Capital de trabajo

Cliff-forming materials and the type of slope failure are also crucial factors to determine the rate and the mode of cliff recession. Along the seashore in the Chengkung littoral cell, the Paliwan bedrock is usually overlain by marine deposits or alluvia (Figure 6.2B), with only exception at Sanhsientai headland where the Tuluanshan formation outcrops. In addition to sub-cell F3 - F5,

old CoastâT iJ ig h w a y -

•• H ig h w a y

marine deposits

fractures caves -

cell. Bedrocks hardly outcrop in those sub-cells where wider beaches are backed by lowlands. The Paliwan Formation is characterised by alternating thin layers o f finer- and coarser- grained sediment, consisting of fine-, medium-, coarse-silt or occasionally very fine sand layers. The overlying deposits are mainly nearshore-facies, beach-facies and subaerial-facies (Figure 6.2A). A particular gravel-rich coarse sandstone sub-stratum (Pis) outcrops in the central and southern stretch of the Chiaping segment (F5). It is, however, better illustrated in the geological map complied by Wang and Chen (1993; Figure 3.5).

6.3.1 Rock resistance

Generally speaking, the resistance of cliff-forming materials is determined by both the properties o f rock mass and the nature and pattern of weak planes (usually termed weak discontinuities in a civil engineering context). In the present study, rock samples were collected where the bedrock outcrops in this cell. All types of bedrock that outcrop in the cell were included in this analysis (i.e. siltstone, conglomerate, coral reef, tuff, tuff-rich breccia and breccia). The rock resistance (i.e. hardness and durability) was tested by the point-load strength test and a slake-durability test. X-ray diffraction was also undertaken to examine the composition of the clay minerals. The weak planes, orientation, spacing and persistence were also measured in the field. All the techniques and apparatus applied for the laboratory analysis and field measurement are described in section 2.3. In total, 61 samples were subjected to the point-load strength test (ISRM 1985); 67 samples were conducted with the slake- durability test (Frankli and Chandra 1972); 40 of them were processed by X-ray diffraction (XRD) and some o f them were also processed for grain-size analysis.

Rock types vj. hardness

The results of point-load strength test are illustrated in Table 6.7, Table 6 . 8 and Figure 6.7. As expected, andésite (Vo) and well-cemented tuff (Tu) are those with the highest point-load Index, (i.e. Is(50) 1> 4 MN m'^). Reef rock and sandstone constitute a second strength category, with all values lying in the range 1 ~ 4 MN m'^. The results for breccia and conglomerates are more varied. This seems to depend on the degree of cementation. In the case of the siltstones, more than two third (33 out of 41) of samples exhibited strength values of < 1 MN m'^, with no samples reading higher than 3 MN m'^. Accordingly, the contrast of the point-load strength index between the siltstone and non-siltstone group is very significant. For the former, more than 37% are categorised as ‘low strength’ and 46% as ‘medium strength’, while for the latter, 84% are as ‘high strength’. It is apparent that the rock hardness of the siltstone samples is much lower than that of the non-siltstone group.

L evel o f strength

Extrem. V ery Low

low low

M edium High Very

high Estrem. High L e v e l o f stren g th I II III IV V V I V II T o ta l Is(50) (M N /m 2) < 0.03 0 .0 3 -0 .1 0 .1 -0 .3 0 .3 -1 1 -3 3 - 1 0 > 1 0 S ilts t o n e 0 1 14 19 7 0 0 41

N ote: based on the op eratin g Instructions, Point Load A pparatus, EL 77 -0 1 1

N o n - s ilt s t o n e 0 0 ] 2 9 7 0 19

T o ta l 0 1 15 2 1 16 7 0 6 0

N ote: L evel I - VII stands for lo w to high p o in t-lo a d strength index (T ab le 6 .7 )

6.00 5.00 4 .0 0 3.00 2.00 .00 0.00

I I

I

I

i

i i

HI

i i i

i

_I

Figure 6.7: Results o f point-load strength test for samples collected from the Chengkung littoral cell (except those noted with "*") (Co: conglomerate. Re: coral reef. Sa: sandstone. Si: siltstone. Tu: tuff, B: (tuff-rich) breccia, Vo: andésite)

It must be noted that the difficulty o f collecting high-hardness rock samples with an ordinary geological hammer has impeded obtaining ‘fresh’ rock samples. Generally speaking, the less-resistant rock samples are usually ‘fresher’ than those more-resistant rock samples, which would be brought back for further test in the laboratory. Under such circumstance, the resistance of the ‘hard’ bedrock may be underestimated and the contrast between the results of the point-load test and slake-durability test between siltstone and non- siltstone group, although still very obvious, is less significant as it should be.

Rock types vs. durability

The slake-durability test is usually applied to measure the capability of geomorphic materials to resist weathering (Frankli and Chandra 1972). The percentage of rock weight retained after the test to the original sample is reported as slake-durability index {Id) (Goodman 1989). The second-cycle slake-durability index (7^^) proposed by Gamble (1971) and Franklin and Chandra (1972) is adopted here. The resulting figures (%) are categorised with Gamble's (1971) slake-durability classification (Table 6.9).

Significant contrasts in durability appear between the siltstone and non-siltstone group (Figure 6 . 8 and Table 6.10). Among the 21 non-siltstone samples, 50% belong to level VI and V (very high and high durability); only two (one conglomerate and the other breccia) are classified as level III (medium durability). In contrast, the durability of siltstone is more variable. Nearly half of the samples belong to the very-low-durability category (level I), however, one-third are classified as level VI (5 samples) and V (15 samples). On the whole, non-siltstone samples exhibit rather uniform resistance to weathering, while siltstone samples are less resistant on average although a few of them show very high durability.

The composition of the bedrock was also examined using X-ray diffraction. O f particular interest is to examine the existence o f the so-called expandable minerals, which can greatly reduce rock durability. The result is expressed in the index of ‘intensity’, which is the relative comparison of the proportion of expandable clay minerals (refer to section 2.3.2 for details). Analysis of 27 samples from the Chengkung cell shows that the expandable minerals are mainly mixed-layered mica/smectite and mixed-layered chlorite/smectite (for details see Appendix IX). Not surprisingly, the intensity of expandable minerals o f siltstone samples is usually higher than those non-siltstone samples on average, since the former contain a much higher percentage of clay-size particles. Among 13 non- siltstone specimens, only two conglomerate specimen have index figures higher than 6, while the index of 8 specimen among 14 siltstones are higher than 6 and half o f them are up

Level o f durability

Very low

Low M edium M edium

h i g h