Influence of faults on mineralogy and geotechnical characteristics of Tropical Volcanic Residual Soils (TVRS) concerning environmental issues and regional development, Case Study: West Lampung Transect Road, Sumatra, Indonesia

Hydrothermal alteration occurs in andesitic rocks in the study area due to the presence of the Great Sumatran Fault. This study shows the consequences of this phenomenon on the characteristics of volcanic residual soils. This study comprises volcanic residual soils formed from the same andesitic rocks in a site located in West Lampung, Sumatra, Indonesia which has a tropical climate. Chemical, physical, and mineralogical characterizations were carried out to describe the materials and the changes imposed by their alteration. Effects of alteration on the soil’s behavior were assessed in terms of plasticity.


Introduction
The West Lampung Districts have complicated geological conditions due to lithotype heterogeneity and a variety of structural characteristics [1].The Great Sumatran Fault, which controls the geomorphology, runs through the study area [2].The occurrence of minor faults and hydrothermal alteration along the Great Sumatran Fault as a result of this regional geotectonic genesis.The characteristics of the volcanic residual soils that exist in the area are highly varied in this situation.Such heterogeneities impose uncertainty in the design of geotechnical models and the prediction of soil behavior.Fault-induced heterogeneities create uncertainty that may go unnoticed without a thorough engineering geology study [3].
This article demonstrates that the fault causes changes that affect the pedogenesis and geotechnical properties of the surrounding material.Soils from the same region were studied for this purpose: tropical volcanic residual soils (TVRS).This study hypothesizes that the Tropical Volcanic Residual Soil (TVRS) has changed its characteristics because the source rock has undergone alteration due to its proximity to the fault.The stress history is not predicted to have a major impact on the soils, because the influence of the stress history is irrelevant to residual soils, according to the research assumptions ( [3]; [4]; [5]; [6]).
The development of volcanic residual soils spanned by fault zones is common along the Liwa-Sumberjaya West Road in West Lampung, Sumatra, Indonesia.Because this zone is closely related to The aim of this study is: a.
Characterizing volcanic residual soil using geotechnical and XRD methods.b.
Explain the influence of faults on the characteristics of tropical volcanic residual soils.

Studied Area
The research area is in a tropical (wet tropical climate category) [7].The average temperature of West Lampung is 24.5oC, with an average humidity level of 86 % [8].The region of West Lampung and its environs is located in the west of the Sumatra island.This location has a rugged and hilly terrain with steep to extremely steep slopes and is located 800 -1100 meters above sea level [7].The research location is located in a region prone to landslides and earthquakes ( [1]; [9]).
Based on the Kotaagung Regional Geological Map [10] and the Baturaja Regional Geological Map [11] (Figure 1) the research area is made up of the following rock formations (from old to young): 1. Hulusimpang Formation (Tomh) Volcanic breccias and andesite-basalt tuff make up the Hulusimpang Formation.It is the oldest rock formation in the research area.This formation has features of quartz veining.
The formation is Miocene in age and was deposited in a terrestrial environment.This formation weathered into brown-red brown soil with sand-clay grains.

Ranau Formation (Qtr)
The Ranau Formation consists of pumice breccia, mica tuff, pumice tuff, and uncoated sand deposits.The Ranau Formation is Pliocene-Pleistocene in age, deposited in a terrestrial environment, and is located inconsistently above the Hulusimpang Formation.This formation weathered into white to brown sand-clay soil.3. Gunungapi Sekincau Formation (Qhv) Breccia, lava, and tuff with andesite-basalt composition are the constituent rocks of the Sekincau Volcano Formation.This formation is Holocene in age, deposited in a terrestrial environment, and lies unconformably above the Ranau Formation.This formation is composed of weathered soil with sand-clay grains and is brownish-red in color.
Figure 1.West Lampung regional geology and sampling map (modified from Iqbal [37], Muslim [12] and www.big.go.id accessed on July 05, 2023 [13]) Indarto investigated the Southern Bukit Barisan Mountains [14].This study finds that the rocks of the Bukit Barisan Selatan Mountains, particularly the Hulusimpang Formation, have been hydrothermally altered to create propylitic, phyllic, and argillic zones.The presence of chlorite, carbonate, epidote, and ore minerals characterizes the propylitic zone.Sericite, adularia, and silica are minerals found in the phyllic zone.The argillic zone is distinguished by the presence of kaolinite and montmorillonite clay minerals.

Material and Methods
Linearity and/or fault mapping is carried out using remote sensing methods by utilizing ALOS-Palsar imagery and geographic information systems.Fault withdrawal is based on technical interpretation in the form of the straightness of the ridge and the presence of sharp river bends combined with regional geological maps.To find out the distance between the observation point and the fault, measurements are made using a ruler in GIS software (ArcGIS/MapInfo) by drawing a perpendicular line from the observation point to the fault ( [15]; [16]; [17]) Samples of disturbed and undisturbed residual soils here are referred to as tropical volcanic residual soils (TVRS).A sample tube and 1kg plastic bag were used to collect up to 12 samples along the Liwa-Sumberjaya West Cross Road in West Lampung.The soil was then tested at the Geological Engineering laboratory, Padjadjaran University, Indonesia, to determine physical properties such as grain size (ASTM D422-63) (ASTM, 2007), Atterberg limit (ASTM D4318 -17e1) (ASTM, 2017), soil type based on ASTM and USCS standards (ASTM D2487-17) (ASTM, 2017), soil activity (AI) [18] and soil plasticity ( [19]; [20]).
Chemical analysis of soils was carried out at the Mineral Chemistry Laboratory at the Research Center for Geotechnology, LIPI, Bandung, Indonesia, utilizing an x-ray diffractor (Shimadzu 7000 tool), SIROQUANT database software [21] and the ICDD PDF-4 Minerals 2018 RDB.The study was carried out to establish the kinds of clay minerals and their abundance.The soil sample utilized to assess the kind of clay mineral in the soil was clay particle size soil (passes through sieve No. 200).Without treatment, samples were examined with diffraction angles (2 Theta) ranging from 10 to 40 degrees.

Geological Fault Interpretation
Based on image analysis and field verification, it is known that the pattern of lineaments and/or faults in the study area is southeast-northwest (Figures 2 a-b; and 3 a-c).This pattern tends to follow the general pattern of the Great Sumatran Fault.

Discussion
The study area is traversed by the active Suoh-Komering Sumatra Fault.This fault is relatively trending northwest-southeast.The Sumatran Fault plays an important role in shaping the terrain of the study area in the form of faults with a direction relatively similar to the direction of the Sumatran Fault which is interpreted as a follow-up fault, namely the Way Robok Fault, Liwa Fault, Sukabumi Fault, and Kenali Fault ( [10]; [11]; [27]; [2]; [1]; [28]; [29]).These faults contribute to changing the rock and soil characteristics of the study area.Based on the geochemical soil data of the study, literature review, and Iqbal [37], the rock in the study area has undergone hydrothermal alteration to form a propylitic-argillic zone [30].
According to the study's findings, there are discrepancies in the analysis of soil plasticity between the Casagrande chart [31] and Prakash and Jain, Jang and Santamarina, and Santamarina ( [22]; [19]; [20]).This becomes intriguing and will be addressed using a geotechnical and geological approach concerning environmental issues and regional development.
According to the findings of the research and a review of the literature, TVRS is a consequence of the weathering of andesitic volcanic rocks.TVRS is not transported and is located near an active fault.TVRS near the fault (+ 300-700 m) (TVRS 1) (sample code X25, S2, S3, DS11, and S4) has low to high plasticity characteristics.While TVRS far from the fault ( > 700 m) (TVRS 2) (sample code S5, X24, X20, X19, S9, S10, G05) has low to medium plasticity characteristics.This shows that faults in the study area influence changes in soil characteristics.
The case that occurred in West Lampung can be explained as follows.Faults will cause weak zones and fractures to form in the source rock [32].This zone causes the fluid around the source rock to get trapped in it and change the chemical composition of the rock [33].Over time, the source rock undergoes a weathering process and becomes soil with various kinds of clay minerals in it.Soils formed close to faults will have a more heterogeneous clay mineral composition than soils formed far from faults.Because the composition of clay minerals is more heterogeneous, soils formed close to faults are more plastic than soils formed far from faults.
It has the following relationship to environmental issues and regional development: due to its characteristics, TVRS 1 is prone to erosion and easily slides [38].This soil has a high swelling capacity and a dispersive property [39].These characteristics imply that these soils may cause soil degradation, as well as negative environmental and economic consequences [34].TVRS 1 is better suited to primary forests, production forests, and conservation areas.Furthermore, if the area is used as a residential area, geological-geotechnical engineering problems must be considered, and construction must be done with extreme caution and following building codes [40].
Meanwhile, TVRS 2 with its characteristics will be more suitable to be used in residential areas, plantations, and for building foundations [37].The area can be considered as an appropriate area, referring that the area receives little potential natural disaster risk, or it can be said that the area does not have geological-geotechnical engineering problems ( [35]; [36])
While TVRS far from the fault (> 700 m) (TVRS 2) (sample code X24, X20, X19, S9, S10, G05) has low to medium plasticity characteristics.The first type (TVRS 1) is better suited to primary forests, production forests, and conservation areas.If the area is used as a residential area, geologicalgeotechnical engineering problems must be considered, and construction must be done with extreme caution and following building codes.The second type (TVRS 2) is more suitable for residential areas, plantations, and building foundations.It can be considered an appropriate area, referring that the area receives little potential natural disaster risk, or it can be said that the area does not have geologicalgeotechnical engineering problems.

4 Figure 2 .Figure 3 .Figure 4 .
Figure 2. The alignment pattern of the geological structure of the study area (red box) interpreted from ALOS-Palsar imagery (a.HV and b.HH) slope instability, variability of comparable soil characteristics may occur throughout the region, posing a challenge to engineering practice.