Karl Birkeland n strength distributins in avalanche paths (Birkeland et ai, 1990), a very large number f strength prfiles (n the rder f 1000 prfiles) was required in the span f a few days, thereby necessitating an instrument which wuld have much larger memry and faster data transfer t permanent strage fr later analysis. This resulted with the develpment f a secnd generatin instrument. The instant plt feature was drpped frm this secnd mdel, since this feature culd nt be develped in time fr the winter seasn. The secnd mdel did include a mre advanced micrprcessr and EPROM (erasable prgrammable read nly memry) mldules which culd be plugged int the DR and culd stre up t 150 prfiles. These culd quickly be unplugged and a' new ne installed t cntinue prfiling. As many as

3 prfiles culd be made in ne day, thereby making this an excellent research instrument. This study was cnducted in rder t determine if a micrprcessr-based penetrmeter such as the DR culd prvide strength prfile infrmatin with gd accuracy and reliability. The standard against which the DR was cmpared was the ram penetrmeter, since this is the mst widely used instrument. Measures f the instrument perfrmance included repeatability, learnability, sensitivity t insertin rate, durability and reliability. These last tw criteria are determined by the way the instrument is engineered s that it perates prperly under a wide variety f cnditins and can withstand sustained use. Repeatability is a measure f hw clsely the instrument can reprduce prfiles when used repeatedly in identical snw cvers. This is nt necessarily a measure f accuracy, since prfiles taken directly adjacent t each ther may give very similar prfiles yet fail t reflect the actual snw cver stratigraphy. At the same time, spatial variability in natural snw cver makes this test smewhat difficult t perfrm with abslute cnfidence that tw adjacent prfiles are actually measuring identical stratigraphies. Learnability is a subjective measure f the ease with which the instrument can be learned. The ram penetrmeter is an instrument that requires practice befre ne can use it in a wide variety f snw cvers and feel cnfident that a gd measure f the actual strength prfile is being btained. Expert instructin, f curse, decreases the learning time, but fr mst field persnnel, mastering this instrument requires practice. As with the measure f repeatability, the assessment f learnability is subjective, and any cmparisn with ther instruments such as the ram penetrmeter will reflect the user's wn preferences. Sensitivity t insertin rate is a measure f hw the instrument readings will change with the rate at which the penetrmeter is pushed thrugh the snw cver. Previus studies by Dwd and Brwn (1986) indicated that the readings prduced with the DR were nt significantly affected by the insertin rate, but this previus study did nt include insertin rates appraching ne meter per secnd. The study in the winter was cnducted late in the winter n Bradley Meadw in the Bridger Range. Due t the winter weather, the snw cver had an uncmplicated stratigraphy with nly a few thin crusts r hard layers. The secnd study was perfrmed late in the winter f just after a large strm event which prduce apprximately ne 21

4 meter f snwfall. The site was at the Bridger research site just nrth f Bridger Bwl Ski Area. This snw cver als did nt have much layering but was substantially deeper than the previus year. FIELD EVALUATION OF THE DIGITAL RESISTOGRAPH Repeatability Studies: An earlier study (Dwd and Brwn, 1986) had shwn that, with an experienced peratr, the DR culd give reprducible results and that it prvided excellent sensitivity t layering. T supplement these earlier studies a user with a mderate amunt f experience (apprximately 30 prfiles ver a five year perid) perfrmed multiple prbes f late winter snw cver in bth 1989 and The results f these evaluatins are shwn in Figures l(a) and 2(a). The first study was dne n Bradley Meadw and cnsisted f making fur prfiles space at 30 cm intervals alng a straight line perpendicular t the fall line. Frm Figure l(a), it can be seen that the three prfiles shwn there give very similar prfiles, althugh sme deviatin begins t emerge belw 140 cm fr ne f the prfiles. It shuld be nted that ne prfile was terminated at 120 cm where a hard crust was situated, giving a false impressin that this was the grund level. A furth prfile which is nt shwn was very similar t the three shwn here. The standard errr fr the three prfiles swn in Figure l(a) was fund t be E = The standard errr is determined by the expressin: N M E =_1_ L L IR ji N M j = 1 i = 1 - R av9i I (1 ) N is the number f prfiles taken, and and M is the number f pints in each prfile where the strength was measured. Rji and R avei are, respectively, the strength index f the /h prfile at the i th data pint and the averaged strength index at the i th pint. Ordinarily this wuld give a gd measure f the repeatability f the strength prfiles. Hwever, if there ccurs a shift in the depths at which specific layers ccur between prfiles, a large deviatin between prfiles can result even if the prfiles lk the same. Such shifts in psitin f layers can ccur fr a number f reasns, including variatins in depsitin, wind scur r drifting. Anther prbable cause wuld be peratr errr in the starting psitin f the penetrmeter cne at the beginning f the prbe. If the readings are averaged ver the depth f the prfile, the crrelatins lk much better, 22

5 BRADLEY MEADOW 4/13/09 DTR samp1 e rate: 0.5 em Prbe speed: Apprx. 30 emls 1 L E 80 u ~< , L.-~--r-.J. '-_-I::::=::a...:::::=a~ ~ ~---- ~ ~ r ~ I ~.-=J I -;:~ L: ===>' ~ ~~ DR Strength Index ~...~~._~=.J Strength Index (a) (b) Fi gure 1. Prf;1es n Breldl ey Meeldw. Part (a) ; 11 ustrates three DR prf;1es taken in 30 cm ;nterva1s. Part (b) cmpares the average f the three DR prflles with tw ram prflles taken d;rectly adjacent t the DR prfiles. 23

6 OR I DGER RESEARCH SITE 3/1/90 40 E C) 80.. &:..Q, 120 C» Q "--, I I 160 z ----, -----~ r-----t RAM Strength Index (0) DR Strength Index (b) Figure 2. DR end rem prfiles teken at the Bridger she. Pert (a) 111 ustrates tw ram prfl1es taken wi thi n 30 cm f each ther. Pert (b) g1 ves 0 cmper1 sn f tw DR prflles with ne f the ram prflles. 24

7 as any shift in depth f layering is averaged ut. The three DR prfiles had an average index f R = 56.9 ver the first 120 cm with a standard deviatin f D = The ram prfiles, which were taken directly adjacent t the DR prfiles, had an average ram index f R = 47.3 with a standard deviatin f 4.88 fr the tp 120 cm f the snw cver. The tp 120 cm was chsen, since fr bth the DR and ram prfiles, ne prfile was terminated at that depth. The lwer average ram index appears t be attributable t the lwer values in the tp 60 cm f the snw cver. Between depths f 60 cm and 120 cm the tw instruments gave very similar values. Fr the Bridger prfiles taken the fllwing year, similar results were btained. The three DR prfiles taken at this site had an average strength index f ver the tp 200 cm with a standard deviatin f The ram prfiles gave an averaged ram index f with a standard deviatin f D = The relative perfrmance f these tw instruments will be discussed later. Speed Studies: Figure 3 illustrates the results f studies t determine the effect f insertin rate n the DR strength readings. Part (a) f that figure shws three prfiles taken at different insertin rates. It can be nted that the general characteristics f the three curves are similar. Over the first 100 cm f penetratin the prfiles are s clse that it is difficult t distinguish the three prfiles. Hwever belw this level there is a significant shift with respect t depth fr the fastest insertin rate. In fact all f the prfiles experience increased deviatins belw 150 cm. This may be due t either spatial variatins due t natural causes, the instrument r the peratr. The mean DR strength indices fr the three prfiles was R = 86.8, 91.8, and 88.4, respectively fr the slw, intermediate and fast speeds. The standard deviatin fr the three prfiles was' D = The speed studies cnducted the fllwing year at the Bridger site are f questinable value, as the DR did nt appear t be functining prperly. During the prfiling it was recrded the lad value nly every 4 cm rather than every 0.5 cm as it was prgramed t d. The reasn fr this is nt clear. Figure 3(b) shws the prfiles fr the three speeds indicated. Only the tp 125 cm are shwn, since the DR stpped perating at that depth n the fastest prfile. As can be seen, the crrelatin between the three prfiles is very gd until a depth f 105 cm is reached, where the fastest 25

8 em!s em!s em!s '1 0 ~ -- aoem/see -- 30e./sec -- aoem/sec IV E u, 100 s=..-i ~ C» Q DR Strength Index (a) \! I' I DR Strength Index (b) Figure 3. Speed studies fr the digital resistgraph. P8rt (a) shws resul ts n Bradley Meadw (4/13/89), and Part (b) illustrates results f simi10r studies t the Bridger site (3/1/90). 26

9 NOVI CE STUDI ES BRIDGER SITE 3/1/90 25 Z E 15 u c,.J a CD Cl DR Strentgh Index DR strength Index (a) Fi rst nvi ce (b) Secnd nvice Figure 4. Prftles tken by tw nvices t ssess vribi1ity f results btained withut frmal instructin. 27

10 prfile deviated substantially frm the ther tw. It shuld be nted that the tp 100 cm f snw cver was new snw frm a recent strm and is therefre fairly unifrm in prperties. Cnsequently the test shwn in figure 3(b) is nt a particularly gd evaluatin, ther than t say that penetratin speed des nt appear t affect the measured strength index under unifrm snw stratigraphy. Nvice Studies: Figure 4 illustrates the prfiles btained by tw novices at the Bridger site. Bth peratrs had never used a DR befre Just prir t making the prfiles shwn here they were given nly abut ne minute f instructin n the use f the instrument. The penetratin speed used by the nvices were rughly abut 30 cm/s, althugh the penetratin speed prbably varied cnsiderably during prfiling and between individual prfiles. The peratr fr part (a) had a mean DR index f R = 49.7 fr the three prfiles with a standard deviatin f D = 2.57, while the peratr fr part (b) btained a mean index f R = 53.5 with D = These values f standard deviatin were larger than btained by the peratr (with mderate experience) fr Figures 1 and 2. DISCUSSION AND CONCLUSIONS It has already been demnstrated that the DR des have the ptential fr. becming a very effective tl fr evaluating slpe stability and fr use as a research tl. Amng the majr advantages are: (a) the ability t rapidly take prfiles repeatedly, (b) instant display f strength index prfiles in the field, and (c) digital strage in the field f a large number f prfiles which can later be quickly ff-laded nt a persnal cmputer fr hard cpies. The study perfrmed here was dne t determine if the DR culd prduce data f a quality similar t the ram penetrmeter. By bserving Figures 1 and 2, ne can see that the DR des have a finer reslutin than the ram penetrmeter fr detecting subtle layering. One reasn fr this is that the DR uses a smaller cne than the ram. It will detect thin layers quite readily, whereas the ram will tend t punch thrugh these small layers undetected, unless the peratr is experienced and careful t wrk his way slwly dwn thrugh the layering. One annying characteristic f the DR in its current cnfiguratin IS the "jitter" displayed in the prfiles. This is prbably due t the fact that as it is pushed dwn thrugh the snw cver, the snw structure experiences intermittent cllapse adjacent t the cne. The current DR is prgrammed 28

11 t measure the resistance every 0.5 cm. Cnsequently the resistance reading may at ne time be made just prir t cllapse and the next time just after the cllapse f the adjining snw, resulting with this jitter. This culd readily be dealt with by prgramming the DR t read the resistance every 0.1 cm and average every five reading t smth ut the data that is actually stred in the DR memry. In terms f repeatability, the DR and ram appeared t be equal in perfrmance. Figures 1 and 2 shw that they prduced similar prfiles, and the tw instruments appeared t achieve similar crrelatin between successive prfiles. Insertin rate fr the DR des nt appear t be a prblem. Figure 3 shws that there is n appreciable affect n perfrmance fr insertin rates between 10 cm/s and 50 cm/s. Abve 70 cm/s there appeared t be sme shift in the utput. Hwever this is a very fast rate f penetratin and requires a cncerted effrt t cnsistently make successive prfiles at this rate. The authrs have fund that rates n the rder f cm/s was mre cmfrtable t use and culd be dne smthly and cnsistently. This rate requires nly a few secnds t cmplete a prfile fr nrmal snw cver depths in the Bridger Range. In terms f learnability, the DR, like the ram penetrmeter, requires a learning curve. We culd nt make a direct cmparisn with the ram, since we did nt perfrm nvice studies with the ram. As a cnsequence ur cnclusins will must be based upn ur past experience in training nvices in the use f the ram penetrmeter. The DR, while appearing t be easier t learn, did shw that ne has t develp the technique f cnsistently inserting the DR thrugh the snw cver at a cnstant and repeatable speed withut any jerks r stps. Figure 4 shws the prfiles f tw nvices wh had n previus experience, whereas Figure l(a) illustrates the prfiles fr an peratr with mderate experience ( abut 30 prfiles ver a five year perid). The tw prfiles shwn in Figure 2(b) were als dne with this same peratr. These results d indicate that the DR can be perated in a reliable manner after a 30 minute practice sessin. In the view f the authrs, this makes it smewhat easier t learn than the ram penetrmeter. A few wrds shuld be said abut durability f the DR. In its current cnfiguratin the DR displayed a prpensity t malfunctin, s much s that it never culd be used fr mre than several days withut sme prblem arising. One prblem was assciated with the penetrmeter tip cntaining the cne and lad cell fr measuring the cne resistance. The assembly wuld wear ut after nly 1000 t 1200 prfiles. While this may 29

12 seem like a cnsiderable number f prfiles, that number can actually be achieved in nly three days f wrk. This significantly limited its value as a research instrument. Anther prblem was assciated with the erratic behavir f the micrprcessr and the liquid crystal display. Often the display wuld nt functin, and this culd be attributed t lw temperatures. The micr prcessr ften did nt recrd data in the apprpriate manner, smetimes skipping data pints. In rder fr the DR t becme accepted as a gd field instrument, these prblems will have t be crrected. This invlves engineering the instrument s that the next design will have the reliability needed in a field instrument. Use f lw temperature liquid crystal displays, mill-spec electrnics and mre durable (and prperly sealed) lad cells shuld crrect these prblems. REFERENCES Birkeland, K., K. Hansen-Bristw, and R. L. Brwn, 1990, "The gegraphic variability f snw strength in ptential avalanche paths," Prceedings f the ISSW '90 Wrkshp, Bigfrk MT Bradley, C. C., 1966, "The snw resistgraph and slab avalanche investigatins," Internatinal Sympsium n Scientific Aspects f S nw and Ice Avalanches, Publicatin N. 69 De L' Assciatin D'Hydrlgie Scientificue, Gentbrugge, Belgium. Dwd, T. and R. L. Brwn, 1986, "A new instrument fr determining strength prfiles in snw cver," Jurnal f Glacil~y, Vl. 32, N. 11l. Perla, R. 1. and M. Matrinelli, Jr., 1975, Avalanche Handbk, U. S. Department f Agriculture, Agriculture Handbk