Pore pres sure pro fil ing in Siercza land slide col lu vium in the Carpathian flysch us ing a Cone Pen e tra tion Test (CPTU)
Jacek STANISZ1, * and Zenon PILECKI2
1 AGH Uni ver sity of Sci ence and Tech nol ogy, Al. A. Mickiewicza 30, 30-059 Kraków, Po land
2 Pol ish Acad emy of Sci ences, Min eral and En ergy Econ omy Re search In sti tute, Józefa Wybickiego 7 A, 31-261 Kraków, Po land
Stanisz, J., Pilecki, Z., 2019. Pore pres sure pro fil ing in Siercza land slide col lu vium in the Carpathian flysch us ing a Cone Pen e tra tion Test (CPTU). Geo log i cal Quar terly, 63 (4): 839–848, doi: 10.7306/gq.1505
As so ci ate Ed i tor: Szymon Uœcinowicz
This study iden ti fies zones with sig nif i cant changes in pore wa ter pres sure in flu enced by land slide pro cesses. Mea sure - ments were taken in the near-sur face strata of the Carpathian flysch, in col lu vium of the Siercza land slide, near Kraków.
Mea sure ment of pore wa ter pres sure in flysch de pos its is com pli cated due to the strongly heterogeneous prop er ties of the me dium and by vari able wa ter con di tions, which are strongly in flu enced by rain fall in ten sity. Pore pres sure pro fil ing was per - formed in six se ries us ing a cone pen e tra tion test with a NOVA Acous tic cone. The tests were car ried out in the col lu vium to a depth of ~6.0 m un der vary ing wa ter con di tions. The cone pore pres sure re sults were com pared to re sults of in cli nom e ter mea sure ments in the re search area. Five zones with sig nif i cant dif fer ences in pore pres sure have been iden ti fied. Changes in both cone pore pres sure and in cli nom e ter dis place ment are ev i dent at a depth range from 1.5 to 2.5 m. Two slip sur faces are likely pres ent in this sec tion. Such in for ma tion can be used in en gi neer ing prac tice for more re li able as sess ment of slope sta bil ity in the Carpathian flysch.
Key words: cone pen e tra tion test, pore pres sure, in cli nom e ter dis place ment, land slide, col lu vium, slip sur face, Carpathian flysch.
INTRODUCTION
Pore wa ter pres sure pro vides es sen tial in for ma tion about the struc ture, prop er ties, and phys i cal-me chan i cal pro cesses oc cur ring in a geo log i cal me dium. One of the pos si bil i ties of its mea sure ments is the Cone Pen e tra tion Test (CPTU) (Lunne et al., 1997; Mayne, 2006; Rob ert son, 2009, 2016; Bajda et al., 2015; Tschuschke et al., 2015; Zawrzykraj, 2017; M³yna - rek et al., 2018). More over, anal y sis of pore pres sure changes may in di cate re la tion to the de vel op ment of the land slide pro - cess (Wang and Sassa, 2003; Take et al., 2004; Schnellmann et al., 2010; Cascini et al., 2013). Stud ies on this is sue have been con ducted in the area of the Carpathian flysch (e.g., Zabuski, 2004; Pilecki et al., 2007; Stanisz, 2013, 2015;
Bednarczyk, 2015; Harba and Pilecki, 2017; Stanisz et al., 2018; Kogut et al., 2018).
Some archil in for ma tion con cern ing stud ies of cone pore wa ter pres sure in the Carpathian flysch de pos its and the de vel - op ment of land slide pro cesses was re ported by Stanisz and Pilecki (2018).
Bednarczyk (2015) made pore pres sure mea sure ments in or der to iden tify slip sur faces in land slides de vel oped on flysch de pos its near Gorlice, south ern Po land. He de vel oped a mon i - tor ing sys tem con sist ing of pneu matic and vi brat ing wire sen - sors of pore pres sure, in cli nom eters, piezometers and an at mo - spheric sta tion. Bednarczyk (2015) noted that the de vel op ment of de for ma tion struc tures was as so ci ated with an in crease in pore pres sure in the range of 50–100 kPa. Pore pres sure in - crease was par tic u larly no tice able af ter in tense rain fall in ex - cess of 200–250 mm.
Zabuski et al. (2004) stud ied changes in wa ter level con tent in flysch de pos its rel a tive to the in ten sity and du ra tion of rain fall for sev eral land slides in the re gion of Szymbark, south ern Po - land. He used 25 open piezometers and sys tems of in cli nom - eters. Zabuski ob served that dis place ments and changes in wa ter level were re lated to the me chan i cal and hy drau lic prop - er ties of the geo log i cal me dium.
Sim i lar stud ies con cern ing land slides in the Szymbark re - gion were also made by Gil et al. (2009). They ob served that the ground wa ter level de creased dur ing brief rain fall ep i sodes of
* Corresponding author, e-mail: jacek.stanisz@gmail.com Received: Frebruary 6, 2019; accepted: May 15, 2019; first published online: December 20, 2019
<30 mm, while for more in tense rain fall >30 mm, the wa ter level in creased by 40–80 cm. An in crease in in cli nom e ter dis place - ment of mass move ment was also re lated to in tense rain fall.
Stanisz and Pilecki (2018) pro vided re sults of four pore pres sure pro files and in cli nom e ter mea sure ments in the near-sur face col lu vium of the Têgoborze – Just land slide per - formed in dry and wet con di tions. Pore pres sure pro fil ing was con ducted by CPTU in the near-sur face me dium up to about 5 m depth. In dry con di tions, the pore pres sure var ied from –82 to 48 kPa. In wet con di tions with a var ied de gree of sat u ra tion, the pore pres sure changed from –85 to 2 kPa. Sig nif i cant changes in pore pres sure val ues were mea sured in wet con di - tions as well as in dry con di tions. Four zones of dis tinct pore pres sure changes to a depth of ~5.0 m were de ter mined. In these zones, in cli nom e ter dis place ments of dif fer ent val ues were reg is tered. This in for ma tion al lows for the de ter mi na tion of weak zones in which the slip sur face of a land slide might de - velop.
This study iden ti fies zones with sig nif i cant changes in piezo cone pore wa ter pres sure in near-sur face strata of Carpa - thian flysch in flu enced by land slide pro cesses. The study was car ried out in the col lu vium of the Siercza land slide near Kraków in the south east ern part of Po land (Fig. 1). As con text, we de scribe the geo log i cal en gi neer ing con di tions in near-sur - face de pos its of the Carpathian flysch in the Siercza land slide.
The meth od ol ogy of pore pres sure pro fil ing with its lim i ta tions is dis cussed. Fi nally, the re sults of piezocone pore wa ter pres sure to gether with in cli nom e ter mea sure ments are ana lysed in terms of the iden ti fi ca tion of a po ten tial land slide slip sur face.
LOCATION AND GEOLOGICAL SETTING
The Siercza land slide is sit u ated on the north ern slope of Siercza Hill that has a long, flat pla teau (350–365 m a.s.l.). This area be longs to the north ern part of the mesoregion of the Wieliczka Foot hills, which ex tends W–E as a range of hills.
Land slide Reg is tra tion Card no. 819 was pre pared for the land - slide in an elec tronic da ta base (Wójcik, 2017).
The Siercza land slide is lo cated at the bound ary be tween the Carpathian flysch and the Pol ish Carpathian Foredeep.
This is the con tact zone of the overthrust of the Subsilesian Nappe onto youn ger, clayey Mio cene de pos its. The syn thetic geo log i cal pro file con sists of (Wójcik, 2017):
–var ie gated marls – Wêglówka marls (Up per Cre ta ceous–
Paleocene),
–sand stone and shale – Grodziskie beds and Up per Cieszyn shales (Valanginian–Hauterivian),
–sand stone, mudstone, shale, gyp sum, anhydrite and ha lite – Wieliczka suc ces sion (Mid dle and Up per Mio cene), –silty clay (Mio cene).
The land slide area is ~22.75 ha, ~621 m wide and 590 m long (Fig. 1A). The az i muth of the move ment di rec tion is close to 3°, and the av er age in cli na tion is 9°. The land slide has a com plex geo log i cal struc ture and it has been clas si fied as of slide type. Ac tu ally, the land slide up per part is con tin u ously ac - tive, while its lower part is in ac tive. The land slide tends to ex - pand to wards its up per part. This is es pe cially vis i ble in its west - ern part. The main scarp has a height of 1–7 m. The land slide is lo cated near to res i den tial build ings.
The ef fects of mass move ments are vis i ble in the area of the land slide as char ac ter is tic mor pho log i cal forms such as bulges, sec ond ary scarps, thresh olds, and bas ins with out out - flow (Fig. 1A, B2–B5). Fold ing and crack ing are also vis i ble on the sur face of the main road 2027K and road 2034K (Fig. 1A).
Along the road, there are cracks in the foun da tions and in the walls of build ings and fenc ing. In the cen tral part of the land - slide, there are many wa ter logged places. The land slide is ad ja - cent on its east ern side with an other pe ri od i cally ac tive land - slide (num ber 820).
The pre cise date of the land slide in cep tion is un known.
Wójcik (2017) re ported that it prob a bly formed dur ing the late gla cial pe riod or at the be gin ning of the Ho lo cene. Mass move - ments usu ally fol low in tense rain fall. The land slide was pe ri od i - cally ac ti vated in 1997, 2007, and 2008. The sat u ra tion of the col lu vium and plas tic be hav iour of the Mio cene silty clays and var ie gated marl lay ers were the main rea sons for mass move - ment. In 2006, soil was dumped in the area of the main scarp, which ad di tion ally loaded the land slide body. In con se quence, many frac tures oc curred along the main scarp. In 2013, fur ther mass move ments took place which ex posed the foun da tions of build ings lo cated near the main road 2034K (Fig. 1A).
In 2008, Geo log i cal Com pany (PG S.A. from Kraków) drilled 15 bore holes to a depth of 15 to 17 m for the geo log i cal en gi - neer ing char ac ter iza tion of the land slide (Jaskólski et al., 2008).
In each of the 12 bore holes se lected, an in cli nom e ter and piezometer were in stalled. The drill ing re sults showed that the flysch strata com prise shale with in ter ca la tions of thin-bed ded sand stones of the Lower Cre ta ceous and var ie gated marls of the Up per Cre ta ceous. These strata are of ten folded with black clays of the Chodenice de pos its (Mio cene). The Qua ter nary is rep re sented by silty clays and dumped soils, of var ied thick ness from 2.8 to 16.0 m in the up per part of the land slide. The col lu - vium con sists of silty-clay de pos its with thin sand stone in ter ca - la tions.
Based on in cli nom e ter mea sure ments, two slip sur faces have been dis tin guished. The first of these was iden ti fied at a depth of 2.5–6.0 m. The sec ond one was iden ti fied at a depth of 7.0–16.0 m (Fig. 2). In the up per part of the land slide, the deeper slip sur face is lo cated at a depth of ~13.6 m, and in the lower part it reaches a depth of 16.0 m (point I–6 in Fig. 2). In the lower parts of the land slide, the col lu vium has an es ti mated thick ness of ~20 m. Phys i cal and me chan i cal pa ram e ters in geo log i cal en gi neer ing terms are dis tin guished on the Siercza land slide are shown in Ta ble 1.
The land slide area is drained by the Serafa Stream, which is the right-bank trib u tary of the Drwina in the Vistula River ba sin.
There is no con tin u ous aqui fer within the col lu vium. Con sid er - able in fil tra tion oc curs within the clayey col lu vium and dumped soils. Wa ter path flows are vari able and de pend able on the oc - cur rence and in ten sity of rain fall as well as on the per me abil ity of the flysch de pos its.
METHODS
The pore wa ter pres sure pro fil ing was car ried out us ing a pen etrom eter with elec tri cal piezocone of NOVA Acous tic of AB Geotech (Swe den) (Fig. 3). The pore wa ter pres sure is mea sured via a po rous el e ment in stalled at the shoul der be - tween the cone tip and the fric tion sleeve (Fig. 3D). Pore wa ter pres sure val ues mea sured in the soil just be hind the cone tip can be dis tin guished from the pore wa ter pres sure in hy dro - static stress con di tions. An ex pla na tion of this ef fect can be given fol low ing Van Baars and Van de Graaf (2007). In a piezocone test, the sur round ing soil is com pressed by the pen - e tra tion of the cone tip, which cre ates ex cess pore wa ter pres - sure. At the same time, the shear ing of the soil by the cone tip may also re sult in dilatant be hav iour, which in duces a neg a tive ex cess pore pres sure. De pend ing on the rel a tive mag ni tudes of
824 Jacek Stanisz and Zenon Pilecki
Fig.1. Lo ca tion of re search area in the Siercza land slide A – map of the Siercza land slide, B1–B5 – land slide forms
the pore wa ter pres sure cre ated by the com pres sion and shear - ing, the re sul tant ex cess pore pres sure may be less or greater than the ini tial hy dro static pres sure. In clean sands and grav els, an es sen tially drained re sponse is ob served and mea sured pore wa ter pres sure is about hy dro static. In most other cases, an ini tial un drained re sponse oc curs that is fol lowed by drain - age. Once pen e tra tion has stopped, the ex cess pres sure will dis si pate with time and tends to the hy dro static value.
Us ing a piezocone we can also mea sure point re sis tance and sleeve fric tion. When the piezocone is pushed into the ground, the to tal force act ing on the cone in duces a cone re sis - tance qc. The to tal force act ing on the fric tion sleeve in duces the sleeve fric tion fs. Based on piezocone pa ram e ters, many meth - ods have been de vel oped for soil type de ter mi na tion (e.g., Senneset et al., 1989; Eslami and Fellenius, 2004). Among them, Rob ert son et al. (1986) in tro duced a pore pres sure ra tio Bq as fol lows:
B u u
q q
t
= -
-
2 0
sv
[1]
where: u2 – pore pres sure mea sured at cone shoul der, u0 – hy dro - static (ini tial) pore wa ter pres sure, qt – cone re sis tance cor rected for pore wa ter pres sure on shoul der, sv – to tal over bur den ver ti cal stress.
The pore pres sure ra tio Bq al lows for the de ter mi na tion of soil type based on a com bi na tion of piezocone pa ram e ters as shown in Fig ure 4. In our study we would con sider only piezocone pore pres sure in the con text of de gree of sat u ra tion and wa ter path flows in the land slide body. Nev er the less, we show the re la tion ship be tween the piezocone pa ram e ters to in - di cate how com plex it is. Anal y sis of the point re sis tance and sleeve fric tion re sponse is be yond the scope of this study.
826 Jacek Stanisz and Zenon Pilecki
Fig. 2. Lon gi tu di nal geo log i cal en gi neer ing cross-sec tion of the Siercza land slide (based on Jaskólski et al., 2008)
Number of geol-eng.
layer
Bulk density [Mg/m ]3
Cohesion
[kPa]
Angle of internal friction
[deg]
Type of geological-engineering
layer
Plasticity index
IIa I
IIIa IIb
IIIb IIIc
soil and rock colluvium: grSi plastic 1.97 45 11
soil colluvium: clSi plastic 1.96 45 10
soil and rock
colluvium: clGr semi-solid 2.10 110 12
plastic/liquid 1.90 10 5
soil and rock
colluvium: Cl+gr 2.08 55 6
soil and rock
colluvium: shales 1.90
soil and rock colluvium: grSi
50 5
plastic plastic
Permeability coefficient
[m/s]
1.79·10-8
–
1.42·10-9
8.13·10-10
1.43·10-9
– T a b l e 1 Phys i cal and me chan i cal pa ram e ters in geo log i cal en gi neer ing lay ers in the area of the Siercza land slide
(based on Jaskólski et al., 2008)
clSi – clayey silt, grSi – grav elly silt, clGr – clayey gravel
The study was con ducted in the cen tral part of the Siercza land slide near the main road 2027K, and near point I–6 (Fig. 5).
This point con sists of two bore holes used for piezometric and in cli nom e ter probes. The in cli nom eters had two com po nents: X in the per pen dic u lar and Y in the par al lel di rec tion to the axis of the land slide. In the cen tral part of the re search area, a bore - hole S-1 was made to a depth of 5 m (Fig. 5). In its vi cin ity, 6 CPTU mea sure ments were made out to a depth of ~6 m. Dur - ing the CPTU test, the wa ter ta ble was mea sured in each of piezometers P1–P5 (Fig. 5A, B).
The mea sure ment pro ce dure con sisted of (Stanisz and Pilecki, 2018):
–pre par ing the mea sur ing set for work – an chor ing and lev el - ling;
–pre par ing a de-aer ated po rous fil ter be fore mea sure ment.
The fil ter was pro tected in a con tainer filled with glyc erol.
Vent ing the mea sur ing cham ber and fil ter as sem bly. Con - nect ing the trans mit ter with the cone and se cur ing the tip with a thin in su lated hous ing;
–Per form ing a “zero” mea sure ment;
–Pro fil ing of the pore wa ter pres sure. Us ing the piezocone we also mea sured point re sis tance and sleeve fric tion;
–Per form ing a “zero” mea sure ment af ter pro fil ing.
The mea sure ment pro ce dure was re peated us ing a new po - rous fil ter each time. The mea sure ment of the pore pres sure was car ried out ev ery 2 cm. The ve loc ity of piezocone ad vance - ment was ~2 cm/s.
Six field mea sure ments were car ried out in the pe riod 2017–2018. The first three were made in dry con di tions – 1a (17 No vem ber, 2017), 2a (8 No vem ber, 2018) and 3a (12 No vem - ber, 2018). The next three were car ried out un der sat u rated con di tions – 1b (20 Sep tem ber, 2017), 2b (12 July, 2018) and 3b (2 No vem ber, 2018). A day be fore mea sure ment 1b, there was in tense rain fall of 336 mm. One day and two days ear lier, it was 2 and 7 mm, re spec tively. Early in the morn ing of 12 July, 2018 (mea sure ment 2b), there was rain fall of 42 mm. The day be fore it was 46 mm. The last mea sure ment (3b) was made Fig. 3. CPTU probe used in mea sure ments (phot. Stanisz and Pilecki, 2018)
A – gen eral view, B – el e ments of the mea sur ing set, C – CPTU probe, D – CPTU piezocone with elas tic hous ing
when the rain fall was low. On No vem ber 2, 2018 it was 2.6 mm, while on the pre vi ous day it was 0.7 mm.
Due to the spe cif ics of the mea sure ment of piezocone pore wa ter pres sure, we ex pected to ob tain in for ma tion con cern ing the lo ca tion of greater sat u ra tion zones or wa ter flow paths. The zones iden ti fied may be re lated to the de vel op ment of slip sur - faces in the geo log i cal me dium. The his tory of sat u ra tion and in so la tion have not been in cluded in the anal y sis.
RESULTS AND ANALYSIS
The re sults of the in cli nom e ter mea sure ments made it pos - si ble to de ter mine the lo ca tion of the slip sur face. At point I–6, ev i dent slip sur faces oc cur at depths of ~2.5 and 16 m. Smaller dis place ments were also ob served at depths of 8.5 and 12 m.
The larg est in creases in in cli nom e ter dis place ments with a max i mum value of 1.1 cm oc curred at a depth of 2.0 m in the NNE di rec tion.
Piezometric mea sure ments showed that the depth of ground wa ter level var ies from 0.4 m to ~1.9 m. In dry con di - tions, the depth of ground wa ter var ied from 1.5 to 1.9 m, and in sat u rated con di tions the depth var ied from 0.4 to 0.6 m.
The re sults of the piezocone pore pres sure pro fil ing to - gether with in cli nom e ter dis place ment mea sure ments are shown in Fig ure 6. Fig ure 6A shows six curves of piezocone pore pres sure pro fil ing and a curve of their rel a tive pore wa ter pres sure u2R. This pa ram e ter u2R was cal cu lated as the nor mal - ized sum of all ab so lute val ues of pore pres sure to to tal ver ti cal stress, sim i larly to geo phys i cal anom aly (Szreder et al., 2008), as fol lows:
u i u
i
R i
2
2 100
= -
å
× ss
v v
% [2]
where: u2i – pore wa ter pres sure mea sured at a given depth, i – num ber of pore wa ter pres sure mea sure ments at a given depth, sv – to tal ver ti cal stress at a given depth.
The pa ram e ter uR2 de fines how great is the pore pres sure anom aly mea sured in dif fer ent con di tions com pared to to tal ver ti cal stress and ex pressed as a per cent age.
Fig ure 6B shows curves of the pa ram e ter u2R and in cli nom e - ter dis place ment in two per pen dic u lar di rec tions. The dis place - ment was mea sured af ter each pore pres sure mea sure ment at the I/6 mea sur ing point. Fig ure 6B also shows the the o ret i cal curve of max i mum hy dro static pres sure changes with depth based on piezometer mea sure ments.
In gen eral, the piezocone pore wa ter pres sure for all six mea sure ments ranged from 102.2 to 59.7 kPa. The max i mum pos i tive value of the pore pres sure was reg is tered in the near-sur face zone (up to 10 cm) and in the up per part of geo log - i cal en gi neer ing layer I. The max i mum neg a tive value was re - corded in dry con di tions at a depth from 3 to 6 m. In this zone, there are clays or clayey gravel in a semi-solid con di tion. The great est val ues of in cli nom e ter dis place ment were re corded at a depth of 2 m.
The re sults of piezocone pore pres sure mea sure ments were ana lysed for sig nif i cant pos i tive or neg a tive changes. We as sumed that such anom a lous changes may be as so ci ated with sig nif i cant changes in wa ter con tent and may in di cate the pres ence of weak zones. In both Fig ure 6A, B, five zones of anom a lous changes in pore pres sure can be dis tin guished.
These are num bered from 1 to 5:
–Zone 1 – this was rec og nized at a depth of 1.5–1.7 m (Fig.
6A). In this zone, an in crease in the piezocone pore pres - sure value was clear. In dry con di tions it reached a max i - mum value of 59.7 kPa. In sat u rated con di tions it reached 48.5 kPa af ter a rain fall of 336 mm. This in crease in the pore pres sure value was ob served in two mea sure ments. In the re main ing four cases, both for dry and sat u rated con di tions, an in crease in pore pres sure in the range of 1.8–8.0 kPa was mea sured. In zone 1, silty clay with rock frag ments in a plas tic con sis tency was pen e trated. This layer was also char ac ter ized by the great est in crease in dis place ment of the X (10 mm) and Y (5 mm) di rec tions. Pre sum ably, this zone con tained an ac tive slip sur face; it was also doc u - mented by Jaskólski et al. (2008).
–Zone 2 – this was rec og nized at a depth of 2.0–2.3 m. For two mea sure ments, a small in crease in pore pres sure was reg is tered. In dry con di tions, it reached a value of 9.6 kPa.
Af ter rain fall of 2.6 mm, the in crease reached a value of 42.3 kPa. In this zone, in cli nom e ter dis place ment took place in the di rec tion down the slope and reached a value of
~2 mm. In this zone, the in crease in pore pres sure was as - so ci ated with the oc cur rence of clay with sand stone frag - ments and var ied with dif fer ent wa ter flow paths.
–Zone 3 – this oc curred at a depth of 2.5–2.6 m. In this zone, an in crease in the piezocone pore pres sure was reg is tered for five mea sure ments. In dry con di tions, the pore pres sure was in the range of –2.2–24.7 kPa. In sat u rated con di tions, the pore pres sure was 31.7 (rain fall at value 46 and 42 mm) – 38.8 kPa (rain fall at value 336 mm). Ar chi val study showed that at this depth the slip sur face in the up per part of the clay layer with the rock frag ments in the plas tic state was oc curred (Jaskólski et al., 2008). This was not clearly con - firmed by in cli nom e ter mea sure ments.
–Zone 4 – this oc curred at a depth of ~3.0 m. In clay in a semi-solid/plas tic con di tion, a suc tion pres sure of –60 kPa was reg is tered. For four mea sure ments, there was an in - crease in pore pres sure in the range of 3.9 to 9.8 kPa for sat u rated con di tions, and –6 to –2 kPa for dry con di tions. In this zone there were small in cli nom e ter dis place ments up to 1 mm for the X di rec tion.
828 Jacek Stanisz and Zenon Pilecki
Fig. 4. Clas si fi ca tion of soil type on the ba sis of the pore pres sure ra tio Bq and cone re sis tance qt
(Senneset et al., 1989) All sym bols are ex plained in the text
Fig. 5. Lo ca tion of re search area in the Siercza land slide with mea sure ment points
A – part of the land slide map, B – lo ca tion of the mea sure ment points
830 Jacek Stanisz and Zenon Pilecki
Fig. 6. Graphs of piezocone pore wa ter pres sure pro files and in cli nom e ter dis place ment along geo log i cal en gi neer ing pro file S-1
A – re sults of six mea sure ments of pore pres sure and av er age of pore wa ter pres sure u2
in dry and sat u rated con di tions; B – re sults of in cli nom e ter dis place ment and rel a tive pore wa ter pres sure uR2; for other ex pla na tions see Fig ure 2
–Zone 5 – this oc curred at a depth of ~3.6 m. Suc tion pres - sure dom i nated in this zone. In dry con di tions, it was from –100 to –65.4 kPa. In sat u rated con di tions, it ranged from –71.8 to –46.1 kPa. The ex cep tion was one mea sure ment, in which there was an in crease in the pore pres sure of 22.7 kPa af ter rain fall of 2.6 and 0.7 mm. Pre sum ably, this in - crease was re lated to the pres ence of clay with shale and sand stone frag ments. In zone 5 there was no in cli nom e ter dis place ment.
Sig nif i cant anom a lies of pore pres sure are ob served in the near-sur face zone at a depth of up to ~1 m, shown in yel low in Fig ure 6B. The cause of these anom a lies may be the high sen - si tiv ity of pore pres sure to rain fall in this zone. The root sys tems of veg e ta tion may also have sig nif i cance re gard ing the ran dom dis tri bu tion of pore pres sure.
CONCLUSIONS
Mea sure ment of piezocone pore wa ter pres sure in the strongly het er o ge neous col lu vium of the Carpathian flysch is dif fi cult to per form and in ter pret. These dif fi cul ties arise from un sta ble wa ter level, strongly change able per me abil ity and com plex wa ter flow paths in a me dium con tain ing rock frag - ments. Wa ter level are also strongly de pend ent on the oc cur - rence and in ten sity of rain fall.
Based on the piezocone pore pres sure and in cli nom e ter dis place ment mea sure ments in the col lu vium of the Siercza land slide, the fol low ing con clu sions may be drawn:
–CPTU tests en abled pore pres sure pro fil ing up to a depth of 6 m. In dry con di tions, the pore pres sure value ranged from
–102.2 to 59.7 kPa. For vary ing wa ter sat u ra tion, pore pres - sure value ranged from –84.2 to 48.5 kPa.
–Five zones of anom a lous changes in pore pres sure were de ter mined, as shown in Fig ure 6. These zones were partly con firmed by in cli nom e ter dis place ments that oc curred up to a depth of 3.5 m with a max i mum of ~10 mm at a depth from 1.5 to 2.5 m.
–Changes in both piezocone pore pres sure and in cli nom e ter dis place ment are ev i dent at a depth range from 1.5 to 2.5 m (zone 1–3). Two slip sur faces are prob a ble in this sec tion (Fig. 6B).
–In the re main ing sec tions, char ac ter is tic changes in pore pres sure were reg is tered, but no dis tinct in cli nom e ter dis - place ment oc curred. Prob a bly, the de for ma tion pro cess has just started in such zones.
The re la tion ship be tween the change in pore wa ter pres - sure and the de vel op ment of land slide pro cesses is com plex.
Piezocone pore pres sure pro fil ing may be use ful to more re li - ably de ter min ing the lo ca tion of the slip sur face. This in for ma - tion can be used in en gi neer ing prac tice for more re li able as - sess ment of slope sta bil ity in the Carpathian flysch.
The study pro vides only qual i ta tive in for ma tion about this re la tion ship and fur ther re search within dif fer ent Carpathian flysch set tings is re quired.
Ac knowl edge ments. We are thank ful to J. Satkunas, an anon y mous re viewer and ed i tors of Geo log i cal Quar terly for valu able com ments. The re search was sup ported by the Fac - ulty of Ge ol ogy, Geo phys ics and En vi ron men tal Pro tec tion, AGH Uni ver sity in Kraków, Pro ject no. 11.11.140.649
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832 Jacek Stanisz and Zenon Pilecki
